Tropospheric aqueous-phase chemistry: kinetics, mechanisms, and its coupling to a changing gas phase

Multiphase Buffering: A Mechanistic Regulator of Aerosol Sulfate Formation and Its Dominant Pathways

Sulfate formation in the aerosol aqueous phase represents a pH-sensitive atmospheric chemical process, with the formation pathways significantly influenced by the fluctuations in aerosol acidity. Buffer capacity, stemming from conjugate acid-base pairs, can resist pH changes in aerosol multiphase systems under external perturbations. However, the regulating role of multiphase buffering in pH-dependent aqueous sulfate formation mechanisms remains unexplored. Here, we propose that multiphase buffering can stabilize aerosol pH and further regulate dominant sulfate formation pathways. In this work, we delve into the instantaneous buffer capacity β and sulfate formation pathways based on field observation and theoretical calculation and further introduce the total buffer capacity α in the aerosol multiphase system to quantify the buffer-constrained pH change after the external acid/base variation during the entire buffering process. The NH4+/NH3 agent (average β 30.8 mol kg-1) shows a superior buffering effect in stabilizing aerosol pH and regulating sulfate formation pathway transition compared with the HNO3/NO3- agent (average β 15.1 mol kg-1). Geos-Chem simulation and machine learning results also validate the buffer capacity as a pivotal factor in sulfate formation. In addition to reactants, the buffer agents and acid/base can also be factors of concern for the sulfate formation mechanism. The diverse sensitivities to acid/base variation and the region-specific responses to pH change can provide insight into regulating acid and base emission measures, modulating regional aerosol acidity, and understanding pH-related atmospheric chemical processes.

Influence of Surfactants with Differently Charged Headgroups on the Surface Propensity of Bromide

Halide ions in oceans and sea-spray aerosol particles are an important source of reactive halogen species in the atmosphere that impact the ozone budget and radiative balance. The multiphase cycling of halogen species is linked to the abundance of halide ions at the aqueous solution-air interface. Ubiquitously present surface-active organic compounds may affect the interfacial abundance of halide ions. Here, we use liquid jet X-ray photoelectron spectroscopy and molecular dynamics (MD) simulations to assess the impact of surfactants with different headgroups on the abundance of bromide and sodium ions at the interface. Core level spectra of Br 3d, Na 2s, and O 1s are reported for solutions containing tetrabutylammonium, hexylamine (HA), and propyl sulfate. We used a photoelectron attenuation model to retrieve the interfacial concentration of bromide in the presence of these different surfactants. The experimental results confirm the previously reported strong enhancement of bromide in the presence of tetrabutylammonium at the interface. In turn, propyl sulfate had a minor impact on the abundance of bromide but led to a significantly enhanced concentration of sodium cations. The MD simulations performed for bromide solutions containing hexylammonium and propyl sulfate show an enhancement of the interfacial bromide and sodium concentrations, respectively, comparable to the experimental results. The difference between the measured enhancement of bromide for HA and the nearly nonexistent effect of HA on bromide in the MD simulations is ascribed to the small amounts of hexylammonium present in the experimental solution. The present work suggests an important role of electrostatic interactions at the interface, which may guide the assessment of anion and cation abundances in atmospheric particles more generally.

Simulating Droplet-Resolved Haze and Cloud Chemistry Forming Secondary Organic Aerosols in Turbulent Conditions within Laboratory and Cloud Parcels

Most models do not explicitly simulate droplet-resolved cloud chemistry and the interactions between turbulence and cloud chemistry due to large associated computational costs. Here, we incorporate the formation of isoprene epoxydiol secondary organic aerosol (IEPOX-SOA) in individual droplets within a one-dimensional explicit mixing parcel model (EMPM-Chem). We apply EMPM-Chem to simulate turbulence and droplet-resolved IEPOX-SOA formation using a laboratory cloud chamber configuration. We find that the dissolution of IEPOX gases is weighted more toward larger cloud droplets due to their large liquid water content (compared to smaller droplets), while the conversion of dissolved IEPOX to IEPOX-SOA is much greater within smaller deliquesced haze particles due to their higher acidity and ionic strengths compared to cloud droplets. We also apply the EMPM-Chem model to simulate how IEPOX-SOA formation evolves in individual cloud droplets within rising cloudy parcels in the atmosphere. We find that as subsaturated air is entrained into and turbulently mixed with the cloud parcel, evaporation causes a reduction in droplet sizes, which leads to corresponding increases in per droplet ionic strength and acidity. Increased droplet acidity, in turn, greatly accelerates the kinetics of IEPOX-SOA formation. Our results provide key insights into single cloud-droplet chemistry, suggesting that entrainment mixing may be an important process that increases SOA formation in the real atmosphere.

A comparative study on the formation of nitrogen-containing organic compounds in cloud droplets and aerosol particles

Nitrogen-containing organic compounds (NOCs) may potentially contribute to aqueous secondary organic aerosols, yet the different formation of NOCs in aerosol particles and cloud droplets remains unclear. With the in-situ measurements performed at a mountain site (1690 m a.s.l.) in southern China, we investigated the formation of NOCs in the cloud droplets and the cloud-free particles, based on their mixing state information of NOCs-containing particles by single particle mass spectrometry. The relative abundance of NOCs in the cloud-free particles was significantly higher than those in cloud residual (cloud RES) particles. NOCs were highly correlated with carbonyl compounds (including glyoxalate and methylglyoxal) in the cloud-free particles, however, limited correlation was observed for cloud RES particles. Analysis of their mixing state and temporal variations highlights that NOCs was mainly formed from the carbonyl compounds and ammonium in the cloud-free particles, rather than in the cloud RES particles. The results support that the formation of NOCs from carbonyl compounds is facilitated in concentrated solutions in wet aerosols, rather than cloud droplets. In addition, we have identified the transport of biomass burning particles that facilitate the formation of NOCs, and that the observed NOCs is most likely contributed to the light absorption. These findings have implications for the evaluation of NOCs formation and their contribution to light absorption.

T- and pH-Dependent Hydroxyl-Radical Reaction Kinetics of Lactic Acid, Glyceric Acid, and Methylmalonic Acid in the Aqueous Phase

Carboxylic acids are a common class of compounds found in atmospheric aerosols and cloud droplets. In this study, the oxidation kinetics of several carboxylic acids in the aqueous phase by the atmospherically relevant •OH radical were investigated to better understand the loss processes for this class of compounds. The rate constants for the reactions of the •OH radical were determined using the thiocyanate competition kinetics method for lactic acid, glyceric acid, and methylmalonic acid as a function of temperature and pH. The Arrhenius equations for oxidation by the •OH radical are as follows (unit in L mol-1 s-1): For lactic acid: k(T, HA) = (1.3 ± 0.1) × 1010 × exp[(-910 ± 160 K)/T] and k(T, A-) = (1.3 ± 0.1) × 1010 × exp[(-800 ± 80 K)/T]; for glyceric acid: k(T, HA) = (6.0 ± 0.2) × 1010 × exp[(-1100 ± 170 K)/T] and k(T, HA±) = (3.6 ± 0.1) × 1010 × exp[(-1500 ± 100 K)/T]; and for methylmalonic acid: k(T, H2A) = (5.5 ± 0.1) × 1010 × exp[(-1760 ± 100 K)/T], k(T, HA-) = (1.4 ± 0.1) × 109 × exp[(-530 ± 80 K)/T] and k(T, A2-) = (9.6 ± 0.4) × 1010 × exp[(-1530 ± 270 K)/T]. The general trend of the •OH rate constant was observed kA2- > kHA- > kH2A. The energy barriers of the •OH radical reaction and thus the most probable site of H atom abstraction were calculated using density functional theory simulations in Gaussian with the M06-2X method and the 6-311++G(3df,2p) basis set. Kinetic data predicted from structure-activity relationships were compared to the measured •OH radical rate constants. •OH radical oxidation in the aqueous phase could serve as an important sink for carboxylic acids, and the pH- and T-dependent rate constants of •OH radical reactions provide a better description of the aqueous-phase sink processes. Hence, the atmospheric lifetime as well as the partitioning of the investigated carboxylic acids was calculated.

Atmospheric Hydroxyl Radical Route Revealed: Interface-Mediated Effects of Mineral-Bearing Microdroplet Aerosol

Hydroxyl radical (·OH) plays a crucial role in atmospheric chemistry, regulating the oxidative potential and aerosol composition. This study reveals an unprecedented source of ·OH in the atmosphere: mineral dust-bearing microdroplet aerosols. We demonstrate that Kaolin clay particles in microdroplet aerosols trigger rapid ·OH production upon solar irradiation, with rates reaching an order of at least 10-3 M s-1. This production rate is several orders of magnitude higher than that of the bulk phase (2.4 × 10-11 M s-1) and previously known pathways. On this basis, the surface-based interfacial ·OH production rate is estimated to be 8.9 × 10-5 mol m-2 s-1 at the air-water-solid interface of 1 μm sized aerosol particles. The enhanced ·OH formation is attributed to the unique features of air-water-solid interfaces, where the lifespan of photoinduced holes was significantly increased due to the presence of strong electric fields at the air-water interface. We further investigated the impacts of various environmental factors and aerosol properties on ·OH production, including light intensity, relative humidity, particle size, and pH. Our findings provide new insights into atmospheric photochemical processes mediated by mineral dust-bearing microdroplet aerosols, which are important contributors to ·OH source in the atmosphere. This work advances our understanding of atmospheric interfacial chemistry and its profound and lasting implications for air quality and climate.

Carbon Nanoparticle Oxidation by NO2 and O2: Chemical Kinetics and Reaction Pathways

Carbon nanoparticle interactions with gases are central to many environmental and technical processes, but the underlying reaction kinetics and mechanisms are not well understood. Here, we investigate the oxidation and gasification of carbon nanoparticles by NO2 and O2 under combustion exhaust conditions. We build on a comprehensive experimental data set and use a kinetic multilayer model (KM-GAP-CARBON) to trace the uptake and release of gas molecules alongside the temporal evolution of particle size and surface composition. The experimental results are captured by a model mechanism that involves different types of carbon atoms (edge/plane-like) and the formation of a reactive oxygen intermediate (activated CO complex) as the rate-limiting step. A transition between distinct chemical regimes driven by NO2 at lower temperatures and O2 at higher temperatures is reflected by an increase in the observable activation energy from ∼ ${ \sim }$ 60 kJ/mol to ∼ ${ \sim }$ 130 kJ/mol. We derive energy profiles for three alternative reaction pathways that involve uni- or bimolecular decomposition of reactive oxygen intermediates.

The influence of pH on the liquid-phase transformation of phenolic compounds driven by nitrite photolysis: Implications for characteristics, products and cytotoxicity

The aqueous-phase conversion of phenolic compounds (PhCs) driven by nitrite photolysis has been recognized as a significant source of secondary brown carbon (BrC). However, the influence of pH on the conversion kinetics and product distribution of PhCs remains unclear. In this study, three representative PhCs with varying functional groups were selected to examine their aqueous-phase conversion kinetics in the presence of nitrite under different pH conditions and simulated sunlight conditions. The results indicate that as the pH increases, the decay rates of PhCs decrease, following first-order reaction kinetics. These varying decay rates also suggest that different substituents on the benzene ring significantly impact the reactivity of PhCs. The molecular composition of the products is pH-dependent, with 4-nitrocatechol (4NC) emerging as the primary reaction product. A range of conversion products were detected across different pH values: nitrification dominated at low pH, while hydroxylation products increased with rising pH, and polymerization products appeared prominently at high pH. Due to the electron-withdrawing effect of the nitro group on the benzene ring, fewer products formed from 4-nitrophenol were observed, and the visible absorption spectrum also showed a decreasing trend as the reaction progressed across various pH conditions. Toxicity assays on human non-small cell lung cancer cells (A549) revealed that the toxicity of the reaction products decreased with increasing pH. Correspondingly, the accumulation of reactive oxygen species (ROS) and apoptosis rates in cells also declined. This may be due to the fact that at lower pH levels, nitrophenols (NPs), which tend to promote ROS accumulation and cell death, dominate the product mix. This study provides valuable insights into the toxicological properties of secondary organic aerosols (SOA) formed from the photo-oxidation products of PhCs under different pH conditions. These findings contribute to a deeper understanding of the environmental and health impacts of SOA in atmospheric chemistry.

Efficient Formation of Secondary Organic Aerosols from the Aqueous Oxidation of Terpenoic 1,2-Diols by OH

Aqueous oxidation of pinanediol (PND) and camphanediol (CND) by hydroxyl radical (OH) was investigated using gas and liquid chromatography coupled with mass spectrometry. The yields of the products formed were measured with authentic and surrogate standards. This approach quantified >97% of the products for both reactions under investigation. For the first time, the formation of 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) and other terpenoic acids (TAs) from the aqueous OH reaction with PND was confirmed with authentic standards. Based on the data acquired, mechanisms of OH oxidation of PND and CND were proposed. The yields of aqSOAs were evaluated by combining kinetic and air-water partitioning models developed for the the precursors, PND and CND, and for the first-generation products: cis-pinonic and camphoric acids. Modeled yields of aqSOAs ranged from 0.05 to 2.5. At liquid water content (LWC) from 1 × 10-4 to 4 × 10-3 (g × m-3, haze, and fogs), oxidized TAs were the major components of aqSOAs. In clouds with LWC > 0.06 (g × m-3), the contribution of nonacidic products to the mass of aqSOAs became dominant. Aqueous OH reaction with PND can produce up to 0.3 (Tg × yr-1) of aqSOA, assuming the average flux of the precursor at 0.5 (Tg × yr-1).

Chemical Differences between Phenolic Secondary Organic Aerosol Formed through Gas-Phase and Aqueous-Phase Reactions

Phenolic compounds, which are significant emissions from biomass burning (BB), undergo rapid photochemical reactions in both gas and aqueous phases to form secondary organic aerosol, namely, gasSOA and aqSOA, respectively. The formation of gasSOA and aqSOA involves different reaction mechanisms, leading to different product distributions. In this study, we investigate the gaseous and aqueous reactions of guaiacol-a representative BB phenol-to elucidate the compositional differences between phenolic aqSOA and gasSOA. Aqueous-phase reactions of guaiacol produce higher SOA yields than gas-phase reactions (e.g., roughly 60 vs 30% at one half-life of guaiacol). These aqueous reactions involve more complex reaction mechanisms and exhibit a more gradual SOA evolution than their gaseous counterparts. Initially, gasSOA forms with high oxidation levels (O/C > 0.82), while aqSOA starts with lower O/C (0.55-0.75). However, prolonged aqueous-phase reactions substantially increase the oxidation state of aqSOA, making its bulk chemical composition closer to that of gasSOA. Additionally, aqueous reactions form a greater abundance of oligomers and high-molecular-weight compounds, alongside a more sustained production of carboxylic acids. AMS spectral signatures representative of phenolic gasSOA have been identified, which, together with tracer ions of aqSOA, can aid in the interpretation of field observation data on aerosol aging within BB smoke. The notable chemical differences between phenolic gasSOA and aqSOA highlighted in this study also underscore the importance of accurately representing both pathways in atmospheric models to better predict the aerosol properties and their environmental impacts.

Positive Feedback between Partitioning of Carbonyl Compounds and Particulate Sulfur Formation during Haze Episodes

Carbonyl compounds are important precursors of aqueous aerosols in the atmosphere, while their gas-particle partitioning behaviors and roles in particulate sulfur formation are poorly understood. In this study, we investigate the partitioning of five carbonyl compounds (formaldehyde, acetaldehyde, acetone, glyoxal, and methylglyoxal) during haze episodes in Beijing, China. On haze days, the values of field-derived effective Henry's law coefficients (KHf) on aerosols for these carbonyl compounds are 106-108 M atm-1, which are significantly higher (102-104 times) than those in pure water. Sulfate is observed to have a pronounced "salting-in" effect on these carbonyl compounds, resulting in at least 1-order-of-magnitude increase in their particle-phase concentrations. Parameterization schemes for their partitioning in the ambient aerosols were provided and applied to the multiphase chemical box model (RACM2-CAPRAM). When incorporated into the field-derived parametrization, the model significantly increased hydroxymethanesulfonate (HMS) production by 50-fold compared to using the parameters obtained in pure water, increasing from 2.6 × 10-2 to 1.23 μg m-3 h-1. The formed HMS can facilitate sulfate formation in turn through further oxidation by OH radicals and enhance aerosol hygroscopicity. These findings indicate a positive feedback loop between the partitioning of carbonyl compounds and particulate sulfur formation during haze episodes, providing new insights for controlling particulate pollution and reducing SO2 levels in urban areas.

OH Radical Oxidation of Organosulfates in the Atmospheric Aqueous Phase

Organosulfates (OS, ROSO3-), ubiquitous constituents of atmospheric particulate matter (PM), influence both the physicochemical and climatic properties of PM. Although the formation pathways of OS have been extensively researched, only a few studies have investigated the atmospheric fate of this class of compounds. Here, to better understand the reactivity and transformation of OS under cloudwater- and aerosol-relevant conditions, we investigate the hydroxyl radical (OH) oxidation bimolecular rate constants (kOS+OHII) and products of five atmospherically relevant OS as a function of pH and ionic strength: methyl sulfate (MeS), ethyl sulfate (EtS), propyl sulfate (PrS), hydroxyacetone sulfate (HaS) and phenyl sulfate (PhS). Our results show that OS are oxidized by OH with kOS+OHII between 108 - 109 M-1 s-1, which corresponds to atmospheric lifetimes of minutes in aqueous aerosol to days in cloudwater. We find that kOS+OHII increases with carbon chain length (MeS < EtS < PrS) and aromaticity (PrS < PhS), but does not depend on solution pH (2, 9). In addition, we find that whereas the OH reactivity of the aliphatic OS studied here decreases by ∼2× with increasing ionic strength (0-15 M), the reactivity of PhS decreases by ∼10×. The oxidation of EtS and PrS produced organic peroxides (ROOH) as first-generation oxidation products, which subsequently photolyzed; the oxidation of PhS resulted in hydroxylated aromatic products. These results highlight the need for inclusion of OS loss pathways in atmospheric models, and suggest caution in using ambient OS concentration measurements alone to estimate their production rates.

Regional differences in molecular characteristics of atmospheric water-soluble organic carbon over northern China: Comparison of remote, rural, and urban environments

Atmospheric water-soluble organic carbon (WSOC) is a critical component of airborne particulates. It significantly affects the Earth's energy balance, air quality, and human health. Despite its importance, the molecular composition and sources of WSOC remain unclear, particularly in non-urban areas. In this study, we collected total suspended particulate (TSP) samples from three sites in northern China: Erenhot (remote site), Zhangbei (rural site), and Jinan (urban site). The WSOC components were analyzed using high-performance liquid chromatography coupled with high-resolution mass spectrometry. The results showed that the formula numbers of identified compounds exhibited a decreasing trend of Jinan (2647) > Zhangbei (2046) > Erenhot (1399). Among the assigned formulas, CHO compounds were the most abundant category for all three sites, accounting for 33 %-38 % of the identified compounds, followed by the CHON compounds with contributions of 27 %-30 %. In the remote site of Erenhot, CHO compounds were dominated by oxidized unsaturated organic compounds, and CHON compounds were mainly low-oxygenated aliphatic compounds, suggesting a significant influence of primary emissions. In contrast, the urban site of Jinan showed higher contributions of CHO and CHON compounds with elevated oxidation degrees, indicating the influence of more extensive secondary oxidation processes. Atmospheric WSOC in Erenhot and Zhangbei had abundant reduced sulfur-containing species, likely from coal or diesel combustion, while that in Jinan was characterized by aliphatic organosulfates and nitrooxy-organosulfates, which are mainly associated with traffic emissions and biogenetic sources, respectively. These findings reveal significant differences in the molecular composition of WSOC in different atmospheric environments and improve our understanding of the chemical properties, potential sources, and transformations of organic aerosols.

Challenges in moving towards fog's contribution to spatial patterns of atmospheric deposition fluxes on a national scale

Fog is an important environmental phenomenon affecting, among other things, geochemical cycles via atmospheric deposition pathways. It is generally accepted that fog contributes substantially to atmospheric deposition fluxes especially in mountain forests. Nevertheless, due to intrinsic constraints, fog pathway has thus far been neglected in the quantification of atmospheric deposition and fog pathway has not been accounted for in nation-wide spatial patterns of atmospheric deposition of air pollutants. In this review we explore the causes as to why it is so complex to create a spatial pattern of fog contribution to atmospheric ion deposition fluxes on a national scale. Physical and chemical principles of fog formation are presented and factors influencing the abrupt temporal and spatial changes in both fog occurrence and fog chemistry are elucidated. The focus is on both constituents essential for fog deposition flux quantification, i.e. (i) hydrological input on fog water and (ii) chemistry of fog water.

Quantifying the Effect of Basic Minerals on Acid- and Ligand-Promoted Dissolution Kinetics of Iron in Simulated Dark Atmospheric Aging of Dust and Coal Fly Ash Particles

The content and multiphase chemistry of iron (Fe) in multicomponent atmospheric aerosols are important to global climate and oceanic models. To date, reported dissolution rates of Fe span orders of magnitude with no quantifiable dependency on the content of basic minerals that coexist with Fe. Here, we report dissolution rates of Fe in simulated dark atmospheric aging of fully characterized multielement particles under acidic conditions (bulk pH 1 or 3) with and without oxalic acid and pyrocatechol. Our main findings are (a) the total amount of Ca and Mg was higher in coal fly ash than in Arizona test dust, (b) Fe dissolution initial rates increased exponentially with %Ca/Al and %Mg/Al below 50%, (c) a reduction in the Fe dissolution initial rate was observed with %Ca/Al higher than 50%, (d) reactive Ca and Mg minerals increased the calculated initial pH at the liquid/solid interface to values higher by only 1.5-2 units than the measured bulk pH, yet interfacial water remained acidic for Fe dissolution to take place, and (e) reactive Ca and Mg minerals enhanced the deprotonation of organics at the interface, aiding in ligand-promoted dissolution of Fe. The impact of these results is discussed within the context of constraining Fe dissolution kinetic models.

Rapid aqueous-phase dark reaction of phenols with nitrosonium ions: Novel mechanism for atmospheric nitrosation and nitration at low pH

Dark aqueous-phase reactions involving the nitrosation and nitration of aromatic organic compounds play a significant role in the production of light-absorbing organic carbon in the atmosphere. This process constitutes a crucial aspect of tropospheric chemistry and has attracted growing research interest, particularly in understanding the mechanisms governing nighttime reactions between phenols and nitrogen oxides. In this study, we present new findings concerning the rapid dark reactions between phenols containing electron-donating groups and inorganic nitrite in acidic aqueous solutions with pH levels <3.5. This reaction generates a substantial amount of nitroso- and nitro-substituted phenolic compounds, known for their light-absorbing properties and toxicity. In experiments utilizing various substituted phenols, we demonstrate that their reaction rates with nitrite depend on the electron cloud density of the benzene ring, indicative of an electrophilic substitution reaction mechanism. Control experiments and theoretical calculations indicate that the nitrosonium ion (NO+) is the reactive nitrogen species responsible for undergoing electrophilic reactions with phenolate anions, leading to the formation of nitroso-substituted phenolic compounds. These compounds then undergo partial oxidation to form nitro-substituted phenols through reactions with nitrous acid (HONO) or other oxidants like oxygen. Our findings unveil a novel mechanism for swift atmospheric nitrosation and nitration reactions that occur within acidic cloud droplets or aerosol water, providing valuable insights into the rapid nocturnal formation of nitrogen-containing organic compounds with significant implications for climate dynamics and human health.

Gas-particle partitioning of low-molecular-weight organic acids in suburban Shanghai: Insight into measured Henry's law constants dependent on relative humidity

Low-molecular-weight (LMW) organic acids are among the most abundant water-soluble organic compounds, but their gas-particle partitioning mechanism remains unclear. In the present study, LMW organic acids were measured using a URG 9000D Ambient Ion Monitor in suburban Shanghai. The average concentrations of formic acid, acetic acid, oxalic acid, and methanesulfonic acid (MSA) in PM2.5 were 405 ± 116, 413 ± 11, 475 ± 266, and 161 ± 54 ng m-3, respectively. The particle fraction exceeded 30 % for formic acid and acetic acid. Model predictions underestimated the particle-phase monocarboxylic acids (MCAs) from the factor of 102 at the highest RH to 107 at the lowest RH. The average measured intrinsic Henry's law constants (Hmea) for formic acid, acetic acid, oxalic acid, and MSA were 3.8 × 107, 4.5 × 107, 8.7 × 108, and 3.4 × 107 mol L-1 atm-1, respectively, approximately four orders of magnitude higher than their literature-based intrinsic Henry's law constants (Hlit) for MCAs and approximately four orders of magnitude lower than Hlit, MSA. The ratio of Hmea /Hlit for MCAs ranged over three orders of magnitude, depending on relative humidity. The strong deviations at low RHs are attributed to the dominance of absorption by the organic phase. The discrepancy at the highest RH possibly relates to surfactant effects and dimer formation. We used Hmea as a model input for the first time to estimate the phase partitioning of particulate MCAs, finding that >80 % of MCAs resided in the organic phase under dry conditions. We propose parameterizing Hmea as model input to predict the multiphase partitioning of MCAs.

Impact of Molecular Chlorine Production from Aerosol Iron Photochemistry on Atmospheric Oxidative Capacity in North China

Elevated levels of atmospheric molecular chlorine (Cl2) have been observed during the daytime in recent field studies in China but could not be explained by the current chlorine chemistry mechanisms in models. Here, we propose a Cl2 formation mechanism initiated by aerosol iron photochemistry to explain daytime Cl2 formation. We implement this mechanism into the GEOS-Chem chemical transport model and investigate its impacts on the atmospheric composition in wintertime North China where high levels of Cl2 as well as aerosol chloride and iron were observed. The new mechanism accounts for more than 90% of surface air Cl2 production in North China and consequently increases the surface air Cl2 abundances by an order of magnitude, improving the model's agreement with observed Cl2. The presence of high Cl2 significantly alters the oxidative capacity of the atmosphere, with a factor of 20-40 increase in the chlorine radical concentration and a 20-40% increase in the hydroxyl radical concentration in regions with high aerosol chloride and iron loadings. This results in an increase in surface air ozone by about 10%. This new Cl2 formation mechanism will improve the model simulation capability for reactive chlorine abundances in the regions with high emissions of chlorine and iron.

Energetics of the OH radical H-abstraction reactions from simple aldehydes and their geminal diol forms

CONTEXT

Carbonyl compounds, especially aldehydes, emitted to the atmosphere, may suffer hydration in aerosols or water droplets in clouds. At the same time, they can react with hydroxyl radicals which may add or abstract hydrogen atoms from these species. The interplay between hydration and hydrogen abstraction is studied using density functional and quantum composite theoretical methods, both in the gas phase and in simulated bulk water. The H-abstraction from the aldehydic and geminal diol forms of formaldehyde, acetaldehyde, glycolaldehyde, glyoxal, methylglyoxal, and acrolein is studied to determine whether the substituent has any noticeable effect in the preference for the abstraction of one form or another. It is found that abstraction of the H-atom adjacent to the carbonyl group gives a more stable radical than same abstraction from the geminal diol in the case of formaldehyde, acetaldehyde, and glycolaldehyde. The presence of a delocalizing group in the Cα (a carbonyl group in glyoxal and methylglyoxal, and a vinyl group in acrolein), reverts this trend, and now the abstraction of the H-atom from the geminal diol gives more stable radicals. A further study was conducted abstracting hydrogen atoms from the other different positions in the species considered, both in the aldehydic and geminal diol forms. Only in the case of glycolaldehyde, the radical formed by H-abstraction from the -CH2OH group is more stable than any of the other radical species. Abstraction of the hydrogen atom in one of the hydroxyl groups in the geminal diol is equivalent to the addition of the •OH radical to the aldehyde. It leads, in some cases, to decomposition into a smaller radical and a neutral molecule. In these cases, some interesting theoretical differences are observed between the results in gas phase and (simulated) bulk solvent, as well as with respect to the method of calculation chosen.

METHODS

DFT (M06-2X, B2PLYP, PW6B95), CCSD(T), and composite (CBS-QB3, jun-ChS, SCVECV-f12) methods using Dunning basis sets and extrapolation to the CBS limit were used to study the energetics of closed shell aldehydes in their keto and geminal-diol forms, as well as the radical derived from them by hydrogen abstraction. Both gas phase and simulated bulk solvent calculations were performed, in the last case using the Polarizable Continuum Model.

Evaluating Possible Formation Mechanisms of Criegee Intermediates during the Heterogeneous Autoxidation of Squalene

Organic molecules in the environment oxidatively degrade by a variety of free radical, microbial, and biogeochemical pathways. A significant pathway is heterogeneous autoxidation, in which degradation occurs via a network of carbon and oxygen centered free radicals. Recently, we found evidence for a new heterogeneous autoxidation mechanism of squalene that is initiated by hydroxyl (OH) radical addition to a carbon-carbon double bond and apparently propagated through pathways involving Criegee Intermediates (CI) produced from β-hydroxy peroxy radicals (β-OH-RO2•). It remains unclear, however, exactly how CI are formed from β-OH-RO2•, which could occur by a unimolecular or bimolecular pathway. Combining kinetic models and multiphase OH oxidation measurements of squalene, we evaluate the kinetic viability of three mechanistic scenarios. Scenario 1 assumes that CI are formed by the unimolecular bond scission of β-OH-RO2•, whereas Scenarios 2 and 3 test bimolecular pathways of β-OH-RO2• to yield CI. Scenario 1 best replicates the entire experimental data set, which includes effective uptake coefficients vs [OH] as well as the formation kinetics of the major products (i.e., aldehydes and secondary ozonides). Although the unimolecular pathway appears to be kinetically viable, future high-level theory is needed to fully explain the mechanistic relationship between CI and β-OH-RO2• in the condensed phase.

Mechanisms of isomerization and hydration reactions of typical β-diketone at the air-droplet interface

Acetylacetone (AcAc) is a typical class of β-diketones with broad industrial applications due to the property of the keto-enol isomers, but its isomerization and chemical reactions at the air-droplet interface are still unclear. Hence, using combined molecular dynamics and quantum chemistry methods, the heterogeneous chemistry of AcAc at the air-droplet interface was investigated, including the attraction of AcAc isomers by the droplets, the distribution of isomers at the air-droplet interface, and the hydration reactions of isomers at the air-droplet interface. The results reveal that the preferential orientation of two AcAc isomers (keto- and enol-AcAc) to accumulate and accommodate at the acidic air-droplet interface. The isomerization of two AcAc isomers at the acidic air-droplet interface is more favorable than that at the neutral air-droplet interface because the "water bridge" structure is destroyed by H3O+, especially for the isomerization from keto-AcAc to enol-AcAc. At the acidic air-droplet interface, the carbonyl or hydroxyl O-atoms of two AcAc isomers display an energetical preference to hydration. Keto-diol is the dominant products to accumulate at the air-droplet interface, and excessive keto-diol can enter the droplet interior to engage in the oligomerization. The photooxidation reaction of AcAc will increase the acidity of the air-droplet interface, which indirectly facilitate the uptake and formation of more keto-diol. Our results provide an insight into the heterogeneous chemistry of β-diketones and their influence on the environment.

Decreases in Epoxide-Driven Secondary Organic Aerosol Production under Highly Acidic Conditions: The Importance of Acid-Base Equilibria

Isoprene has the highest atmospheric emissions of any nonmethane hydrocarbon, and isoprene epoxydiols (IEPOX) are well-established oxidation products and the primary contributors forming isoprene-derived secondary organic aerosol (SOA). Highly acidic particles (pH 0-3) widespread across the lower troposphere enable acid-driven multiphase chemistry of IEPOX, such as epoxide ring-opening reactions forming methyltetrol sulfates through nucleophilic attack of sulfate (SO42-). Herein, we systematically demonstrate an unexpected decrease in SOA formation from IEPOX on highly acidic particles (pH < 1). While IEPOX-SOA formation is commonly assumed to increase at low pH when more [H+] is available to protonate epoxides, we observe maximum SOA formation at pH 1 and less SOA formation at pH 0.0 and 0.4. This is attributed to limited availability of SO42- at pH values below the acid dissociation constant (pKa) of SO42- and bisulfate (HSO4-). The nucleophilicity of HSO4- is 100× lower than SO42-, decreasing SOA formation and shifting particulate products from low-volatility organosulfates to higher-volatility polyols. Current model parameterizations predicting SOA yields for IEPOX-SOA do not properly account for the SO42-/HSO4- equilibrium, leading to overpredictions of SOA formation at low pH. Accounting for this underexplored acidity-dependent behavior is critical for accurately predicting SOA concentrations and resolving SOA impacts on air quality.

Photo-Oxidation of α-Pinene Oxidation Products in Atmospheric Waters - pH- and Temperature-Dependent Kinetic Studies

The atmospheric α-pinene oxidation leads to three carboxylic acids: norpinonic acid (NPA), pinic acid (PA), and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA). In this study, the OH radical kinetics in the aqueous phase of these carboxylic acids were investigated at different temperatures and pH values of solutions. Activation parameters and the corresponding atmospheric lifetimes of the acids in the troposphere were derived. The overall second-order rate constants for the individual speciation forms of the acids (AH and A- for NPA; AH2, AH- and A2- for PA; and AH3, AH2-, AH2- and A3- for MBTCA) were determined. At 298 K, the rate constants for reactions of protonated forms (AHx) of NPA, PA, and MBTCA with •OH, were (1.5 ± 0.2) × 109 L mol-1 s-1, (2.4 ± 0.1) × 109 L mol-1 s-1, and (4.1 ± 0.6) × 108 L mol-1 s-1, respectively. For the fully deprotonated forms (Ax-) of studied acids, the second-order rate constants were (2.2 ± 0.2) × 109 L mol-1 s-1, (2.8 ± 0.1) × 109 L mol-1 s-1, and (10.2 ± 0.7) × 108 L mol-1 s-1 at 298 K, respectively. It was found that the reactions of NPA and PA with OH radicals are faster than with MBTCA. For MBTCA, the reaction rate depends on pH more strongly at elevated temperatures (>298 K). The atmospheric lifetimes of the acids considered due to their reactivity with •OH were calculated for different model scenarios at a temperature of 283 K and pH = 2 in the aqueous phase. For this purpose, liquid water content (LWC) was used for aerosols and clouds under storm conditions and at various aqueous-phase concentrations of OH radicals. The lifetimes decreased with increasing LWC (from 10-12 m3 m-3 in aerosol to 10-5 m3 m-3 in storms), indicating that the acids undergo significant aqueous processing under realistic atmospheric conditions. Besides, the aerosol systems appeared less effective in removing PA and NPA, with lifetimes ranging from hundreds of days to tens and hundreds of hours, respectively. Clouds were more effective, with lifetimes ranging from tens of hours to a single second or less. MBTCA, which dissolves better in water, was effectively removed in all systems, with the longest lifetime of approximately 90 min.

Subsurface wastewater infiltration systems for nitrogen pollution control

Subsurface wastewater infiltration systems (SWISs) are suggested to be a cost-effective and environmentally friendly method for sewage treatment. However, a comprehensive summary of the relevant mechanisms and optimization methods for nitrogen (N) removal in SWIS is currently lacking. In this review, we first summarize the N transformation mechanisms in SWIS. The impact of operational parameters on the N removal efficiency is then delineated. To enhance pollutant removal and minimize resource wastage, it is advisable to maintain a wet-dry ratio of 1:1 and a hydraulic loading rate of 8-10 cm/day. The organic load should be determined based on influent characteristics to optimize the balance between sewage treatment and nitrous oxide (N2O) emission. Finally, various strategies and modifications have been suggested to enhance pollutant removal efficiency and reduce N2O emissions in SWIS, such as artificial aeration, supply electron donors, and well-designed structures. Overall, greater emphasis should be placed on the design and management of SWIS to optimize their co-benefits while effectively controlling N pollution. PRACTITIONER POINTS: SWISs are often considered black boxes with their efficiency depending on hydraulic characteristics, biological characteristics, and substrate properties. Biological nitrification coupled with denitrification is considered to be the major N removal process. Increasing the reduction of N2O to the inert N2 form is a potential mechanism to mitigate global warming. Strategies such as artificial aeration, supply electron donors, and well-designed structures are suggested to improve N removal performance.

Photooxidation-Initiated Aqueous-Phase Formation of Organic Peroxides: Delving into Formation Mechanisms

Formation of highly oxygenated molecules (HOMs) such as organic peroxides (ROOR, ROOH, and H2O2) is known to degrade food and organic matter. Gas-phase unimolecular autoxidation and bimolecular RO2 + HO2/RO2 reactions are prominently renowned mechanisms associated with the formation of peroxides. However, the reaction pathways and conditions favoring the generation of peroxides in the aqueous phase need to be evaluated. Here, we identified bulk aqueous-phase ROOHs in varying organic precursors, including a laboratory model compound and monoterpene oxidation products. Our results show that formation of ROOHs is suppressed at enhanced oxidant concentrations but exhibits complex trends at elevated precursor concentrations. Furthermore, we observed an exponential increase in the yield of ROOHs when UV light with longer wavelengths was used in the experiment, comparing UVA, UVB, and UVC. Water-soluble organic compounds represent a significant fraction of ambient cloud-water components (up to 500 μM). Thus, the reaction pathways facilitating the formation of HOMs (i.e., ROOHs) during the aqueous-phase oxidation of water-soluble species add to the climate and health burden of atmospheric particulate matter.

Aqueous-phase formation of N-containing secondary organic compounds affected by the ionic strength

The reaction of carbonyl-to-imine/hemiaminal conversion in the atmospheric aqueous phase is a critical pathway to produce the light-absorbing N-containing secondary organic compounds (SOC). The formation mechanism of these compounds has been wildly investigated in bulk solutions with a low ionic strength. However, the ionic strength in the aqueous phase of the polluted atmosphere may be higher. It is still unclear whether and to what extent the inorganic ions can affect the SOC formation. Here we prepared the bulk solution with certain ionic strength, in which glyoxal and ammonium were mixed to mimic the aqueous-phase reaction. Molecular characterization by High-resolution Mass Spectrometry was performed to identify the N-containing products, and the light absorption of the mixtures was measured by ultraviolet-visible spectroscopy. Thirty-nine N-containing compounds were identified and divided into four categories (N-heterocyclic chromophores, high-molecular-weight compounds with N-heterocycle, aliphatic imines/hemiaminals, and the unclassified). It was observed that the longer reaction time and higher ionic strength led to the formation of more N-heterocyclic chromophores and the increasing of the light-absorbance of the mixture. The added inorganic ions were proposed to make the aqueous phase somewhat viscous so that the molecules were prone to undergo consecutive and intramolecular reactions to form the heterocycles. In general, this study revealed that the enhanced ionic strength and prolonged reaction time had the promotion effect on the light-absorbing SOC formation. It implies that the aldehyde-derived aqueous-phase SOC would contribute more light-absorbing particulate matter in the industrial or populated area where inorganic ions are abundant.

Secondary inorganic aerosols and aerosol acidity at different PM2.5 pollution levels during winter haze episodes in the Sichuan Basin, China

Wintertime fine particle (PM2.5) pollution remains to be perplexing air quality problems in many parts of China. In this study, PM2.5 compositions and aerosol acidity at different pollution levels at an urban cite in the southwest China's Sichuan Basin were investigated during a sustained winter haze episode. Organic matter was the most abundant component of PM2.5, followed by nitrate, sulfate and ammonium. Shares of organic aerosol in PM2.5 mass decreased with the elevated PM2.5 levels, while the enhancements of sulfate and secondary organic aerosol were much less than that of nitrate and ammonium during heavy pollution with increased ratios of nitrate to sulfate, implying a significant role of nitrate in the haze formation. Results also suggest the nighttime chemistry might contribute substantially to the formation of nitrate under severe pollutions. The daily average aerosol pH showed a decreasing trend with the elevated levels of PM2.5, and this increased aerosl acidity was mainly due to the fast rising secondary inorganic aerosol (SIA) concentration, with the increase in hydronium ion concentration in air (Hair+) surpassing the dilution effect of elevated aerosol liquid water content (LWC). Thermodynamic model calculations revealed that the air environment was NH3-rich with total NHx (NH3 + NH4+) greater than required NHx, and the aerosol pH exponentially declined with the decreasing excess NHx (p < 0.01). This study demonstrated that under air stagnation and NH3-rich environment during winter, the raised relative humidity (RH) would lead to an increase in LWC and thereby facilitate the aqueous chemistry processes with the neutralization capacity of NH3 to form sulfate and nitrate, which would further increase the LWC and lower the pH. This self-amplifying SIA formation might be crucial to the severe PM2.5 pollution and haze events during winter, and therefore cutting both NOx and NH3 emissions would benefit stopping the self-amplification.

Influence of Fuel Properties on the Light Absorption of Fresh and Laboratory-Aged Atmospheric Brown Carbon Produced from Realistic Combustion of Boreal Peat and Spruce Foliage

Climate change has exacerbated fire activity in the boreal region. Consequently, smoldering boreal peatland fires are an increasingly important source of light-absorbing atmospheric organic carbon ("brown carbon"; BrC). To date, however, BrC from this source remains largely unstudied, which limits our ability to predict its climate impact. Here, we use size-exclusion chromatography coupled with diode array UV-vis detection to examine the molecular-size-dependent light absorption properties of fresh and photoaged aqueous BrC extracts collected during laboratory combustion of boreal peat and live spruce foliage. The atmospheric stability of BrC extracts varies with chromophore molecular size and fuel type: in particular, the high-molecular-weight fractions of both peat- and spruce-BrC are more resistant to photobleaching than their corresponding low-molecular-weight fractions, and total light absorption by peat-BrC persists over longer illumination timescales than that of spruce-BrC. Importantly, the BrC molecular size distribution itself varies with fuel properties (e.g., moisture content) and to an even greater extent with fuel type. Overall, our findings suggest that the accurate estimation of BrC radiative forcing, and the overall climate impact of wildfires, will require atmospheric models to consider the impact of regional diversity in vegetation/fuel types.

Synthesis and Characterization of Organic Peroxides from Monoterpene-Derived Criegee Intermediates in Secondary Organic Aerosol

Ozonolysis of alkenes is known to produce reactive intermediates─stabilized Criegee intermediates (SCIs), and their subsequent bimolecular reactions with various carboxylic acids can form α-acyloxyalkyl hydroperoxides (AAHPs), which is considered a major class of organic peroxides in secondary organic aerosol (SOA). Despite their atmospheric and health importance, the molecular-level identification of organic peroxides in atmospheric aerosols is highly challenging, preventing further assessment of their environmental fate. Here, we synthesize 20 atmospherically relevant AAHPs through liquid-phase ozonolysis, in which two types of monoterpene-derived SCIs from either α-pinene or 3-carene are scavenged by 10 different carboxylic acids to form AAHPs with diverse structures. These AAHPs are identified individually by liquid chromatography coupled with high-resolution mass spectrometry. AAHPs were previously thought to decompose quickly in an aqueous environment such as cloud droplets, but we demonstrate here that AAHPs hydrolysis rates are highly compound-dependent with rate constants differing by 2 orders of magnitude. In contrast, the aqueous-phase formation rate constants between SCI and various carboxylic acids vary only within a factor of 2-3. Finally, we identified two of the 20 synthesized AAHPs in α-pinene SOA and two in 3-carene SOA, contributing ∼0.3% to the total SOA mass. Our results improve the current molecular-level understanding of organic peroxides and are useful for a more accurate assessment of their environmental fate and health impact.

Optical Variation and Molecular Transformation of Brown Carbon During Oxidation by NO3• in the Aqueous Phase

The NO3•-driven nighttime aging of brown carbon (BrC) is known to greatly impact its atmospheric radiative forcing. However, the impact of oxidation by NO3• on the optical properties of BrC in atmospheric waters as well as the associated reaction mechanism remain unclear. In this work, we found that the optical variation of BrC proxies under environmentally relevant NO3• exposure depends strongly on their sources, with enhanced light absorptivity for biomass-burning BrC but bleaching for urban aerosols and humic substances. High-resolution mass spectrometry using FT-ICR MS shows that oxidation by NO3• leads to the formation of light-absorbing species (e.g., nitrated organics) for biomass-burning BrC while destroying electron donors (e.g., phenols) within charge transfer complexes in urban aerosols and humic substances, as evidenced by transient absorption spectroscopy and NaBH4 reduction experiments as well. Moreover, we found that the measured rate constants between NO3• with real BrCs (k = (1.8 ± 0.6) × 107 MC-1s-1, expressed as moles of carbon) are much higher than those of individual model organic carbon (OC), suggesting the reaction with OCs may be a previously ill-quantified important sink of NO3• in atmospheric waters. This work provides insights into the kinetics and molecular transformation of BrC during the oxidation by NO3•, facilitating further evaluation of BrC's climatic effects and atmospheric NO3• levels.

Iron content in aerosol particles and its impact on atmospheric chemistry

Atmospheric aerosol effects on ecological and human health remain uncertain due to their highly complex and evolving nature when suspended in air. Atmospheric chemistry, global climate/oceanic and health exposure models need to incorporate more realistic representations of aerosol particles, especially their bulk and surface chemistry, to account for the evolution in aerosol physicochemical properties with time. (Photo)chemistry driven by iron (Fe) in atmospheric aerosol particles from natural and anthropogenic sources remains limited in these models, particularly under aerosol liquid water conditions. In this feature article, recent advances from our work on Fe (photo)reactivity in multicomponent aerosol systems are highlighted. More specifically, reactions of soluble Fe with aqueous extracts of biomass burning organic aerosols and proxies of humic like substances leading to brown carbon formation are presented. Some of these reactions produced nitrogen-containing gaseous and condensed phase products. For comparison, results from these bulk aqueous phase chemical studies were compared to those from heterogeneous reactions simulating atmospheric aging of Fe-containing reference materials. These materials include Arizona test dust (AZTD) and combustion fly ash particles. Also, dissolution of Fe and other trace elements is presented from simulated human exposure experiments to highlight the impact of aerosol aging on levels of trace metals. The impacts of these chemical reactions on aerosol optical, hygroscopic and morphological properties are also emphasized in light of their importance to aerosol-radiation and aerosol-cloud interactions, in addition to biogeochemical processes at the sea/ocean surface microlayer upon deposition. Future directions for laboratory studies on Fe-driven multiphase chemistry are proposed to advance knowledge and encourage collaborations for efficient utilization of expertise and resources among climate, ocean and health scientific communities.

Residential Wood Combustion in Germany: A Twin-Site Study of Local Village Contributions to Particulate Pollutants and Their Potential Health Effects

Residential wood combustion contributing to airborne particulate matter (PM10) was studied for 1 year at two sites in the village of Melpitz. Significant excess pollution was observed at the Melpitz center compared to that at the TROPOS research station Melpitz reference site, situated only 700 m away. Local concentration increments at the village site for the combustion PM constituents organic carbon, elemental carbon, levoglucosan, and benzo[a]pyrene were determined under appropriate wind directions, and their winter mean values were 0.7 μg m-3, 0.3 μg m-3, 0.1 μg m-3, and 0.4 ng m-3, representing relative increases over the regional background concentration of 24, 70, 61, and 107%, respectively. Yearly, weekly, and diurnal profiles of village increments suggest residential heating as the dominant source of this excess pollution, mainly originating from wood combustion. Receptor modeling using positive matrix factorization quantified 4.5 μg m-3 wood combustion PM at the village site, representing an increment of 1.9 μg m-3 and an increase of ∼75% over the 2.6 μg m-3 regional background wood combustion PM. This increment varied with season, temperature, and boundary layer height and reached daily mean values of 4-6 μg m-3 during unfavorable meteorological conditions. Potential health effects were estimated and resulted in an all-cause mortality from short-term exposure to wood combustion PM of 2.1 cases per 100,000 inhabitants and year for areas with similar wood smoke levels as observed in Melpitz. The excess cancer risk from the concentrations of polycyclic aromatic hydrocarbons was 6.4 per 100,000. For both health metrics, the very local contributions from the village itself were about 40-50%, indicating a strong potential for mitigation through local-scale policies. A compilation of literature data demonstrates wood combustion to represent a major source of PM pollution in Germany, with average winter-time contributions of 10-20%. The present study quantifies the negative impacts of heating with wood in rural residential areas, where the continuous monitoring of air quality is typically lacking. Further regulation of this PM source is warranted in order to protect human health.

Multiple Sulfur Isotopic Evidence for Sulfate Formation in Haze Pollution

The mechanism of sulfate formation during winter haze events in North China remains largely elusive. In this study, the multiple sulfur isotopic composition of sulfate in different grain-size aerosol fractions collected seasonally from sampling sites in rural, suburban, urban, industrial, and coastal areas of North China are used to constrain the mechanism of SO2 oxidation at different levels of air pollution. The Δ33S values of sulfate in aerosols show an obvious seasonal variation, except for those samples collected in the rural area. The positive Δ33S signatures (0‰ < Δ33S < 0.439‰) observed on clean days are mainly influenced by tropospheric SO2 oxidation and stratospheric SO2 photolysis. The negative Δ33S signatures (-0.236‰ < Δ33S < ∼0‰) observed during winter haze events (PM2.5 > 200 μg/m3) are mainly attributed to SO2 oxidation by H2O2 and transition metal ion catalysis (TMI) in the troposphere. These results reveal that both the H2O2 and TMI pathways play critical roles in sulfate formation during haze events in North China. Additionally, these new data provide evidence that the tropospheric oxidation of SO2 can produce significant negative Δ33S values in sulfate aerosols.

Beyond the formation: unveiling the atmospheric transformation of organosulfates via heterogeneous OH oxidation

Organosulfates (OSs), characterized with a sulfate ester group (R-OSO3-), are abundant constituents in secondary organic aerosols. Recent laboratory-based investigations have revealed that OSs can undergo efficient chemical transformation through heterogeneous oxidation by hydroxyl radicals (˙OH, interchangeably termed as OH in this article), which freshly derives functionalized and fragmented OSs. The reaction not only contributes to the presence of structurally transformed OSs in the atmosphere of which sources were unidentified, but it also leads to the formation of inorganic sulfates (e.g., SO42-) with profound implication on the form of aerosol sulfur. In this article, we review the current state of knowledge regarding the heterogeneous OH oxidation of OSs based on state-of-the-art designs of experiments, computational approaches, and chemical analytical techniques. Here, we discuss the formation potential of new OSs and SO42-, in light of the influence of diverse OS structures on the relative importance of different reaction pathways. We propose future research directions to advance our mechanistic understanding of these reactions, taking into account aerosol matrix effects, interactions with other atmospheric pollutants, and the incorporation of experimental findings into atmospheric chemical transport models.

Chemical characterization of microplastic particles formed in airborne waste discharged from sewer pipe repairs

Microplastic particles are of increasing environmental concern due to the widespread uncontrolled degradation of various commercial products made of plastic and their associated waste disposal. Recently, common technology used to repair sewer pipes was reported as one of the emission sources of airborne microplastics in urban areas. This research presents results of the multi-modal comprehensive chemical characterization of the microplastic particles related to waste discharged in the pipe repair process and compares particle composition with the components of uncured resin and cured plastic composite used in the process. Analysis of these materials employs complementary use of surface-enhanced Raman spectroscopy, scanning transmission X-ray spectro-microscopy, single particle mass spectrometry, and direct analysis in real-time high-resolution mass spectrometry. It is shown that the composition of the relatively large (100 μm) microplastic particles resembles components of plastic material used in the process. In contrast, the composition of the smaller (micrometer and sub-micrometer) particles is significantly different, suggesting their formation from unintended polymerization of water-soluble components occurring in drying droplets of the air-discharged waste. In addition, resin material type influences the composition of released microplastic particles. Results are further discussed to guide the detection and advanced characterization of airborne microplastics in future field and laboratory studies pertaining to sewer pipe repair technology.

Diversity of organic components in airborne waste discharged from sewer pipe repairs

Air-discharged waste from commonly used trenchless technologies of sewer pipe repairs is an emerging and poorly characterized source of urban pollution. This study reports on the molecular-level characterization of the atmospherically discharged aqueous-phase waste condensate samples collected at four field sites of the sewer pipe repairs. The molecular composition of organic species in these samples was investigated using reversed-phase liquid chromatography coupled with a photodiode array detector and a high-resolution mass spectrometer equipped with interchangeable atmospheric pressure photoionization and electrospray ionization sources. The waste condensate components comprise a complex mixture of organic species that can partition between gas-, aqueous-, and solid-phases when water evaporates from the air-discharged waste. Identified organic species have broad variability in molecular weight, molecular structures, and carbon oxidation state, which also varied between the waste samples. All condensates contained complex mixtures of oxidized organics, N- and S-containing organics, condensed aromatics, and their functionalized derivatives that are directly released to the atmospheric environment during installations. Furthermore, semi-volatile, low volatility, and extremely low volatility organic compounds comprise 75-85% of the total compounds identified in the waste condensates. Estimates of the component-specific viscosities suggest that upon evaporation of water waste material would form the semi-solid and solid phases. The low volatilities and high viscosities of chemical components in these waste condensates will contribute to the formation of atmospheric secondary organic aerosols and atmospheric solid nanoplastic particles. Lastly, selected components expected in the condensates were quantified and found to be present at high concentrations (1-20 mg L-1) that may exceed regulatory limits.

In silico Investigation of the Thermochemistry and Photoactivity of Pyruvic Acid in an Aqueous Solution of NaCl

The photochemistry of oxocarboxylic acids contributes significantly to the complex chemistry occurring in the atmosphere. In this regard, pyruvic acid undergoes photoreactions that lead to many diverse products. The presence of sodium cation near pyruvic acid in an aqueous solution, or its conjugate base in non-acidic conditions, influences the hydration equilibrium and the photosensitivity to UV-visible light of the oxocarboxylic acid. We performed an ab initio metadynamics simulation which serves two purposes: first, it unveils the mechanisms of the reversible hydration reaction between the keto and the diol forms, with a free-energy difference of only 2 kJ/mol at 300 K, which shows the influence of sodium on the keto/diol ratio; second, it provides solvent-shared ion pairing (SSIP) and contact ion pairing (CIP) structures, including Na+ coordinated to carbonyl, for the calculations of the electronic transition energies to an antibonding π* orbital, which sheds light on the photoactivity of these two forms in the actinic region.

Role of Carbon Dioxide, Ammonia, and Organic Acids in Buffering Atmospheric Acidity: The Distinct Contribution in Clouds and Aerosols

Acidity is one central parameter in atmospheric multiphase reactions, influencing aerosol formation and its effects on climate, health, and ecosystems. Weak acids and bases, mainly CO2, NH3, and organic acids, are long considered to play a role in regulating atmospheric acidity. However, unlike strong acids and bases, their importance and influencing mechanisms in a given aerosol or cloud droplet system remain to be clarified. Here, we investigate this issue with new insights provided by recent advances in the field, in particular, the multiphase buffer theory. We show that, in general, aerosol acidity is primarily buffered by NH3, with a negligible contribution from CO2 and a potential contribution from organic acids under certain conditions. For fogs, clouds, and rains, CO2, organic acids, and NH3 may all provide certain buffering under higher pH levels (pH > ∼4). Despite the 104to 107 lower abundance of NH3 and organic weak acids, their buffering effect can still be comparable to that of CO2. This is because the cloud pH is at the very far end of the CO2 multiphase buffering range. This Perspective highlights the need for more comprehensive field observations under different conditions and further studies in the interactions among organic acids, acidity, and cloud chemistry.

Temperature-Dependent Oxidation of Hydroxylated Aldehydes by •OH, SO4•-, and NO3• Radicals in the Atmospheric Aqueous Phase

T-dependent aqueous-phase rate constants were determined for the oxidation of the hydroxy aldehydes, glyceraldehyde, glycolaldehyde, and lactaldehyde, by the hydroxyl radicals (•OH), the sulfate radicals (SO4•-), and the nitrate radicals (NO3•). The obtained Arrhenius expressions for the oxidation by the •OH radical are: k(T,GLYCERALDEHYDE+OH•) = (3.3 ± 0.1) × 1010 × exp((-960 ± 80 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+OH•) = (4.3 ± 0.1) × 1011 × exp((-1740 ± 50 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+OH•) = (1.6 ± 0.1) × 1011 × exp((-1410 ± 180 K)/T)/L mol-1 s-1; for the SO4•- radical: k(T,GLYCERALDEHYDE+SO4•-) = (4.3 ± 0.1) × 109 × exp((-1400 ± 50 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+SO4•-) = (10.3 ± 0.3) × 109 × exp((-1730 ± 190 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+SO4•-) = (2.2 ± 0.1) × 109 × exp((-1030 ± 230 K)/T)/L mol-1 s-1; and for the NO3• radical: k(T,GLYCERALDEHYDE+NO3•) = (3.4 ± 0.2) × 1011 × exp((-3470 ± 460 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+NO3•) = (7.8 ± 0.2) × 1011 × exp((-3820 ± 240 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+NO3•) = (4.3 ± 0.2) × 1010 × exp((-2750 ± 340 K)/T)/L mol-1 s-1, respectively. Targeted simulations of multiphase chemistry reveal that the oxidation by OH radicals in cloud droplets is important under remote and wildfire influenced continental conditions due to enhanced partitioning. There, the modeled average aqueous •OH concentration is 2.6 × 10-14 and 1.8 × 10-14 mol L-1, whereas it is 7.9 × 10-14 and 3.5 × 10-14 mol L-1 under wet particle conditions. During cloud periods, the aqueous-phase reactions by •OH contribute to the oxidation of glycolaldehyde, lactaldehyde, and glyceraldehyde by about 35 and 29%, 3 and 3%, and 47 and 37%, respectively.

Molecular-Scale Investigation on the Formation of Brown Carbon Aerosol via Iron-Phenolic Compound Reactions in the Dark

Brown carbon (BrC) is one of the most mysterious aerosol components responsible for global warming and air pollution. Iron (Fe)-induced catalytic oxidation of ubiquitous phenolic compounds has been considered as a potential pathway for BrC formation in the dark. However, the reaction mechanism and product composition are still poorly understood. Herein, 13 phenolic precursors were employed to react with Fe under environmentally relevant conditions. Using Fourier transform ion cyclotron resonance mass spectrometry, a total of 764 unique molecular formulas were identified, and over 85% of them can be found in atmospheric aerosols. In particular, products derived from precursors with catechol-, guaiacol-, and syringol-like-based structures can be distinguished by their optical and molecular characteristics, indicating the structure-dependent formation of BrC from phenolic precursors. Multiple pieces of evidence indicate that under acidic conditions, the contribution of either autoxidation or oxygen-induced free radical oxidation to BrC formation is extremely limited. Ligand-to-Fe charge transfer and subsequent phenoxy radical coupling reactions were the main mechanism for the formation of polymerization products with high molecular diversity, and the efficiency of BrC generation was linearly correlated with the ionization potential of phenolic precursors. The present study uncovered how chemically diverse BrC products were formed by the Fe-phenolic compound reactions at the molecular level and also provide a new paradigm for the study of the atmospheric aerosol formation mechanism.

Characterization of water-soluble brown carbon in atmospheric fine particles over Xi'an, China: Implication of aqueous brown carbon formation from biomass burning

Brown carbon (BrC) aerosols can affect not only the climate but also human health, however, the light absorption, chemical compositions, and formation mechanisms of BrC are still uncertain, which leads to uncertainties in the accurate estimation of its climate and health impacts. In this study, highly time - resolved brown carbon (BrC) in fine particles was investigated in Xi'an using offline aerosol mass spectrometer analysis. The light absorption coefficient (b_abs365) and mass absorption efficiency (MAE₃₆₅) at 365 nm of water-soluble organic aerosol (WSOA) generally increased with oxygen-to-carbon (O/C) ratios, indicating that oxidized OA could have more impacts on BrC light absorption. Meanwhile, the light absorption appeared to increase generally with the increases of nitrogen-to-carbon (N/C) ratios and water-soluble organic nitrogen; strong correlations (R of 0.76 for C_xH_yN_p⁺ and R of 0.78 for C_xH_yO_zN_p⁺) between b_abs365 and the N - containing organic ion families were observed, suggesting that the N - containing compounds are the effective BrC chromophores. b_abs365 correlated relatively well with BBOA (r of 0.74) and OOA (R of 0.57), but weakly correlated with CCOA (R of 0.33), indicating that BrC in Xi'an was likely to be associated with biomass burning and secondary sources. A multiple linear regression model was applied to apportion b_abs365 to contributions of different factors resolved from positive matrix factorization on water-soluble organic aerosols (OA) and obtained MAE₃₆₅ values of different OA factors. We found that biomass-burning organic aerosol (BBOA) dominated the b_abs365 (48.3 %), followed by oxidized organic aerosol (OOA, 33.6 %) and coal combustion organic aerosol (CCOA, 18.1 %). We further observed that nitrogen-containing organic matter (i.e., C_xH_yN_p⁺ and C_xH_yO_zN_p⁺) increased with the increase of OOA/WSOA and the decrease of BBOA/WSOA, especially under high ALWC conditions. Our work offered proper observation evidence that BBOA is oxidized through the aqueous formation to produce BrC in Xi'an, China.

Molecular characteristics, sources and transformation of water-insoluble organic matter in cloud water

Studies have shown that water-insoluble organic matter (WIOM) accounts for a large part of the organic components in cloud water and significantly contributes to brown carbon. However, the molecular characteristics of WIOM in cloud droplets remain unclear, hampering the understanding of their climate effects. In this study, cloud water was collected at a remote mountain site in South China during the winter of 2020, and WIOM was separated by membrane filtration, extracted by methanol, and characterized using Fourier transform ion cyclotron resonance mass spectrometry coupled with an electrospray ionization source. A total of 697-1637 molecules were identified in WIOM. WIOM is characterized by lower oxidation states of carbon atoms (-1.10 ∼ -0.84 in WIOM vs. -0.58 ∼ -0.51 in water-soluble organic matter (WSOM) on average), higher carbon number (14.12-20.59 vs. 9.87-10.56) and lower unsaturation (double-bond equivalent 4.55-4.95 vs. 4.84-5.23) relative to WSOM. More abundant lipid-like compounds (12.2-41.9% in WIOM vs. <2% in WSOM) but less highly oxygenated compounds (<7% vs. 28.6-35.3%) exist in WIOM. More than 30% of WIOM molecules in cloud water are common with interstitial particles, implying that WIOM in cloud water may originate from aerosol activation and/or collision. Some unique molecules in WIOM in cloud water are identified as aqueous-phase oligomerization products, indicating the aqueous-phase formation of WIOM. Further analysis of the intermolecular relationship shows that WIOM has the potential to transform into WSOM by partitioning into the dissolved phase, oxidation and functionalization by heteroatom-containing groups, representing a previously unidentified pathway for WSOM formation in cloud water. The results provide new insights into the in-cloud chemistry, which would assist in the understanding of the aqueous formation and evolution of WIOM.

Ionic Strength Enhances the Multiphase Oxidation Rate of Sulfur Dioxide by Ozone in Aqueous Aerosols: Implications for Sulfate Production in the Marine Atmosphere

Multiphase oxidation of sulfur dioxide (SO₂) by ozone (O₃) in alkaline sea salt aerosols is an important source of sulfate aerosols in the marine atmosphere. However, a recently reported low pH of fresh supermicron sea spray aerosols (mainly sea salt) would argue against the importance of this mechanism. Here, we investigated the impact of ionic strength on the kinetics of multiphase oxidation of SO₂ by O₃ in proxies of aqueous acidified sea salt aerosols with buffered pH of ∼4.0 via well-controlled flow tube experiments. We find that the sulfate formation rate for the O₃ oxidation pathway proceeds 7.9 to 233 times faster under high ionic strength conditions of 2-14 mol kg^-1 compared to the dilute bulk solutions. The ionic strength effect is likely to sustain the importance of multiphase oxidation of SO₂ by O₃ in sea salt aerosols in the marine atmosphere. Our results indicate that atmospheric models should consider the ionic strength effects on the multiphase oxidation of SO₂ by O₃ in sea salt aerosols to improve the predictions of the sulfate formation rate and the sulfate aerosol budget in the marine atmosphere.

Aging of α-Pinene Secondary Organic Aerosol by Hydroxyl Radicals in the Aqueous Phase: Kinetics and Products

The reaction of hydroxyl radicals (OH) with a water-soluble fraction of the α-pinene secondary organic aerosol (SOA) was investigated using liquid chromatography coupled with negative electrospray ionization mass spectrometry. The SOA was generated by the dark ozonolysis of α-pinene, extracted into the water, and subjected to chemical aging by the OH. Bimolecular reaction rate coefficients (k_OH) for the oxidation of terpenoic acids by the OH were measured using the relative rate method. The unaged SOA was dominated by the cyclobutyl-ring-retaining compounds, primarily cis-pinonic, cis-pinic, and hydroxy-pinonic acids. Aqueous oxidation by the OH resulted in the removal of early-stage products and dimers, including well-known oligomers with MW = 358 and 368 Da. Furthermore, a 2- to 5-fold increase in the concentration of cyclobutyl-ring-opening products was observed, including terpenylic and diaterpenylic acids and diaterpenylic acid acetate as well as some of the newly identified OH aging markers. At the same time, results obtained from the kinetic box model showed a high degree of SOA fragmentation following the reaction with the OH, which indicates that non-radical reactions occurring during the evaporation of water likely contribute to the high yields of terpenoic _aqSOAs reported previously. The estimated atmospheric lifetimes showed that in clouds, terpenoic acids react with the OH exclusively in the aqueous phase. Aqueous OH aging of the α-pinene SOA results in a 10% increase of the average O/C ratio and a 3-fold decrease in the average k_OH value, which is likely to affect the cloud condensation nuclei activity of the _aqSOA formed after the evaporation of water.

Spontaneous dark formation of OH radicals at the interface of aqueous atmospheric droplets

Hydroxyl radical (OH) is a key oxidant that triggers atmospheric oxidation chemistry in both gas and aqueous phases. The current understanding of its aqueous sources is mainly based on known bulk (photo)chemical processes, uptake from gaseous OH, or related to interfacial O3 and NO3 radical-driven chemistry. Here, we present experimental evidence that OH radicals are spontaneously produced at the air-water interface of aqueous droplets in the dark and the absence of known precursors, possibly due to the strong electric field that forms at such interfaces. The measured OH production rates in atmospherically relevant droplets are comparable to or significantly higher than those from known aqueous bulk sources, especially in the dark. As aqueous droplets are ubiquitous in the troposphere, this interfacial source of OH radicals should significantly impact atmospheric multiphase oxidation chemistry, with substantial implications on air quality, climate, and health.

Assessing the Use of Probes and Quenchers for Understanding the Reactive Species in Advanced Oxidation Processes

Advanced oxidation processes (AOPs) are increasingly applied in water and wastewater treatment. Understanding the role of reactive species using probes and quenchers is one of the main requirements for good process design. However, much fundamental kinetic data for the reactions of probes and quenchers with reactive species is lacking, probably leading to inappropriate probe and quencher selection and dosing. In this work, second-order rate constants for over 150 reactions of probes and quenchers with reactive species such as ^•OH, SO₄^•-, and Cl^• and chemical oxidants such as free chlorine and persulfate were determined. Some previously ill-quantified reactions (e.g., furfuryl alcohol and methyl phenyl sulfoxide reactions with certain chemical oxidants, nitrobenzene and 1,4-dioxane reactions with certain halogen radicals) were found to be kinetically favorable. The selection of specific probes can be guided by the improved kinetic database. The criteria for properly choosing dosages of probes and quenchers were proposed along with a procedure for quantifying reactive species free of interference from probe addition. The limitations of probe and quencher approaches were explicated, and possible solutions (e.g., the combination with other tools) were proposed. Overall, the kinetic database and protocols provided in this work benefit future research in understanding the radical chemistry in AOPs as well as other radical-involved processes.

Kinetics of the nitrate-mediated photooxidation of monocarboxylic acids in the aqueous phase

The photooxidation of organic compounds by hydroxyl radicals (·OH) in atmospheric aqueous phases contributes to both the formation and aging of secondary organic aerosols (SOAs), which usually include carboxylic acids. Hydrogen peroxide (H₂O₂) and inorganic nitrate are two important ·OH photochemical sources in atmospheric aqueous phases. The aqueous phase pH is an important factor that not only controls the dissociation of carboxylic acids and consequently their ·OH reactivities, but also the production of ·OH and other reactive species from the photolysis of some ·OH photochemical precursors, particularly inorganic nitrate. While many studies have reported on the aqueous pH-dependent photodegradation rates of carboxylic acids with ·OH produced by H₂O₂ photolysis, the aqueous pH-dependent photodegradation rates of carboxylic acids with ·OH produced by inorganic nitrate photolysis have not been studied. In this work, we investigated the pH-dependent (pH 2 to 7) aqueous photooxidation of formic acid (FA), glycolic acid (GA), and pyruvic acid (PA) initiated by the photolysis of ammonium nitrate (NH₄NO₃). The observed reaction rates of the three carboxylic acids were controlled by the [NH₄NO₃]/[carboxylic acid] concentration ratio. Higher [NH₄NO₃]/[carboxylic acid] concentration ratios resulted in faster photodegradation rates, which could be attributed to the higher concentrations of ·OH produced from the photolysis of higher concentrations of NH₄NO₃. In addition, the observed photodegradation rates of the three carboxylic acids strongly depended on the pH. The highest photodegradation rate was observed at pH 4 for FA, whereas the highest photodegradation rates were observed at pH 2 for GA and PA. The observed pH-dependent FA and GA photodegradation rates were due to the combined effects of the pH-dependent ·OH formation from NH₄NO₃ photolysis and the differences in ·OH reactivities of dissociated vs. undissociated FA and GA. In contrast, the observed pH-dependent PA photodegradation rate was due primarily to the pH-dependent decarboxylation of PA initiated by light. These results highlight how the aqueous phase pH and inorganic nitrate photolysis can combine to influence the degradation rates of carboxylic acids, which can have significant implications for how the atmospheric fates of carboxylic acids are modeled for regions with substantial concentrations of inorganic nitrate in cloud water and aqueous aerosols.

Rapid Redox Cycling of Fe(II)/Fe(III) in Microdroplets during Iron-Citric Acid Photochemistry

Fe(III) and carboxylic acids are common compositions in atmospheric microdroplet systems like clouds, fogs, and aerosols. Although photochemical processes of Fe(III)-carboxylate complexes have been extensively studied in bulk aqueous solution, relevant information on the dynamic microdroplet system, which may be largely different from the bulk phase, is rare. With the help of the custom-made ultrasonic-based dynamic microdroplet photochemical system, this study examines the photochemical process of Fe(III)-citric acid complexes in microdroplets for the first time. We find that when the degradation extent of citric acid is similar between the microdroplet system and the bulk solution, the significantly lower Fe(II) ratio is present in microdroplet samples due to the rapider reoxidation of photogenerated Fe(II). However, by replacing citric acid with benzoic acid, no much difference in the Fe(II) ratio between microdroplets and bulk solution is observed, which indicates distinct reoxidation pathways of Fe(II). Moreover, the presence of ^•OH scavenger, namely, methanol, greatly accelerates the reoxidation of photogenerated Fe(II) in both citric acid and benzoic acid situations. Further experiments reveal that the high availability of O₂ and the citric acid- or methanol-derived carbon-centered radicals are responsible for the rapider reoxidation of Fe(II) in iron-citric acid microdroplets by prolonging the length of HO₂^•- and H₂O₂-involved radical reaction chains. The results in this study may provide a new understanding about iron-citric acid photochemistry in atmospheric liquid particles, which can further influence the photoactivity of particles and the formation of secondary organic aerosols.

Molecular investigation of the multi-phase photochemistry of Fe(III)-citrate in aqueous solution

Iron (Fe) is ubiquitous in nature and found as Fe^II or Fe^III in minerals or as dissolved ions Fe²⁺ or Fe³⁺ in aqueous systems. The interactions of soluble Fe have important implications for fresh water and marine biogeochemical cycles, which have impacts on global terrestrial and atmospheric environments. Upon dissolution of Fe^III into natural aquatic systems, organic carboxylic acids efficiently chelate Fe^III to form [Fe^III-carboxylate]²⁺ complexes that undergo a wide range of photochemistry-induced radical reactions. The chemical composition and photochemical transformations of these mixtures are largely unknown, making it challenging to estimate their environmental impact. To investigate the photochemical process of Fe^III-carboxylates at the molecular level, we conduct a comprehensive experimental study employing UV-visible spectroscopy, liquid chromatography coupled to photodiode array and high-resolution mass spectrometry detection, and oil immersion flow microscopy. In this study, aqueous solutions of Fe^III-citrate were photolyzed under 365 nm light in an experimental setup with an apparent quantum yield of (φ) ∼0.02, followed by chemical analyses of reacted mixtures withdrawn at increment time intervals of the experiment. The apparent photochemical reaction kinetics of Fe³⁺-citrates (aq) were expressed as two generalized consecutive reactions of with the experimental rate constants of j₁ ∼ 0.12 min^-1 and j₂ ∼ 0.05 min^-1, respectively. Molecular characterization results indicate that R and I consist of both water-soluble organic and Fe-organic species, while P compounds are a mixture of water-soluble and colloidal materials. The latter were identified as Fe-carbonaceous colloids formed at long photolysis times. The carbonaceous content of these colloids was identified as unsaturated organic species with low oxygen content and carbon with a reduced oxidation state, indicative of their plausible radical recombination mechanism under oxygen-deprived conditions typical for the extensively photolyzed mixtures. Based on the molecular characterization results, we discuss the comprehensive reaction mechanism of Fe^III-citrate photochemistry and report on the formation of previously unexplored colloidal reaction products, which may contribute to atmospheric and terrestrial light-absorbing materials in aquatic environments.

pH affects the aqueous-phase nitrate-mediated photooxidation of phenolic compounds: implications for brown carbon formation and evolution

Brown carbon (BrC) is known to have important impacts on atmospheric chemistry and climate. Phenolic compounds are a prominent class of BrC precursors that are emitted in large quantities from biomass burning and fossil fuel combustion. Inorganic nitrate is a ubiquitous component of atmospheric aqueous phases such as cloudwater, fog, and aqueous aerosols. The photolysis of inorganic nitrate can lead to BrC formation via the photonitration of phenolic compounds in the aqueous phase. However, the acidity of the atmospheric aqueous phase adds complexity to these photonitration processes and needs to be considered when investigating BrC formation from the nitrate-mediated photooxidation of phenolic compounds. In this study, we investigated the influence of pH on the formation and evolution of BrC from the aqueous-phase photooxidation of guaiacol, catechol, 5-nitroguaiacol, and 4-nitrocatechol initiated by inorganic nitrate photolysis. The reaction rates, BrC composition and quantities were found to depend on the aqueous phase pH. Guaiacol, catechol, and 5-nitroguaiacol reacted substantially faster at lower pH. In contrast, 4-nitrocatechol reacted at slower rates at lower pH. For all four phenolic compounds, the initial stages of photooxidation resulted in an increase in light absorption (i.e., photo-enhancement) in the near-UV and visible range due to the formation of light absorbing products formed via the addition of nitro and/or hydroxyl groups to the phenolic compound. Greater photo-enhancement was observed at lower pH during the nitrate-mediated photooxidation of guaiacol and catechol. In contrast, greater photo-enhancement was observed at higher pH during the nitrate-mediated photooxidation of 5-nitroguaiacol and 4-nitrocatechol. This indicated that the effect that the aqueous phase pH has on the composition and yields of BrC formed is not universal, and will depend on the initial phenolic compound. These results provide new insights into how the atmospheric aqueous phase acidity influences the reactivities of different phenolic compounds and BrC formation/evolution during photooxidation initiated by inorganic nitrate photolysis, which will have significant implications for how the atmospheric fates of phenolic compounds and BrC formation/evolution are modeled for areas with high levels of inorganic nitrate.