The Acidity of Atmospheric Particles and Clouds

Photochemistry of Imidazole-2-carbaldehyde in Droplets as a Potential Source of H2O2 and Its Oxidation of SO2

Hydrogen peroxide (H2O2) plays a crucial role as an oxidizing agent within the tropospheric environment, making a substantial contribution to sulfate formation in hydrated aerosols and cloud and fog droplets. Field observations show that high levels of H2O2 are often observed in heavy haze events and polluted air. However, the source of H2O2 remains unclear. Here, using the droplets formed in situ by the deliquescence of hygroscopic compounds under a high relative humidity (RH), the formation of H2O2 by the photochemistry of imidazole-2-carbaldehyde (2-IC) under ultraviolet irradiation was explored. The results indicate that 2-IC produces IM-C•-OH and IM-C•═O radicals via H transfer itself to its excited triplet state and generates H2O2 and organic peroxides in the presence of O2, which has an evident oxidizing effect on SO2, suggesting the potential involvement of this pathway in the formation of atmospheric sulfate. H2O2 formation is limited in acidic droplets or droplets containing ammonium ions, and no H2O2 is detected in droplets containing nitrate, whereas droplets containing citric acid have an obvious promotion effect on H2O2 formation. These findings provide valuable insights into the behaviors of atmospheric photosensitizers, the source of H2O2, and the formation of sulfate in atmospheric droplets.

Regime shift in secondary inorganic aerosol formation and nitrogen deposition in the rural United States

Secondary inorganic aerosols play an important role in air pollution and climate change, and their formation modulates the atmospheric deposition of reactive nitrogen (including oxidized and reduced nitrogen), thus impacting the nitrogen cycle. Large-scale and long-term analyses of secondary inorganic aerosol formation based on model simulations have substantial uncertainties. Here we improve constraints on secondary inorganic aerosol formation using decade-long in situ observations of aerosol composition and gaseous precursors from multiple monitoring networks across the United States. We reveal a shift in the secondary inorganic aerosol formation regime in the rural United States between 2011 and 2020, making rural areas less sensitive to changes in ammonia concentrations and shortening the effective atmospheric lifetime of reduced forms of reactive nitrogen. This leads to potential increases in reactive nitrogen deposition near ammonia emission hotspots, with ecosystem impacts warranting further investigation. Ammonia (NH3), a critical but not directly regulated precursor of fine particulate matter in the United States, has been increasingly scrutinized to improve air quality. Our findings, however, show that controlling NH3 became significantly less effective for mitigating fine particulate matter in the rural United States. We highlight the need for more collocated aerosol and precursor observations for better characterization of secondary inorganic aerosols formation in urban areas.

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.

Atmospheric reduced nitrogen: Sources, transformations, effects, and management

Human activities have increased atmospheric emissions and deposition of oxidized and reduced forms of nitrogen, but emission control programs have largely focused on oxidized nitrogen. As a result, in many regions of the world emissions of oxidized nitrogen are decreasing while emissions of reduced nitrogen are increasing. Emissions of reduced nitrogen largely originate from livestock waste and fertilizer application, with contributions from transportation sources in urban areas. Observations suggest a discrepancy between trends in emissions and deposition of reduced nitrogen in the U.S., likely due to an underestimate in emissions. In the atmosphere, ammonia reacts with oxides of sulfur and nitrogen to form fine particulate matter that impairs health and visibility and affects climate forcings. Recent reductions in emissions of sulfur and nitrogen oxides have limited partitioning with ammonia, decreasing long-range transport. Continuing research is needed to improve understanding of how shifting emissions alter formation of secondary particulates and patterns of transport and deposition of reactive nitrogen. Satellite remote sensing has potential for monitoring atmospheric concentrations and emissions of ammonia, but there remains a need to maintain and strengthen ground-based measurements and continue development of chemical transport models. Elevated nitrogen deposition has decreased plant and soil microbial biodiversity and altered the biogeochemical function of terrestrial, freshwater, and coastal ecosystems. Further study is needed on differential effects of oxidized versus reduced nitrogen and pathways and timescales of ecosystem recovery from elevated nitrogen deposition. Decreases in deposition of reduced nitrogen could alleviate exceedances of critical loads for terrestrial and freshwater indicators in many U.S. areas. The U.S. Environmental Protection Agency should consider using critical loads as a basis for setting standards to protect public welfare and ecosystems. The U.S. and other countries might look to European experience for approaches to control emissions of reduced nitrogen from agricultural and transportation sectors.Implications: In this Critical Review we synthesize research on effects, air emissions, environmental transformations, and management of reduced forms of nitrogen. Emissions of reduced nitrogen affect human health, the structure and function of ecosystems, and climatic forcings. While emissions of oxidized forms of nitrogen are regulated in the U.S., controls on reduced forms are largely absent. Decreases in emissions of sulfur and nitrogen oxides coupled with increases in ammonia are shifting the gas-particle partitioning of ammonia and decreasing long-range atmospheric transport of reduced nitrogen. Effort is needed to understand, monitor, and manage emissions of reduced nitrogen in a changing environment.

Hydrogen peroxide serves as pivotal fountainhead for aerosol aqueous sulfate formation from a global perspective

Traditional atmospheric chemistry posits that sulfur dioxide (SO2) can be oxidized to sulfate (SO42-) through aqueous-phase reactions in clouds and gas-phase oxidation. Despite adequate knowledge of traditional mechanisms, several studies have highlighted the potential for SO2 oxidation within aerosol water. Given the widespread presence of tropospheric aerosols, SO42- production through aqueous-phase oxidation in aerosol water could have a pervasive global impact. Here, we quantify the potential contributions of aerosol aqueous pathways to global sulfate formation based on the GEOS-Chem simulations and subsequent theoretical calculations. Hydrogen peroxide (H2O2) oxidation significantly influences continental regions both horizontally and vertically. Over the past two decades, shifts in the formation pathways within typical cities reveal an intriguing trend: despite reductions in SO2 emissions, the increased atmospheric oxidation capacities, like rising H2O2 levels, prevent a steady decline in SO42- concentrations. Abating oxidants would facilitate the benefit of SO2 reduction and the positive feedback in sulfate mitigation.

Inversion of Stabilized Large Droplet Clusters

We investigate the spontaneous rearrangement of microdroplets in a self-assembled droplet cluster levitating over a thin locally heated water layer. The center-to-periphery droplet diameter ratio (the "inversion coefficient") controls the onset of the inversion. Larger droplets can squeeze between smaller ones due to increased drag force on them from the air-vapor flow. In smaller clusters, the rotation of the droplets plays an important role since larger droplets rotating with the same angular velocity (dependent on the rotor of the airflow field) have higher viscous friction force with the liquid layer. It is desirable to avoid cluster inversion in experiments where individual droplet positions should be traced.

Aqueous-Phase Photoreactions of Mixed Aromatic Carbonyl Photosensitizers Yield More Oxygenated, Oxidized, and less Light-Absorbing Secondary Organic Aerosol (SOA) than Single Systems

Aromatic carbonyls have been mainly probed as photosensitizers for aqueous secondary organic aerosol (aqSOA) and light-absorbing organic aerosol (i.e., brown carbon or BrC) formation, but due to their organic nature, they can also undergo oxidation to form aqSOA and BrC. However, photochemical transformations of aromatic carbonyl photosensitizers, particularly in multicomponent systems, are understudied. This study explored aqSOA formation from the irradiation of aromatic carbonyl photosensitizers in mixed and single systems under cloud/fog conditions. Mixed systems consisting of phenolic carbonyls only (VL + ActSyr + SyrAld: vanillin [VL] + acetosyringone [ActSyr] + syringaldehyde [SyrAld]) and another composed of both nonphenolic and phenolic carbonyls (DMB + ActSyr + SyrAld: 3,4-dimethoxybenzaldehyde [DMB], a nonphenolic carbonyl, + ActSyr + SyrAld) were compared to single systems of VL (VL*) and DMB (DMB*), respectively. In mixed systems, the shorter lifetimes of VL and DMB indicate their diminished capacity to trigger the oxidation of other organic compounds (e.g., guaiacol [GUA], a noncarbonyl phenol). In contrast to the slow decay and minimal photoenhancement for DMB*, the rapid photodegradation and significant photoenhancement for VL* indicate efficient direct photosensitized oxidation (i.e., self-photosensitization). Relative to single systems, the increased oxidant availability promoted functionalization in VL + ActSyr + SyrAld and accelerated the conversion of early generation aqSOA in DMB + ActSyr + SyrAld. Moreover, the increased availability of oxidizable substrates countered by stronger oxidative capacity limited the contribution of mixed systems to aqSOA light absorption. This suggests a weaker radiative effect of BrC from mixed photosensitizer systems than BrC from single photosensitizer systems. Furthermore, more oxygenated and oxidized aqSOA was observed with increasing complexity of the reaction systems (e.g., VL* < VL + ActSyr + SyrAld < VL + ActSyr + SyrAld + GUA). This work offers new insights into aqSOA formation by emphasizing the dual role of organic photosensitizers as oxidant sources and oxidizable substrates.

Direct Observation of Particle-To-Particle Variability in Ambient Aerosol pH Using a Novel Analytical Approach Based on Surface-Enhanced Raman Spectroscopy

The pH of atmospheric aerosols is a key characteristic that profoundly influences their impacts on climate change, human health, and ecosystems. Despite widely performed aerosol pH research, determining the pH levels of individual atmospheric aerosol particles has been a challenge. This study presents a novel analytical technique that utilizes surface-enhanced Raman spectroscopy to assess the pH of individual ambient PM2.5-10 aerosol particles in conjunction with examining their hygroscopic behavior, morphology, and elemental compositions. The results revealed a substantial pH variation among simultaneously collected aerosol particles, ranging from 3.3 to 5.7. This variability is likely related to each particle's unique reaction and aging states. The extensive particle-to-particle pH variability suggests that atmospheric aerosols present at the same time and location can exhibit diverse reactivities, reaction pathways, phase equilibria, and phase separation properties. This pioneering study paves the way for in-depth investigations into particle-to-particle variability, size dependency, and detailed spatial and temporal variations of aerosol pH, thus deepening our understanding of atmospheric chemistry and its environmental implications.

Formation of Chromophores from cis-Pinonaldehyde Aged in Highly Acidic Conditions

Sulfuric acid in the atmosphere can participate in acid-catalyzed and acid-driven reactions, including those within secondary organic aerosols (SOA). Previous studies have observed enhanced absorption at visible wavelengths and significant changes in the chemical composition when SOA was exposed to sulfuric acid. However, the specific chromophores responsible for these changes could not be identified. The goals of this study are to identify the chromophores and determine the mechanism of browning in highly acidified α-pinene SOA by following the behavior of specific common α-pinene oxidation products, namely, cis-pinonic acid and cis-pinonaldehyde, when they are exposed to highly acidic conditions. The products of these reactions were analyzed with ultra-performance liquid chromatography coupled with photodiode array spectrophotometry and high-resolution mass spectrometry, UV-vis spectrophotometry, and nuclear magnetic resonance spectroscopy. cis-Pinonic acid (2) was found to form homoterpenyl methyl ketone (4), which does not absorb visible radiation, while cis-pinonaldehyde (3) formed weakly absorbing 1-(4-(propan-2-ylidene)cyclopent-1-en-1-yl)ethan-1-one (5) and 1-(4-isopropylcyclopenta-1,3-dien-1-yl)ethan-1-one (6) via an acid-catalyzed aldol condensation. This chemistry could be relevant for environments characterized by high sulfuric acid concentrations, for example, during the transport of organic compounds from the lower to the upper atmosphere by fast updrafts.

Trimethylamine from Subtropical Forests Rival Total Farmland Emissions in China

Many types of living plants release gaseous trimethylamine (TMA), making it a potentially important contributor to new particle formation (NPF) in remote areas. However, a panoramic view of the importance of forest biogenic TMA at the regional scale is lacking. Here, we pioneered nationwide mobile measurements of TMA across a transect of contiguous farmland in eastern China and a transect of subtropical forests in southern China. In contrast to the farmland route, TMA concentrations measured during the subtropical forest route correlated significantly with isoprene, suggesting potential TMA emissions from leaves. Our high time-resolved concentrations obtained from a weak photo-oxidizing atmosphere reflected freshly emitted TMA, indicating the highest emission intensity from irrigated dryland (set as the baseline of 10), followed by paddy field (7.1), subtropical evergreen forests (5.9), and subtropical broadleaf and mixed forests (4.3). Extrapolating their proportions roughly to China, subtropical forests alone, which constitute half of the total forest area, account for nearly 70% of the TMA emissions from the nation's total farmland. Our estimates, despite the uncertainties, take the first step toward large-scale assessment of forest biogenic amines, highlighting the need for observational and modeling studies to consider this hitherto overlooked source of TMA.

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.

Comparison of acidity and chemical composition of summertime cloud water and aerosol at an alpine site in Northwest China: Implications for the neutral property of clouds in the free troposphere

Aerosol and cloud acidity are essential to human health, ecosystem health and productivity, as well as climate effects. The main chemical composition of cloud water greatly varies in different regions, resulting in substantial differences in the pH of cloud water. However, the influences of the anthropogenic emissions of acidic gases and substances, alkaline dust components, and dicarboxylic acids (diacids) on the ground concerning the acidity of cloud water in the free troposphere of the Guanzhong Plain, China, remain clear. In this study, cloud water and PM2.5 samples were simultaneously collected in the troposphere (Mt. Hua, 2060 m a.s.l). The results indicated that the cloud water was alkaline (pH = 7.6) and PM2.5 was acidic (pH = 3.2). These results showed the neutral property of clouds collected in the heavily polluted Guanzhong Plain, although most previous studies always considered acidity as a marker of pollution. The sulfate (SO42-), nitrate (NO3-), and ammonium (NH4+) (SNA) of particulate matter and cloud water in the same period were compared. SO42- was dominant in particulate matters (accounting for 63.4 % of the total SNA) but substantially decreased in cloud water (only 30.1 % of the total SNA), whereas NO3- and NH4+ increased from 28.5 % and 8.2 % to 39.8 % and 30.2 %, respectively. This could be attributed to the complex formation mechanism and sources of SO42- and NO3- in the cloud. The results of ion balance indicated that a significant deficit of inorganic anion equivalents was observed in the cloud water samples. The high concentration of diacids in the cloud phase (1237.4 μg L-1) may facilitate the formation of salt complexes with NH4+, thus influencing the acidity of the cloud water. The pH of cloud water has increased in recent decades due to the sustained reduction of sulfur dioxide, which may also affect the acidity of future precipitation.

Mysteriously rapid rise in Legionnaires' disease incidence correlates with declining atmospheric sulfur dioxide

Legionnaires' disease (LD) is a severe form of pneumonia (∼10-25% fatality rate) caused by inhalation of aerosols containing Legionella, a pathogenic gram-negative bacteria. These bacteria can grow, spread, and aerosolize through building water systems. A recent dramatic increase in LD incidence has been observed globally, with a 9-fold increase in the United States from 2000 to 2018, and with disproportionately higher burden for socioeconomically vulnerable subgroups. Despite the focus of decades of research since the infamous 1976 outbreak, substantial knowledge gaps remain with regard to source of exposure and the reason(s) for the dramatic increase in LD incidence. Here, we rule out factors indicated in literature to contribute to its long-term increases and identify a hitherto unexplored explanatory factor. We also provide an epidemiological demonstration that the occurrence of LD is linked with exposure to cooling towers (CTs). Our results suggest that declining sulfur dioxide air pollution, which has many well-established health benefits, results in reduced acidity of aerosols emitted from CTs, which may prolong the survival duration of Legionella in contaminated CT droplets and contribute to the increase in LD incidence. Mechanistically associating decreasing aerosol acidity with this respiratory disease has implications for better understanding its transmission, predicting future risks, and informed design of preventive and interventional strategies that consider the complex impacts of continued sulfur dioxide changes.

Dissociation constants of relevant secondary organic aerosol components in the atmosphere

The presented studies focus on measuring the determination of the acidity constant (pKa) of relevant secondary organic aerosol components. For our research, we selected important oxidation products (mainly carboxylic acids) of the most abundant terpene compounds, such as α-pinene, β-pinene, β-caryophyllene, and δ-3-carene. The research covered the synthesis and determination of the acidity constant of selected compounds. We used three methods to measure the acidity constant, i.e., 1H NMR titration, pH-metric titration, Bates-Schwarzenbach spectrophotometric method. Moreover, the pKa values were calculated with Marvin 21.17.0 software to compare the experimentally derived values with those calculated from the chemical structure. pKa values measured with 1H NMR titration ranged from 3.51 ± 0.01 for terebic acid to 5.18 ± 0.06 for β-norcaryophyllonic acid. Moreover, the data determined by the 1H NMR method revealed a good correlation with the data obtained with the commonly used potentiometric and UV-spectroscopic methods (R2 = 0.92). In contrast, the comparison with in silico results exhibits a relatively low correlation (R2Marvin = 0.66). We found that most of the values calculated with the Marvin Program are lower than experimental values obtained with pH-metric titration with an average difference of 0.44 pKa units. For di- and tricarboxylic acids, we obtained two and three pKa values, respectively. A good correlation with the literature values was observed, for example, Howell and Fisher (1958) used pH-metric titration and measured pKa1 and pKa2 to be 4.48 and 5.48, while our results are 4.24 ± 0.10 and 5.40 ± 0.02, respectively.

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.

Surfaces of Atmospheric Droplet Models Probed with Synchrotron XPS on a Liquid Microjet

ConspectusThe atmosphere is a key part of the earth system comprising myriad chemical species in all basic forms of matter. Ubiquitous nano- and microscopic aerosol particles and cloud droplets suspended in the air play crucial roles in earth's climate and the formation of air pollution. Surfaces are a prominent part of aerosols and droplets, due to the high surface area to bulk volume ratios, but very little is known about their specific properties. Many atmospheric compounds are surface-active, leading to enhanced surface concentrations in aqueous solutions. Their distribution between the surface and bulk may determine heterogeneous chemistry and many other properties of aerosol and cloud droplets, but has not been directly observed.We used X-ray photoelectron spectroscopy (XPS) to obtain direct molecular-level information on the surface composition and structure of aqueous solutions of surface-active organics as model systems for atmospheric aerosol and cloud droplets. XPS is a vacuum-based technique enabled for volatile aqueous organic samples by the application of a high-speed liquid microjet. In combination with brilliant synchrotron X-rays, the chemical specificity of XPS allows distinction between elements in different chemical states and positions within molecular structures. We used core-level C 1s and N 1s signals to identify the alkyl and hydrophilic groups of atmospheric carboxylic acids, alkyl-amines, and their conjugate acids and bases. From this, we infer changes in the orientation of surface-adsorbed species and quantify their relative abundances in the surface. XPS-derived surface enrichments of the organics follow trends expected from their surface activities and we observed a preferential orientation at the surface with the hydrophobic alkyl chains pointing increasingly outward from the solution at higher concentrations. This provides a first direct experimental observation of well-established concepts of surface adsorption and confirms the soundness of the method.We mapped relative abundances of conjugate acid-base pairs in the aqueous solution surfaces from the respective intensities of distinctive XPS signals. For each pair, the protonation equilibrium was significantly shifted toward the neutral form in the surface, compared to the bulk solution, across the full pH range. This represents an apparent shift of the pKa in the surface, which may be toward either higher or lower pH, depending on whether the acid or base form of the pair is the neutral species. The surface shifts are broadly consistent with the relative differences in surface enrichment of the individual acid and base conjugates in binary aqueous solutions, with additional contributions from nonideal interactions in the surface. In aqueous mixtures of surface-active carboxylate anions with ammonium salts at near-neutral pH, we found that the conjugate carboxylic acids were further strongly enhanced. This occurs as the coadsorption of weakly basic carboxylate anions and weakly acidic ammonium cations forms ion-pair surface layers with strongly enhanced local abundances, increasing the probability of net proton transfer according to Le Chatelier's principle. The effect is stronger when the evaporation of ammonia from the surface further contributes to irreversibly perturb the protonation equilibrium, leaving a surplus of carboxylic acid. These surface-specific effects may profoundly influence atmospheric chemistry mediated by aqueous aerosols and cloud droplets but are currently not taken into account in atmospheric models.

Unraveling the Unimolecular Ion Chemistry of Protonated Isoprene and Prenol

The atmospheric chemistries of isoprene and prenol have been studied extensively; however, much of that research has focused on neutral or radical chemistry. Recent studies have demonstrated that under acidic conditions, isoprene and prenol can become protonated in the atmosphere, and we have explored the unimolecular chemistry of protonated isoprene and prenol with tandem mass spectrometry (using a triple-quadrupole mass spectrometer) and density functional theory. The collision-induced dissociation of protonated isoprene revealed two product ion channels: the neutral losses of C2H4 and H2, the former dominating over the latter. Protonated prenol dissociates by four product ion channels: the neutral losses of water, formaldehyde, methanol, and propene, with the former two being minor channels and the latter two being major channels. Density functional theory supplemented with CBS-QB3 single-point calculations revealed the underlying mechanisms to explain the breakdown behavior. The two competing channels from protonated isoprene could easily be rationalized due to the relative energy difference between key transition states along the reaction coordinates. However, in the case of protonated prenol, it was revealed that the minor products observed in the breakdown of protonated prenol had significantly lower reaction barriers when compared to the major products, an apparent contradiction. This could be rationalized if the initial ion population entering the collision cell is comproed of several isomeric species on the minimum energy reaction pathway, species populated by collisional excitation in the ion source region.

Influence of aerosol acidity and organic ligands on transition metal solubility and oxidative potential of fine particulate matter in urban environments

The adverse health effects of air pollution around the world have been associated with the inhalation of fine particulate matter (PM2.5). Such outcomes are thought to be related to the induction of oxidative stress due to the excess formation of reactive oxygen species (ROS) in the respiratory and cardiovascular systems. The ability of airborne chemicals to deplete antioxidants and to form ROS is known as oxidative potential (OP). Here we studied the influence of aerosol acidity and organic ligands on the solubility of transition metals, in particular iron (Fe) and copper (Cu), and on the OP of PM2.5 from Canadian National Air Pollution Surveillance urban sites in Toronto, Vancouver, and Hamilton. Using chemical assays and model simulations of the lung redox chemistry, we quantified ROS formation in the lung lining fluid, targeting superoxide anion (O2•-), hydrogen peroxide (H2O2), and hydroxyl radical (•OH), as well as the PM2.5 redox potential (RP). Experimental •OH formation (OPOH) showed high correlations with RP and model-predicted ROS metrics. Both aerosol acidity and oxalate content enhanced the solubility of transition metals, with oxalate showing a stronger association. While experimental OP metrics were primarily associated with species of primary origin such as elemental carbon, Fe, and Cu, model-predicted ROS were associated with secondary processes including proton- and ligand-mediated dissolution of Fe. Model simulations showed that water-soluble Cu was the main contributor to O2•- formation, while water-soluble Fe dominated the formation of highly reactive •OH radical, particularly at study sites with highly acidic aerosol and elevated levels of oxalate. This study underscores the importance of reducing transition metal emissions in urban environments to improve population health.

Comparison of chemical composition and acidity of size-resolved inorganic aerosols at the top and foot of Mt. Hua, Northwest China: The role of the gas-particle distribution of ammonia

Aerosol pH is not only a diagnostic indicator of secondary aerosol formation, but also a key factor in the specific chemical reaction routes that produce sulfate and nitrate. To understand the characteristics of aerosol acidity in the Mt. Hua, the chemical fractions of water-soluble inorganic ions in the atmospheric PM2.5 and size-resolved particle at the top and foot of Mt. Hua in summer 2020 were studied. The results showed the mass concentrations of PM2.5 and water-soluble ions at the foot were 2.0-2.6 times higher than those at the top. The secondary inorganic ions, i.e., SO42-, NO3-, and NH4+ (SNA) were 56 %-61 % higher by day than by night. SO42- was mainly distributed in the fine particles (Dp < 2.1 μm). NO3- showed a unimodal size distribution (peaking at 0.7-1.1 μm) at the foot and a bimodal (0.7-1.1 μm and 4.7-5.8 μm) size distribution at the top. At the top site, the distribution of NO3- in coarse particles (> 2.1 μm) was mainly attributed to the gaseous HNO3 volatilized from fine particles reacting with cations in coarse particles to form non-volatile salts (such as Ca(NO3)2). The pH values of PM2.5 were 2.7 ± 1.3 and 3.3 ± 0.42 at the top and foot, respectively. NH4+/NH3(g) plays a decisive role in stabilizing aerosol acidity. In addition, the increase of the liquid water content (LWC) at the foot facilitates the gas-particle conversion of NH3, while the H+ concentration was diluted, resulting in a decrease in acidity at the foot. NH4+/NH3 had good linear correlations with SO42-, NO3-, and LWC during the daytime at both sites, indicating that SO42-, NO3-, and LWC together affect the gas-particle distribution of ammonia by day: however, the effect of LWC at night was not evident.

Long-term trends in inorganic aerosol chemical composition and chemistry at an urban and rural site in the northeastern US

Atmospheric nitrate and sulfate are major inorganic particulate matter components that impact human and ecosystem health and air quality. Over the last several decades, emissions of the precursor gases, nitrogen oxides (NOx = NO + NO2) and sulfur dioxide (SO2), have dramatically decreased in the US in response to federal regulations. However, the response in concentrations of particulate nitrate (pNO3) and sulfate (pSO4) have not followed predictions due to complex non-linear chemistry feedbacks that may differ amongst environments (i.e., urban vs. rural). In this study, we explored the long-term response of particle chemistry for urban and rural environments in southern New England, a region historically impacted by NOx and SO2 emissions. Particulate matter (PM10) samples collected via the same method from 2005 to 2015 at urban and rural locations in Rhode Island were analyzed for their major inorganic components, and air mass trajectories and statistical analysis were used to identify source regions over time. Our results indicated a significant urban-rural aerosol chemical composition gradient for sampling locations within 40 km. Over time, as anthropogenic influences have decreased, the relative contribution of marine and crustal sources has increased greatly, impacting fine and coarse particle chemistry in recent years. Total mass concentrations of chemical species, particularly anthropogenic pSO4 and particulate ammonium (pNH4), have shown dramatic decreases over the ten years at both the urban and rural sites; however, pNO3 concentration increased by 95 % and 57 % in the urban and rural sites, respectively, despite significant NOx emission reductions. Our results demonstrate that changes in chemical mechanisms due to the decrease in SO2 emissions contributed to decreases in pNH4, along with enhanced pNO3 concentration. Furthermore, the change in SO2 emissions has significantly impacted the atmospheric lifetime and transport distance of pNH4, favoring more localized contributions in recent years.

Anthropogenic pollutants induce enhancement of aerosol acidity at a mountainous background atmosphere in southern China

Aerosol acidity plays a crucial role in atmospheric physicochemical processes, climate change and human health, particularly in the formation of secondary organic aerosols (SOA). However, understanding the characteristics and driving factors of aerosol acidity in background mountains has been limited. In this study, we conducted intensive field measurements in the Nanling mountains during the dry and wet seasons to analyze aerosol pH characteristics and their driving factors using sensitivity tests. The mean aerosol pH in the background mountains was found to be 2.68 ± 0.55, with values ranging from 0.38 to 4.44, significantly lower than predicted values in northern China. Sensitivity tests revealed that aerosol acidity in the background atmosphere was more responsive to dominant chemical species (T-NH3 (= NH4+ + NH3) and SO42-) rather than relative humidity and temperature. Additionally, we observed that sulfate and ammonium, transported occasionally by dryer northern air masses, had a substantial impact on decreasing aerosol pH at the site. Similar to the southeastern United States, NH4+/NH3 also dominated the total buffer capacity of aerosol acidity in the Nanling mountains. The strong aerosol acidity in this area is expected to have adverse effects on regional air quality and climate by enhancing SOA formation and regulating the dry deposition of inorganic reactive nitrogen.

Acidity in rainwater and airborne suspended particles in the southwestern coast of the East Sea (Sea of Japan): Their potential impact on seawater total alkalinity

Our understanding of the impact of atmospheric acid deposition on marine carbonate system remains limited, largely due to a lack of data regarding acidity present in atmospheric particles and precipitation. Previous research has relied on the electroneutrality-based ion balance method for indirect estimation of atmospheric acidity. In this study, atmospheric samples collected at a coastal site of South Korea were mixed with seawater to measure the change in seawater total alkalinity (ΔTAAPL) associated with atmospheric proton loading. For the precipitation samples, the measured ΔTAAPL and electroneutrality-based estimates showed a significant correlation. However, we did not observe similar results for the atmospheric particle samples. Furthermore, the decrease in oceanic TA due to ΔTAAPL was substantially smaller than that in dissolved inorganic carbon from concurrent nitrogen fertilization. Consequently, the adverse impact of acid deposition on ocean acidification or air-sea exchange of CO2 appears to be insignificant on a short-term scale.

Conversion of Catechol to 4-Nitrocatechol in Aqueous Microdroplets Exposed to O3 and NO2

Catechol is a widespread atmospheric dihydroxybenzene present in vehicle emissions, biomass burning, and combustion pollution plumes. Although the daytime reactivity of catechol is controlled by ozone (O3) and hydroxyl radicals (HO), the action of nitrate radicals (NO3) on the surface of aqueous atmospheric particles should become significant at night. This work simulates nighttime interfacial chemistry between hydrated catechol and adsorbed NO3 to form 4-nitrocatechol during experiments lasting ≤1 μs. Surface-sensitive online electrospray ionization mass spectrometry (OESI-MS) examines the reaction on the water surface under variable ratios of [NO2] and [O3]. The produced 4-nitrocatechol is quantified by a standard addition in real-time experiments under [NO2]:[O3] ratios of 1:1, 2:1, 3:1, and 4:1. Three mechanisms contribute to produce 4-nitrocatechol: (1) electron and proton transfers from catechol to NO3, forming a semiquinone radical, (2) electrophilic NO3 attack to the ring to yield a cyclohexadienyl radical intermediate, and (3) electrophilic attack to the ring by nitronium ion (NO2+) formed at the interface of water by colliding N2O5(g) at low pH. Ozonolysis competes strongly with nitration when using [NO2]:[O3] ratios 1:1 or smaller. Instead, nighttime chemistry under higher molar ratios proceeds mainly by nitration with a maximum yield of 0.90 for [NO2]:[O3] = 4:1.

The role of NO2:O3 molar ratios on the interfacial nitration of catechol emitted from combustion processes remains unexplored. The work reports that nitration becomes prevalent for molar ratios of NO2:O3 ≥ 2:1.

How the Acidity of Water Droplets and Films Is Controlled by the Air-Water Interface

Acidity is a key determinant of chemical reactivity in atmospheric aqueous aerosols and water microdroplets used for catalysis. However, many fundamental questions about these systems have remained elusive, including how their acidity differs from that of bulk solutions, the degree of heterogeneity between their core and surface, and how the acid-base properties are affected by their size. Here, we perform hybrid density functional theory (DFT)-quality neural network-based molecular simulations with explicit nuclear quantum effects and combine them with an analytic model to describe the pH and self-ion concentrations of droplets and films for sizes ranging from nm to μm. We determine how the acidity of water droplets and thin films is controlled by the properties of the air-water interface and by their surface-to-volume ratio. We show that while the pH is uniform in each system, hydronium and hydroxide ions exhibit concentration gradients that span the two outermost molecular layers, enriching the interface with hydronium cations and depleting it with hydroxide anions. Acidity depends strongly on the surface-to-volume ratio for system sizes below a few tens of nanometers, where the core becomes enriched in hydroxide ions and the pH increases as a result of hydronium stabilization at the interface. These results obtained for pure water systems have important implications for our understanding of chemical reactivity in atmospheric aerosols and for catalysis in aqueous microdroplets.

Abundant nitrogenous secondary organic aerosol formation accelerated by cloud processing

Nitrogenous organic (CHON), crucial for secondary organic aerosol (SOA), forms through poorly studied mechanisms in clouds. Our study explores CHON transformation during cloud processes (CPs). These processes play a vital role in enhancing the variety of CHONs, leading to the formation of CHONs with oxygen atom counts ranging from 1 to 10 and double bond equivalent (DBE) values spanning from 2 to 10. We proposed that the CHONs formed during CPs are formed through aqueous phase reactions with CHO compound precursors via nucleophilic attacks by NH3. This scheme can be account for roughly three-quarters of the CHONs by number in cloud water, and near two-thirds of all CHONs are formed through reactions between NH3 and carbonyl-containing biogenic volatile organic compound (BVOC) ozonolysis intermediates. This study provides the first insights into the evolution of CHONs during CPs and reveals the significant roles of CPs in the formation of CHONs.

Impact of pH on Gas-Particle Partitioning of Semi-Volatile Organics in Multicomponent Aerosol

The partitioning of semivolatile organic compounds (SVOCs) between the condensed and gas phases can have significant implications for the properties of aerosol particles. In addition to affecting size and composition, this partitioning can alter radiative properties and impact cloud activation processes. We present measurements and model predictions on how activity and pH influence the evaporation of SVOCs from particles to the gas phase, specifically investigating aqueous inorganic particles containing dicarboxylic acids (DCAs). The aerosols are studied at the single-particle level by using optical trapping and cavity-enhanced Raman spectroscopy. Optical resonances in the spectra enable precise size tracking, while vibrational bands allow real-time monitoring of pH. Results are compared to a Maxwell-type model that accounts for volatile and nonvolatile solutes in aqueous droplets that are held at a constant relative humidity. The aerosol inorganic-organic mixture functional group activity coefficients thermodynamic model and Debye-Hückel theory are both used to calculate the activities of the species present in the droplet. For DCAs, we find that the evaporation rate is highly sensitive to the particle pH. For acidity changes of approximately 1.5 pH units, we observe a shift from a volatile system to one that is completely nonvolatile. We also observe that the pH itself is not constant during evaporation; it increases as DCAs evaporate, slowing the rate of evaporation until it eventually ceases. Whether a DCA evaporates or remains a stable component of the droplet is determined by the difference between the lowest pKa of the DCA and the pH of the droplet.

Significant Reductions in Secondary Aerosols after the Three-Year Action Plan in Beijing Summer

Air quality in China has continuously improved during the Three-Year Action Plan (2018-2020); however, the changes in aerosol composition, properties, and sources in Beijing summer remain poorly understood. Here, we conducted real-time measurements of aerosol composition in five summers from 2018 to 2022 along with WRF-Community Multiscale Air Quality simulations to characterize the changes in aerosol chemistry and the roles of meteorology and emission reductions. Largely different from winter, secondary inorganic aerosol and photochemical-related secondary organic aerosol (SOA) showed significant decreases by 55-67% in summer, and the most decreases occurred in 2021. Comparatively, the decreases in the primary aerosol species and gaseous precursors were comparably small. While decreased atmospheric oxidation capacity as indicated by ozone changes played an important role in changing SOA composition, the large decrease in aerosol liquid water and small increase in particle acidity were critical for nitrate changes by decreasing gas-particle partitioning substantially (∼28%). Analysis of meteorological influences demonstrated clear and similar transitions in aerosol composition and formation mechanisms at a relative humidity of 50-60% in five summers. Model simulations revealed that emission controls played the decisive role in reducing sulfate, primary OA, and anthropogenic SOA during the Three-Year Action Plan, while meteorology affected more nitrate and biogenic SOA.

Multiphase Reactions between Organic Peroxides and Sulfur Dioxide in Internally Mixed Inorganic and Organic Particles: Key Roles of Particle Phase Separation and Acidity

Organic peroxides (POs) are ubiquitous in the atmosphere and particularly reactive toward dissolved sulfur dioxide (SO2), yet the reaction kinetics between POs and SO2, especially in complex inorganic-organic mixed particles, remain poorly constrained. Here, we report the first investigation of the multiphase reactions between SO2 and POs in monoterpene-derived secondary organic aerosol internally mixed with different inorganic salts (ammonium sulfate, ammonium bisulfate, or sodium nitrate). We find that when the particles are phase-separated, the PO-S(IV) reactivity is consistent with that measured in pure SOA and depends markedly on the water content in the organic shell. However, when the organic and inorganic phases are miscible, the PO-S(IV) reactivity varies substantially among different aerosol systems, mainly driven by their distinct acidities (not by ionic strength). The second-order PO-S(IV) rate constant decreases monotonically from 5 × 105 to 75 M-1 s-1 in the pH range of 0.1-5.6. Both proton catalysis and general acid catalysis contribute to S(IV) oxidation, with their corresponding third-order rate constants determined to be (6.4 ± 0.7) × 106 and (6.9 ± 4.6) × 104 M-2 s-1 at pH 2-6, respectively. The measured kinetics imply that the PO-S(IV) reaction in aerosol is an important sulfate formation pathway, with the reaction kinetics dominated by general acid catalysis at pH > 3 under typical continental atmospheric conditions.

High Chlorine Concentrations in an Unconventional Oil and Gas Development Region and Impacts on Atmospheric Chemistry

Growth in unconventional oil and gas development (UOGD) in the United States has increased airborne emissions, raising environmental and human health concerns. To assess the potential impacts on air quality, we deployed instrumentation in Karnes City, Texas, a rural area in the middle of the Eagle Ford Shale. We measured several episodes of elevated Cl2 levels, reaching maximum hourly averages of 800 ppt, the highest inland Cl2 concentration reported to date. Concentrations peak during the day, suggesting a strong local source (given the short photolysis lifetime of Cl2) and/or a photoinitiated production mechanism. Well preproduction activity near the measurement site is a plausible source of these high Cl2 levels via direct emission and photoactive chemistry. ClNO2 is also observed, but it peaks overnight, consistent with well-known nocturnal formation processes. Observations of organochlorines in the gas and particle phases reflect the contribution of chlorine chemistry to the formation of secondary pollutants in the area. Box modeling results suggest that the formation of ozone at this location is influenced by chlorine chemistry. These results suggest that UOGD can be an important source of reactive chlorine in the atmosphere, impacting radical budgets and the formation of secondary pollutants in these regions.

Changed mercury speciation in clouds driven by changing cloud water chemistry and impacts on photoreduction: Field evidence at Mt. Tai in eastern China

Chemical speciation of mercury (Hg) in clouds largely determines the photochemistry of Hg in the atmosphere and consequently influences Hg deposition on the surface through precipitation. Cloud water chemistry has notably changed over the last decade in response to global changes, however, the effects on Hg speciation remain poorly understood. During summer 2021, we collected sixty cloud water samples at Mt. Tai in eastern China and compared the cloud chemistry and Hg speciation with our previous findings during summer 2015. The results showed that although there were no statistically significant differences in the concentrations of total Hg (THg), dissolved Hg (DHg), and particulate Hg (PHg), there was a distinct shift in DHg species from the predominated Hg-DOM (78.6% in 2015 campaign) to the more homogeneously distributed Hg(OH)2 (28.4% in 2021 campaign), HgBr2 (26.5%), Hg-DOM (17.3%) and HgBrOH (17.0%). Changes in cloud water chemistry, particularly the significant increase in pH values to 6.49 ± 0.27 and unexpectedly high levels of bromide ions (Br-, 0.19 ± 0.22 mg L-1), were found to drive the changing of Hg speciation by enhancing Hg(II) hydrolysis and binding by Br-. Elevated Br- originating primarily from the continent likely caused noticeable differences in the dominating DHg species between cloud water sourced from marine and continental regions. The changes in chemical speciation of DHg were estimated to result in a 2.6-fold decrease in Hg(II) photoreduction rate between 2015 and 2021 campaigns (0.178 ± 0.054 h-1 vs. 0.067 ± 0.027 h-1), implying a shortened lifetime of atmospheric Hg and increased ecological risks associated with Hg wet deposition.

Characterization and dark oxidation of the emissions of a pellet stove

Pellet combustion in residential heating stoves has increased globally during the last decade. Despite their high combustion efficiency, the widespread use of pellet stoves is expected to adversely impact air quality. The atmospheric aging of pellet emissions has received even less attention, focusing mainly on daytime conditions, while the degree to which pellet emissions undergo night-time aging as well as the role of relative humidity remain poorly understood. In this study, environmental simulation chamber experiments were performed to characterize the fresh and aged organic aerosol (OA) emitted by a pellet stove. The fresh pellet stove PM1 (particulate matter with an aerodynamic diameter less than 1 μm) emissions consisted mainly of OA (93 ± 4%, mean ± standard deviation) and black carbon (5 ± 3%). The primary OA (POA) oxygen-to-carbon ratio (O : C) was 0.58 ± 0.04, higher than that of fresh logwood emissions. The fresh OA at a concentration of 70 μg m-3 (after dilution and equilibration in the chamber) consisted of semi-volatile (68%), low and extremely low volatility (16%) and intermediate-volatility (16%) compounds. The oxidation of pellet emissions under dark conditions was investigated by injecting nitrogen dioxide (NO2) and ozone (O3) into the chamber, at different (10-80%) relative humidity (RH) levels. In all dark aging experiments secondary organic aerosol (SOA) formation was observed, increasing the OA levels after a few hours of exposure to NO3 radicals. The change in the aerosol composition and the extent of oxidation depended on RH. For low RH, the SOA mass formed was up to 30% of the initial OA, accompanied by a moderate change in both O : C levels (7-8% increase) and the OA spectrum. Aging under higher RH conditions (60-80%) led to a more oxygenated aerosol (increase in O : C of 11-18%), but only a minor (1-10%) increase in OA mass. The increase in O : C at high RH indicates the importance of heterogeneous aqueous reactions in this system, that oxidize the original OA with a relatively small net change in the OA mass. These results show that the OA in pellet emissions can chemically evolve under low photochemical activity (e.g. the wintertime period) with important enhancement in SOA mass under certain conditions.

Inversion model for extracting chemically resolved depth profiles across liquid interfaces of various configurations from XPS data: PROPHESY

PROPHESY, a technique for the reconstruction of surface-depth profiles from X-ray photoelectron spectroscopy data, is introduced. The inversion methodology is based on a Bayesian framework and primal-dual convex optimization. The acquisition model is developed for several geometries representing different sample types: plane (bulk sample), cylinder (liquid microjet) and sphere (droplet). The methodology is tested and characterized with respect to simulated data as a proof of concept. Possible limitations of the method due to uncertainty in the attenuation length of the photo-emitted electron are illustrated.

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.

Thermodynamical framework for effective mitigation of high aerosol loading in the Indo-Gangetic Plain during winter

The Indo-Gangetic Plain (IGP) experiences severe air pollution every winter, with ammonium chloride and ammonium nitrate as the major inorganic fractions of fine aerosols. Many past attempts to tackle air pollution in the IGP were inadequate, as they targeted a subset of the primary pollutants in an environment where the majority of the particulate matter burden is secondary in nature. Here, we provide new mechanistic insight into aerosol mitigation by integrating the ISORROPIA-II thermodynamical model with high-resolution simultaneous measurements of precursor gases and aerosols. A mathematical framework is explored to investigate the complex interaction between hydrochloric acid (HCl), nitrogen oxides (NOx), ammonia (NH3), and aerosol liquid water content (ALWC). Aerosol acidity (pH) and ALWC emerge as governing factors that modulate the gas-to-particle phase partitioning and mass loading of fine aerosols. Six "sensitivity regimes" were defined, where PM1 and PM2.5 fall in the "HCl and HNO3 sensitive regime", emphasizing that HCl and HNO3 reductions would be the most effective pathway for aerosol mitigation in the IGP, which is ammonia-rich during winter. This study provides evidence that precursor abatement for aerosol mitigation should not be based on their descending mass concentrations but instead on their sensitivity to high aerosol loading.

Global environmental implications of atmospheric methane removal through chlorine-mediated chemistry-climate interactions

Atmospheric methane is both a potent greenhouse gas and photochemically active, with approximately equal anthropogenic and natural sources. The addition of chlorine to the atmosphere has been proposed to mitigate global warming through methane reduction by increasing its chemical loss. However, the potential environmental impacts of such climate mitigation remain unexplored. Here, sensitivity studies are conducted to evaluate the possible effects of increasing reactive chlorine emissions on the methane budget, atmospheric composition and radiative forcing. Because of non-linear chemistry, in order to achieve a reduction in methane burden (instead of an increase), the chlorine atom burden needs to be a minimum of three times the estimated present-day burden. If the methane removal target is set to 20%, 45%, or 70% less global methane by 2050 compared to the levels in the Representative Concentration Pathway 8.5 scenario (RCP8.5), our modeling results suggest that additional chlorine fluxes of 630, 1250, and 1880 Tg Cl/year, respectively, are needed. The results show that increasing chlorine emissions also induces significant changes in other important climate forcers. Remarkably, the tropospheric ozone decrease is large enough that the magnitude of radiative forcing decrease is similar to that of methane. Adding 630, 1250, and 1880 Tg Cl/year to the RCP8.5 scenario, chosen to have the most consistent current-day trends of methane, will decrease the surface temperature by 0.2, 0.4, and 0.6 °C by 2050, respectively. The quantity and method in which the chlorine is added, its interactions with climate pathways, and the potential environmental impacts on air quality and ocean acidity, must be carefully considered before any action is taken.

Aqueous-phase photo-oxidation of selected green leaf volatiles initiated by OH radicals: Products and atmospheric implications

C₅- and C₆- unsaturated oxygenated organic compounds emitted by plants under stress like cutting, freezing or drying, known as Green Leaf Volatiles (GLVs), may clear some of the existing uncertainties in secondary organic aerosol (SOA) budget. The transformations of GLVs are a potential source of SOA components through photo-oxidation processes occurring in the atmospheric aqueous phase. Here, we investigated the aqueous photo-oxidation products from three abundant GLVs (1-penten-3-ol, (Z)-2-hexen-1-ol, and (E)-2-hexen-1-al) induced by OH radicals, carried out in a photo-reactor under simulated solar conditions. The aqueous reaction samples were analyzed using advanced hyphenated mass spectrometry techniques: capillary gas chromatography mass spectrometry (c-GC-MS); and reversed-phase liquid chromatography high resolution mass spectrometry (LC-HRMS). Using carbonyl-targeted c-GC-MS analysis, we confirmed the presence of propionaldehyde, butyraldehyde, 1-penten-3-one, and 2-hexen-1-al in the reaction samples. The LC-HRMS analysis confirmed the presence of a new carbonyl product with the molecular formula C₆H₁₀O₂, which probably bears the hydroxyhexenal or hydroxyhexenone structure. Density functional theory (DFT)-based quantum calculations were used to evaluate the experimental data and obtain insight into the formation mechanism and structures of the identified oxidation products via the addition and hydrogen-abstraction pathways. DFT calculations highlighted the importance of the hydrogen abstraction pathway leading to the new product C₆H₁₀O₂. Atmospheric relevance of the identified products was evaluated using a set of physical property data like Henry's law constant (HLC) and vapor pressure (VP). The unknown product of molecular formula C₆H₁₀O₂ has higher HLC and lower VP than the parent GLV and thus has potential to remain in the aqueous phase leading to possible aqueous SOA formation. Other observed carbonyl products are likely first stage oxidation products and precursors of aged SOA.

Brown Carbon from Photo-Oxidation of Glyoxal and SO2 in Aqueous Aerosol

Aqueous-phase dark reactions during the co-oxidation of glyoxal and S(IV) were recently identified as a potential source of brown carbon (BrC). Here, we explore the effects of sunlight and oxidants on aqueous solutions of glyoxal and S(IV), and on aqueous aerosol exposed to glyoxal and SO₂. We find that BrC is able to form in sunlit, bulk-phase, sulfite-containing solutions, albeit more slowly than in the dark. In more atmospherically relevant chamber experiments where suspended aqueous aerosol particles are exposed to gas-phase glyoxal and SO₂, the formation of detectable amounts of BrC requires an OH radical source and occurs most rapidly after a cloud event. From these observations we infer that this photobrowning is caused by radical-initiated reactions as evaporation concentrates aqueous-phase reactants and aerosol viscosity increases. Positive-mode electrospray ionization mass spectrometric analysis of aerosol-phase products reveals a large number of C_xH_yO_z oligomers that are reduced rather than oxidized (relative to glyoxal), with the degree of reduction increasing in the presence of OH radicals. This again suggests a radical-initiated redox mechanism where photolytically produced aqueous radical species trigger S(IV)-O₂ auto-oxidation chain reactions, and glyoxal-S(IV) redox reactions especially if aerosol-phase O₂ is depleted. This process may contribute to daytime BrC production and aqueous-phase sulfur oxidation in the atmosphere. The BrC produced, however, is about an order of magnitude less light-absorbing than wood smoke BrC at 365 nm.

Abundant nitrogen oxide and weakly acidic environment synergistically promote daytime particulate nitrate pollution

Nitrate is formed through the chemical production of gas-phase nitric acid and subsequent partitioning to the aerosol phase during the daytime. Many studies in the past separated these two aspects, even though they occur simultaneously in the atmosphere. To better understand the nitrate formation mechanism and effectively mitigate its production, it is necessary to consider the synergy between these two mechanisms. For this, we analyze hourly-speciated ambient observations data, with EK&TMA (Empirical Kinetic & Thermodynamic Modeling Approach) map to comprehensively explore the factors controlling nitrate production. Results show that precursor NO₂ concentration and aerosol pH, which are related to anthropogenic activities, are the two major factors for chemical kinetics production and gas/particle thermodynamic partitioning processes respectively. Abundant NO₂ and weakly acidic environments are favorable conditions for daytime particulate nitrate pollution, thus collaborative control of coal source, vehicle source, and dust source is needed to alleviate nitrate pollution.

Imaging of pH distribution inside individual microdroplet by stimulated Raman microscopy

Aerosol microdroplets as microreactors for many important atmospheric reactions are ubiquitous in the atmosphere. pH largely regulates the chemical processes within them; however, how pH and chemical species spatially distribute within an atmospheric microdroplet is still under intense debate. The challenge is to measure pH distribution within a tiny volume without affecting the chemical species distribution. We demonstrate a method based on stimulated Raman scattering microscopy to visualize the three-dimensional pH distribution inside single microdroplets of varying sizes. We find that the surface of all microdroplets is more acidic, and a monotonic trend of pH decreasing is observed in the 2.9-μm aerosol microdroplet from center to edge, which is well supported by molecular dynamics simulation. However, bigger cloud microdroplet differs from small aerosol for pH distribution. This size-dependent pH distribution in microdroplets can be related to the surface-to-volume ratio. This work presents noncontact measurement and chemical imaging of pH distribution in microdroplets, filling the gap in our understanding of spatial pH in atmospheric aerosol.

Characterization of fine particulate matter water-soluble inorganic ions and estimation of aerosol acidity at three COALESCE network sites - Mysuru, Bhopal, and Mesra - in India

The study was carried out to understand the chemical, spatiotemporal characteristics of water-soluble inorganic ions (WSIIs), their association with PM_2.5 mass, and aerosol acidity in three COALESCE (carbonaceous aerosol emissions, source apportionment, and climate impacts) network sites of India (Mesra - Eastern India, Bhopal - Central India and Mysuru - Southern India). Alternate-day 24-h integrated bulk PM_2.5 samples were collected during 2019 along with on-site meteorological parameters. Annual average PM_2.5 concentrations were 67 ± 46 µg m^-3, 54 ± 47 µg m^-3, and 30 ± 24 µg m^-3 at Mesra, Bhopal, and Mysuru, respectively. PM_2.5 concentrations exceeded the annual mean (40 µg m^-3) recommended by the National Ambient Air Quality Standards (NAAQS) at Mesra and Bhopal. WSIIs existed in PM_2.5 mass at Mesra (50.5%), Bhopal (39.6%), and Mysuru (29.2%). SO₄^2-, NO₃^-, and NH₄⁺ (SNA) were major secondary inorganic ions in total WSIIs, with an annual average of 88.4% in Mesra and 82.0% in Bhopal 78.4% in Mysuru. Low NO₃^-/SO₄^2- ratios annually at Mesra (0.41), Bhopal (0.44), and Mysuru (0.24) indicated that stationary sources dominated vehicular emissions (1.0). Aerosol acidity varied from region to region and season to season depending on the presence of NH₄⁺, the dominant counter-ion to neutralize anions. Aerosols were near-neutral or alkaline at all three sites, except during the pre-monsoon season in Mysuru. An assessment of neutralization pathways for major anions [SO₄^2-  + NO₃^-] suggests that they mainly existed as sulfate and nitrate salts such as ammonium sulfate ((NH₄)₂SO₄) and ammonium bisulfate (NH₄HSO₄) in conjunction with ammonium nitrate (NH₄NO₃).

Evaluation of Probes to Measure Oxidizing Organic Triplet Excited States in Aerosol Liquid Water

Oxidizing triplet excited states of organic matter (³C*) drive numerous reactions in fog/cloud drops and aerosol liquid water (ALW). Quantifying oxidizing triplet concentrations in ALW is difficult because ³C* probe loss can be inhibited by the high levels of dissolved organic matter (DOM) and copper in particle water, leading to an underestimate of triplet concentrations. In addition, illuminated ALW contains high concentrations of singlet molecular oxygen (¹O₂*), which can interfere with ³C* probes. Our overarching goal is to find a triplet probe that has low inhibition by DOM and Cu(II) and low sensitivity to ¹O₂*. To this end, we tested 12 potential probes from a variety of compound classes. Some probes are strongly inhibited by DOM, while others react rapidly with ¹O₂*. One of the probe candidates, (phenylthiol)acetic acid (PTA), seems well suited for ALW conditions, with mild inhibition and fast rate constants with triplets, but it also has weaknesses, including a pH-dependent reactivity. We evaluated the performance of both PTA and syringol (SYR) as triplet probes in aqueous extracts of particulate matter. While PTA is less sensitive to inhibition than SYR, it results in lower triplet concentrations, possibly because it is less reactive with weakly oxidizing triplets.

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.

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.

Counterintuitive Oxidation of Alcohols at Air-Water Interfaces

This study shows that the oxidation of alcohols can rapidly occur at air-water interfaces. It was found that methanediols (HOCH₂OH) orient at air-water interfaces with a H atom of the -CH₂- group pointing toward the gaseous phase. Counterintuitively, gaseous hydroxyl radicals do not prefer to attack the exposed -CH₂- group but the -OH group that forms hydrogen bonds with water molecules at the surface via a water-promoted mechanism, leading to the formation of formic acids. Compared with gaseous oxidation, the water-promoted mechanism at the air-water interface significantly lowers free-energy barriers from ∼10.7 to ∼4.3 kcal·mol^-1 and therefore accelerates the formation of formic acids. The study unveils a previously overlooked source of environmental organic acids that are bound up with aerosol formation and water acidity.

Observational evidence for the non-suppression effect of atmospheric chemical modification on the ice nucleation activity of East Asian dust

Airborne mineral dust triggers ice formation in clouds and alters cloud microphysical properties by acting as ice-nucleating particles (INPs), potentially influencing weather and climate at regional and global scales. Anthropogenic pollution would modify natural mineral dust during the atmospheric transport process. However, the effects of anthropogenic pollution aging on the ice nucleation activity (INA) of mineral dust remain not well-understood. In this study, we investigated the immersion mode ice nucleation properties and particle chemical characterizations of collected size-resolved Asian dust samples (eight particle size classes ranging from 0.18 to 10.0 μm), and testified the chemical modification of aged dust particles via particle chemistry and morphology analyses including the mass concentrations of particulate matter, the water-soluble ion concentrations, the mental element concentrations, and single-particle morphology. The mass fraction of Ca²⁺ in element Ca and the mean relative mass proportions of supermicron Ca²⁺ increased by 67.0 % and 3.5-11.2 % in aged Asian dust particles, respectively, suggesting the occurrence of heterogeneous reactions. On the other hand, the total INP concentrations (total N_INP) and total ice nucleation active site densities (total n_s(T)) were consistent between aged and normal dust particles (0.62-1.18 times) without a statistically significant difference. And the N_INP and n_s(T) of chemically aged supermicron dust (1.0-10.0 μm) in each particle size class were nearly equal to or slightly higher than those of normal Asian dust, which were 0.70-2.45 times and 0.64-4.34 times at -18 °C, respectively. These results reveal that anthropogenic air pollution does not notably change the INP concentrations and does not impair the INA of Asian dust. Our work provides direct observational evidence and clarifies the non-suppression effect of anthropogenic air pollution on the INA of East Asian dust, advancing the understanding of the ice nucleation of airborne aged mineral dust.

Kinetics of oligomer-forming reactions involving the major functional groups present in atmospheric secondary organic aerosol particles

Atmospheric organic aerosol particles impact climate as well as human and environmental health. Secondary organic aerosol (SOA), which is formed by the gas-to-particle partitioning of products of the oxidation of volatile organic compounds (VOCs) emitted from biogenic or anthropogenic sources, contributes a large fraction of this material. In the particle phase, these products can undergo accretion reactions to form oligomers that impact the formation, composition, and chemical-physical properties of aerosols. While these reactions are known to occur in the atmosphere, data and models describing their kinetics and equilibria are sparse. Here, reactions of compounds containing potentially reactive hydroperoxide, hydroxyl, carboxyl, aldehyde, and ketone groups were investigated in single and phase-separated organic/aqueous mixtures in the absence and presence of a sulfuric acid catalyst. Compounds containing these groups and a nonreactive UV-absorbing nitrate group were synthesized and their reactions and products were monitored and characterized using high-performance liquid chromatography with UV detection (HPLC-UV), electrospray ionization-mass spectrometry (ESI-MS), and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Reactions were observed between hydroperoxides and aldehydes to form peroxyhemiacetals, and between carboxylic acids and alcohols to form esters, and their rate and equilibrium constants were determined. No reactions were observed in other mixtures, indicating that under the conditions of these experiments only a few reaction pathways form oligomers. Reactions were also conducted with probe compounds and SOA formed in an environmental chamber reaction of α-pinene with O₃. Whereas in a previous study we observed a rapid hydroperoxide reaction in this SOA, among the other compounds studied here only alcohols reacted. These results provide insight into the types of accretion reactions that are likely to occur in atmospheric aerosols, and the rate and equilibrium constants can be used to better model SOA chemistry.

A perspective on iron (Fe) in the atmosphere: air quality, climate, and the ocean

As scientists engage in research motivated by climate change and the impacts of pollution on air, water, and human health, we increasingly recognize the need for the scientific community to improve communication and knowledge exchange across disciplines to address pressing and outstanding research questions holistically. Our professional paths have crossed because our research activities focus on the chemical reactivity of Fe-containing minerals in air and water, and at the air-sea interface. (Photo)chemical reactions driven by Fe can take place at the surface of the particles/droplets or within the condensed phase. The extent and rates of these reactions are influenced by water content and biogeochemical activity ubiquitous in these systems. One of these reactions is the production of reactive oxygen species (ROS) that cause damage to respiratory organs. Another is that the reactivity of Fe and organics in aerosol particles alter surficial physicochemical properties that impact aerosol-radiation and aerosol-cloud interactions. Also, upon deposition, aerosol particles influence ocean biogeochemical processes because micronutrients such as Fe or toxic elements such as copper become bioavailable. We provide a perspective on these topics and future research directions on the reactivity of Fe in atmospheric aerosol systems, from sources to short- and long-term impacts at the sinks with emphasis on needs to enhance the predictive power of atmospheric and ocean models.

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.

Exploring controls on perfluorocarboxylic acid (PFCA) gas-particle partitioning using a model with observational constraints

The atmospheric fate of perfluorocarboxylic acids (PFCAs) has attracted much attention in recent decades due to the role of the atmosphere in global transport of these persistent chemicals. There is a gap in our understanding of gas-particle partitioning, limited by availability of reliable atmospheric measurements, partitioning properties, and models of gas-particle interactions. The gas-particle equilibrium phase partitioning of C2-C16 PFCAs in the atmosphere were modeled here by taking account of both deprotonation and phase partitioning equilibria among air, aerosol liquid water, and particulate water-insoluble organic matter using a range of available PFCA partitioning properties. We systematically varied water and organic matter content to simulate the full range of atmospheric conditions. Except in severe organic matter pollution episodes, shorter-chain PFCAs are predicted to mainly partition between air and aqueous phase, while for PFCAs with carbon chains longer than 12, organic matter is more likely to be the dominant particle phase reservoir. The model framework underestimated the particle fraction of C2-C8 PFCAs compared with several ambient observations, with larger discrepancies observed for longer-chain PFCAs. The discrepancy could result from externally mixed dust components, non-ideality of aerosol liquid water, surfactant descriptions at phase boundaries, and missed interactions between organic matter and charged PFCA molecules. Reliable measurements of ambient PFCAs with high time resolution and the measurement of uptake parameters by particle-relevant components will be beneficial to more reliable environmental fate modeling of ambient PFCAs.