Particle phase acidity and oligomer formation in secondary organic aerosol

Investigation of pyruvic acid photolysis at the air-liquid interface as a source of aqueous secondary organic aerosols

Pyruvic acid (PA) is a ubiquitous 2-oxocarboxylic acid in the atmosphere. Its photochemical process at the air-liquid (a-l) interface has been suggested as an important source of aqueous secondary organic aerosols. We investigated the photochemical reaction pathways of PA at the a-l interface using synchrotron-based vacuum ultraviolet single-photon ionization mass spectrometry (VUV SPI-MS) coupled with the System for Analysis at the Liquid Vacuum Interface (SALVI) microreactor. Results from mass spectral analysis and the determination of appearance energies (AEs) indicate that photolysis of PA can generate radicals, then they recombine with carboxylic acids and simple molecular oligomers. Furthermore, the preliminary products could form larger oligomers via radical reaction or esterification in the presence of hydroxyl and carboxyl functional groups. Mass spectral comparison shows that most photochemical reactions would complete within 4 h. The expanded photochemistry-driven reaction flowchart of PA is proposed based on the newly discovered products. Our results reveal that the interfacial PA photochemical reactions have different mechanisms from the bulk liquid due to the interfacial properties, such as molecular density, composition, and ion concentration. Our findings show that in situ mass spectral analysis with bright photon ionization is useful to elucidate the contribution of a-l interfacial reactions leading to aqSOA formation.

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.

Toward Modeling the Growth of Large Atmospheric Sulfuric Acid-Ammonia Clusters

Studying large atmospheric molecular clusters is needed to understand the transition between clusters and aerosol particles. In this work, we studied the (SA)n(AM)n clusters with n up to 30 and the (SA)m(AM)m±2 clusters, with m = 6-20. The cluster configurations are sampled using the ABCluster program, and the cluster geometries and thermochemical parameters are calculated using GFN1-xTB. The cluster binding energies are calculated using B97-3c. We find that the addition of sulfuric acid is preferred to the addition of ammonia. The addition free energies were found to have large uncertainties, which could potentially be attributed to errors in the applied level of theory. Based on DLPNO-CCSD(T0)/aug-cc-pVTZ benchmarks of the binding energies of the large (SA)8-9(AM)10 and (SA)10(AM)10-11 clusters, we find that ωB97X-D3BJ with a large basis set is required to yield accurate binding and addition energies. However, based on recalculations of the single-point energy at r2SCAN-3c and ωB97X-D3BJ/6-311++G(3df,3pd), we show that the single-point energy contribution is not the primary source of error. We hypothesize that a larger source of error might be present in the form of insufficient configurational sampling. Finally, we train Δ machine learning model on (SA)n(AM)n clusters with n up to 5 and show that we can predict the binding energies of clusters up to sizes of (SA)30(AM)30 with a binding energy error below 0.6 %. This is an encouraging approach for accurately modeling the binding energies of large acid-base clusters in the future.

Low-NO-like Oxidation Pathway Makes a Significant Contribution to Secondary Organic Aerosol in Polluted Urban Air

Anthropogenic pollutants can greatly mediate formation pathways and chemical compositions of secondary organic aerosol (SOA) in urban atmospheres. We investigated the molecular tracers for different types of SOA in PM2.5 under varying NO/NO2 conditions in Guangzhou using source analysis of particle-phase speciated organics obtained from an iodide chemical ionization mass spectrometer with a Filter Inlet for Gases and AEROsols (FIGAERO-I-CIMS). Results show that low-NO-like pathways (when NO/NO2 < 0.2) explained ∼75% of the total measured FIGAERO-OA during regional transport periods, which was enriched in more-oxidized C4-C6 non-nitrogenous compounds over ozone accumulation. Daytime high-NO chemistry played larger roles (38%) in local pollution episodes, with organic nitrates (ONs) and nitrophenols increasing with enhanced aerosol water content and nitrate fraction. Nighttime NO3-initiated oxidation, characterized by monoterpene-derived ONs, accounted for comparable percentages (10-12%) of FIGAERO-OA for both two periods. Furthermore, the presence of organosulfates (OSs) improves the understanding of the roles of aqueous-phase processes in SOA production. Carbonyl-derived OSs exhibited a preferential formation under conditions of high aerosol acidity and/or abundant sulfate, which correlated well with low-NO-like SOA. Our results demonstrate the importance of NO/NO2 ratios in controlling SOA compositions, as well as interactions between water content, aerosol acidity, and inorganic salts in gas-to-particle partitioning of condensable organics.

Organic synthesis in the study of terpene-derived oxidation products in the atmosphere

Covering: 1997 up to 2022Volatile biogenic terpenes involved in the formation of secondary organic aerosol (SOA) particles participate in rich atmospheric chemistry that impacts numerous aspects of the earth's complex climate system. Despite the importance of these species, understanding their fate in the atmosphere and determining their atmospherically-relevant properties has been limited by the availability of authentic standards and probe molecules. Advances in synthetic organic chemistry directly aimed at answering these questions have, however, led to exciting discoveries at the interface of chemistry and atmospheric science. Herein we provide a review of the literature regarding the synthesis of commercially unavailable authentic standards used to analyze the composition, properties, and mechanisms of SOA particles in the atmosphere.

Heterogeneous Formation of Organonitrates (ON) and Nitroxy-Organosulfates (NOS) from Adsorbed α-Pinene-Derived Organosulfates (OS) on Mineral Surfaces

Organonitrates (ON) and nitroxy-organosulfates (NOS) are important components of secondary organic aerosols (SOAs). Gas-phase reactions of α-pinene (C₁₀H₁₆), a primary precursor for several ON compounds, are fairly well understood although formation pathways for NOS largely remain unknown. NOS formation may occur via reactions of ON and organic peroxides with sulfates as well as through radical-initiated photochemical processes. Despite the fact that organosulfates (OS) represent a significant portion of the organic aerosol mass, ON and NOS formation from OS is less understood, especially through nighttime heterogeneous and multiphase chemistry pathways. In the current study, surface reactions of adsorbed α-pinene-derived OS with nitrogen oxides on hematite and kaolinite surfaces, common components of mineral dust, have been investigated. α-Pinene reacts with sulfated mineral surfaces, forming a range of OS compounds on the surface. These OS compounds when adsorbed on mineral surfaces can further react with HNO₃ and NO₂, producing several ON and NOS compounds as well as several oxidation products. Overall, this study reveals the complexity of reactions of prevalent organic compounds leading to the formation of OS, ON, and NOS via heterogeneous and multiphase reaction pathways on mineral surfaces. It is also shown that this chemistry is mineralogy-specific.

Highly Acidic Conditions Drastically Alter the Chemical Composition and Absorption Coefficient of α-Pinene Secondary Organic Aerosol

Secondary organic aerosols (SOA), formed through the gas-phase oxidation of volatile organic compounds (VOCs), can reside in the atmosphere for many days. The formation of SOA takes place rapidly within hours after VOC emissions, but SOA can undergo much slower physical and chemical processes throughout their lifetime in the atmosphere. The acidity of atmospheric aerosols spans a wide range, with the most acidic particles having negative pH values, which can promote acid-catalyzed reactions. The goal of this work is to elucidate poorly understood mechanisms and rates of acid-catalyzed aging of mixtures of representative SOA compounds. SOA were generated by the ozonolysis of α-pinene in a continuous flow reactor and then collected using a foil substrate. SOA samples were extracted and aged by exposure to varying concentrations of aqueous H₂SO₄ for 1-2 days. Chemical analysis of fresh and aged samples was conducted using ultra-performance liquid chromatography coupled with photodiode array spectrophotomety and high-resolution mass spectrometry. In addition, UV-vis spectrophotometry and fluorescence spectrophotometry were used to examine the changes in optical properties before and after aging. We observed that SOA that aged in moderately acidic conditions (pH from 0 to 4) experienced small changes in composition, while SOA that aged in a highly acidic environment (pH from -1 to 0) experienced more dramatic changes in composition, including the formation of compounds containing sulfur. Additionally, at highly acidic conditions, light-absorbing and fluorescent compounds appeared, but their identities could not be ascertained due to their small relative abundance. This study shows that acidity is a major driver of SOA aging, resulting in a large change in the chemical composition and optical properties of aerosols in regions where high concentrations of H₂SO₄ persist, such as upper troposphere and lower stratosphere.

Ozonolysis of α-Pinene and Δ3-Carene Mixtures: Formation of Dimers with Two Precursors

The formation of secondary organic aerosol (SOA) from the structurally similar monoterpenes, α-pinene and Δ³-carene, differs substantially. The aerosol phase is already complex for a single precursor, and when mixtures are oxidized, products, e.g., dimers, may form between different volatile organic compounds (VOCs). This work investigates whether differences in SOA formation and properties from the oxidation of individual monoterpenes persist when a mixture of the monoterpenes is oxidized. Ozonolysis of α-pinene, Δ³-carene, and a 1:1 mixture of them was performed in the Aarhus University Research on Aerosol (AURA) atmospheric simulation chamber. Here, ∼100 ppb of monoterpene was oxidized by 200 ppb O₃ under dark conditions at 20 °C. The particle number concentration and particle mass concentration for ozonolysis of α-pinene exceed those from ozonolysis of Δ³-carene alone, while their mixture results in concentrations similar to α-pinene ozonolysis. Detailed offline analysis reveals evidence of VOC-cross-product dimers in SOA from ozonolysis of the monoterpene mixture: a VOC-cross-product dimer likely composed of the monomeric units cis-caric acid and 10-hydroxy-pinonic acid and a VOC-cross-product dimer ester likely from the monomeric units caronaldehyde and terpenylic acid were tentatively identified by liquid chromatography-mass spectrometry. To improve the understanding of chemical mechanisms determining SOA, it is relevant to identify VOC-cross-products.

Monoterpene Photooxidation in a Continuous-Flow Chamber: SOA Yields and Impacts of Oxidants, NOx, and VOC Precursors

Monoterpene photooxidation plays an important role in secondary organic aerosol (SOA) formation in the atmosphere. The low-volatility products can enhance new particle formation and particle growth and thus influence climate feedback. Here, we present the results of α-pinene and Δ-3-carene photooxidation experiments conducted in continuous-flow mode in an environmental chamber under several reaction conditions. The roles of oxidants, addition of NO, and VOC molecular structure in influencing SOA yield are illustrated. SOA yield from α-pinene photooxidation shows a weak dependence on H₂O₂ concentration, which is a proxy for HO₂ concentration. The high O/C ratios observed in the α-pinene photooxidation products suggest the production of highly oxygenated organic molecules (HOM). Addition of ozone to the chamber during low-NO_x photooxidation experiments leads to higher SOA yield. With the addition of NO, the production of N-containing HOMs is enhanced and the SOA yield shows a modest, nonlinear dependence on the input NO concentration. Carene photooxidation leads to higher SOA yield than α-pinene under similar reaction conditions, which agrees with the lower volatility retrieved from evaporation kinetics experiments. These results improve the understanding of SOA formation from monoterpene photooxidation and could be applied to refine the representation of biogenic SOA formation in models.

Roles of semivolatile and intermediate-volatility organic compounds in secondary organic aerosol formation and its implication: A review

Secondary organic aerosol (SOA) is a very important component of fine particulate matter (PM_2.5) in the atmosphere. However, the simulations of SOA, which could help to elucidate the detailed mechanism of SOA formation and quantify the roles of various precursors, remains unsatisfactory, as SOA levels are frequently underestimated. It has been found that the performance of SOA formation models can be significantly improved by incorporating the emission and evolution of semivolatile and intermediate-volatility organic compounds (S/IVOCs). In order to explore the roles of S/IVOCs in SOA formation, this study reviews some simulation models which could consider S/IVOCs for SOA formation as well as the development of emission inventories of S/IVOCs and S/IVOC modules for SOA formation. In addition, the future research directions for simulations of the effect of S/IVOCs on SOA formation are suggested.

Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds

The acidity of aqueous atmospheric solutions is a key parameter driving both the partitioning of semi-volatile acidic and basic trace gases and their aqueous-phase chemistry. In addition, the acidity of atmospheric aqueous phases, e.g., deliquesced aerosol particles, cloud, and fog droplets, is also dictated by aqueous-phase chemistry. These feedbacks between acidity and chemistry have crucial implications for the tropospheric lifetime of air pollutants, atmospheric composition, deposition to terrestrial and oceanic ecosystems, visibility, climate, and human health. Atmospheric research has made substantial progress in understanding feedbacks between acidity and multiphase chemistry during recent decades. This paper reviews the current state of knowledge on these feedbacks with a focus on aerosol and cloud systems, which involve both inorganic and organic aqueous-phase chemistry. Here, we describe the impacts of acidity on the phase partitioning of acidic and basic gases and buffering phenomena. Next, we review feedbacks of different acidity regimes on key chemical reaction mechanisms and kinetics, as well as uncertainties and chemical subsystems with incomplete information. Finally, we discuss atmospheric implications and highlight the need for future investigations, particularly with respect to reducing emissions of key acid precursors in a changing world, and the need for advancements in field and laboratory measurements and model tools.

Multiphase buffer theory explains contrasts in atmospheric aerosol acidity

Aerosol acidity largely regulates the chemistry of atmospheric particles, and resolving the drivers of aerosol pH is key to understanding their environmental effects. We find that an individual buffering agent can adopt different buffer pH values in aerosols and that aerosol pH levels in populated continental regions are widely buffered by the conjugate acid-base pair NH4+/NH3 (ammonium/ammonia). We propose a multiphase buffer theory to explain these large shifts of buffer pH, and we show that aerosol water content and mass concentration play a more important role in determining aerosol pH in ammonia-buffered regions than variations in particle chemical composition. Our results imply that aerosol pH and atmospheric multiphase chemistry are strongly affected by the pervasive human influence on ammonia emissions and the nitrogen cycle in the Anthropocene.

Molecular characteristics and stable carbon isotope compositions of dicarboxylic acids and related compounds in the urban atmosphere of the North China Plain: Implications for aqueous phase formation of SOA during the haze periods

In the past five years, Chinese government has promulgated stringent measures to mitigate air pollution. However, PM_2.5 levels in the China North Plain (NCP), which is one of the regions with the heaviest air pollution in the world, are still far beyond the World Health Organization (WHO) standard. To improve our understanding on the sources and formation mechanisms of haze in the NCP, PM_2.5 samples were collected during the winter of 2017 on a day/night basis at the urban site of Liaocheng, which is one of the most polluted cities in the NCP. The samples were determined for molecular distributions and stable carbon isotope compositions of dicarboxylic acids and their precursors (ketocarboxylic acids and α-dicarbonyls), levoglucosan, elemental carbon (EC), organic carbon (OC) and water-soluble organic carbon (WSOC). Our results showed that oxalic acid (C₂) is the dominant dicarboxylic acid, followed by succinic acid (C₄) and malonic acid (C₃), and glyoxylic acid (ωC₂) is the most abundant ketocarboxylic acids. Concentrations of C₂, glyoxal (Gly) and methylglyoxal (mGly) presented robust correlations with levoglucosan, suggesting that biomass burning is a significant source of PM_2.5 in the NCP. Moreover, C₂ and Gly and mGly linearly correlated with SO₄^2-, relative humidity (RH), aerosol liquid water content (LWC) as well as particle in-situ pH (pH_is), indicating that aqueous-phase oxidation is the major formation pathway of these SOA, and is driven by acid-catalyzed oxidation. Concentrations and relative abundances of secondary species including SNA (SO₄^2-, NO₃^- and NH₄⁺), dicarboxylic acids, and aerosol LWC in PM_2.5 are much higher in the haze periods than in the clean periods, suggesting that aqueous reaction is a vital role in the haze formation. In comparison with those in the clean periods, stable carbon isotopic compositions (δ¹³C) of major dicarboxylic acids and related SOA and the mass ratios of C₂/diacids, C₂/Gly and C₂/mGly are higher in the haze periods, indicating that haze particles were more aged and enriched in secondary species.

Direct measurement of aerosol pH in individual malonic acid and citric acid droplets under different relative humidity conditions via Raman spectroscopy

Acidity of aerosol particles plays important roles in atmospheric chemistry, in turn, impacting climate system and public health. Current knowledge of acidity in atmosphere aerosols remains fairly scarce largely because of difficulty in direct measurement. On the other hand, indirect methods for estimating aerosol pH are often inconsistent with pH values predicted by thermodynamic models. Recently, a direct Raman spectroscopy method has been reported to determine pH values of acid-conjugate base equilibria systems based on Raman intensity of distinct characteristic peaks of conjugate acid-base pair. Nevertheless, for pure carboxylic acid aerosol particles, such as malonic acid (MA), characteristic peak of its conjugate base cannot be clearly observed in Raman spectra owing to small K_a value (weak acid dissociation constant), which leads to little dissociation of weak acid and low concentration of its conjugated base. As a result, pH of carboxylic acid particles cannot be directly determined by calibrating concentrations of acid and its conjugated base using the above-mentioned method. To address such an issue, we demonstrate a new approach for determining pH values of malonic acid (MA) and citric acid (CA) droplets under different relative humidity (RH) based on calibration curves. We measure Raman intensity ratios of acid solutions at different concentrations and their pH values to establish a calibration curve, and then using the intensity ratio of MA and CA droplets under different RH to determine aerosol particle pH based on calibration curves. Results have shown that aerosol pH of MA droplet decreases with a decreasing RH and pH values ranges from 1.03 to -0.12, when RH value is reduced from 90% to 26%, in good agreement with model prediction values. In addition, we also, for the first time, report pH values of CA droplets under different RH conditions and its pH values range from 1.13 to -0.74 when RH is reduced from 91% to 28%.

Radical chemistry in oxidation flow reactors for atmospheric chemistry research

We summarize the studies on the chemistry in oxidation flow reactor and discuss its atmospheric relevance.

Monoterpene and Sesquiterpene α-Hydroxy Organosulfates: Synthesis, MS/MS Characteristics, and Ambient Presence

Organosulfates (OSs) derived from biogenic volatile organic compounds are important compounds signifying interactions between anthropogenic sulfur pollution and natural emission. In this work, we substantially expand the OS standard library through the chemical synthesis of 26 α-hydroxy OS standards from eight monoterpenes (i.e., α- and β-pinene, limonene, sabinene, Δ³-carene, terpinolene, and α- and γ-terpinene) and two sesquiterpenes (i.e., α-humulene and β-caryophyllene). The sulfation of unsymmetrically substituted 1,2-diol intermediates produced a regioisomeric mixture of two OSs. The major regioisomeric OSs were isolated and purified for full NMR characterization, while the minor regioisomers could only be determined by liquid chromatograph-mass spectrometer (MS). The tandem mass spectra of the molecular ion formed through electrospray ionization confirmed the formation of abundant bisulfate ion fragments (m/z 97) and certain minor ion fragments characteristic of the carbon backbone. A knowledge of the MS/MS spectra and chromatographic retention times for authentic standards allows us to identify α-hydroxy OSs derived from six monoterpenes and β-caryophyllene in ambient samples. Notably, among two possible regioisomers of α-hydroxy OSs, we only detected the isomers with the sulfate group at the less substituted carbon position derived from α-pinene, limonene, sabinene, Δ³-carene, and terpinolene in the ambient samples. This observation sheds light on the atmospheric OS formation mechanisms.

100 Years of Progress in Gas-Phase Atmospheric Chemistry Research

Remarkable progress has occurred over the last 100 years in our understanding of atmospheric chemical composition, stratospheric and tropospheric chemistry, urban air pollution, acid rain, and the formation of airborne particles from gas-phase chemistry. Much of this progress was associated with the developing understanding of the formation and role of ozone and of the oxides of nitrogen, NO and NO2, in the stratosphere and troposphere. The chemistry of the stratosphere, emerging from the pioneering work of Chapman in 1931, was followed by the discovery of catalytic ozone cycles, ozone destruction by chlorofluorocarbons, and the polar ozone holes, work honored by the 1995 Nobel Prize in Chemistry awarded to Crutzen, Rowland, and Molina. Foundations for the modern understanding of tropospheric chemistry were laid in the 1950s and 1960s, stimulated by the eye-stinging smog in Los Angeles. The importance of the hydroxyl (OH) radical and its relationship to the oxides of nitrogen (NO and NO2) emerged. The chemical processes leading to acid rain were elucidated. The atmosphere contains an immense number of gas-phase organic compounds, a result of emissions from plants and animals, natural and anthropogenic combustion processes, emissions from oceans, and from the atmospheric oxidation of organics emitted into the atmosphere. Organic atmospheric particulate matter arises largely as gas-phase organic compounds undergo oxidation to yield low-volatility products that condense into the particle phase. A hundred years ago, quantitative theories of chemical reaction rates were nonexistent. Today, comprehensive computer codes are available for performing detailed calculations of chemical reaction rates and mechanisms for atmospheric reactions. Understanding the future role of atmospheric chemistry in climate change and, in turn, the impact of climate change on atmospheric chemistry, will be critical to developing effective policies to protect the planet.

Role of pH in Aerosol Processes and Measurement Challenges

pH is one of the most basic chemical properties of aqueous solution, but its measurement in nanoscale aerosol particles presents many challenges. The pH of aerosol particles is of growing interest in the atmospheric chemistry community because of its demonstrated effects on heterogeneous chemistry and human health, as well as potential effects on climate. The authors have shown that phase transitions of aerosol particles are sensitive to pH, focusing on systems that undergo liquid-liquid phase separation. Currently, aerosol pH is calculated indirectly from knowledge of species present in the gas and aerosol phases through the use of thermodynamic models. From these models, ambient aerosol is expected to be highly acidic (pH ∼ 0-3). Direct measurements have focused on model systems due to the difficulty of this measurement. This area is one in which physical chemists should be encouraged to contribute because of the potential consequences for aerosol processes in the environment.

Formation and Evolution of aqSOA from Aqueous-Phase Reactions of Phenolic Carbonyls: Comparison between Ammonium Sulfate and Ammonium Nitrate Solutions

We investigate the effects of sulfate and nitrate on the formation and evolution of secondary organic aerosol formed in the aqueous phase (aqSOA) from photooxidation of two phenolic carbonyls emitted from wood burning. AqSOA was formed efficiently from the photooxidation of both syringaldehyde (C₉H₁₀O₄) and acetosyringone (C₁₀H₁₂O₄) in ammonium sulfate and ammonium nitrate solutions, with mass yields ranging from 30% to 120%. Positive matrix factorization on the organic mass spectra acquired by an Aerosol Mass Spectrometer revealed a combination of functionalization, oligomerization, and fragmentation processes in the chemical evolution of aqSOA. Functionalization and oligomerization dominated in the first 4 h of reaction, with phenolic oligomers and their derivatives significantly contributing to aqSOA formation; and oxidation of the first-generation products led to an abundance of oxygenated ring-opening products. Degradation rates of syringaldehyde and acetosyringone in nitrate solutions were 1.5 and 3.5 times faster than rates in sulfate solutions, and aqSOA yields in nitrate experiments are twice as high as in sulfate experiments. Nitrate likely promoted the reactions because it is a photolytic source of OH radicals, while sulfate is not, highlighting the importance of aerosol-phase nitrate in the formation of aqSOA by facilitating the photooxidation of organic precursors.

Formation mechanism of secondary organic aerosol from ozonolysis of gasoline vehicle exhaust

Gasoline vehicles are a major source of anthropogenic secondary organic aerosols (SOAs). However, current models based on known precursors fail to explain the substantial SOAs from vehicle emissions due to the inadequate understanding of the formation mechanism. To provide more information on this issue, the formation of SOAs from ozonolysis of four light-duty gasoline vehicle exhaust systems was investigated with a vacuum ultraviolet photoionization mass spectrometer (VUV-PIMS). Remarkable SOAs formation was observed and the SOAs were primarily aliphatic alkenes. PI mass spectra of the SOAs from all vehicles exhibited similar spectral patterns (a regular mass group with m/z at 98, 112, 126 …). Interestingly, most carbonyl products of aliphatic alkenes observed as major gaseous products have specific molecular weights, and the main formation pathway of SOAs can be explained well using aldol condensation reactions of these carbonyls. This is a direct observation of the aldol condensation as a dominated pathway for SOAs formation, and the first report on the composition and formation mechanism of the SOAs from the ozonolysis of gasoline vehicle exhaust is given. The study reveals that low molecular weight alkenes may play a more significant role in vehicle-induced SOAs formation than previously believed. More importantly, the PI mass spectra of SOAs from vehicles show similarities to the field aerosol sample mass spectra, suggesting the possible significance of the aldol condensation reactions in ambient aerosol formation. Since carbonyls are a major degradation product of biogenic and anthropogenic VOCs through atmospheric oxidation processes, the mechanism proposed in this study can be applied more generally to explain aerosol formation from the oxidation of atmospheric hydrocarbons.

Closed-Shell Organic Compounds Might Form Dimers at the Surface of Molecular Clusters

The role of covalently bound dimer formation is studied using high-level quantum chemical methods. Reaction free energy profiles for dimer formation between common oxygen-containing functional groups are calculated, and based on the Gibbs free energy differences between transition states and reactants, we show that none of the studied two-component gas-phase reactions are kinetically feasible at 298.15 K and 1 atm. Therefore, the catalyzing effect of water, base, or acid molecules is calculated, and sulfuric acid is identified to lower the activation free energies significantly. We find that the reactions yielding hemiacetal, peroxyhemiacetal, α-hydroxyester, and geminal diol products occur with activation free energies of less than 10 kcal/mol with sulfuric acid as a catalyst, indicating that these reactions could potentially take place on the surface of sulfuric acid clusters. Additionally, the formed dimer products bind stronger onto the pre-existing cluster than the corresponding reagent monomers do. This implies that covalent dimerization reactions stabilize the existing cluster thermodynamically and make it less likely to evaporate. However, the studied small organic compounds, which contain only one functional group, are not able to form dimer products that are stable against evaporation at atmospheric conditions. Calculations of dimer formation onto a cluster surface and the clustering ability of dimer products should be extended to large terpene oxidation products in order to estimate the real atmospheric significance.

The effect of sub-zero temperature on the formation and composition of secondary organic aerosol from ozonolysis of alpha-pinene

This study presents a newly constructed temperature controlled cold-room smog chamber at Aarhus University, Denmark. The chamber is herein utilized to study the effect of sub-zero temperature on the formation and chemical composition of secondary organic aerosol (SOA) from ozone initiated oxidation of α-pinene. The chemical composition of α-pinene SOA formed from dark ozonolysis of α-pinene at 293 K and 258 K was investigated using High-Resolution Time-of-Flight Aerosol Mass Spectrometry (HR-ToF-AMS) and Ultra-High Performance Liquid Chromatography/Electrospray Ionization Quadrupole Time-of-Flight Mass Spectrometry (UHPLC/ESI-qToF-MS). For comparison, an OH-initiated oxidation experiment was performed at 293 K. In ozonolysis experiments it was found that oxygen-to-carbon (O : C) ratios were higher in the particles formed at 293 K compared to 258 K. A total of 16 different organic acids and 30 dimers esters were quantified in the collected particles composing up to 34% of the total α-pinene SOA mass with increased mass fraction of carboxylic acids in particles from α-pinene ozonolysis at 258 K compared to 293 K. In contrast, dimer esters showed suppressed formation at the sub-zero reaction temperature, thus contributing 3% to SOA mass at 258 K while contributing 9% at 293 K. SOA formed in the OH-initiated oxidation of α-pinene at 293 K resulted in low concentrations of dimer esters supporting Criegee intermediates as a possible pathway to dimer ester formation. Vapour pressure estimates of the identified carboxylic acids and dimer esters are presented and show how otherwise semi-volatile carboxylic acids at sufficiently low temperatures may classify as low or even extremely low volatile organic compounds (ELVOC), thus may add to an enhanced particle formation observed at the sub-zero temperature through gas-to-particle conversion. The change in chemical composition of the SOA particles with temperature is ascribed to a combination of effects: the decreased vapour pressures and hence increased condensation of carboxylic acids from the gas phase to the particle phase along with suppressed formation of the high molecular weight dimer esters and different gas and particle phase chemistry results in particles of different chemical composition as a consequence of low reaction temperatures.

The effect of gas-phase polycyclic aromatic hydrocarbons on the formation and properties of biogenic secondary organic aerosol particles

When secondary organic aerosol (SOA) particles are formed by ozonolysis in the presence of gas-phase polycyclic aromatic hydrocarbons (PAHs), their formation and properties are significantly different from SOA particles formed without PAHs. For all SOA precursors and all PAHs, discussed in this study, the presence of the gas-phase PAHs during SOA formation significantly affects particle mass loadings, composition, growth, evaporation kinetics, and viscosity. SOA particles formed in the presence of PAHs have, as part of their compositions, trapped unreacted PAHs and products of heterogeneous reactions between PAHs and ozone. Compared to 'pure' SOA particles, these particles exhibit slower evaporation kinetics, have higher fractions of non-volatile components, like oligomers, and higher viscosities, assuring their longer atmospheric lifetimes. In turn, the increased viscosity and decreased volatility provide a shield that protects PAHs from chemical degradation and evaporation, allowing for the long-range transport of these toxic pollutants. The magnitude of the effect of PAHs on SOA formation is surprisingly large. The presence of PAHs during SOA formation increases mass loadings by factors of two to five, and particle number concentrations, in some cases, by more than a factor of 100. Increases in SOA mass, particle number concentrations, and lifetime have important implications to many atmospheric processes related to climate, weather, visibility, and human health, all of which relate to the interactions between biogenic SOA and anthropogenic PAHs. The synergistic relationship between SOA and PAHs presented here are clearly complex and call for future research to elucidate further the underlying processes and their exact atmospheric implications.

Synthesis of Four Monoterpene-Derived Organosulfates and Their Quantification in Atmospheric Aerosol Samples

Monoterpenes, a major class of biogenic volatile organic compounds, are known to produce oxidation products that further react with sulfate to form organosulfates. The accurate quantification of monoterpene-derived organosulfates (OSs) is necessary for quantifying this controllable aerosol source; however, it has been hampered by a lack of authentic standards. Here we report a unified synthesis strategy starting from the respective monoterpene through Upjohn dihydroxylation or Sharpless asymmetric dihydroxylation followed by monosulfation with the sulfur trioxide-pyridine complex. We demonstrate the successful synthesis of four monoterpene-derived OS compounds, including α-pinene OS, β-pinene OS, limonene OS, and limonaketone OS. Quantification of OSs is commonly achieved using liquid chromatography-mass spectrometry (LC-MS) by either monitoring the [M-H]^- ion or through multiple reaction monitoring (MRM) of mass transitions between the [M-H]^- and m/z 97 ions. Comparison between the synthesized standards and previously adopted quantification surrogates reveals that camphor-10-sulfonic acid is a better quantification surrogate using [M-H]^- as the quantification ion, while the highly compound-specific nature of MRM quantification makes it difficult to choose a suitable surrogate. Both could be rationalized in accordance to their respective MS quantification mechanisms. The in-house availability of the authentic standards enables us to discover that β-pinene OS, due to the sulfate group at the primary carbon, partially degrades to a dehydrogenated OS compound during LC/MS analysis and a hydroperoxy OS over a prolonged storage period (>5 month) and forms a regioisomer through intermolecular isomerization. Limonene OS was positively identified for the first time in ambient samples and found to be more abundant than α-/β-pinene OS in the Pearl River Delta, China. This work highlights the critical importance of having authentic standards in advancing our understanding of the interactions between biogenic VOC emissions and anthropogenic sulfur pollution.

Influence of metal-mediated aerosol-phase oxidation on secondary organic aerosol formation from the ozonolysis and OH-oxidation of α-pinene

The organic component is the most abundant fraction of atmospheric submicron particles, while the formation mechanisms of secondary organic aerosol (SOA) are not fully understood. The effects of sulfate seed aerosols on SOA formation were investigated with a series of experiments carried out using a 9 m³ smog chamber. The presence of FeSO₄ or Fe₂(SO₄)₃ seed aerosols decreased SOA yields and increased oxidation levels in both ozonolysis and OH-oxidation of α-pinene compared to that in the presence of ZnSO₄ or (NH₄)₂SO₄. These findings were explained by metal-mediated aerosol-phase oxidation of organics: reactive radicals were generated on FeSO₄ or Fe₂(SO₄)₃ seed aerosols and reacted further with the organic mass. This effect would help to explain the high O/C ratios of organics in ambient particles that thus far cannot be reproduced in laboratory and model studies. In addition, the gap in the SOA yields between experiments with different seed aerosols was more significant in OH-oxidation experiments compared to ozonolysis experiments, while the gap in estimated O/C ratios was less obvious. This may have resulted from the different chemical compositions and oxidation levels of the SOA generated in the two systems, which affect the branching ratio of functionalization and fragmentation during aerosol oxidation.

Influence of Functional Groups on the Viscosity of Organic Aerosol

Organic aerosols can exist in highly viscous or glassy phase states. A viscosity database for organic compounds with atmospherically relevant functional groups is compiled and analyzed to quantify the influence of number and location of functional groups on viscosity. For weakly functionalized compounds the trend in viscosity sensitivity to functional group addition is carboxylic acid (COOH) ≈ hydroxyl (OH) > nitrate (ONO₂) > carbonyl (CO) ≈ ester (COO) > methylene (CH₂). Sensitivities to group addition increase with greater levels of prior functionalization and decreasing temperature. For carboxylic acids a sharp increase in sensitivity is likely present already at the second addition at room temperature. Ring structures increase viscosity relative to linear structures. Sensitivities are correlated with analogously derived sensitivities of vapor pressure reduction. This may be exploited in the future to predict viscosity in numerical models by piggybacking on schemes that track the evolution of organic aerosol volatility with age.

Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol

Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO₃) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO₃-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO₃-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO₃ radical, the difficulty of characterizing the spatial distributions of BVOC and NO₃ within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry-climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO₃-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO₃-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

High molecular weight organic compounds (HMW-OCs) in severe winter haze: Direct observation and insights on the formation mechanism

High molecular weight organic compounds (HMW-OCs), formed as secondary organic aerosols (SOA), have been reported in many laboratory studies. However, little evidence of HMW-OCs formation, in particular during winter season in the real atmosphere, has been reported. In January 2013, Beijing faced historically severe haze pollution, in which the hourly PM_2.5 concentration reached as high as 974 μg m^-3. Four typical haze events (HE1 to HE4) were identified, and HE2 (Jan. 9-16) was the most serious of these. Based on the hourly observed chemical composition of PM_2.5 and the daily organic composition analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), we found that abundant ion peaks in m/z 200-850 appeared on heavy haze days, whereas these were negligible on a clear day, indicating the existence of HMW-OCs in the wintertime haze. A negative nonlinear correlation between HMW-OCs and O₃ suggested that gas oxidation was not likely to be the dominant mechanism for HMW-OCs formation. During the heavy haze events, the relative humidity and mass ratio of H₂O/PM_2.5 reached as high as 80% and 0.2, respectively. The high water content and its good positive correlation with HMW-OCs indicated that an aqueous-phase process may be a significant pathway in wintertime. The evidence that acidity was much higher during HE2 (0.37 μg m^-3) than on other days, as well as its strong correlation with HMW-OCs, indicated that acid-catalyzed reactions likely resulted in HMW-OCs formation during the heavy winter haze in Beijing.

pH Dependence of Liquid-Liquid Phase Separation in Organic Aerosol

Atmospheric aerosol particles influence climate through their direct and indirect effects. These impacts depend in part on the morphology of the particles, which is determined by their composition. The effect of pH on morphology was investigated using particles composed of 3-methylglutaric acid and ammonium sulfate by manipulating the starting pH of the bulk solution through the addition of aqueous sodium hydroxide. Efflorescence, deliquescence, phase separation, and mixing transitions were observed with optical microscopy. Due to changes in its protonation states, the solubility of the organic component increases with increasing pH, which shifts the location of the separation relative humidity (SRH) from 78.7% for the fully protonated acid to 63.9% for the fully deprotonated acid. Surprisingly, this shift in the SRH leads to hysteresis between the SRH and the mixing relative humidity (MRH). Particle pH has the greatest effect on phase transitions that require nucleus formation, that is, efflorescence and SRH.

Ambient Gas-Particle Partitioning of Tracers for Biogenic Oxidation

Exchange of atmospheric organic compounds between gas and particle phases is important in the production and chemistry of particle-phase mass but is poorly understood due to a lack of simultaneous measurements in both phases of individual compounds. Measurements of particle- and gas-phase organic compounds are reported here for the southeastern United States and central Amazonia. Polyols formed from isoprene oxidation contribute 8% and 15% on average to particle-phase organic mass at these sites but are also observed to have substantial gas-phase concentrations contrary to many models that treat these compounds as nonvolatile. The results of the present study show that the gas-particle partitioning of approximately 100 known and newly observed oxidation products is not well explained by environmental factors (e.g., temperature). Compounds having high vapor pressures have higher particle fractions than expected from absorptive equilibrium partitioning models. These observations support the conclusion that many commonly measured biogenic oxidation products may be bound in low-volatility mass (e.g., accretion products, inorganic-organic adducts) that decomposes to individual compounds on analysis. However, the nature and extent of any such bonding remains uncertain. Similar conclusions are reach for both study locations, and average particle fractions for a given compound are consistent within ∼25% across measurement sites.

Comparison of ionic and carbonaceous compositions of PM2.5 in 2009 and 2012 in Shanghai, China

Daily PM2.5 samples were obtained at the site of East China University of Science and Technology (ECUST) in urban Shanghai during 2009-2010 and 2011-2012. The temporal variations of PM2.5 and its chemical compositions including secondary inorganic aerosol (SIA) and carbonaceous components were studied. The concentrations of PM2.5 were (94.04±52.17) μg/m3 and (68.44±41.57) μg/m3 in 2009 and 2012, respectively. The concentrations and contributions of SIA to PM2.5 were significantly higher in 2012 than those in 2009. Sulfate took up above 50% of SIA in 2009 and the corresponding value decreased to 41% in 2012. The increasing trend of NO3-/SO4(2-) mass ratio implied that the contributions of mobile sources were more and more important. The molar ratios of [NH4+]/(2[SO4(2-)]+[NO3-]) were 0.57 and 0.70, lower than 1, which demonstrated that most aerosol samples were ammonium-poor. But the neutralization process of ammonium might affect the formation of sulfate and nitrate, which was indicated by the strong correlation between [NH4+]-[SO4(2-)] and [NH4+]-[NO3-]. The average molar ratio of ammonium to sulfate was 1.74 in 2009, so there was not enough ammonium to neutralize sulfate sufficiently. The higher value of 2.30 in 2012 indicated that sulfate was sufficiently neutralized by ammonium and the predominant production was (NH4)2SO4. Aerosol samples had higher char-EC/soot-EC ratios and lower OC/EC ratios in 2009 than those of samples in 2012. The higher K+/OC values demonstrated that biomass burning made important contributions to carbonaceous components in both 2009 and 2012. All backward trajectories were grouped into four clusters, mainly from the Northwest, the Circum-Bohai-Sea Region (CBSR), the Southwest and the Southeast. The cluster from the Northwest was the most polluted pathway in 2009, while the cluster from the CBSR had more effects in 2012. PSCF model for PM2.5 and carbonaceous components except soot-EC suggested that the Yangtze River Delta Region (YRDR) made significant contributions by short-range transportation in 2009 and 2012. The Northwest, the CBSR and the Pearl River Delta Region (PRDR) were also the potential source areas. However, the source pattern of soot-EC was significantly different from those of other carbonaceous components.

Insights into the Formation and Evolution of Individual Compounds in the Particulate Phase during Aromatic Photo-Oxidation

Secondary organic aerosol (SOA) is well-known to have adverse effects on air quality and human health. However, the dynamic mechanisms occurring during SOA formation and evolution are poorly understood. The time-resolved SOA composition formed during the photo-oxidation of three aromatic compounds, methyl chavicol, toluene and 4-methyl catechol, were investigated at the European Photoreactor. SOA was collected using a particle into liquid sampler and analyzed offline using state-of-the-art mass spectrometry to produce temporal profiles of individual photo-oxidation products. In the photo-oxidation of methyl chavicol, 70 individual compounds were characterized and three distinctive temporal profile shapes were observed. The calculated mass fraction (Ci,aer/COA) of the individual SOA compounds showed either a linear trend (increasing/decreasing) or exponential decay with time. Substituted nitrophenols showed an exponential decay, with the nitro-group on the aromatic ring found to control the formation and loss of these species in the aerosol phase. Nitrophenols from both methyl chavicol and toluene photo-oxidation experiments showed a strong relationship with the NO2/NO (ppbv/ppbv) ratio and were observed during initial SOA growth. The location of the nitrophenol aromatic substitutions was found to be critically important, with the nitrophenol in the photo-oxidation of 4-methyl catechol not partitioning into the aerosol phase until irradiation had stopped; highlighting the importance of studying SOA formation and evolution at a molecular level.

Mining in subarctic Canada: airborne PM2.5 metal concentrations in two remote First Nations communities

Airborne particulate matter arising from upwind mining activities is a concern for First Nations communities in the western James Bay region of Ontario, Canada. Aerosol chemical components were collected in 2011 from two communities in northern Ontario. The chemical and mass concentration data of particulate matter collected during this study shows a significant difference in PM2.5 in Attawapiskat compared to Fort Albany. Elemental profiles indicate enhanced levels of some tracers thought to arise from mining activities, such as, K, Ni, and crustal materials. Both communities are remote and isolated from urban and industrial pollution sources, however, Attawapiskat First Nation has significantly enhanced levels of particulate matter, and it is likely that some of this arises from upwind mining activities.

PM2.5 acidity at a background site in the Pearl River Delta region in fall-winter of 2007-2012

Based on field observations and thermodynamic model simulation, the annual trend of PM2.5 acidity and its characteristics on non-hazy and hazy days in fall-winter of 2007-2012 in the Pearl River Delta region were investigated. Total acidity ([H(+)](total)) and in-situ acidity ([H(+)](in-situ)) of PM2.5 significantly decreased (F-test, p < 0.05) at a rate of -32 ± 1.5 nmol m(-3)year(-1) and -9 ± 1.7 nmol m(-3) year(-1), respectively. The variation of acidity was mainly caused by the change of the PM2.5 component, i.e., the decreasing rates of [H(+)](total) and [H(+)](in-situ) due to the decrease of sulfate (SO4(2-)) exceeded the increasing rate caused by the growth of nitrate (NO3(-)). [H(+)](total), [H(+)](in-situ) and liquid water content on hazy days were 0.9-2.2, 1.2-3.5 and 2.0-3.0 times those on non-hazy days, respectively. On hazy days, the concentration of organic matter (OM) showed significant enhancement when [H(+)](in-situ) increased (t-test, p < 0.05), while this was not observed on non-hazy days. Moreover, when the acidity was low (i.e., R = [NH4(+)]/(2 × [SO4(2-)]+[NO3(-)])>0.6), NH4NO3 was most likely formed via homogenous reaction. When the acidity was high (R ≤ 0.6), the gas-phase formation of NH4NO3 was inhibited, and the proportion of NO3(-) produced via heterogeneous reaction of N2O5 became significant.

Complex chemical composition of colored surface films formed from reactions of propanal in sulfuric acid at upper troposphere/lower stratosphere aerosol acidities

Particles in the upper troposphere and lower stratosphere (UT/LS) consist mostly of concentrated sulfuric acid (40-80 wt %) in water. However, airborne measurements have shown that these particles also contain a significant fraction of organic compounds of unknown chemical composition. Acid-catalyzed reactions of carbonyl species are believed to be responsible for significant transfer of gas phase organic species into tropospheric aerosols and are potentially more important at the high acidities characteristic of UT/LS particles. In this study, experiments combining sulfuric acid (H₂SO₄) with propanal and with mixtures of propanal with glyoxal and/or methylglyoxal at acidities typical of UT/LS aerosols produced highly colored surface films (and solutions) that may have implications for aerosol properties. In order to identify the chemical processes responsible for the formation of the surface films, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) and ¹H nuclear magnetic resonance (NMR) spectroscopies were used to analyze the chemical composition of the films. Films formed from propanal were a complex mixture of aldol condensation products, acetals and propanal itself. The major aldol condensation products were the dimer (2-methyl-2-pentenal) and 1,3,5-trimethylbenzene that was formed by cyclization of the linear aldol condensation trimer. Additionally, the strong visible absorption of the films indicates that higher-order aldol condensation products must also be present as minor species. The major acetal species were 2,4,6-triethyl-1,3,5-trioxane and longer-chain linear polyacetals which are likely to separate from the aqueous phase. Films formed on mixtures of propanal with glyoxal and/or methylglyoxal also showed evidence of products of cross-reactions. Since cross-reactions would be more likely than self-reactions under atmospheric conditions, similar reactions of aldehydes like propanal with common aerosol organic species like glyoxal and methylglyoxal have the potential to produce significant organic aerosol mass and therefore could potentially impact chemical, optical and/or cloud-forming properties of aerosols, especially if the products partition to the aerosol surface.

The formation of highly oxidized multifunctional products in the ozonolysis of cyclohexene

The prompt formation of highly oxidized organic compounds in the ozonolysis of cyclohexene (C6H10) was investigated by means of laboratory experiments together with quantum chemical calculations. The experiments were performed in borosilicate glass flow tube reactors coupled to a chemical ionization atmospheric pressure interface time-of-flight mass spectrometer with a nitrate ion (NO3(-))-based ionization scheme. Quantum chemical calculations were performed at the CCSD(T)-F12a/VDZ-F12//ωB97XD/aug-cc-pVTZ level, with kinetic modeling using multiconformer transition state theory, including Eckart tunneling corrections. The complementary investigation methods gave a consistent picture of a formation mechanism advancing by peroxy radical (RO2) isomerization through intramolecular hydrogen shift reactions, followed by sequential O2 addition steps, that is, RO2 autoxidation, on a time scale of seconds. Dimerization of the peroxy radicals by recombination and cross-combination reactions is in competition with the formation of highly oxidized monomer species and is observed to lead to peroxides, potentially diacyl peroxides. The molar yield of these highly oxidized products (having O/C > 1 in monomers and O/C > 0.55 in dimers) from cyclohexene ozonolysis was determined as (4.5 ± 3.8)%. Fully deuterated cyclohexene and cis-6-nonenal ozonolysis, as well as the influence of water addition to the system (either H2O or D2O), were also investigated in order to strengthen the arguments on the proposed mechanism. Deuterated cyclohexene ozonolysis resulted in a less oxidized product distribution with a lower yield of highly oxygenated products and cis-6-nonenal ozonolysis generated the same monomer product distribution, consistent with the proposed mechanism and in agreement with quantum chemical modeling.

Light-absorbing oligomer formation in secondary organic aerosol from reactive uptake of isoprene epoxydiols

Secondary organic aerosol (SOA) produced from reactive uptake and multiphase chemistry of isoprene epoxydiols (IEPOX) has been found to contribute substantially (upward of 33%) to the fine organic aerosol mass over the Southeastern U.S. Brown carbon (BrC) in rural areas of this region has been linked to secondary sources in the summer when the influence of biomass burning is low. We demonstrate the formation of light-absorbing (290 < λ < 700 nm) SOA constituents from reactive uptake of trans-β-IEPOX onto preexisting sulfate aerosols as a potential source of secondary BrC. IEPOX-derived BrC generated in controlled chamber experiments under dry, acidic conditions has an average mass absorption coefficient of ∼ 300 cm(2) g(-1). Chemical analyses of SOA constituents using UV-visible spectroscopy and high-resolution mass spectrometry indicate the presence of highly unsaturated oligomeric species with molecular weights separated by mass units of 100 (C5H8O2) and 82 (C5H6O) coincident with the observations of enhanced light absorption, suggesting such oligomers as chromophores, and potentially explaining one source of humic-like substances (HULIS) ubiquitously present in atmospheric aerosol. Similar light-absorbing oligomers were identified in fine aerosol collected in the rural Southeastern U.S., supporting their atmospheric relevance and revealing a previously unrecognized source of oligomers derived from isoprene that contributes to ambient fine aerosol mass.

Molecular characterization of organosulfates in organic aerosols from Shanghai and Los Angeles urban areas by nanospray-desorption electrospray ionization high-resolution mass spectrometry

Fine aerosol particles in the urban areas of Shanghai and Los Angeles were collected on days that were characterized by their stagnant air and high organic aerosol concentrations. They were analyzed by nanospray-desorption electrospray ionization mass spectrometry with high mass resolution (m/Δm = 100,000). Solvent mixtures of acetonitrile and water and acetonitrile and toluene were used to extract and ionize polar and nonpolar compounds, respectively. A diverse mixture of oxygenated hydrocarbons, organosulfates, organonitrates, and organics with reduced nitrogen were detected in the Los Angeles sample. A majority of the organics in the Shanghai sample were detected as organosulfates. The dominant organosulfates that were detected at two locations have distinctly different molecular characteristics. Specifically, the organosulfates in the Los Angeles sample were dominated by biogenic products, while the organosulfates of a yet unknown origin found in the Shanghai sample had distinctive characteristics of long aliphatic carbon chains and low degrees of oxidation and unsaturation. The use of the acetonitrile and toluene solvent facilitated the observation of this type of organosulfates, which suggests that they could have been missed in previous studies that relied on sample extraction using common polar solvents. The high molecular weight and low degree of unsaturation and oxidization of the uncommon organosulfates suggest that they may act as surfactants and plausibly affect the surface tension and hygroscopicity of atmospheric particles. We propose that direct esterification of carbonyl or hydroxyl compounds by sulfates or sulfuric acid in the liquid phase could be the formation pathway of these special organosulfates. Long-chain alkanes from vehicle emissions might be their precursors.

Seasonal characteristics of oxalic acid and related SOA in the free troposphere of Mt. Hua, central China: implications for sources and formation mechanisms

PM10 aerosols from the summit of Mt. Hua (2060 m a.s.l) in central China during the winter and summer of 2009 were analyzed for dicarboxylic acids, ketocarboxylic acids and α-dicarbonyls. Molecular composition of dicarboxylic acids (C2-C11) in the free tropospheric aerosols reveals that oxalic acid (C2, 399 ± 261 ng m(-3) in winter and 522 ± 261 ng m(-3) in summer) is the most abundant species in both seasons, followed by malonic (C3) and succinic (C4) acids, being consistent with that on ground levels. Most of the diacids are more abundant in summer than in winter, but adipic (C6) and phthalic (Ph) acids are twice lower in summer, suggesting more significant impact of anthropogenic pollution on the wintertime alpine atmosphere. Moreover, glyoxal (Gly) and methylglyoxal (mGly) are also lower in summer (12 ± 6.1 ng m(-3)) than in winter (22 ± 13 ng m(-3)). As both dicarbonyls are a major precursor of C2, their seasonal variation patterns, which are opposite to those of the diacids, indicate that the mountain troposphere is more oxidative in summer. C2 showed strong linear correlations with levoglucosan in winter and oxidation products of isoprene and monoterpene in summer. PCA analysis further suggested that the wintertime C2 and related SOA in the Mt. Hua troposphere mostly originate from photochemical oxidations of anthropogenic pollutants emitted from biofuel and coal combustion in lowland regions. On contrast, the summertime C2 and related SOA mostly originate from further oxidation of the mountainous isoprene and monoterpene oxidation products. The AIM model calculation results showed that oxalic acid concentration well correlated with particle acidity (R(2)=0.60) but not correlated with particle liquid water content, indicating that particle acidity favors the organic acid formation because aqueous-phase C2 production is the primary mechanism of C2 formation in ambient aerosols and is driven by acid-catalyzed oxidation.

New insights into secondary organic aerosol from the ozonolysis of α-pinene from combined infrared spectroscopy and mass spectrometry measurements

Thermograms of desorbing species from size-fractionated SOA.

Hygroscopicity of particles generated from photooxidation of alpha-pinene under different oxidation conditions in the presence of sulfate seed aerosols

Smog chamber experiments were conducted to investigate the hygroscopicity of particles generated from photooxidation of alpha-pinene/NO(x) with different sulfate seed aerosols or oxidation conditions. Hygroscopicity of particles was measured by a tandem differential mobility analyzer (TDMA) in terms of hygroscopic growth factor (Gf), with a relative humidity of 85%. With sulfate seed aerosols present, Gf of the aerosols decreased very fast before notable secondary organic aerosols (SOA) formation was observed, indicating a heterogeneous process between inorganic seeds and organic products might take place as soon as oxidation begins, rather than only happening after gas-aerosol partition of organic products starts. The final SOA-coated sulfate particles had similar or lower Gf than seed-free SOA. The hygroscopicity of the final particles was not dependent on the thickness but on the hygroscopicity properties of the SOA, which were influenced by the initial sulfate seed particles. In the two designed aging processes, Gf of the particles increased more significantly with introduction of OH radical than with ozone. However, the hygroscopicity of SOA was very low even after a long time of aging, implying that either SOA aging in the chamber was very slow or the Gf of SOA did not change significantly in aging. Using an aerosol composition speciation monitor (ACSM) and matrix factorization (PMF) method, two factors for the components of SOA were identified, but the correlation between SOA hygroscopicity and the proportion of the more highly oxidized factor could be either positive or negative depending on the speciation of seed aerosols present.

The effects of α-pinene versus toluene-derived secondary organic aerosol exposure on the expression of markers associated with vascular disease

To investigate the toxicological effects of biogenic- versus anthropogenic-source secondary organic aerosol (SOA) on the cardiovascular system, the Secondary Particulate Health Effects Research program irradiation chamber was used to expose atherosclerotic apolipoprotein E null (Apo E-/-) mice to SOA from the oxidation of either α-pinene or toluene for 7 days. SOA atmospheres were produced to yield 250-300 μg/m(3) of particulate matter and ratios of 10:1:1 α-pinene:nitrogen oxide (NOx):ammonia (NH3); 10:1:1:1 α-pinene:NOx:NH3:sulfur dioxide (SO2) or 10:1:1 toluene:NOx:NH3; and 10:1:1:1 toluene:NOx:NH3:SO2. Resulting effects on the cardiovascular system were assessed by measurement of vascular lipid peroxidation (thiobarbituric acid reactive substance (TBARS)), as well as quantification of heme-oxygenase (HO)-1, endothelin (ET)-1, and matrix metalloproteinase (MMP)-9 mRNA expression for comparison to previous program exposure results. Consistent with similar previous studies, vascular TBARS were not increased significantly with any acute SOA exposure. However, vascular HO-1, MMP-9, and ET-1 observed in Apo E-/- mice exposed to α-pinene + NOx + NH3 + SO2 increased statistically, while α-pinene + NOx + NH3 exposure to either toluene + NOx + NH3 or toluene +NOx + NH3 + SO2 resulted in a decreased expression of these vascular factors. Such findings suggest that the specific chemistry created by the presence or absence of acidic components may be important in SOA-mediated toxicity in the cardiovascular system and/or progression of cardiovascular disease.