Contribution of isoprene-derived organosulfates to free tropospheric aerosol mass

Computer aided therapeutic tripeptide design, in alleviating the pathogenic proclivities of nocuous α-synuclein fibrils

Parkinson's disorder (PD) exacerbates neuronal degeneration of motor nerves, thereby effectuating uncoordinated movements and tremors. Aberrant alpha-synuclein (α-syn) is culpable of triggering PD, wherein cytotoxic amyloid aggregates of α-syn get deposited in motor neurons to instigate neuro-degeneration. Amyloid aggregates, typically rich in beta sheets are cardinal targets to mitigate their neurotoxic effects. In this analysis, owing to their interaction specificity, we formulated an efficacious tripeptide out of the aggregation-prone region of α-syn protein. With the help of a proficient computational pipeline, systematic peptide shortening and an adept molecular simulation platform, we formulated a tripeptide, VAV from α-syn structure based hexapeptide KISVRV. Indeed, the VAV tripeptide was able to effectively mitigate the α-syn amyloid fibrils' dynamic rate of beta-sheet formation. Additional trajectory analyses of the VAV- α-syn complex indicated that, upon its dynamic interaction, VAV efficiently altered the distinct pathogenic structural dynamics of α-syn, further advocating its potential in alleviating aberrant α-syn's amyloidogenic proclivities. Consistent findings from various computational analyses have led us to surmise that VAV could potentially re-alter the pathogenic conformational orientation of α-syn, essential to mitigate its cytotoxicity. Hence, VAV tripeptide could be an efficacious therapeutic candidate to efficiently ameliorate aberrant α-syn amyloid mediated neurotoxicity, eventually attenuating the nocuous effects of PD.Communicated by Ramaswamy H. Sarma.

Field observations of C2 and C3 organosulfates and insights into their formation mechanisms at a suburban site in Hong Kong

Organosulfates (OSs) are formed from volatile organic compounds (VOCs) and their oxidation products in the presence of sulfate particles. While OSs represent an important component in secondary organic aerosol, the knowledge of their formation driving force, mechanisms, and environmental impact remain inadequately understood. In this study, we report ambient observations of C2-3 oxygenated VOCs derived OSs (C2-3 OSs) at a suburban location of Hong Kong during autumn 2016. The C2-3 OSs, including glycolaldehyde sulfate (GS), hydroxyacetone sulfate (HAS), glycolic acid sulfate (GAS), and lactic acid sulfate (LAS), were quantified/semi-quantified using offline liquid chromatography-mass spectrometry analysis of aerosol filter samples. The average sum concentration of C2-3 OSs was 36 ng/m3. Correlation analysis revealed that sulfate, surface area, and liquid water content were important factors influencing C2-3 OS formation. Online measurement with an iodide High-Resolution Time-of-Flight Chemical-Ionization Mass Spectrometer (HR-ToF-CIMS) coupled with the Filter Inlet for Gases and AEROsols (FIGAERO) was also conducted to monitor C2-3 OSs, and their potential oxygenated VOC precursors in both gas- and particle-phase, and aerosol acidity tracer simultaneously. Our measurements support that glycolaldehyde/glyoxal, hydroxyacetone, glycolic acid/glyoxal, and lactic acid/methylglyoxal are likely precursors for GS, HAS, GAS, and LAS, respectively. Additionally, we found strong correlation between C2-3 OSs and H3S2O8-, a marker for aerosol acidity, providing field observational evidence for acid-catalyzed formation of small OSs. Based on both online and offline measurements, acid-catalyzed formation mechanisms in particle/aqueous phase are proposed. Specifically, the unique structure of adjacent carbonyl and hydroxyl groups in the C2-3 oxygenated VOC precursors can facilitate the formation of (1) a five-member ring intermediate via intramolecular hydrogen bond to react with sulfur trioxide through heterogenous reaction or (2) cyclic sulfate intermediate via particle-phase reaction with sulfuric acid to generate C2-3 OSs. These proposed mechanisms provide an alternative pathway for the liquid-phase production of C2-3 OSs.

Physical properties of short chain aqueous organosulfate aerosol

Organosulfates comprise up to 30% of the organic fraction of aerosol. Organosulfate aerosol physical properties, such as water activity, density, refractive index, and surface tension, are key to predicting their impact on global climate. However, current understanding of these properties is limited. Here, we measure the physical properties of aqueous solutions containing sodium methyl or ethyl sulfate and parameterise the data as a function of solute concentration. The experimental data are compared to available literature data for organosulfates, as well as salts (sodium sulfate and sodium bisulfate) and organics (short alkyl chain length alcohols and carboxylic acids) to determine if the physical properties of organosulfates can be approximated by molecules of similar functionality. With the exception of water activity, we find that organosulfates have intermediate physical properties between those of the salts and short alkyl chain organics. This work highlights the importance of measuring and developing models for the physical properties of abundant atmospheric organosulfates in order to better describe aerosol's impact on climate.

Environmental factors driving the formation of water-soluble organic aerosols: A comparative study under contrasting atmospheric conditions

Water-soluble organic carbon (WSOC), as major fractions of atmospheric aerosols, have gained attention due to their light-absorption properties. To illustrate the sources and key environmental factors driving WSOC formation under different atmospheric conditions, a comparative study was conducted by summarizing the results obtained from five field campaigns at inland (urban, suburban or regional) sites and a coastal site during different seasons. Organic carbon concentrations varied from 8.5 μg/m³ at the summer regional site to 17.5 μg/m³ at the winter urban site, with 46 %- 89 % of the mass as WSOC. Based on correlation analysis, primary combustion emissions were more important in winter than in summer, and secondary formation was an important source of WSOC during winter, summer and autumn. Atmospheric oxidants (NO₂, O₃), aerosol liquid water (ALW) and ambient RH were important factors influencing the WSOC formation, while their roles varied in different atmospheres. We observed a seasonal transition of atmospheric oxidants dominating the WSOC formation from O₃ and NO₂-driven conditions in summer to NO₂-driven conditions in winter. Elevated ALW or ambient RH generally favor the WSOC formation, while the WSOC dependence of ALW varied among different ALW ranges. As the increasing of ALW or ambient RH, a transition of WSOC formation from "RH/ALW-limited regime" under low-ALW conditions, to "RH/ALW and precursor-driven regime" under medium-ALW/RH, and to "precursor-limited (RH/ALW-excess) regime" were observed for the inland atmospheric conditions. Under the high-RH and ALW conditions in coastal areas, ALW or ambient RH was generally not a limiting factor for WSOC formation.

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.

Distinct diurnal chemical compositions and formation processes of individual organic-containing particles in Beijing winter

Organic aerosols (OA) are major components of fine particulate matter, yet their formation mechanism remains unclear, especially in polluted environments. In this study, we investigated the diurnal chemical compositions and formation processes of OA in carbonaceous particles during winter in Beijing using aerosol time-of-flight mass spectrometry. We found that 84.5% of the measured carbonaceous particles underwent aging processes, characterized by larger diameter and more secondary species compared to fresh carbonaceous particles, and presented different chemical compositions of OA in the daytime and nighttime. During the day, under high O₃ concentrations, organosulfates and oligomers existed in the aged carbonaceous particles, which were mixed with a higher signal of nitrate compared with sulfate. At night, under high relative humidity, distinct spectral signatures of hydroxymethanesulfonate and organic nitrogen compounds, and minor signals of other hydroxyalkylsulfonates and high molecular weight organic compounds were present in the aged carbonaceous particles, which were mixed with a higher signal of sulfate compared with nitrate. Our results indicated that photochemistry contributed to OA formation in the daytime, while aqueous chemistry played an important role in OA formation in the nighttime. The findings can help improve the performance of air quality and climate models on OA simulation.

The light absorbing and molecule characteristic of PM2.5 brown carbon observed in urban Shanghai

Both brown carbon (BrC) and the non-absorbing components coated on black carbon (BC) aerosols can enhance the light absorption of BC aerosols. BrC is a complicated mixture of organic compounds and not well characterized, which hinders exploring the links between BrC and optical properties. We conducted an in-depth field study on optical properties of ambient aerosols at a monitoring site in Shanghai, China via real-time monitoring and offline analysis. Results showed that BrC caused light absorption coefficients were 3.3 ± 3.3 Mm^-1, 2.2 ± 5.0 Mm^-1, 1.2 ± 1.2 Mm^-1 at λ = 370, 470 and 520 nm, respectively, accounting for 11%, 10%, 6% of the total aerosol absorption for the corresponding wavelengths. A larger proportion of long-chain aliphatic organosulfates (OSs, C_nH_2n+2O₄S, (CH₂)_nO₅S, (CH₂)_nO₆S) with double bond equivalent (DBE) values of 0 or 1 accounted for 5-20% of the light absorption (λ = 365 nm) for soluble brown carbon (BrC), which were dominating for the days with less N-containing aromatic compounds appearing. Furthermore, the structure of C_nH_2n+2O₄S, (CH₂)_nO₅S, (CH₂)_nO₆S were explored using target MS/MS of HPLC-Q-ToF-MS: (CH₂)_nO₅S series, the most abundant family of OSs, were constructed by functionalizing a saturated hydrocarbon with one sulfate and one carbonyl group. C_nH_2n+2O₄S series were oxidized with only one sulfate group in the aliphatic chain R. (CH₂)_nO₆S series were proposed as aliphatic OSs with one ester group. We speculated aliphatic OSs were formed via acid catalyzed perhydrolysis of hydroperoxides derived from long-chain alkanes releasing from diesel fueled vehicles, followed by the reaction with sulfate anion radicals. Therefore, relevant technologies should be further explored to reduce the impacts from vehicle emissions.

Isoprene Epoxydiol-Derived Sulfated and Nonsulfated Oligomers Suppress Particulate Mass Loss during Oxidative Aging of Secondary Organic Aerosol

Acid-driven multiphase chemistry of isoprene epoxydiols (IEPOX) with inorganic sulfate aerosols contributes substantially to secondary organic aerosol (SOA) formation, which constitutes a large mass fraction of atmospheric fine particulate matter (PM_2.5). However, the atmospheric chemical sinks of freshly generated IEPOX-SOA particles remain unclear. We examined the role of heterogeneous oxidation of freshly generated IEPOX-SOA particles by gas-phase hydroxyl radical (^•OH) under dark conditions as one potential atmospheric sink. After 4 h of gas-phase ^•OH exposure (∼3 × 10⁸ molecules cm^-3), chemical changes in smog chamber-generated IEPOX-SOA particles were assessed by hydrophilic interaction liquid chromatography coupled with electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). A comparison of the molecular-level compositional changes in IEPOX-SOA particles during aging with or without ^•OH revealed that decomposition of oligomers by heterogeneous ^•OH oxidation acts as a sink for ^•OH and maintains a reservoir of low-volatility compounds, including monomeric sulfate esters and oligomer fragments. We propose tentative structures and formation mechanisms for previously uncharacterized SOA constituents in PM_2.5. Our results suggest that this ^•OH-driven renewal of low-volatility products may extend the atmospheric lifetimes of particle-phase IEPOX-SOA by slowing the production of low-molecular weight, high-volatility organic fragments and likely contributes to the large quantities of 2-methyltetrols and methyltetrol sulfates reported in PM_2.5.

Computational chemistry of cluster: Understanding the mechanism of atmospheric new particle formation at the molecular level

New particle formation (NPF), which exerts significant influence over human health and global climate, has been a hot topic and rapidly expands field of research in the environmental and atmospheric chemistry recent years. Generally, NPF contains two processes: formation of critical nucleus and further growth of the nucleus. However, due to the complexity of the atmospheric nucleation, which is a multicomponent process, formation of critical clusters as well as their growth is still connected to large uncertainties. Detection limits of instruments in measuring specific gaseous aerosol precursors and chemical compositions at the molecular level call for computational studies. Computational chemistry could effectively compensate the deficiency of laboratory experiments as well as observations and predict the nucleation mechanisms. We review the present theoretical literatures that discuss nucleation mechanism of atmospheric clusters. Focus of this review is on different nucleation systems involving sulfur-containing species, nitrogen-containing species and iodine-containing species. We hope this review will provide a deep insight for the molecular interaction of nucleation precursors and reveal nucleation mechanism at the molecular level.

OH Group Orientation Leads to Organosulfate Formation at the Liquid Aerosol Surface

Organosulfates (OSs) are well-known and ubiquitous constituents of atmospheric aerosol particles and have been used as secondary organic aerosol markers in many field studies. Hence, it is imperative to understand the formation of OS species in the atmosphere. Recently, hydroxy acids (HAs) and hydroxy acid sulfates have been extensively detected in the atmospheric environment. However, the reaction mechanism of HAs to form OSs is much less understood. In this work, we have mainly investigated the reaction of typical α-HAs, including glycolic acid (GA) and lactic acid (LA), and SO₃ at the liquid aerosol surface using quantum chemistry calculations and Born-Oppenheimer molecular dynamics simulations. The OH group orientation of α-HAs at the air-water interface is found to exert a significant impact on the formation of OSs. The OH group pointing to the gas phase is obviously beneficial to the formation of OSs. Two key factors are discovered important to the reaction of α-HAs adsorbed on the liquid surface with SO₃: (a) the exposure position of the active site to the gas phase and (b) the reactivity of the exposed site to the attracted SO₃ molecule. Moreover, we found that the air-water interface exerts a significant influence on the physicochemical behaviors of GA and LA, especially on their OH group orientation, and thus leads to their different properties for the SO₃ colliding reaction. The presented reaction mechanism provides a new feasible pathway for the production of OSs at the liquid aerosol surface, which may have important impacts on the formation of organic aerosols.

Second-Order Kinetic Rate Coefficients for the Aqueous-Phase Sulfate Radical (SO4•-) Oxidation of Some Atmospherically Relevant Organic Compounds

The sulfate anion radical (SO₄^•–) is a reactive oxidant formed in the autoxidation chain of sulfur dioxide, among other sources. Recently, new formation pathways toward SO₄^•– and other reactive sulfur species have been reported. This work investigated the second-order rate coefficients for the aqueous SO₄^•– oxidation of the following important organic aerosol compounds (k_SO4): 2-methyltetrol, 2-methyl-1,2,3-trihydroxy-4-sulfate, 2-methyl-1,2-dihydroxy-3-sulfate, 1,2-dihydroxyisoprene, 2-methyl-2,3-dihydroxy-1,4-dinitrate, 2-methyl-1,2,4-trihydroxy-3-nitrate, 2-methylglyceric acid, 2-methylglycerate, lactic acid, lactate, pyruvic acid, pyruvate. The rate coefficients of the unknowns were determined against that of a reference in pure water in a temperature range of 298–322 K. The decays of each reagent were measured with nuclear magnetic resonance (NMR) and high-performance liquid chromatography–high-resolution mass spectrometry (HPLC-HRMS). Incorporating additional SO₄^•– reactions into models may aid in the understanding of organosulfate formation, radical propagation, and aerosol mass sinks.

Sulfur radical formation from the tropospheric irradiation of aqueous sulfate aerosols

It was found that shining natural or artificial sunlight on concentrated solutions of sulfate ions mixed with organics, a mixture commonly found in atmospheric aerosol particles, can generate sulfur-containing radicals under a variety of conditions. This reaction has not previously been characterized in atmospheric chemistry. These reactive radicals can subsequently degrade organic compounds in atmospheric particles, forming a variety of products that stay in the particle water and small molecules that are volatile enough to partition to the gas phase. In particular, this source of sulfur radicals can produce surface-active organosulfates and organic acids.

The sulfate anion radical (SO₄^•–) is known to be formed in the autoxidation chain of sulfur dioxide and from minor reactions when sulfate or bisulfate ions are activated by OH radicals, NO₃ radicals, or iron. Here, we report a source of SO₄^•–, from the irradiation of the liquid water of sulfate-containing organic aerosol particles under natural sunlight and laboratory UV radiation. Irradiation of aqueous sulfate mixed with a variety of atmospherically relevant organic compounds degrades the organics well within the typical lifetime of aerosols in the atmosphere. Products of the SO₄^•– + organic reaction include surface-active organosulfates and small organic acids, alongside other products. Scavenging and deoxygenated experiments indicate that SO₄^•– radicals, instead of OH, drive the reaction. Ion substitution experiments confirm that sulfate ions are necessary for organic reactivity, while the cation identity is of low importance. The reaction proceeds at pH 1–6, implicating both bisulfate and sulfate in the formation of photoinduced SO₄^•–. Certain aromatic species may further accelerate the reaction through synergy. This reaction may impact our understanding of atmospheric sulfur reactions, aerosol properties, and organic aerosol lifetimes when inserted into aqueous chemistry model mechanisms.

Study on heterogeneous OH oxidation of 3-methyltetraol sulfate in the atmosphere under high NO conditions

Organosulfates (OSs), also known as organic sulfate esters, are ubiquitous in atmospheric particles and used as secondary organic aerosol (SOA) markers. However, the chemical transformation mechanism of these OSs remains unclear. Therefore, we investigated the heterogeneous OH oxidation of 3-methyltetraol sulfate (3-MTS), which is one of the most abundant particulate organosulfates, by using quantum chemical and kinetic calculations. 3-MTS can easily undergo abstraction reaction with OH radicals, and the reaction rate constant is about 7.87 × 10^-12 cm³ per molecule per s. The generated HCOOH, CH₃COOH, HCHO, CH₃CHO and 2-methyl-2,3-dihydroxypropionic acid are low-volatility species with increased water solubility, which are the main components of SOA. In addition, the OH radicals obtained from the reaction can continue to promote the oxidation reaction. The results of this study provide insights into the heterogeneous OH reactivity of other organosulfates in atmospheric aerosols, and it also provides a new understanding of the conversion of sulfur (S) between its organic and inorganic forms during the heterogeneous OH oxidation of organic sulfates.

Direct Measurement of pH Evolution in Aerosol Microdroplets Undergoing Ammonium Depletion: A Surface-Enhanced Raman Spectroscopy Approach

Accurately measuring the pH of atmospheric aerosols is a prerequisite for understanding the multiphase chemistry that profoundly affects the environment and climate systems. Despite the advancements of experimental techniques for in situ pH measurements in aerosols, current studies are limited to measuring the static pH of aerosol microdroplets with an unperturbed composition. This steady-state scenario, however, deviates from the real-world aerosols undergoing atmospheric aging reactions, specifically, those characterized with a spontaneous displacement of strong bases (or acids) with high volatility. Here, we introduce a continuous and in situ measurement of aerosol pH by using a 4-mercaptopyridine-functionalized silver nanoparticle probe and surface-enhanced Raman spectroscopy. We find that the ammonium depletion─a spontaneous displacement of ammonium by dicarboxylic acid salts─continuously acidifies aerosol water over time. The decaying trends of pH in the aerosols under various humidity conditions can be unified with a universal exponential function. Such an exponentially decaying function further indicates that the ammonium depletion reaction is a self-limiting process. Our technique can be applied to study the dynamic change of aerosol acidity during the complex atmospheric aging processes, toward elucidating their implications on atmospheric chloride, nitrate, and ammonium cycles.

A review of secondary organic aerosols formation focusing on organosulfates and organic nitrates

Secondary organic aerosols (SOA) are crucial constitution of fine particulate matter (PM), which are mainly derived from photochemical oxidation products of primary organic matter and volatile organic compounds (VOCs), and can induce terrible impacts to human health, air quality and climate change. As we know, organosulfates (OSs) and organic nitrates (ON) are important contributors for SOA formation, which could be possibly produced through various pathways, resulting in extremely complex formation mechanism of SOA. Although plenty of research has been focused on the origins, spatial distribution and formation mechanisms of SOA, a comprehensive and systematic understanding of SOA formation in the atmosphere remains to be detailed explored, especially the most important OSs and ON dedications. Thus, in this review, we systematically summarize the recent research about origins and formation mechanisms of OSs and ON, and especially focus on their contribution to SOA, so as to have a clearer understanding of the origin, spatial distribution and formation principle of SOA. Importantly, we interpret the complex interaction with coexistence effect of SO_x and NO_x on SOA formation, and emphasize the future insights for SOA research to expect a more comprehensive theory and practice to alleviate SOA burden.

Analytical methods for organosulfate detection in aerosol particles: Current status and future perspectives

Organosulfates (OSs) are well-known water-soluble constituents of atmospheric aerosol particles. They are formed from multiphase reactions between volatile organic compounds (VOCs) and their photooxidation products, and acidic sulfate originating from biogenic and anthropogenic sources in the atmosphere. Although the analytical procedures used to measure OSs, including sampling, pre-treatment, and instrumental detection, have advanced substantially in the last decade, there is still a need for accurate and standardized analysis procedures for the identification, quantification, and comparison of OSs in different regions. Additionally, there has no study focused on the health effects of OSs. This review outlines the analytical methods developed for OS detection during the last decade, highlighting both improvements and drawbacks. It also considers the future development of analytical methods for OS detection, and proposes the establishment of OSs screening method from the perspective of health effects to solve the problem of unknown health related OSs identification.

A biogenic secondary organic aerosol source of cirrus ice nucleating particles

Atmospheric ice nucleating particles (INPs) influence global climate by altering cloud formation, lifetime, and precipitation efficiency. The role of secondary organic aerosol (SOA) material as a source of INPs in the ambient atmosphere has not been well defined. Here, we demonstrate the potential for biogenic SOA to activate as depositional INPs in the upper troposphere by combining field measurements with laboratory experiments. Ambient INPs were measured in a remote mountaintop location at -46 °C and an ice supersaturation of 30% with concentrations ranging from 0.1 to 70 L-1. Concentrations of depositional INPs were positively correlated with the mass fractions and loadings of isoprene-derived secondary organic aerosols. Compositional analysis of ice residuals showed that ambient particles with isoprene-derived SOA material can act as depositional ice nuclei. Laboratory experiments further demonstrated the ability of isoprene-derived SOA to nucleate ice under a range of atmospheric conditions. We further show that ambient concentrations of isoprene-derived SOA can be competitive with other INP sources. This demonstrates that isoprene and potentially other biogenically-derived SOA materials could influence cirrus formation and properties.

Global Importance of Hydroxymethanesulfonate in Ambient Particulate Matter: Implications for Air Quality

Sulfur compounds are an important constituent of particulate matter, with impacts on climate and public health. While most sulfur observed in particulate matter has been assumed to be sulfate, laboratory experiments reveal that hydroxymethanesulfonate (HMS), an adduct formed by aqueous phase chemical reaction of dissolved HCHO and SO2, may be easily misinterpreted in measurements as sulfate. Here we present observational and modeling evidence for a ubiquitous global presence of HMS. We find that filter samples collected in Shijiazhuang, China, and examined with ion chromatography within 9 days show as much as 7.6 μg m-3 of HMS, while samples from Singapore examined 9-18 months after collection reveal ~0.6 μg m-3 of HMS. The Shijiazhuang samples show only minor traces of HMS 4 months later, suggesting that HMS had decomposed over time during sample storage. In contrast, the Singapore samples do not clearly show a decline in HMS concentration over 2 months of monitoring. Measurements from over 150 sites, primarily derived from the IMPROVE network across the United States, suggest the ubiquitous presence of HMS in at least trace amounts as much as 60 days after collection. The degree of possible HMS decomposition in the IMPROVE observations is unknown. Using the GEOS-Chem chemical transport model, we estimate that HMS may account for 10% of global particulate sulfur in continental surface air and over 25% in many polluted regions. Our results suggest that reducing emissions of HCHO and other volatile organic compounds may have a co-benefit of decreasing particulate sulfur.

Effects of NOx and SO2 on the secondary organic aerosol formation from the photooxidation of 1,3,5-trimethylbenzene: A new source of organosulfates

1,3,5-Trimethylbeneze (TMB) is an important constituent of anthropogenic volatile organic compounds that contributes to the formation of secondary organic aerosol (SOA). A series of chamber experiments were performed to probe the effects of NO_x and SO₂ on SOA formation from TMB photooxidation. The molecular composition of TMB SOA was investigated by ultra-high performance liquid chromatography/electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS). We found that the SOA yield increases notably with elevated NO_x concentrations under low-NO_x condition ([TMB]₀/[NO_x]₀ > 10 ppbC ppb^-1), while an opposite trend is observed in high-NO_x experiments ([TMB]₀/[NO_x]₀ < 10 ppbC ppb^-1). The increase in SOA yield in low-NO_x regime is attributed to the increase of NO_x-induced OH concentrations. The formation of low-volatility species might be suppressed, thereby leading to a lower SOA yield in high-NO_x conditions. Moreover, SOA formation was promoted in experiment with SO₂ addition. Multifunctional products containing carbonyl, acid, alcohol, and nitrate functional groups were characterized in TMB/NO_x photooxidation, whereas several organosulfates (OSs) and nitrooxy organosulfates were identified in TMB/NO_x/SO₂ photooxidation based on HR-Q-TOFMS analysis. The formation mechanism relevant to the detected compounds in SOA were proposed. Based on our measurements, the photooxidation of TMB in the presence of SO₂ may be a new source of OSs in the atmosphere. The results presented here also deepen the understanding of SOA formation under relatively complex polluted environments.

Tracing the biogenic secondary organic aerosol markers in rain, snow and hail

The molecular characterization of secondary organic aerosol (SOA) is based mainly on LC-MS analyses of particulate matter (PM) samples collected with aerosol samplers. Several studies have analyzed atmospheric waters, including rain and cloud water, for the presence of SOA components, however, no separation techniques were used making identification of the individual components in these complex mixtures impossible. We have applied our improved UHPLC-HR-MS methodology to analyze atmospheric precipitates (hailstone, rain and snow), as well as SOA collected with high-volume samplers. We achieved sensitivity levels and separation efficiencies that were sufficient for molecular-level identification of individual compounds. Tracing commonly known SOA markers such as organosulfates (OS), C₄-C₆ dicarboxylic acids and terpenoic acids revealed that the chromatographic profiles for both atmospheric precipitate and PM samples were very similar, with both giving similar component ratios, especially for OS. We also demonstrated that SOA markers can be detected directly from raw rain samples. Our results show that LC-MS techniques are suitable for the convenient analysis of atmospheric precipitates containing SOA markers at the molecular level. It complements traditional SOA analyses and provides additional sampling opportunities which will no doubt allow for better elucidation of chemical transformations of volatile organic compounds in the atmosphere.

Natural and Anthropogenically Influenced Isoprene Oxidation in Southeastern United States and Central Amazon

Anthropogenic emissions alter secondary organic aerosol (SOA) formation chemistry from naturally emitted isoprene. We use correlations of tracers and tracer ratios to provide new perspectives on sulfate, NO_x, and particle acidity influencing isoprene-derived SOA in two isoprene-rich forested environments representing clean to polluted conditions-wet and dry seasons in central Amazonia and Southeastern U.S. summer. We used a semivolatile thermal desorption aerosol gas chromatograph (SV-TAG) and filter samplers to measure SOA tracers indicative of isoprene/HO₂ (2-methyltetrols, C₅-alkene triols, 2-methyltetrol organosulfates) and isoprene/NO_x (2-methylglyceric acid, 2-methylglyceric acid organosulfate) pathways. Summed concentrations of these tracers correlated with particulate sulfate spanning three orders of magnitude, suggesting that 1 μg m^-3 reduction in sulfate corresponds with at least ∼0.5 μg m^-3 reduction in isoprene-derived SOA. We also find that isoprene/NO_x pathway SOA mass primarily comprises organosulfates, ∼97% in the Amazon and ∼55% in Southeastern United States. We infer under natural conditions in high isoprene emission regions that preindustrial aerosol sulfate was almost exclusively isoprene-derived organosulfates, which are traditionally thought of as representative of an anthropogenic influence. We further report the first field observations showing that particle acidity correlates positively with 2-methylglyceric acid partitioning to the gas phase and negatively with the ratio of 2-methyltetrols to C₅-alkene triols.

The Acidity of Atmospheric Particles and Clouds

Acidity, defined as pH, is a central component of aqueous chemistry. In the atmosphere, the acidity of condensed phases (aerosol particles, cloud water, and fog droplets) governs the phase partitioning of semi-volatile gases such as HNO3, NH3, HCl, and organic acids and bases as well as chemical reaction rates. It has implications for the atmospheric lifetime of pollutants, deposition, and human health. Despite its fundamental role in atmospheric processes, only recently has this field seen a growth in the number of studies on particle acidity. Even with this growth, many fine particle pH estimates must be based on thermodynamic model calculations since no operational techniques exist for direct measurements. Current information indicates acidic fine particles are ubiquitous, but observationally-constrained pH estimates are limited in spatial and temporal coverage. Clouds and fogs are also generally acidic, but to a lesser degree than particles, and have a range of pH that is quite sensitive to anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient ammonia. Historical measurements indicate that cloud and fog droplet pH has changed in recent decades in response to controls on anthropogenic emissions, while the limited trend data for aerosol particles indicates acidity may be relatively constant due to the semi-volatile nature of the key acids and bases and buffering in particles. This paper reviews and synthesizes the current state of knowledge on the acidity of atmospheric condensed phases, specifically particles and cloud droplets. It includes recommendations for estimating acidity and pH, standard nomenclature, a synthesis of current pH estimates based on observations, and new model calculations on the local and global scale.

Organosulfates in Ambient Aerosol: State of Knowledge and Future Research Directions on Formation, Abundance, Fate, and Importance

Organosulfates (OSs), also referred to as organic sulfate esters, are well-known and ubiquitous constituents of atmospheric aerosol particles. Commonly, they are assumed to form upon mixing of air masses of biogenic and anthropogenic origin, that is, through multiphase reactions between organic compounds and acidic sulfate particles. However, in contrast to this simplified picture, recent studies suggest that OSs may also originate from purely anthropogenic precursors or even directly from biomass and fossil fuel burning. Moreover, besides classical OS formation pathways, several alternative routes have been discovered, suggesting that OS formation possibly occurs through a wider variety of formation mechanisms in the atmosphere than initially expected. During the past decade, OSs have reached a constantly growing attention within the atmospheric science community with evermore studies reporting on large numbers of OS species in ambient aerosol. Nonetheless, estimates on OS concentrations and implications on atmospheric physicochemical processes are still connected to large uncertainties, calling for combined field, laboratory, and modeling studies. In this Critical Review, we summarize the current state of knowledge in atmospheric OS research, discuss unresolved questions, and outline future research needs, also in view of reductions of anthropogenic sulfur dioxide (SO₂) emissions. Particularly, we focus on (1) field measurements of OSs and measurement techniques, (2) formation pathways of OSs and their atmospheric relevance, (3) transformation, reactivity, and fate of OSs in atmospheric particles, and (4) modeling efforts of OS formation and their global abundance.

Structural Characterization of Lactone-Containing MW 212 Organosulfates Originating from Isoprene Oxidation in Ambient Fine Aerosol

Isoprene (C₅H₈) is the main non-methane hydrocarbon emitted into the global atmosphere. Despite intense research, atmospheric transformations of isoprene leading to secondary organic aerosol (SOA) are still not fully understood, including its multiphase chemical reactions. Herein, we report on the detailed structural characterization of atmospherically relevant isoprene-derived organosulfates (OSs) with a molecular weight (MW) of 212 (C₅H₈SO₇), which are abundantly present in both ambient fine aerosol (PM_2.5) and laboratory-generated isoprene SOA. The results obtained from smog chamber-generated isoprene SOA and aqueous-phase laboratory experiments coupled to the S(IV)-autooxidation chemistry of isoprene, 3-methyl-2(5H)-furanone, and 4-methyl-2(5H)-furanone, allowed us for the first time to fully elucidate the isomeric structures of the MW 212 OSs. By applying liquid chromatography interfaced to electrospray ionization high-resolution mass spectrometry, we firmly confirmed six positional isomers of the MW 212 OSs in PM_2.5 collected from different sites in Europe and the United States. Our results also show that despite the low solubility of isoprene in water, aqueous-phase or multiphase chemistry can play an important role in the formation of OSs from isoprene. Possible formation mechanisms for the MW 212 OSs are also tentatively proposed.

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%.

Atmospheric Chemistry in a Box or a Bag

Environmental chambers have proven to be essential for atmospheric photochemistry research. This historical perspective summarizes chamber research characterizing smog. Experiments with volatile organic compounds (VOCs)-nitrogen oxides (NOx) have characterized O3 and aerosol chemistry. These led to the creation and evaluation of complex reaction mechanisms adopted for various applications. Gas-phase photochemistry was initiated and developed using chamber studies. Post-1950s study of photochemical aerosols began using smog chambers. Much of the knowledge about the chemistry of secondary organic aerosols (SOA) derives from chamber studies complemented with specially designed atmospheric studies. Two major findings emerge from post-1990s SOA experiments: (1) photochemical SOAs hypothetically involve hydrocarbons and oxygenates with carbon numbers of 2, and (2) SOA evolves via more than one generation of reactions as condensed material exchanges with the vapor phase during “aging”. These elements combine with multiphase chemistry to yield mechanisms for aerosols. Smog chambers, like all simulators, are limited representations of the atmosphere. Translation to the atmosphere is complicated by constraints in reaction times, container interactions, influence of precursor injections, and background species. Interpretation of kinetics requires integration into atmospheric models addressing the combined effects of precursor emissions, surface exchange, hydrometeor interactions, air motion and sunlight.

Secondary organic aerosols in Jinan, an urban site in North China: Significant anthropogenic contributions to heavy pollution

Secondary organic aerosols (SOAs) are an important component of particulates, but whether biogenic SOAs (BSOAs) or anthropogenic SOAs (ASOAs) are the dominant contributors to haze pollution remains poorly characterized. In this study, particulate samples were collected from September 2014 to August 2015 at an urban site in Jinan, which is the capital of Shandong Province and a typical city in the North China Plain. The PM_2.5 samples were analyzed for BSOA (isoprene (SOA_I) and monoterpenes (SOA_M)) and ASOA (aromatic (SOA_A)) tracers. The concentrations of the SOA_A tracer (1.1 ± 1.0 ng/m³) were lowest, and those of SOA_I tracers (41.8 ± 86.2 ng/m³) were highest, with the concentrations of SOA_M tracers (19.4 ± 9.9 ng/m³) being intermediate. The SOA_I tracers were more abundant in the summer and less abundant in the winter. Both SOA_I and SOA_M increased with increasing ozone level but decreased with increasing NOx level. Correlation analysis revealed a good correlation between 2,3-dihydroxy-4-oxopentanoic acid and levoglucosan levels in three seasons. These results suggested that biomass burning activities occurring in the NCP can enhance the emissions of aromatics and should be controlled, especially in the autumn and winter. SOA tracers were classified according to pollution degree, and the results showed that as pollution increases, the contributions of SOA_A increase. These results indicate that reducing anthropogenic emissions is necessary to prevent SOA pollution, especially during heavy pollution episodes.

Response of the Aerodyne Aerosol Mass Spectrometer to Inorganic Sulfates and Organosulfur Compounds: Applications in Field and Laboratory Measurements

Organosulfur compounds are important components of secondary organic aerosols (SOA). While the Aerodyne high-resolution time-of-flight aerosol mass spectrometer (AMS) has been extensively used in aerosol studies, the response of the AMS to organosulfur compounds is not well-understood. Here, we investigated the fragmentation patterns of organosulfurs and inorganic sulfates in the AMS, developed a method to deconvolve total sulfate into components of inorganic and organic origins, and applied this method in both laboratory and field measurements. Apportionment results from laboratory isoprene photooxidation experiment showed that with inorganic sulfate seed, sulfate functionality of organic origins can contribute ∼7% of SOA mass at peak growth. Results from measurements in the Southeastern U.S. showed that 4% of measured sulfate is from organosulfur compounds. Methanesulfonic acid was estimated for measurements in the coastal and remote marine boundary layer. We explored the application of this method to unit mass-resolution data, where it performed less well due to interferences. Our apportionment results demonstrate that organosulfur compounds could be a non-negligible source of sulfate fragments in AMS laboratory and field data sets. A reevaluation of previous AMS measurements over the full range of atmospheric conditions using this method could provide a global estimate/constraint on the contribution of organosulfur compounds.

Effects of NOx, SO2 and RH on the SOA formation from cyclohexene photooxidation

We performed a laboratory investigation of the secondary organic aerosol (SOA) formation from cyclohexene photooxidation with different initial NOx and SO₂ concentrations at low and high relative humidity (RH). Both SOA yield and number concentration first increase drastically and then, decreased when the [VOC]₀/[NOx]₀ ratio changed from 30 to 10 and from 10 to 3. Though the presence of SO₂ could increase the SOA number concentration, the SOA yield could only increase under [VOC]₀/[NOx]₀ = 10 and high RH, and [VOC]₀/[NOx]₀ = 3 and low RH experimental conditions, while decreasing under [VOC]₀/[NOx]₀ = 10 and low RH conditions. In the presence of SO₂, the high RH and high NOx conditions were keys to efficient sulfate formation and could promote the SOA formation. The chemical composition of SOA was characterized using hybrid quadrupole-orbitrap mass spectrometer equipped with electrospray ionization (ESI-Q-Orbitrap-HRMS), and few organosulfates were identified. A visible enhancement of organosulfates and the formation of high molecular weight organic compounds were observed at high RH conditions, and this seemed to be the reason for the SOA yield increase at high RH.

Chemical composition of isoprene SOA under acidic and non-acidic conditions: effect of relative humidity

The effect of acidity and relative humidity on bulk isoprene aerosol parameters has been investigated in several studies; however, few measurements have been conducted on individual aerosol compounds. The focus of this study has been the examination of the effect of acidity and relative humidity on secondary organic aerosol (SOA) chemical composition from isoprene photooxidation in the presence of nitrogen oxide (NO x ). A detailed characterization of SOA at the molecular level was also investigated. Experiments were conducted in a 14.5 m3 smog chamber operated in flow mode. Based on a detailed analysis of mass spectra obtained from gas chromatography-mass spectrometry of silylated derivatives in electron impact and chemical ionization modes, ultra-high performance liquid chromatography/electrospray ionization/time-of-flight high-resolution mass spectrometry, and collision-induced dissociation in the negative ionization modes, we characterized not only typical isoprene products but also new oxygenated compounds. A series of nitroxy-organosulfates (NOSs) were tentatively identified on the basis of high-resolution mass spectra. Under acidic conditions, the major identified compounds include 2-methyltetrols (2MT), 2-methylglyceric acid (2mGA), and 2MT-OS. Other products identified include epoxydiols, mono- and dicarboxylic acids, other organic sulfates, and nitroxy- and nitrosoxy-OS. The contribution of SOA products from isoprene oxidation to PM2.5 was investigated by analyzing ambient aerosol collected at rural sites in Poland. Methyltetrols, 2mGA, and several organosulfates and nitroxy-OS were detected in both the field and laboratory samples. The influence of relative humidity on SOA formation was modest in non-acidic-seed experiments and stronger under acidic seed aerosol. Total secondary organic carbon decreased with increasing relative humidity under both acidic and non-acidic conditions. While the yields of some of the specific organic compounds decreased with increasing relative humidity, others varied in an indeterminate manner from changes in the relative humidity.

Aqueous-Phase Oxidation of Terpene-Derived Acids by Atmospherically Relevant Radicals

Terpene-derived acids formed through the atmospheric gas-phase oxidation of terpenes are able to efficiently undergo a phase transfer into the aqueous phase. The subsequent aqueous-phase oxidation of such compounds has not been intensely studied. Accordingly, the aqueous-phase second-order rate constants of the oxidation reactions of cis-pinonic acid (CPA) and (+)-camphoric acid (+CA) with hydroxyl radicals (^•OH), nitrate radicals (NO₃^•), and sulfate radicals (SO₄^•-) were investigated as a function of temperature and pH in the present study. For CPA and +CA the following ^•OH reaction rate constants at T = 298 K are determined: k_second(CPA, pH<2) = (2.8 ± 0.1) × 10⁹ L mol^-1 s^-1, k_second(CPA, pH>8) = (2.7 ± 0.3) × 10⁹ L mol^-1 s^-1, k_second(+CA, pH<2) = (2.1 ± 0.1) × 10⁹ L mol^-1 s^-1, k_second(+CA, pH=5.3) = (2.7 ± 0.3) × 10⁹ L mol^-1 s^-1, k_second(+CA, pH>8) = (2.7 ± 0.1) × 10⁹ L mol^-1 s^-1. In order to assess the atmospheric impact of the aqueous-phase oxidation of such compounds, atmospheric aqueous-phase lifetimes were calculated for two model scenarios based on CAPRAM 3.0i. The aqueous-phase oxidation under remote conditions emerges to be the most favored pathway with lifetimes of 5 ± 1 h.

Seasonal cycles of secondary organic aerosol tracers in rural Guangzhou, Southern China: The importance of atmospheric oxidants

Thirteen secondary organic aerosol (SOA) tracers of isoprene (SOA_I), monoterpenes (SOA_M), sesquiterpenes (SOA_S) and aromatics (SOA_A) in fine particulate matter (PM_2.5) were measured at a Pearl River Delta (PRD) regional site for one year. The characteristics including their seasonal cycles and the factors influencing their formation in this region were studied. The seasonal patterns of SOA_I, SOA_M and SOA_S tracers were characterized over three enhancement periods in summer (I), autumn (II) and winter (III), while the elevations of SOA_A tracer (i.e., 2,3-dihydroxy-4-oxopentanoic acid, DHOPA) were observed in Periods II and III. We found that SOA formed from different biogenic precursors could be driven by several factors during a one-year seasonal cycle. Isoprene emission controlled SOA_I formation throughout the year, while monoterpene and sesquiterpene emissions facilitated SOA_M and SOA_S formation in summer rather than in other seasons. The influence of atmospheric oxidants (O_x) was found to be an important factor of the formation of SOA_M tracers during the enhancement periods in autumn and winter. The formation of SOA_S tracer was influenced by the precursor emissions in summer, atmospheric oxidation in autumn and probably also by biomass burning in both summer and winter. In this study, we could not see the strong contribution of biomass burning to DHOPA as suggested by previous studies in this region. Instead, good correlations between observed DHOPA and O_x as well as [NO₂][O₃] suggest the involvement of both ozone (O₃) and nitrogen dioxide (NO₂) in the formation of DHOPA. The results showed that regional air pollution may not only increase the emissions of aromatic precursors but also can greatly promote the formation processes.

Gas-Phase Reactions of Isoprene and Its Major Oxidation Products

Isoprene carries approximately half of the flux of non-methane volatile organic carbon emitted to the atmosphere by the biosphere. Accurate representation of its oxidation rate and products is essential for quantifying its influence on the abundance of the hydroxyl radical (OH), nitrogen oxide free radicals (NO _x), ozone (O₃), and, via the formation of highly oxygenated compounds, aerosol. We present a review of recent laboratory and theoretical studies of the oxidation pathways of isoprene initiated by addition of OH, O₃, the nitrate radical (NO₃), and the chlorine atom. From this review, a recommendation for a nearly complete gas-phase oxidation mechanism of isoprene and its major products is developed. The mechanism is compiled with the aims of providing an accurate representation of the flow of carbon while allowing quantification of the impact of isoprene emissions on HO _x and NO _x free radical concentrations and of the yields of products known to be involved in condensed-phase processes. Finally, a simplified (reduced) mechanism is developed for use in chemical transport models that retains the essential chemistry required to accurately simulate isoprene oxidation under conditions where it occurs in the atmosphere-above forested regions remote from large NO _x emissions.

Simulating Aqueous-Phase Isoprene-Epoxydiol (IEPOX) Secondary Organic Aerosol Production During the 2013 Southern Oxidant and Aerosol Study (SOAS)

The lack of statistically robust relationships between IEPOX (isoprene epoxydiol)-derived SOA (IEPOX SOA) and aerosol liquid water and pH observed during the 2013 Southern Oxidant and Aerosol Study (SOAS) emphasizes the importance of modeling the whole system to understand the controlling factors governing IEPOX SOA formation. We present a mechanistic modeling investigation predicting IEPOX SOA based on Community Multiscale Air Quality (CMAQ) model algorithms and a recently introduced photochemical box model, simpleGAMMA. We aim to (1) simulate IEPOX SOA tracers from the SOAS Look Rock ground site, (2) compare the two model formulations, (3) determine the limiting factors in IEPOX SOA formation, and (4) test the impact of a hypothetical sulfate reduction scenario on IEPOX SOA. The estimated IEPOX SOA mass variability is in similar agreement (r2 ∼ 0.6) with measurements. Correlations of the estimated and measured IEPOX SOA tracers with observed aerosol surface area (r2 ∼ 0.5-0.7), rate of particle-phase reaction (r2 ∼ 0.4-0.7), and sulfate (r2 ∼ 0.4-0.5) suggest an important role of sulfate in tracer formation via both physical and chemical mechanisms. A hypothetical 25% reduction of sulfate results in ∼70% reduction of IEPOX SOA formation, reaffirming the importance of aqueous phase chemistry in IEPOX SOA production.

Ambient observations of sub-1.0 hygroscopic growth factor and f(RH) values: Case studies from surface and airborne measurements

This study reports on the first set of ambient observations of sub-1.0 hygroscopicity values (i.e., growth factor, ratio of humidified-to-dry diameter, GF=D _p,wet /D _p,dry and f(RH), ratio of humidified-to-dry scattering coefficients, less than 1) with consistency across different instruments, regions, and platforms. We utilized data from (i) a shipboard humidified tandem differential mobility analyzer (HTDMA) during Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) in 2011, (ii) multiple instruments on the DC-8 aircraft during Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC⁴RS) in 2013, as well as (iii) the Differential Aerosol Sizing and Hygroscopicity Spectrometer Probe (DASH-SP) during measurement intensives during Summer 2014 and Winter 2015 in Tucson, Arizona. Sub-1.0 GFs were observed across the range of relative humidity (RH) investigated (75-95%), and did not show a RH-dependent trend in value below 1.0 or frequency of occurrence. A commonality between suppressed hygroscopicity in these experiments, including sub-1.0 GF, was the presence of smoke. Evidence of externally mixed aerosol, and thus multiple GFs, was observed during smoke periods resulting in at least one mode with GF < 1. Time periods during which the DASH-SP detected externally mixed aerosol coincide with sub-1.0 f(RH) observations. Mechanisms responsible for sub-1.0 hygroscopicity are discussed and include refractive index (RI) modifications due to aqueous processing, particle restructuring, and volatilization effects. To further investigate ambient observations of sub-1.0 GFs, f(RH), and particle restructuring, modifying hygroscopicity instruments with pre-humidification modules is recommended.

Recent Discoveries and Future Challenges in Atmospheric Organic Chemistry

Earth's atmosphere contains a multitude of organic compounds, which differ by orders of magnitude regarding fundamental properties such as volatility, reactivity, and propensity to form cloud droplets, affecting their impact on global climate and human health. Despite recent major research efforts and advances, there are still substantial gaps in understanding of atmospheric organic chemistry, hampering efforts to understand, model, and mitigate environmental problems such as aerosol formation in both polluted urban and more pristine regions. The analytical toolbox available for chemists to study atmospheric organic components has expanded considerably during the past decade, opening new windows into speciation, time resolution and detection of reactive and semivolatile compounds at low concentrations. This has provided unprecedented opportunities, but also unveiled new scientific challenges. Specific groundbreaking examples include the role of epoxides in aerosol formation especially from isoprene, the importance of highly oxidized, reactive organics in air-surface processes (whether atmosphere-biosphere exchange or aerosols), as well as the extent of interactions of anthropogenic and biogenic emissions and the resulting impact on atmospheric organic chemistry.

Direct Measurement of pH in Individual Particles via Raman Microspectroscopy and Variation in Acidity with Relative Humidity

Atmospheric aerosol acidity is an important characteristic of aqueous particles, which has been linked to the formation of secondary organic aerosol by catalyzing reactions of oxidized organic compounds that have partitioned to the particle phase. However, aerosol acidity is difficult to measure and traditionally estimated using indirect methods or assumptions based on composition. Ongoing disagreements between experiments and thermodynamic models of particle acidity necessitate improved fundamental understanding of pH and ion behavior in high ionic strength atmospheric particles. Herein, Raman microspectroscopy was used to determine the pH of individual particles (H2SO4+MgSO4) based on sulfate and bisulfate concentrations determined from νs(SO4(2-)) and νs(HSO4(-)), the acid dissociation constant, and activity coefficients from extended Debye-Hückel calculations. Shifts in pH and peak positions of νs(SO4(2-)) and νs(HSO4(-)) were observed as a function of relative humidity. These results indicate the potential for direct spectroscopic determination of pH in individual particles and the need to improve fundamental understanding of ion behavior in atmospheric particles.

Kinetics and Products of the Reaction of the First-Generation Isoprene Hydroxy Hydroperoxide (ISOPOOH) with OH

The atmospheric oxidation of isoprene by the OH radical leads to the formation of several isomers of an unsaturated hydroxy hydroperoxide, ISOPOOH. Oxidation of ISOPOOH by OH produces epoxydiols, IEPOX, which have been shown to contribute mass to secondary organic aerosol (SOA). We present kinetic rate constant measurements for OH + ISOPOOH using synthetic standards of the two major isomers: (1,2)- and (4,3)-ISOPOOH. At 297 K, the total OH rate constant is 7.5 ± 1.2 × 10(-11) cm(3) molecule(-1) s(-1) for (1,2)-ISOPOOH and 1.18 ± 0.19 × 10(-10) cm(3) molecule(-1) s(-1) for (4,3)-ISOPOOH. Abstraction of the hydroperoxy hydrogen accounts for approximately 12% and 4% of the reactivity for (1,2)-ISOPOOH and (4,3)-ISOPOOH, respectively. The sum of all H-abstractions account for approximately 15% and 7% of the reactivity for (1,2)-ISOPOOH and (4,3)-ISOPOOH, respectively. The major product observed from both ISOPOOH isomers was IEPOX (cis-β and trans-β isomers), with a ∼ 2:1 preference for trans-β IEPOX and similar total yields from each ISOPOOH isomer (∼ 70-80%). An IEPOX global production rate of more than 100 Tg C each year is estimated from this chemistry using a global 3D chemical transport model, similar to earlier estimates. Finally, following addition of OH to ISOPOOH, approximately 13% of the reactivity proceeds via addition of O2 at 297 K and 745 Torr. In the presence of NO, these peroxy radicals lead to formation of small carbonyl compounds. Under HO2 dominated chemistry, no products are observed from these channels. We suggest that the major products, highly oxygenated organic peroxides, are lost to the chamber walls. In the atmosphere, formation of these compounds may contribute to organic aerosol mass.

Surface and Airborne Measurements of Organosulfur and Methanesulfonate Over the Western United States and Coastal Areas

This study reports on ambient measurements of organosulfur (OS) and methanesulfonate (MSA) over the western United States and coastal areas. Particulate OS levels are highest in summertime, and generally increase as a function of sulfate (a precursor) and sodium (a marine tracer) with peak levels at coastal sites. The ratio of OS to total sulfur (TS) is also highest at coastal sites, with increasing values as a function of Normalized Difference Vegetation Index (NDVI) and the ratio of organic carbon to elemental carbon. Correlative analysis points to significant relationships between OS and biogenic emissions from marine and continental sources, factors that coincide with secondary production, and vanadium due to a suspected catalytic role. A major OS species, methanesulfonate (MSA), was examined with intensive field measurements and the resulting data support the case for vanadium's catalytic influence. Mass size distributions reveal a dominant MSA peak between aerodynamic diameters of 0.32-0.56 μm at a desert and coastal site with nearly all MSA mass (≥ 84%) in sub-micrometer sizes; MSA:non-sea salt sulfate ratios vary widely as a function of particle size and proximity to the ocean. Airborne data indicate that relative to the marine boundary layer, particulate MSA levels are enhanced in urban and agricultural areas, and also the free troposphere when impacted by biomass burning. Some combination of fires and marine-derived emissions leads to higher MSA levels than either source alone. Finally, MSA differences in cloud water and out-of-cloud aerosol are discussed.

Aqueous Processing of Atmospheric Organic Particles in Cloud Water Collected via Aircraft Sampling

Cloudwater and below-cloud atmospheric particle samples were collected onboard a research aircraft during the Southern Oxidant and Aerosol Study (SOAS) over a forested region of Alabama in June 2013. The organic molecular composition of the samples was studied to gain insights into the aqueous-phase processing of organic compounds within cloud droplets. High resolution mass spectrometry (HRMS) with nanospray desorption electrospray ionization (nano-DESI) and direct infusion electrospray ionization (ESI) were utilized to compare the organic composition of the particle and cloudwater samples, respectively. Isoprene and monoterpene-derived organosulfates and oligomers were identified in both the particles and cloudwater, showing the significant influence of biogenic volatile organic compound oxidation above the forested region. While the average O:C ratios of the organic compounds were similar between the atmospheric particle and cloudwater samples, the chemical composition of these samples was quite different. Specifically, hydrolysis of organosulfates and formation of nitrogen-containing compounds were observed for the cloudwater when compared to the atmospheric particle samples, demonstrating that cloud processing changes the composition of organic aerosol.

Airborne measurements of organosulfates over the continental U.S

Organosulfates are important secondary organic aerosol (SOA) components and good tracers for aerosol heterogeneous reactions. However, the knowledge of their spatial distribution, formation conditions, and environmental impact is limited. In this study, we report two organosulfates, an isoprene-derived isoprene epoxydiols (IEPOX) (2,3-epoxy-2-methyl-1,4-butanediol) sulfate and a glycolic acid (GA) sulfate, measured using the NOAA Particle Analysis Laser Mass Spectrometer (PALMS) on board the NASA DC8 aircraft over the continental U.S. during the Deep Convective Clouds and Chemistry Experiment (DC3) and the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS). During these campaigns, IEPOX sulfate was estimated to account for 1.4% of submicron aerosol mass (or 2.2% of organic aerosol mass) on average near the ground in the southeast U.S., with lower concentrations in the western U.S. (0.2-0.4%) and at high altitudes (<0.2%). Compared to IEPOX sulfate, GA sulfate was more uniformly distributed, accounting for about 0.5% aerosol mass on average, and may be more abundant globally. A number of other organosulfates were detected; none were as abundant as these two. Ambient measurements confirmed that IEPOX sulfate is formed from isoprene oxidation and is a tracer for isoprene SOA formation. The organic precursors of GA sulfate may include glycolic acid and likely have both biogenic and anthropogenic sources. Higher aerosol acidity as measured by PALMS and relative humidity tend to promote IEPOX sulfate formation, and aerosol acidity largely drives in situ GA sulfate formation at high altitudes. This study suggests that the formation of aerosol organosulfates depends not only on the appropriate organic precursors but also on emissions of anthropogenic sulfur dioxide (SO2), which contributes to aerosol acidity.

KEY POINTS

IEPOX sulfate is an isoprene SOA tracer at acidic and low NO conditions Glycolic acid sulfate may be more abundant than IEPOX sulfate globally SO2 impacts IEPOX sulfate by increasing aerosol acidity and water uptake.

Aqueous organic chemistry in the atmosphere: sources and chemical processing of organic aerosols

Over the past decade, it has become clear that aqueous chemical processes occurring in cloud droplets and wet atmospheric particles are an important source of organic atmospheric particulate matter. Reactions of water-soluble volatile (or semivolatile) organic gases (VOCs or SVOCs) in these aqueous media lead to the formation of highly oxidized organic particulate matter (secondary organic aerosol; SOA) and key tracer species, such as organosulfates. These processes are often driven by a combination of anthropogenic and biogenic emissions, and therefore their accurate representation in models is important for effective air quality management. Despite considerable progress, mechanistic understanding of some key aqueous processes is still lacking, and these pathways are incompletely represented in 3D atmospheric chemistry and air quality models. In this article, the concepts, historical context, and current state of the science of aqueous pathways of SOA formation are discussed.

Uptake of epoxydiol isomers accounts for half of the particle-phase material produced from isoprene photooxidation via the HO2 pathway

The oxidation of isoprene is a globally significant source of secondary organic material (SOM) of atmospheric particles. The relative importance of different parallel pathways, however, remains inadequately understood and quantified. SOM production from isoprene photooxidation was studied under hydroperoxyl-dominant conditions for <5% relative humidity and at 20 °C in the presence of highly acidic to completely neutralized sulfate particles. Isoprene photooxidation was separated from SOM production by using two continuously mixed flow reactors connected in series and operated at steady state. Two online mass spectrometers separately sampled the gas and particle phases in the reactor outflow. The loss of specific gas-phase species as contributors to the production of SOM was thereby quantified. The produced SOM mass concentration was directly proportional to the loss of isoprene epoxydiol (IEPOX) isomers from the gas phase. IEPOX isomers lost from the gas phase accounted for (46 ± 11)% of the produced SOM mass concentration. The IEPOX isomers comprised (59 ± 21)% (molecular count) of the loss of monitored gas-phase species. The implication is that for the investigated reaction conditions the SOM production pathways tied to IEPOX isomers accounted for half of the SOM mass concentration.

Acid-catalyzed reactions of epoxides for atmospheric nanoparticle growth

Although new particle formation accounts for about 50% of the global aerosol production in the troposphere, the chemical species and mechanism responsible for the growth of freshly nucleated nanoparticles remain largely uncertain. Here we show large size growth when sulfuric acid nanoparticles of 4-20 nm are exposed to epoxide vapors, dependent on the particle size and relative humidity. Composition analysis of the nanoparticles after epoxide exposure reveals the presence of high molecular weight organosulfates and polymers, indicating the occurrence of acid-catalyzed reactions of epoxides. Our results suggest that epoxides play an important role in the growth of atmospheric newly nucleated nanoparticles, considering their large formation yields from photochemical oxidation of biogenic volatile organic compounds.