Mechanism of the hydroxyl radical oxidation of methacryloyl peroxynitrate (MPAN) and its pathway toward secondary organic aerosol formation in the atmosphere

Size distributions of molecular markers for biogenic secondary organic aerosol in urban Beijing

To understand the source, formation, and seasonality of biogenic secondary organic aerosol (BSOA), a nine-stage cascade impactor was utilized to collect size-segregated particulate samples from April 2017 to January 2018 in Beijing, China. BSOA tracers derived from isoprene, monoterpene, and sesquiterpene were measured with gas chromatography-mass spectrometry. Isoprene and monoterpene SOA tracers exhibited significant seasonal variations, with a summer maximum and a winter minimum. Dominance of 2-methyltetrols (isoprene SOA tracers) with a good correlation with levoglucosan (a biomass burning tracer), which was combined with the detection of methyltartaric acids (possible indicators for aged isoprene) in summer, implies possible biomass burning and long-range transport. In contrast, sesquiterpene SOA tracer (β-caryophyllinic acid) was dominant in winter and was probably associated with the local burning of biomass. Bimodal size distributions were observed for most isoprene SOA tracers, consistent with previous laboratory experiments and field studies showing that they can be formed not only in the aerosol phase but also in the gas phase. Monoterpene SOA tracers cis-pinonic acid and pinic acid showed a coarse-mode peak (5.8-9.0 μm) in four seasons due to their volatile nature. Sesquiterpene SOA tracer β-caryophyllinic acid showed a unimodal pattern with a major fine-mode peak (1.1-2.1 μm), which is linked to local biomass burning. The tracer-yield method was used to quantify the contributions of isoprene, monoterpene, and sesquiterpene to secondary organic carbon (SOC) and SOA. The highest isoprene SOC and SOA concentrations occurred in summer (2.00 μgC m^-3 and 4.93 μg m^-3, respectively), contributing to 1.61% of OC and 5.22% of PM_2.5, respectively. These results suggest that BSOA tracers are promising tracers for understanding the source, formation, and seasonality of BSOA.

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.

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.

Pathogenic Mechanisms of Secondary Organic Aerosols

Air pollution represents a major health problem and an economic burden. In recent years, advances in air pollution research has allowed particle fractionation and identification of secondary organic aerosol (SOA). SOA is formed from either biogenic or anthropogenic emissions, through a mass transfer from the gaseous mass to the particulate phase in the atmosphere. They can have deleterious impact on health and the mortality of individuals with chronic inflammatory diseases. The pleiotropic effects of SOA could involve different and interconnected pathogenic mechanisms ranging from oxidative stress, inflammation, and immune system dysfunction. The purpose of this review is to present recent findings about SOA pathogenic roles and potential underlying mechanisms focusing on the lungs; the latter being the primary exposed organ to atmospheric pollutants.

PM2.5-bound organosulfates in two Eastern Mediterranean cities: The dominance of isoprene organosulfates

PM_2.5 samples were collected during 2017-2018 at two Eastern Mediterranean urban sites in Greece, Athens and Patra, in order to study the abundances, the seasonal trends, the sources and the possible impact of gas phase pollutants on organosulfate formation. Each of the studied groups, except that of aromatic organosulfates, presented higher concentrations in Patra compared to those measured in Athens, from 1.1 (nitro-oxy organosulfates) to 3.6 times (isoprene organosulfates). At both sites, isoprene organosulfates was the dominant group which accounted on average for more than 50% of the total measured organosulfates, with the contribution being more than 80% during summer. Strong seasonality was observed at both sites, regarding the isoprene organosulfates, with an almost 21-fold increase from winter to summer. The same pattern, but to a lesser extent, was also observed for monoterpenes organosulfates at both sites. Alkyl organosulfates followed an identical seasonal trend with the highest mean concentrations observed during spring followed by autumn. The seasonality of anthropogenic organosulfates, multisource organosulfates and nitro-oxy organosulfates differed among the two sites or presented a more compound-specific variation. The isoprene-epoxydiol pathway appeared to be the dominant pathway of isoprene transformation, with the compounds iOS211, iOS213 and iOS215 being the major isoprene organosulfate compounds at both sites. Organosulfate contribution to the concentration of particulate matter presented common variation at both sites, ranging from 0.20 ± 0.14% (winter) to 2.5 ± 1.2% (summer) and from 0.21 ± 0.13% (winter) to 5.0 ± 2.5% (summer) for Athens and Patra, respectively. The increased NO_x levels in Athens, appeared to affect isoprene organosulfate formation as well as the formation of monoterpene and decalin nitro-oxy organosulfates. Principal component analysis followed by multiple linear regression analysis highlighted the dominance of isoprene organosulfates. In Athens, the possible impact of transportation emissions on the formation of monoterpene nitro-oxy organosulfates is indicated while the correlation of naphthalene organosulfates with low molecular weight polycyclic aromatic hydrocarbons suggests that vehicle emissions may be a significant source. In Patra, the possible contribution of sea on methyl sulfate levels is denoted.

Observational Insights into Isoprene Secondary Organic Aerosol Formation through the Epoxide Pathway at Three Urban Sites from Northern to Southern China

Isoprene is the most abundant precursor of global secondary organic aerosol (SOA). The epoxide pathway plays a critical role in isoprene SOA (iSOA) formation, in which isoprene epoxydiols (IEPOX) and/or hydroxymethyl-methyl-α-lactone (HMML) can react with nucleophilic sulfate and water producing isoprene-derived organosulfates (iOSs) and oxygen-containing tracers (iOTs), respectively. This process is complicated and highly influenced by anthropogenic emissions, especially in the polluted urban atmospheres. In this study, we took a 1-year measurement of the paired iOSs and iOTs formed through the IEPOX and HMML pathways at the three urban sites from northern to southern China. The annual average concentrations of iSOA products at the three sites ranged from 14.6 to 36.5 ng m^-3. We found that the nucleophilic-addition reaction of isoprene epoxides with water dominated over that with sulfate in the polluted urban air. A simple set of reaction rate constant could not fully describe iOS and iOT formation everywhere. We also found that the IEPOX pathway was dominant over the HMML pathway over urban regions. Using the kinetic data of IEPOX to estimate the reaction parameters of HMML will cause significant underestimation in the importance of HMML pathway. All these findings provide insights into iSOA formation over polluted areas.

Impacts of biogenic emissions from urban landscapes on summer ozone and secondary organic aerosol formation in megacities

The impact of biogenic emissions on ozone and secondary organic aerosol (SOA) has been widely acknowledged; nevertheless, biogenic emissions emitted from urban landscapes have been largely ignored. We find that including urban isoprene in megacities like Beijing improves not only the modeled isoprene concentrations but also its diurnal cycle. Specifically, the mean bias of the simulated isoprene concentrations is reduced from 87% to 39% by adding urban isoprene emissions while keeping the diurnal cycle the same as that in non-urban or rural areas. Further adjusting the diurnal cycle of isoprene emissions to the urban profile steers the original early morning peak of the isoprene concentration to a double quasi-peak, i.e., bell shape, consistent with observations. The efficiency of ozone generation caused by isoprene emissions in urban Beijing is found to be twice as large as those in rural areas, indicative of vital roles of urban BVOC emissions in modulating the ozone formation. Our study also shows that in the future along with NOx emission reduction, isoprene emissions from urban landscapes will become more important for the formation of ozone in urban area, and their contributions may exceed that of isoprene caused by transport from rural areas. Finally, the impact of biogenic emissions on SOA is examined, revealing that biogenic induced SOA accounts for 16% of the total SOA in urban Beijing. The effect of isoprene on SOA (iSOA) is modulated through two pathways associated with the abundance of NOx emissions, and the effect can be amplified in future when NOx emissions are reduced. The findings of our study are not limited to Beijing but also apply to other megacities or densely populated regions, suggesting an urgent need to construct an accurate emission inventory for urban landscapes and evaluate their impact on ozone and SOA in air quality planning and management.

Second-Order Kinetic Rate Coefficients for the Aqueous-Phase Hydroxyl Radical (OH) Oxidation of Isoprene-Derived Secondary Organic Aerosol Compounds at 298 K

The hydroxyl radical (OH) oxidation of the most abundant nonmethane volatile organic compound emitted to the atmosphere, isoprene (C₅H₈), produces a number of chemical species that partition to the condensed phase via gas-particle partitioning or form condensed-phase compounds via multiphase/heterogeneous chemistry to generate secondary organic aerosols (SOA). The SOA species in aerosol water or cloud/fog droplets may oxidize further via aqueous reaction with OH radicals, among other fates. Rate coefficients for compounds in isoprene's photochemical cascade are well constrained in the gas phase; however, a gap of information exists for the aqueous OH rate coefficients of the condensed-phased products, precluding the atmospheric modeling of the oxidative fate of isoprene-derived SOA. This work investigated the OH-initiated oxidation kinetic rate coefficients (k_OH) for six major SOA compounds formed from the high-NO and low-NO channels of isoprene's atmospheric oxidation and one analog, most of which were synthesized and purified for study: (k₁) 2-methyltetrol [MT: 1.14 (±0.17) × 10⁹ M^-1 s^-1], (k₂) 2-methyl-1,2,3-trihydroxy-4-sulfate [MT-4-S: 1.52 (±0.25) × 10⁹ M^-1 s^-1], (k₃) 2-methyl-1,2-dihydroxy-3-sulfate [MD-3-S: 0.56 (±0.15) × 10⁹ M^-1 s^-1], (k₄) 2-methyl-1,2-dihydroxy-but-3-ene [MDE: 4.35 (±1.16) × 10⁹ M^-1 s^-1], (k₅) 2-methyl-2,3-dihydroxy-1,4-dinitrate [MD-1,4-DN: 0.24 (±0.04) × 10⁹ M^-1 s^-1], (k₆) 2-methyl-1,2,4-trihydroxy-3-nitrate [MT-3-N: 1.12 (±0.15) × 10⁹ M^-1 s^-1], and (k₇) 2-methylglyceric acid [MGA: pH 2:1.41 (±0.49) × 10⁹ M^-1 s^-1; pH 5:0.97 (±0.42) × 10⁹ M^-1 s^-1]. The second-order rate coefficients are determined against the known k_OH of erythritol in pure water. The decays of each reagent were measured with nuclear magnetic resonance (NMR) and high-performance liquid chromatography-high resolution mass spectrometry (HPLC-HRMS). The aqueous photooxidation fates of isoprene-derived SOA compounds are substantial and may impact the SOA budget when implemented into global models.

Effect of relative humidity on SOA formation from aromatic hydrocarbons: Implications from the evolution of gas- and particle-phase species

Relative humidity (RH) plays a significant role in secondary organic aerosol (SOA) formation, but the mechanisms remain uncertain. Using a 30 m³ indoor smog chamber, the influences of RH on SOA formation from two conventional anthropogenic aromatics (toluene and m-xylene) were investigated from the perspective of both the gas- and particle- phases based on the analysis of multi-generation gas-phase products and the chemical composition of SOA, which clearly distinguishes from many previous works mainly focused on the particle-phase. Compared to experiments with RH of 2.0%, SOA yields increased by 11.1%-133.4% and 4.0%-64.5% with higher RH (30.0%-90.0%) for toluene and m-xylene, respectively. The maximum SOA concentration always appeared at 50.0% RH, which is consistent with the change trend of SOA concentration with RH in the summertime field observation. The most plausible reason is that the highest gas-phase OH concentration was observed at 50.0% RH, when the increases in gas-phase OH formation and OH uptake to aerosols and chamber walls with increasing RH reached a balance. The maximum OH concentration was accompanied by a notable decay of second-generation products and formation of third-generation products at 50.0% RH. With further increasing RH, more second-generation products with insufficient oxidation degree will be partitioned into the aerosol phase, and the aqueous-phase oxidation process will also be promoted due to the enhanced uptake of OH. These processes concurrently caused the O/C and oxidation state of carbon (OSc) to first increase and then slightly decrease. This work revealed the complex influence of RH on SOA formation from aromatic VOCs through affecting the OH concentration, partitioning of advanced gas-phase oxidation products as well as aqueous-phase oxidation processes. Quantitative studies to elucidate the role of RH in the partitioning of oxidation products should be conducted to further clarify the mechanism of the influence of RH on SOA formation.

Low-Molecular-Weight Carboxylic Acids in the Southeastern U.S.: Formation, Partitioning, and Implications for Organic Aerosol Aging

While carboxylic acids are important components in both particle and gas phases in the atmosphere, their sources and partitioning are not fully understood. In this study, we present real-time measurements of both particle- and gas-phase concentrations for five of the most common and abundant low-molecular-weight carboxylic acids (LMWCA) in a rural region in the southeastern U.S. in Fall 2016. Through comparison with secondary organic aerosol (SOA) tracers, we find that isoprene was the most important local precursor for all five LMWCA but via different pathways. We propose that monocarboxylic acids (formic and acetic acids) were mainly formed through gas-phase photochemical reactions, while dicarboxylic acids (oxalic, malonic, and succinic acids) were predominantly from aqueous processing. Unexpectedly high concentrations of particle-phase formic and acetic acids (in the form of formate and acetate, respectively) were observed and likely the components of long-range transport organic aerosol (OA), decoupled from their gas-phase counterparts. In addition, an extraordinarily strong correlation (R² = 0.90) was observed between a particulate LMWCA and aged SOA, which we tentatively attribute to boundary layer dynamics.

Low molecular weight dicarboxylic acids, oxocarboxylic acids and α-dicarbonyls as ozonolysis products of isoprene: Implication for the gaseous-phase formation of secondary organic aerosols

Oxidation of isoprene, a major biogenic volatile organic compound emitted from forest canopies, is a potential source of oxalic acid; the dominant species in organic aerosols. We evaluated here ozonolysis of isoprene in dry darkness as a source of oxalic (C₂), malonic (C₃) and succinic (C₄) acids. We found that oxalic acid and methylglyoxal are dominant products within 10 min of reaction followed by glyoxylic, malonic or succinic acids. Interestingly, molecular distributions of oxidation products from early reactions (9-29 min) were characterized by the predominance of methylglyoxal followed by C₂, which became dominant after 30 min. The isoprene-derived secondary organic aerosols (SOAs) showed chemical evolution with reaction time towards the molecular characteristics of dicarboxylic acids similar to those of ambient aerosols (C₂>C₃≥C₄). The carbon-based relative abundances of methylglyoxal decreased steadily (40%→30%), while those of C₂ increased with reaction time (15%→25%), but no such variations persisted for glyoxal (6-10%). This finding means that methylglyoxal is more important intermediate of oxalic acid than glyoxal. In contrast, smaller variability and lower concentrations of pyruvic and glyoxylic acids than other intermediates indicate that oxalic acid formation under dry conditions follows a different pathway than in aqueous-phase heterogeneous chemistry usually invoked for cloud/fog/atmospheric waters. Here, we propose new reaction schemes for high levels of methylglyoxal and oxalic acid via gas-phase chemical reactions with ozone and OH radicals to better interpret the ambient SOA composition. Furthermore, the relative abundances of C₂ exhibit small variability from 1 to 8 h, suggesting its stable character towards the oxidation by hydroxyl radicals.

Key Role of NO3 Radicals in the Production of Isoprene Nitrates and Nitrooxyorganosulfates in Beijing

The formation of isoprene nitrates (IsN) can lead to significant secondary organic aerosol (SOA) production and they can act as reservoirs of atmospheric nitrogen oxides. In this work, we estimate the rate of production of IsN from the reactions of isoprene with OH and NO₃ radicals during the summertime in Beijing. While OH dominates the loss of isoprene during the day, NO₃ plays an increasingly important role in the production of IsN from the early afternoon onwards. Unusually low NO concentrations during the afternoon resulted in NO₃ mixing ratios of ca. 2 pptv at approximately 15:00, which we estimate to account for around a third of the total IsN production in the gas phase. Heterogeneous uptake of IsN produces nitrooxyorganosulfates (NOS). Two mono-nitrated NOS were correlated with particulate sulfate concentrations and appear to be formed from sequential NO₃ and OH oxidation. Di- and tri-nitrated isoprene-related NOS, formed from multiple NO₃ oxidation steps, peaked during the night. This work highlights that NO₃ chemistry can play a key role in driving biogenic-anthropogenic interactive chemistry in Beijing with respect to the formation of IsN during both the day and night.

Aerosol Acidity: Novel Measurements and Implications for Atmospheric Chemistry

The pH of a solution is one of its most fundamental chemical properties, impacting reaction pathways and kinetics across every area of chemistry. The atmosphere is no different, with the pH of the condensed phase driving key chemical reactions that ultimately impact global climate in numerous ways. The condensed phase in the atmosphere is comprised of suspended liquid or solid particles, known as the atmospheric aerosol, which are differentiated from cloud droplets by their much smaller size (primarily <10 μm). The pH of the atmospheric aerosol can enhance certain chemical reactions leading to the formation of additional condensed phase mass from lower volatility species (secondary aerosol), alter the optical and water uptake properties of particles, and solubilize metals that can act as key nutrients in nutrient-limited ecosystems or cause oxidative stress after inhalation. However, despite the importance of aerosol acidity for climate and health, our fundamental understanding of pH has been limited due to aerosol size (by number >99% of particles are <1 μm) and complexity. Within a single atmospheric particle, there can be hundreds to thousands of distinct chemical species, varying water content, high ionic strengths, and different phases (liquid, semisolid, and solid). Making aerosol analysis even more challenging, atmospheric particles are constantly evolving through heterogeneous reactions with gases and multiphase chemistry within the condensed phase. Based on these challenges, traditional pH measurements are not feasible, and, for years, indirect and proxy methods were the most common way to estimate aerosol pH, with mixed results. However, aerosol pH needs to be incorporated into climate models to accurately determine which chemical reactions are dominant in the atmosphere. Consequently, experimental measurements that probe pH in atmospherically relevant particles are sorely needed to advance our understanding of aerosol acidity.This Account describes recent advances in measurements of aerosol particle acidity, specifically three distinct methods we developed for experimentally determining particle pH. Our acid-conjugate base method uses Raman microspectroscopy to probe an acid (e.g., HSO₄^-) and its conjugate base (e.g., SO₄^2-) in individual micrometer-sized particles. Our second approach is a field-deployable colorimetric method based on pH indicators (e.g., thymol blue) and cell phone imaging to provide a simple, low-cost approach to ensemble average (or bulk) pH for particles in distinct size ranges down to a few hundred nanometers in diameter. In our third method, we monitor acid-catalyzed polymer degradation of a thin film (∼23 nm) of poly(ε-caprolactone) (PCL) on silicon by individual particles with atomic force microscopy (AFM) after inertially impacting particles of different pH. These measurements are improving our understanding of aerosol pH from a fundamental physical chemistry perspective and have led to initial atmospheric measurements. The impact of aerosol pH on key atmospheric processes, such as secondary organic aerosol (SOA) formation, is discussed. Some unique findings, such as an unexpected size dependence to aerosol pH and kinetic limitations, illustrate that particles are not always in thermodynamic equilibrium with the surrounding gas. The implications of our limited, but improving, understanding of the fundamental chemical concept of pH in the atmospheric aerosol are critical for connecting chemistry and climate.

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.

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.

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.

Development of a hydrophilic interaction liquid chromatography (HILIC) method for the chemical characterization of water-soluble isoprene epoxydiol (IEPOX)-derived secondary organic aerosol

Acid-catalyzed multiphase chemistry of isoprene epoxydiols (IEPOX) on sulfate aerosol produces substantial amounts of water-soluble secondary organic aerosol (SOA) constituents, including 2-methyltetrols, methyltetrol sulfates, and oligomers thereof in atmospheric fine particulate matter (PM2.5). These constituents have commonly been measured by gas chromatography interfaced to electron ionization mass spectrometry (GC/EI-MS) with prior derivatization or by reverse-phase liquid chromatography interfaced to electrospray ionization high-resolution mass spectrometry (RPLC/ESI-HR-MS). However, both techniques have limitations in explicitly resolving and quantifying polar SOA constituents due either to thermal degradation or poor separation. With authentic 2-methyltetrol and methyltetrol sulfate standards synthesized in-house, we developed a hydrophilic interaction liquid chromatography (HILIC)/ESI-HR-quadrupole time-of-flight mass spectrometry (QTOFMS) protocol that can chromatographically resolve and accurately measure the major IEPOX-derived SOA constituents in both laboratory-generated SOA and atmospheric PM2.5. 2-Methyltetrols were simultaneously resolved along with 4-6 diastereomers of methyltetrol sulfate, allowing efficient quantification of both major classes of SOA constituents by a single non-thermal analytical method. The sum of 2-methyltetrols and methyltetrol sulfates accounted for approximately 92%, 62%, and 21% of the laboratory-generated β-IEPOX aerosol mass, laboratory-generated δ-IEPOX aerosol mass, and organic aerosol mass in the southeastern U.S., respectively, where the mass concentration of methyltetrol sulfates was 171-271% the mass concentration of methyltetrol. Mass concentrations of methyltetrol sulfates were 0.39 and 2.33 μg m-3 in a PM2.5 sample collected from central Amazonia and the southeastern U.S., respectively. The improved resolution clearly reveals isomeric patterns specific to methyltetrol sulfates from acid-catalyzed multiphase chemistry of β- and δ-IEPOX. We also demonstrate that conventional GC/EI-MS analyses overestimate 2-methyltetrols by up to 188%, resulting (in part) from the thermal degradation of methyltetrol sulfates. Lastly, C5-alkene triols and 3-methyltetrahydrofuran-3,4-diols are found to be largely GC/EI-MS artifacts formed from thermal degradation of 2-methyltetrol sulfates and 3-methyletrol sulfates, respectively, and are not detected with HILIC/ESI-HR-QTOFMS.

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.

Effect of Oxidation Process on Complex Refractive Index of Secondary Organic Aerosol Generated from Isoprene

Oxidation of isoprene by hydroxyl radical (OH), ozone (O₃), or nitrate radical (NO₃) leads to the formation of secondary organic aerosol (SOA) in the atmosphere. This SOA contributes to the radiation balance by scattering and absorbing solar radiation. In this study, the effect of oxidation processes on the wavelength-dependent complex refractive index (RI) of SOA generated from isoprene was examined. Oxidation conditions did not have a large effect on magnitude and wavelength dependence of the real part of the RI. In the case of SOA generated in the presence of sulfur dioxide (SO₂), significant light absorption at short visible and ultraviolet wavelengths with the imaginary part of the RI, up to 0.011 at 375 nm, was observed during oxidation with OH. However, smaller and negligible values were observed during oxidation with O₃ and NO₃, respectively. Moreover, in the absence of SO₂, light absorption was not observed regardless of the oxidation process. There was an empirical correlation between the imaginary part of the RI and the average degree of unsaturation of organic molecules. The results obtained herein demonstrate that oxidation processes should be considered for estimating the radiative effect of isoprene-derived SOA.

Effect of secondary organic aerosol from isoprene-derived hydroxyhydroperoxides on the expression of oxidative stress response genes in human bronchial epithelial cells

Isoprene-derived secondary organic aerosol (SOA), which comprise a large portion of atmospheric fine particulate matter (PM_2.5), can be formed through various gaseous precursors, including isoprene epoxydiols (IEPOX), methacrylic acid epoxide (MAE), and isoprene hydroxyhydroperoxides (ISOPOOH). The composition of the isoprene-derived SOA affects its reactive oxygen species (ROS) generation potential and its ability to alter oxidative stress-related gene expression. In this study we assess effects of isoprene SOA derived solely from ISOPOOH oxidation on human bronchial epithelial cells by measuring the gene expression changes in 84 oxidative stress-related genes. In addition, the thiol reactivity of ISOPOOH-derived SOA was measured through the dithiothreitol (DTT) assay. Our findings show that ISOPOOH-derived SOA alter more oxidative-stress related genes than IEPOX-derived SOA but not as many as MAE-derived SOA on a mass basis exposure. More importantly, we found that the different types of SOA derived from the various gaseous precursors (MAE, IEPOX, and ISOPOOH) have unique contributions to changes in oxidative stress-related genes that do not total all gene expression changes seen in exposures to atmospherically relevant compositions of total isoprene-derived SOA mixtures. This study suggests that amongst the different types of known isoprene-derived SOA, MAE-derived SOA are the most potent inducer of oxidative stress-related gene changes but highlights the importance of considering isoprene-derived SOA as a total mixture for pollution controls and exposure studies.

Characterization of isoprene-derived secondary organic aerosols at a rural site in North China Plain with implications for anthropogenic pollution effects

Isoprene is the most abundant non-methane volatile organic compound (VOC) and the largest contributor to secondary organic aerosol (SOA) burden on a global scale. In order to examine the influence of high concentrations of anthropogenic pollutants on isoprene-derived SOA (SOA_i) formation, summertime PM_2.5 filter samples were collected with a three-hour sampling interval at a rural site in the North China Plain (NCP), and determined for SOA_i tracers and other chemical species. RO₂+NO pathway derived 2-methylglyceric acid presented a relatively higher contribution to the SOA_i due to the high-NOx (~20 ppb) conditions in the NCP that suppressed the reactive uptake of RO₂+HO₂ reaction derived isoprene epoxydiols. Compared to particle acidity and water content, sulfate plays a dominant role in the heterogeneous formation process of SOA_i. Diurnal variation and correlation of 2-methyltetrols with ozone suggested an important effect of isoprene ozonolysis on SOA_i formation. SOA_i increased linearly with levoglucosan during June 10–18, which can be attributed to an increasing emission of isoprene caused by the field burning of wheat straw and a favorable aqueous SOA formation during the aging process of the biomass burning plume. Our results suggested that isoprene oxidation is highly influenced by intensive anthropogenic activities in the NCP.

Trimethoprim degradation by Fenton and Fe(II)-activated persulfate processes

Trimethoprim is a pollutant ubiquitous in the environment due to its extensive application, and it cannot be effectively removed by conventional wastewater treatment processes. In this study, the Fenton and the Fe(II)-activated persulfate processes were employed to degrade trimethoprim in an aqueous solution. The results showed that the concentration of persulfate, H₂O₂ and Fe(II) a had significant influence on the degradation of trimethoprim in both processes. De-ionized water spiked with trimethoprim resulted in the complete degradation of trimethoprim (0.05 mM) by the mineralization of 54.9% of Fenton's reagent when the concentrations of H₂O₂ and Fe(II) were 1 mM and 0.05 mM, respectively. In contrast, 73.4% of trimethoprim was degraded by the mineralization of 40.5% of the Fe(II)-activated persulfate process when the concentration of persulfate and Fe(II) were each 4 mM. Intermediate compounds with different m/z were detected for the Fenton and the Fe(II)-activated persulfate processes, indicating alternative degradation pathways. In the actual wastewater spiked with trimethoprim, the removal efficiency of trimethoprim decreased to 35.8% and 43.6%, respectively, for the Fenton and the Fe(II)-activated persulfate processes. In addition, the decomposition efficiencies for hydrogen peroxide and persulfate were 43.8% and 92.5%, respectively, which was lower than those in the de-ionized water system. These results demonstrated that wastewater components had a negative influence on trimethoprim degradation and the decomposition of the oxidants (persulfate and H₂O₂). In summary, the Fe(II)-activated persulfate process could be used as an alternative technology for treating trimethoprim-containing wastewater.

Cloud Processing of Secondary Organic Aerosol from Isoprene and Methacrolein Photooxidation

Aerosol-cloud interaction contributes to the largest uncertainties in the estimation and interpretation of the Earth's changing energy budget. The present study explores experimentally the impacts of water condensation-evaporation events, mimicking processes occurring in atmospheric clouds, on the molecular composition of secondary organic aerosol (SOA) from the photooxidation of methacrolein. A range of on- and off-line mass spectrometry techniques were used to obtain a detailed chemical characterization of SOA formed in control experiments in dry conditions, in triphasic experiments simulating gas-particle-cloud droplet interactions (starting from dry conditions and from 60% relative humidity (RH)), and in bulk aqueous-phase experiments. We observed that cloud events trigger fast SOA formation accompanied by evaporative losses. These evaporative losses decreased SOA concentration in the simulation chamber by 25-32% upon RH increase, while aqueous SOA was found to be metastable and slowly evaporated after cloud dissipation. In the simulation chamber, SOA composition measured with a high-resolution time-of-flight aerosol mass spectrometer, did not change during cloud events compared with high RH conditions (RH > 80%). In all experiments, off-line mass spectrometry techniques emphasize the critical role of 2-methylglyceric acid as a major product of isoprene chemistry, as an important contributor to the total SOA mass (15-20%) and as a key building block of oligomers found in the particulate phase. Interestingly, the comparison between the series of oligomers obtained from experiments performed under different conditions show a markedly different reactivity. In particular, long reaction times at high RH seem to create the conditions for aqueous-phase processing to occur in a more efficient manner than during two relatively short cloud events.

Gene Expression Profiling in Human Lung Cells Exposed to Isoprene-Derived Secondary Organic Aerosol

Secondary organic aerosol (SOA) derived from the photochemical oxidation of isoprene contributes a substantial mass fraction to atmospheric fine particulate matter (PM2.5). The formation of isoprene SOA is influenced largely by anthropogenic emissions through multiphase chemistry of its multigenerational oxidation products. Considering the abundance of isoprene SOA in the troposphere, understanding mechanisms of adverse health effects through inhalation exposure is critical to mitigating its potential impact on public health. In this study, we assessed the effects of isoprene SOA on gene expression in human airway epithelial cells (BEAS-2B) through an air-liquid interface exposure. Gene expression profiling of 84 oxidative stress and 249 inflammation-associated human genes was performed. Our results show that the expression levels of 29 genes were significantly altered upon isoprene SOA exposure under noncytotoxic conditions (p < 0.05), with the majority (22/29) of genes passing a false discovery rate threshold of 0.3. The most significantly affected genes belong to the nuclear factor (erythroid-derived 2)-like 2 (Nrf2) transcription factor network. The Nrf2 function is confirmed through a reporter cell line. Together with detailed characterization of SOA constituents, this study reveals the impact of isoprene SOA exposure on lung responses and highlights the importance of further understanding its potential health outcomes.

Assessing the impact of anthropogenic pollution on isoprene-derived secondary organic aerosol formation in PM2.5 collected from the Birmingham, Alabama, ground site during the 2013 Southern Oxidant and Aerosol Study

In the southeastern US, substantial emissions of isoprene from deciduous trees undergo atmospheric oxidation to form secondary organic aerosol (SOA) that contributes to fine particulate matter (PM_2.5). Laboratory studies have revealed that anthropogenic pollutants, such as sulfur dioxide (SO₂), oxides of nitrogen (NO _x ), and aerosol acidity, can enhance SOA formation from the hydroxyl radical (OH)-initiated oxidation of isoprene; however, the mechanisms by which specific pollutants enhance isoprene SOA in ambient PM_2.5 remain unclear. As one aspect of an investigation to examine how anthropogenic pollutants influence isoprene-derived SOA formation, high-volume PM_2.5 filter samples were collected at the Birmingham, Alabama (BHM), ground site during the 2013 Southern Oxidant and Aerosol Study (SOAS). Sample extracts were analyzed by gas chromatography-electron ionization-mass spectrometry (GC/EI-MS) with prior trimethylsilylation and ultra performance liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-QTOFMS) to identify known isoprene SOA tracers. Tracers quantified using both surrogate and authentic standards were compared with collocated gas- and particle-phase data as well as meteorological data provided by the Southeastern Aerosol Research and Characterization (SEARCH) network to assess the impact of anthropogenic pollution on isoprene-derived SOA formation. Results of this study reveal that isoprene-derived SOA tracers contribute a substantial mass fraction of organic matter (OM) (~ 7 to ~ 20 %). Isoprene-derived SOA tracers correlated with sulfate ( SO42- ) (r² = 0.34, n = 117) but not with NO _x . Moderate correlations between methacrylic acid epoxide and hydroxymethyl-methyl-α-lactone (together abbreviated MAE/HMML)-derived SOA tracers with nitrate radical production (P[NO₃]) (r² = 0.57, n = 40) were observed during nighttime, suggesting a potential role of the NO₃ radical in forming this SOA type. However, the nighttime correlation of these tracers with nitrogen dioxide (NO₂) (r² = 0.26, n = 40) was weaker. Ozone (O₃) correlated strongly with MAE/HMML-derived tracers (r² = 0.72, n = 30) and moderately with 2-methyltetrols (r² = 0.34, n = 15) during daytime only, suggesting that a fraction of SOA formation could occur from isoprene ozonolysis in urban areas. No correlation was observed between aerosol pH and isoprene-derived SOA. Lack of correlation between aerosol acidity and isoprene-derived SOA is consistent with the observation that acidity is not a limiting factor for isoprene SOA formation at the BHM site as aerosols were acidic enough to promote multiphase chemistry of isoprene-derived epoxides throughout the duration of the study. All in all, these results confirm previous studies suggesting that anthropogenic pollutants enhance isoprene-derived SOA formation.

A framework for expanding aqueous chemistry in the Community Multiscale Air Quality (CMAQ) model version 5.1

This paper describes the development and implementation of an extendable aqueous-phase chemistry option (AQCHEM -KMT(I)) for the Community Multiscale Air Quality (CMAQ) modeling system, version 5.1. Here, the Kinetic PreProcessor (KPP), version 2.2.3, is used to generate a Rosenbrock solver (Rodas3) to integrate the stiff system of ordinary differential equations (ODEs) that describe the mass transfer, chemical kinetics, and scavenging processes of CMAQ clouds. CMAQ's standard cloud chemistry module (AQCHEM) is structurally limited to the treatment of a simple chemical mechanism. This work advances our ability to test and implement more sophisticated aqueous chemical mechanisms in CMAQ and further investigate the impacts of microphysical parameters on cloud chemistry. Box model cloud chemistry simulations were performed to choose efficient solver and tolerance settings, evaluate the implementation of the KPP solver, and assess the direct impacts of alternative solver and kinetic mass transfer on predicted concentrations for a range of scenarios. Month-long CMAQ simulations for winter and summer periods over the US reveal the changes in model predictions due to these cloud module updates within the full chemical transport model. While monthly average CMAQ predictions are not drastically altered between AQCHEM and AQCHEM-KMT, hourly concentration differences can be significant. With added in-cloud secondary organic aerosol (SOA) formation from biogenic epoxides (AQCHEM-KMTI), normalized mean error and bias statistics are slightly improved for 2-methyltetrols and 2-methylglyceric acid at the Research Triangle Park measurement site in North Carolina during the Southern Oxidant and Aerosol Study (SOAS) period. The added in-cloud chemistry leads to a monthly average increase of 11-18 % in "cloud" SOA at the surface in the eastern United States for June 2013.

Gas Phase Oxidation of Campholenic Aldehyde and Solution Phase Reactivity of its Epoxide Derivative

The rate constant for the OH reaction with campholenic aldehyde (CA) was measured using the flow tube-chemical ionization mass spectrometry method with a relative rate kinetics technique and was found to be (6.54 ± 0.52) × 10^-11 cm³ molecule^-1 s^-1 at 100 Torr pressure and 298 K. A mechanism for the formation of the observed products was developed for both NO-free and NO-present conditions. On the basis of measurements of the pressure dependent yields of the products, between 5 and 20% of the CA oxidation at atmospheric pressure is predicted to lead to campholenic aldehyde epoxide (CAE). The aqueous solution reaction rate constants for CAE were determined via NMR spectroscopy and were found to be (2.241 ± 0.036) × 10^-5 s^-1 for neutral conditions and 0.0989 ± 0.0053 M^-1 s^-1 for acid-catalyzed conditions at 298 K. The products of the CAE aqueous solution reaction were identified as an isomer of CAE and the aldehyde group hydrated form of this isomer. Unlike the isoprene-derived epoxide, IEPOX, a nucleophilic addition mechanism was not observed. On the basis of the rate constants determined for CA and CAE, it is likely that these species are reactive on atmospherically relevant time scales in the gas and aerosol phases, respectively. The results of the present study largely support a previous supposition that α-pinene-derived secondary organic aerosol may be influenced by the multiphase processing of various intermediate species, including those with epoxide functionality.

A review of stereochemical implications in the generation of secondary organic aerosol from isoprene oxidation

The atmospheric reactions leading to the generation of secondary organic aerosol (SOA) from the oxidation of isoprene are generally assumed to produce only racemic mixtures, but aspects of the chemical reactions suggest this may not be the case. In this review, the stereochemical outcomes of published isoprene-degradation mechanisms contributing to high amounts of SOA are evaluated. Despite evidence suggesting isoprene first-generation oxidation products do not contribute to SOA directly, this review suggests the stereochemistry of first-generation products may be important because their stereochemical configurations may be retained through to the second-generation products which form SOA. Specifically, due to the stereochemistry of epoxide ring-opening mechanisms, the outcome of the reactions involving epoxydiols of isoprene (IEPOX), methacrylic acid epoxide (MAE) and hydroxymethylmethyl-α-lactone (HMML) are, in principle, stereospecific which indicates the stereochemistry is predefined from first-generation precursors. The products from these three epoxide intermediates oligomerise to form macromolecules which are proposed to form chiral structures within the aerosol and are considered to be the largest contributors to SOA. If conditions in the atmosphere such as pH, aerosol water content, relative humidity, pre-existing aerosol, aerosol coatings and aerosol cation/anion content (and other) variables acting on the reactions leading to SOA affect the tacticity (arrangement of chiral centres) in the SOA then they may influence its physical properties, for example its hygroscopicity. Chamber studies of SOA formation from isoprene encompass particular sets of controlled conditions of these variables. It may therefore be important to consider stereochemistry when upscaling from chamber study data to predictions of SOA yields across the range of ambient atmospheric conditions. Experiments analysing the stereochemistry of the reactions under varying conditions of the above variables would help elucidate whether there is stereoselectivity in SOA formation from isoprene and if the rates of SOA formation are affected.

Spatial and seasonal variations of isoprene secondary organic aerosol in China: Significant impact of biomass burning during winter

Isoprene is a substantial contributor to global secondary organic aerosol (SOA). The formation of isoprene SOA (SOA_I) is highly influenced by anthropogenic emissions. Currently, there is rare information regarding SOA_I in polluted regions. In this study, one-year concurrent observation of SOA_I tracers was undertaken at 12 sites across China for the first time. The tracers formed from the HO₂-channel exhibited higher concentrations at rural sites, while the tracer formed from the NO/NO₂-channel showed higher levels at urban sites. 3-Methyltetrahydrofuran-3,4-diols exhibited linear correlations with their ring-opening products, C₅-alkenetriols. And the slopes were steeper in the southern China than the northern China, indicating stronger ring-opening reactions there. The correlation analysis of SOA_I tracers with the factor determining biogenic emission and the tracer of biomass burning (levoglucosan) implied that the high level of SOA_I during summer was controlled by biogenic emission, while the unexpected increase of SOA_I during winter was largely due to the elevated biomass burning emission. The estimated secondary organic carbon from isoprene (SOC_I) exhibited the highest levels in Southwest China. The significant correlations of SOC_I between paired sites implied the regional impact of SOA_I in China. Our findings implicate that isoprene origins and SOA_I formation are distinctive in polluted regions.

Aqueous-phase mechanism for secondary organic aerosol formation from isoprene: application to the Southeast United States and co-benefit of SO2 emission controls

Isoprene emitted by vegetation is an important precursor of secondary organic aerosol (SOA), but the mechanism and yields are uncertain. Aerosol is prevailingly aqueous under the humid conditions typical of isoprene-emitting regions. Here we develop an aqueous-phase mechanism for isoprene SOA formation coupled to a detailed gas-phase isoprene oxidation scheme. The mechanism is based on aerosol reactive uptake coefficients (γ) for water-soluble isoprene oxidation products, including sensitivity to aerosol acidity and nucleophile concentrations. We apply this mechanism to simulation of aircraft (SEAC⁴RS) and ground-based (SOAS) observations over the Southeast US in summer 2013 using the GEOS-Chem chemical transport model. Emissions of nitrogen oxides (NO_x ≡ NO + NO₂) over the Southeast US are such that the peroxy radicals produced from isoprene oxidation (ISOPO₂) react significantly with both NO (high-NO_x pathway) and HO₂ (low-NO_x pathway), leading to different suites of isoprene SOA precursors. We find a mean SOA mass yield of 3.3 % from isoprene oxidation, consistent with the observed relationship of total fine organic aerosol (OA) and formaldehyde (a product of isoprene oxidation). Isoprene SOA production is mainly contributed by two immediate gas-phase precursors, isoprene epoxydiols (IEPOX, 58% of isoprene SOA) from the low-NO_x pathway and glyoxal (28%) from both low- and high-NO_x pathways. This speciation is consistent with observations of IEPOX SOA from SOAS and SEAC⁴RS. Observations show a strong relationship between IEPOX SOA and sulfate aerosol that we explain as due to the effect of sulfate on aerosol acidity and volume. Isoprene SOA concentrations increase as NO_x emissions decrease (favoring the low-NO_x pathway for isoprene oxidation), but decrease more strongly as SO₂ emissions decrease (due to the effect of sulfate on aerosol acidity and volume). The US EPA projects 2013-2025 decreases in anthropogenic emissions of 34% for NO_x (leading to 7% increase in isoprene SOA) and 48% for SO₂ (35% decrease in isoprene SOA). Reducing SO₂ emissions decreases sulfate and isoprene SOA by a similar magnitude, representing a factor of 2 co-benefit for PM_2.5 from SO₂ emission controls.

Aqueous-phase mechanism for secondary organic aerosol formation from isoprene: application to the Southeast United States and co-benefit of SO2 emission controls

Isoprene emitted by vegetation is an important precursor of secondary organic aerosol (SOA), but the mechanism and yields are uncertain. Aerosol is prevailingly aqueous under the humid conditions typical of isoprene-emitting regions. Here we develop an aqueous-phase mechanism for isoprene SOA formation coupled to a detailed gas-phase isoprene oxidation scheme. The mechanism is based on aerosol reactive uptake coefficients (γ) for water-soluble isoprene oxidation products, including sensitivity to aerosol acidity and nucleophile concentrations. We apply this mechanism to simulation of aircraft (SEAC⁴RS) and ground-based (SOAS) observations over the Southeast US in summer 2013 using the GEOS-Chem chemical transport model. Emissions of nitrogen oxides (NO_x ≡ NO + NO₂) over the Southeast US are such that the peroxy radicals produced from isoprene oxidation (ISOPO₂) react significantly with both NO (high-NO_x pathway) and HO₂ (low-NO_x pathway), leading to different suites of isoprene SOA precursors. We find a mean SOA mass yield of 3.3 % from isoprene oxidation, consistent with the observed relationship of total fine organic aerosol (OA) and formaldehyde (a product of isoprene oxidation). Isoprene SOA production is mainly contributed by two immediate gas-phase precursors, isoprene epoxydiols (IEPOX, 58% of isoprene SOA) from the low-NO_x pathway and glyoxal (28%) from both low- and high-NO_x pathways. This speciation is consistent with observations of IEPOX SOA from SOAS and SEAC⁴RS. Observations show a strong relationship between IEPOX SOA and sulfate aerosol that we explain as due to the effect of sulfate on aerosol acidity and volume. Isoprene SOA concentrations increase as NO_x emissions decrease (favoring the low-NO_x pathway for isoprene oxidation), but decrease more strongly as SO₂ emissions decrease (due to the effect of sulfate on aerosol acidity and volume). The US EPA projects 2013-2025 decreases in anthropogenic emissions of 34% for NO_x (leading to 7% increase in isoprene SOA) and 48% for SO₂ (35% decrease in isoprene SOA). Reducing SO₂ emissions decreases sulfate and isoprene SOA by a similar magnitude, representing a factor of 2 co-benefit for PM_2.5 from SO₂ emission controls.