Secondary organic aerosol formation from isoprene photooxidation

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.

Photolysis Controls Atmospheric Budgets of Biogenic Secondary Organic Aerosol

Secondary organic aerosol (SOA) accounts for a large fraction of the tropospheric particulate matter. Although SOA production rates and mechanisms have been extensively investigated, loss pathways remain uncertain. Most large-scale chemistry and transport models account for mechanical deposition of SOA but not chemical losses such as photolysis. There is also a paucity of laboratory measurements of SOA photolysis, which limits how well photolytic losses can be modeled. Here, we show, through a combined experimental and modeling approach, that photolytic loss of SOA mass significantly alters SOA budget predictions. Using environmental chamber experiments at variable relative humidity between 0 and 60%, we find that SOA produced from several biogenic volatile organic compounds undergoes photolysis-induced mass loss at rates between 0 and 2.2 ± 0.4% of nitrogen dioxide (NO₂) photolysis, equivalent to average atmospheric lifetimes as short as 10 h. We incorporate our photolysis rates into a regional chemical transport model to test the sensitivity of predicted SOA mass concentrations to photolytic losses. The addition of photolysis causes a ∼50% reduction in biogenic SOA loadings over the Amazon, indicating that photolysis exerts a substantial control over the atmospheric SOA lifetime, with a likely dependence upon the SOA molecular composition and thus production mechanisms.

Exposure to Dimethyl Selenide (DMSe)-Derived Secondary Organic Aerosol Alters Transcriptomic Profiles in Human Airway Epithelial Cells

Dimethyl selenide (DMSe) is one of the major volatile organoselenium compounds released from aquatic and terrestrial environments through microbial transformation and plant metabolism. The detailed processes of DMSe leading to secondary organic aerosol (SOA) formation and the pulmonary health effects induced by inhalation of DMSe-derived SOA remain largely unknown. In this study, we characterized the chemical composition and formation yields of SOA produced from the oxidation of DMSe with OH radicals and O₃ in controlled chamber experiments. Further, we profiled the transcriptome-wide gene expression changes in human airway epithelial cells (BEAS-2B) after exposure to DMSe-derived SOA. Our analyses indicated a significantly higher SOA yield resulting from the OH-initiated oxidation of DMSe. The oxidative potential of DMSe-derived SOA, as measured by the dithiothreitol (DTT) assay, suggested the presence of oxidizing moieties in DMSe-derived SOA at levels higher than typical ambient aerosols. Utilizing RNA sequencing (RNA-Seq) techniques, gene expression profiling followed by pathway enrichment analysis revealed several major biological pathways perturbed by DMSe-derived SOA, including elevated genotoxicity, DNA damage, and p53-mediated stress responses, as well as downregulated cholesterol biosynthesis, glycolysis, and interleukin IL-4/IL-13 signaling. This study highlights the significance of DMSe-derived SOA as a stressor in human airway epithelial cells.

Understanding the Impact of High-NOx Conditions on the Formation of Secondary Organic Aerosol in the Photooxidation of Oil Sand-Related Precursors

Oil sands (OS) are an important type of heavy oil deposit, for which operations in Alberta, Canada, were recently found to be a large source of secondary organic aerosol (SOA). However, SOA formation from the OS mining, processing, and subsequent tailings, especially in the presence of NO_x, remains unclear. Here, photooxidation experiments for OS-related precursors under high-NO_x conditions were performed using an oxidation flow reactor, in which ∼95% of peroxy radicals (RO₂) react with NO. The SOA yields under high-NO_x conditions were found to be lower than yields under low-NO_x conditions for all precursors, which is likely due to the higher volatilities of the products from the RO₂ + NO pathway compared with RO₂ + HO₂. The SOA yields under high-NO_x conditions show a strong dependence on pre-existing surface area (not observed in previous low-NO_x experiments), again attributed to the higher product volatilities. Comparing the mass spectra of SOA formed from different precursors, we conclude that the fraction of m/z > 80 (F₈₀) can be used as a parameter to separate different types of SOA in the region. In addition, particle-phase organic nitrate was found to be an important component (9-23%) of OS SOA formed under high-NO_x conditions. These results have implications for better understanding the atmospheric processing of OS emissions.

Combining ozone with UV and H2O2 for the degradation of micropollutants from different origins: lab-scale analysis and optimization

The degradation of micropollutants (MPs), including pesticides, herbicides, pharmaceuticals and endocrine disrupting compounds, by ozone-based advanced oxidation techniques (AOP) was investigated in this study. The effect of different factors, such as ozone concentration, hydrogen peroxide concentration and initial pH, on the removal rate was studied in detail. The combination of UV with ozone/ H₂O₂ increased the MPs degradation. For example, atrazine removal increased from 12.6% to 66.9%. Increasing the concentration of ozone and H₂O₂ can enhance the degradation efficiency of MPs, while excess H₂O₂ plays a role as a scavenger for ^•OH. In addition, the optimizing conditions of degradation of MPs by an ozone-based AOP were investigated in this study. The optimal dosages of ozone for atrazine (ATZ), alachlor (ALA), carbamazepine (CBZ), 17-α-ethinylestradiol (EE2) and pentachlorophenol (PCP), were in the range of 0.6-0.75, while for ATZ a much higher dosage (5.4 mg/l) is needed. The optimal dosages of H₂O₂ concentration were at 0.75, 0.2, 0.47, 0.75 and 0.63 mM, and pH were at 10, 10, 7, 10 and 10, and reaction time at 38.5, 33.5 43, 6 and 6 min, respectively. Ozone-based AOP and in particular combination of UV with ozone and H₂O₂ is efficient to degrade atrazine, alachlor, carbamazepine, 17-α-ethinylestradiol and pentachlorophenol, and is attractive for future application of real wastewater treatment.

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

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

Influence of relative humidity on cyclohexene SOA formation from OH photooxidation

Secondary organic aerosol (SOA) is a complex mixture consisting of a variety of oxidation products. In this study, the role of relative humidity (RH) on SOA formation with different [H₂O₂]₀/[cyclohexene]₀ was investigated in a smog chamber. It was found that the cyclohexene SOA yield increases with increasing initial OH concentration at both high and low RH conditions. The increased rate of SOA formation was lower at wet conditions than that at dry conditions. For [H₂O₂]₀/[cyclohexene]₀ = 0.4 and 0.8, the SOA yield increased from 1.5% to 8% at dry condition to 7% and 12% at wet condition, respectively. In contrast, at high RH the SOA yield fell from 54% to 52% for [H₂O₂]₀/[cyclohexene]₀ = 1.3. The SOA mass loss was higher at high RH at the same OH exposure. The chemical composition of SOA was characterized using hybrid quadrupole-orbitrap mass spectrometer equipped with electrospray ionization (ESI-Q-Orbitrap-HRMS). Oligomers, which were responsible for the increase of the SOA yield, were detected in the SOA formed at high RH conditions. The esterification reaction between dicarboxylic acids and HOC₆H₁₀-O-O-C₆H₁₀OH was the pathway of oligomers formation. All the oligomers have cyclic molecular structures. For [H₂O₂]₀/[cyclohexene]₀ = 1.3, the relative intensity of both low and high molecular weight substances reduced more at wet conditions. This indicates that at sufficient OH level, the inhibition of oligomers formation and the further reaction of SOA with OH result in a slightly lower SOA yield at wet condition.

Photolytic Aging of Secondary Organic Aerosol: Evidence for a Substantial Photo-Recalcitrant Fraction

Photolytic aging has been proposed as a major mass loss mechanism for atmospheric secondary organic aerosol (SOA). However, estimated mass loss rates vary by orders of magnitude, and their impacts on modeled SOA loadings and properties are highly uncertain. In this study, photolysis rates and composition changes of α-pinene SOA are analyzed in situ over multiple days in an environmental chamber. After an initial exponential decay (τ ∼ 22 h), the mass loss rate slows dramatically, with more than ∼70-90% of the SOA particulate mass undergoing an essentially negligible photolytic degradation. Scaled to ambient conditions, SOA undergoes rapid photolysis over only its first day in the atmosphere; beyond this, the remaining SOA is photo-recalcitrant, and photolysis ceases to be a major sink compared to wet deposition time scales. Thus, extrapolation of the initial photolysis loss rate to the entire aerosol mass may significantly overestimate the role of photolysis in the removal of atmospheric SOA.

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.

Reassimilation of Leaf Internal CO2 Contributes to Isoprene Emission in the Neotropical Species Inga edulis Mart

Isoprene (C5H8) is a hydrocarbon gas emitted by many tree species and has been shown to protect photosynthesis under abiotic stress. Under optimal conditions for photosynthesis, ~70%–90% of carbon used for isoprene biosynthesis is produced from recently assimilated atmospheric CO2. While the contribution of alternative carbon sources that increase with leaf temperature and other stresses have been demonstrated, uncertainties remain regarding the biochemical source(s) of isoprene carbon. In this study, we investigated leaf isoprene emissions (Is) from neotropical species Inga edulis Mart. as a function of light and temperature under ambient (450 µmol m−2 s−1) and CO2-free (0 µmol m−2 s−1) atmosphere. Is under CO2-free atmosphere showed light-dependent emission patterns similar to those observed under ambient CO2, but with lower light saturation point. Leaves treated with the photosynthesis inhibitor DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) failed to produce detectable Is in normal light under a CO2-free atmosphere. While strong temperature-dependent Is were observed under CO2-free atmosphere in the light, dark conditions failed to produce detectable Is even at the highest temperatures studied (40 °C). Treatment of leaves with 13C-labeled sodium bicarbonate under CO2-free atmosphere resulted in Is with over 50% containing at least one 13C atom. Is under CO2-free atmosphere and standard conditions of light and leaf temperature represented 19% ± 7% of emissions under ambient CO2. The results show that the reassimilation of leaf internal CO2 contributes to Is in the neotropical species I. edulis. Through the consumption of excess photosynthetic energy, our results support a role of isoprene biosynthesis, together with photorespiration, as a key tolerance mechanism against high temperature and high light in the tropics.

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.

Urban pollution greatly enhances formation of natural aerosols over the Amazon rainforest

One of the least understood aspects in atmospheric chemistry is how urban emissions influence the formation of natural organic aerosols, which affect Earth's energy budget. The Amazon rainforest, during its wet season, is one of the few remaining places on Earth where atmospheric chemistry transitions between preindustrial and urban-influenced conditions. Here, we integrate insights from several laboratory measurements and simulate the formation of secondary organic aerosols (SOA) in the Amazon using a high-resolution chemical transport model. Simulations show that emissions of nitrogen-oxides from Manaus, a city of ~2 million people, greatly enhance production of biogenic SOA by 60-200% on average with peak enhancements of 400%, through the increased oxidation of gas-phase organic carbon emitted by the forests. Simulated enhancements agree with aircraft measurements, and are much larger than those reported over other locations. The implication is that increasing anthropogenic emissions in the future might substantially enhance biogenic SOA in pristine locations like the Amazon.

The influence of OH concentration on SOA formation from isoprene photooxidation

The formation of secondary organic aerosol (SOA) from isoprene photooxidation was investigated to reveal the influence of OH concentration on SOA formation through varying the concentrations of isoprene and H₂O₂ in a smog chamber. The results indicated that the higher the OH concentration was, the less the isoprene consumed for the detectable SOA mass concentration, for example, the lowest isoprene consumption for the detectable SOA was about 14.4 ppb under the OH concentration of about 1.65 × 10⁷ molecules cm^-3, whereas tens ppb of isoprene consumption were needed under the OH concentrations <1.0 × 10⁷ molecules cm^-3, and even no detectable SOA was observed with isoprene consumption of 75.1 ppb under OH concentration of 7.2 × 10⁵ molecules cm^-3. SOA yield was also found to increase with increasing OH concentration for a given aerosol loading (M₀) at atmospherically relevant conditions, confirming that OH concentration played important role in SOA formation from isoprene photooxidation. The maximal SOA yields (5.8-42.8%) obtained by this study were a factor of 1.5-3.1 greater than those reported by previous study for the almost the same initial reactant concentrations of isoprene and H₂O₂, and the difference was mainly ascribed to the higher OH concentrations in the reaction systems of this study than those of previous study. The OH concentrations adopted in this study closed to those in the real atmosphere around noontime, and hence the SOA yield obtained from the isoprene photooxidation might be representative.

Chemical Characterization of Highly Functionalized Organonitrates Contributing to Night-Time Organic Aerosol Mass Loadings and Particle Growth

Reactions of volatile organic compounds (VOC) with NO₃ radicals and of reactive intermediates of oxidized VOC with NO _x can lead to the formation of highly functionalized organonitrates (ON). We present quantitative and chemical information on ON contributing to high night-time organic aerosol (OA) mass concentrations measured during July-August 2016 in a rural area in southwest Germany. A filter inlet for gases and aerosols coupled to a high-resolution time-of-flight chemical ionization mass spectrometer (FIGAERO-HR-ToF-CIMS) was used to analyze the molecular composition of ON in both the gas and particle phase. We find larger contributions of ON to OA mass during the night. Identified ON are highly functionalized, with 4 to 12 oxygen atoms. The diel patterns of ON compounds with 5, 7, 10, or 15 carbon atoms per molecule vary, indicating a corresponding behavior of their potential precursor VOC. The temporal behavior of ON after sunset correlates with that of the number concentration of ultrafine particles, indicating a potential role of ON in night-time new particle formation (NPF) regularly observed at this location. We estimate an ON contribution of 18-25% to the mass increase of newly formed particles after sunset. Our study provides insights into the chemical composition of highly functionalized ON in the rural atmosphere and the role of anthropogenic emissions for night-time SOA formation in an area where biogenic VOC emissions dominate.

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.

Coral reef aerosol emissions in response to irradiance stress in the Great Barrier Reef, Australia

We investigate the correlation between stress-related compounds produced by corals of the Great Barrier Reef (GBR) and local atmospheric properties-an issue that goes to the core of the coral ecosystem's ability to survive climate change. We relate the variability in a satellite decadal time series of fine-mode aerosol optical depth (AOD) to a coral stress metric, formulated as a function of irradiance, water clarity, and tide, at Heron Island in the southern GBR. We found that AOD was correlated with the coral stress metric, and the correlation increased at low wind speeds, when horizontal advection of air masses was low and the production of non-biogenic aerosols was minimal. We posit that coral reefs may be able to protect themselves from irradiance stress during calm weather by affecting the optical properties of the atmosphere and local incident solar radiation.

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.

Additional Benefits of Federal Air-Quality Rules: Model Estimates of Controllable Biogenic Secondary Organic Aerosol

Atmospheric models that accurately describe the fate and transport of trace species for the right reasons aid in the development of effective air-quality management strategies that safeguard human health. Controllable emissions facilitate the formation of biogenic secondary organic aerosol (BSOA) to enhance the atmospheric fine particulate matter (PM2.5) burden. Previous modeling with the EPA's Community Multiscale Air Quality (CMAQ) model predicted that anthropogenic primary organic aerosol (POA) emissions had the greatest impact on BSOA. That experiment included formation processes involving semivolatile partitioning but not aerosol liquid water (ALW), a ubiquitous PM constituent. We conduct 17 summertime CMAQ simulations with updated chemistry and evaluate changes in BSOA due to the removal of individual pollutants and source sectors for the contiguous U.S. CMAQ predicts SO2 from electricity generating units, and mobile source NOX emissions have the largest impacts on BSOA. The removal of anthropogenic NOX, SO2, and POA emissions during the simulation reduces the nationally averaged BSOA by 23, 14, and 8% and PM2.5 by 9.2, 14, and 5.3%, respectively. ALW mass concentrations decrease by 10 and 35% in response to the removal of NOX and SO2 emissions. This work contributes chemical insight into ancillary benefits of Federal NOX and SO2 rules that concurrently reduce organic PM2.5 mass.

Parameterized Yields of Semivolatile Products from Isoprene Oxidation under Different NO x Levels: Impacts of Chemical Aging and Wall-Loss of Reactive Gases

We developed a parametrizable box model to empirically derive the yields of semivolatile products from VOC oxidation using chamber measurements, while explicitly accounting for the multigenerational chemical aging processes (such as the gas-phase fragmentation and functionalization and aerosol-phase oligomerization and photolysis) under different NO _x levels and the loss of particles and gases to chamber walls. Using the oxidation of isoprene as an example, we showed that the assumptions regarding the NO _x-sensitive, multigenerational aging processes of VOC oxidation products have large impacts on the parametrized product yields and SOA formation. We derived sets of semivolatile product yields from isoprene oxidation under different NO _x levels. However, we stress that these product yields must be used in conjunction with the corresponding multigenerational aging schemes in chemical transport models. As more mechanistic insights regarding SOA formation from VOC oxidation emerge, our box model can be expanded to include more explicit chemical aging processes and help ultimately bridge the gap between the process-based understanding of SOA formation from VOC oxidation and the bulk-yield parametrizations used in chemical transport models.

Sources Profiles of Volatile Organic Compounds (VOCs) Measured in a Typical Industrial Process in Wuhan, Central China

Industrial emission is an important source of ambient volatile organic compounds (VOCs) in Wuhan City, Hubei Province, China. We collected 53 VOC samples from petrochemical, surface coating, electronic manufacturing, and gasoline evaporation using stainless canisters to develop localized source profiles. Concentrations of 86 VOC species, including hydrocarbons, halocarbons, and oxygenated VOCs, were quantified by a gas chromatography–flame ionization detection/mass spectrometry system. Alkanes were the major constituents observed in the source profile from the petrochemical industry. Aromatics (79.5~81.4%) were the largest group in auto-painting factories, while oxygenated VOCs (82.0%) and heavy alkanes (68.7%) were dominant in gravure printing and offset printing factories, respectively. Acetone was the largest contributor and the most frequently monitored species in printed circuit board (PCB) manufacturing, while VOC species emitted from integrated chip (IC) were characterized by high contents of isopropanol (56.4–98.3%) and acetone (30.8%). Chemical compositions from vapor of gasoline 92#, 93#, and 98# were almost identical. Alkanes were the dominant VOC group, with i-pentane being the most abundant species (31.4–37.7%), followed by n-butane and n-pentane. However, high loadings of heavier alkanes were observed in the profile of diesel evaporation.

Observations of sesquiterpenes and their oxidation products in central Amazonia during the wet and dry seasons

Biogenic volatile organic compounds (BVOCs) from the Amazon forest region represent the largest source of organic carbon emissions to the atmosphere globally. These BVOC emissions dominantly consist of volatile and intermediate-volatility terpenoid compounds that undergo chemical transformations in the atmosphere to form oxygenated condensable gases and secondary organic aerosol (SOA). We collected quartz filter samples with 12 h time resolution and performed hourly in situ measurements with a semi-volatile thermal desorption aerosol gas chromatograph (SV-TAG) at a rural site ("T3") located to the west of the urban center of Manaus, Brazil as part of the Green Ocean Amazon (GoAmazon2014/5) field campaign to measure intermediate-volatility and semi-volatile BVOCs and their oxidation products during the wet and dry seasons. We speciated and quantified 30 sesquiterpenes and 4 diterpenes with mean concentrations in the range 0.01-6.04 ngm^-3 (1-670ppq_v). We estimate that sesquiterpenes contribute approximately 14 and 12% to the total reactive loss of O₃ via reaction with isoprene or terpenes during the wet and dry seasons, respectively. This is reduced from ~ 50-70 % for within-canopy reactive O3 loss attributed to the ozonolysis of highly reactive sesquiterpenes (e.g., β-caryophyllene) that are reacted away before reaching our measurement site. We further identify a suite of their oxidation products in the gas and particle phases and explore their role in biogenic SOA formation in the central Amazon region. Synthesized authentic standards were also used to quantify gas- and particle-phase oxidation products derived from β-caryophyllene. Using tracer-based scaling methods for these products, we roughly estimate that sesquiterpene oxidation contributes at least 0.4-5 % (median 1 %) of total submicron OA mass. However, this is likely a low-end estimate, as evidence for additional unaccounted sesquiterpenes and their oxidation products clearly exists. By comparing our field data to laboratory-based sesquiterpene oxidation experiments we confirm that more than 40 additional observed compounds produced through sesquiterpene oxidation are present in Amazonian SOA, warranting further efforts towards more complete quantification.

Emissions of volatile organic compounds (VOCs) from cooking and their speciation: A case study for Shanghai with implications for China

Cooking emission is one of sources for ambient volatile organic compounds (VOCs), which is deleterious to air quality, climate and human health. These emissions are especially of great interest in large cities of East and Southeast Asia. We conducted a case study in which VOC emissions from kitchen extraction stacks have been sampled in total 57 times in the Megacity Shanghai. To obtain representative data, we sampled VOC emissions from kitchens, including restaurants of seven common cuisine types, canteens, and family kitchens. VOC species profiles and their chemical reactivities have been determined. The results showed that 51.26%±23.87% of alkane and 24.33±11.69% of oxygenated VOCs (O-VOCs) dominate the VOC cooking emissions. Yet, the VOCs with the largest ozone formation potential (OFP) and secondary organic aerosol potential (SOAP) were from the alkene and aromatic categories, accounting for 6.8-97.0% and 73.8-98.0%, respectively. Barbequing has the most potential of harming people's heath due to its significant higher emissions of acetaldehyde, hexanal, and acrolein. Methodologies for calculating VOC emission factors (EF) for restaurants that take into account VOCs emitted per person (EF_person), per kitchen stove (EF_{kitchen stove}) and per hour (EF_hour) are developed and discussed. Methodologies for deriving VOC emission inventories (S) from restaurants are further defined and discussed based on two categories: cuisine types (S_type) and restaurant scales (S_scale). The range of S_type and S_scale are 4124.33-7818.04t/year and 1355.11-2402.21t/year, respectively. We also found that S_type and S_scale for 100,000 people are 17.07-32.36t/year and 5.61-9.95t/year, respectively. Based on Environmental Kuznets Curve, the annual total amount of VOCs emissions from catering industry in different provinces in China was estimated, which was 5680.53t/year, 6122.43t/year, and 66,244.59t/year for Shangdong and Guangdong provinces and whole China, respectively. Large and medium-scaled restaurants should be paid more attention with respect to regulation of VOCs.

Gas-phase kinetics modifies the CCN activity of a biogenic SOA

Our current knowledge of cloud condensation nuclei (CCN) activity and the hygroscopicity of secondary organic aerosol (SOA) depends on the particle size and composition, explicitly, the thermodynamic properties of the aerosol solute and subsequent interactions with water. Here, we examine the CCN activation of 3 SOA systems (2 biogenic single precursor and 1 mixed precursor SOA system) in relation to gas-phase decay. Specifically, the relationship between time, gas-phase precursor decay and CCN activity of 100 nm SOA is studied. The studied SOA systems exhibit a time-dependent growth of CCN activity at an instrument supersaturation of ∼0.2%. As such, we define a critical activation time, t₅₀, above which a 100 nm SOA particle will activate. The critical activation time for isoprene, longifolene and a mixture of the two precursor SOA is 2.01 hours, 2.53 hours and 3.17 hours, respectively. The activation times are then predicted with gas-phase kinetic data inferred from measurements of precursor decay. The gas-phase prediction of t₅₀ agrees well with CCN measured t₅₀ (within 0.05 hours of the actual critical times) and suggests that the gas-to-particle phase partitioning may be more significant for SOA CCN prediction than previously thought.

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.

The effects of isoprene and NO x on secondary organic aerosols formed through reversible and irreversible uptake to aerosol water

Isoprene oxidation produces water-soluble organic gases capable of partitioning to aerosol liquid water. The formation of secondary organic aerosols through such aqueous pathways (aqSOA) can take place either reversibly or irreversibly; however, the split between these fractions in the atmosphere is highly uncertain. The aim of this study was to characterize the reversibility of aqSOA formed from isoprene at a location in the eastern United States under substantial influence from both anthropogenic and biogenic emissions. The reversible and irreversible uptake of water-soluble organic gases to aerosol water was characterized in Baltimore, Maryland, USA, using measurements of particulate water-soluble organic carbon (WSOCp) in alternating dry and ambient configurations. WSOCp evaporation with drying was observed systematically throughout the late spring and summer, indicating reversible aqSOA formation during these times. We show through time lag analyses that WSOCp concentrations, including the WSOCp that evaporates with drying, peak 6 to 11h after isoprene concentrations, with maxima at a time lag of 9h. The absolute reversible aqSOA concentrations, as well as the relative amount of reversible aqSOA, increased with decreasing NO x /isoprene ratios, suggesting that isoprene epoxydiol (IEPOX) or other low-NO x oxidation products may be responsible for these effects. The observed relationships with NO x and isoprene suggest that this process occurs widely in the atmosphere, and is likely more important in other locations characterized by higher isoprene and/or lower NO x levels. This work underscores the importance of accounting for both reversible and irreversible uptake of isoprene oxidation products to aqueous particles.

Regional Similarities and NOx-related Increases in Biogenic Secondary Organic Aerosol in Summertime Southeastern U.S

During the 2013 Southern Oxidant and Aerosol Study, Fourier Transform Infrared Spectroscopy (FTIR) and Aerosol Mass Spectrometer (AMS) measurements of submicron mass were collected at Look Rock (LRK), Tennessee, and Centreville (CTR), Alabama. Carbon monoxide and submicron sulfate and organic mass concentrations were 15-60% higher at CTR than at LRK but their time series had moderate correlations (r~0.5). However, NOx had no correlation (r=0.08) between the two sites with nighttime-to-early-morning peaks 3~10 times higher at CTR than at LRK. Organic mass (OM) sources identified by FTIR Positive Matrix Factorization (PMF) had three very similar factors at both sites: Fossil Fuel Combustion (FFC) related organic aerosols, Mixed Organic Aerosols (MOA), and Biogenic Organic Aerosols (BOA). The BOA spectrum from FTIR is similar (cosine similarity > 0.6) to that of lab-generated particle mass from the photochemical oxidation of both isoprene and monoterpenes under high NOx conditions from chamber experiments. The BOA mass fraction was highest during the night at CTR but in the afternoon at LRK. AMS PMF resulted in two similar pairs of factors at both sites and a third nighttime NOx-related factor (33% of OM) at CTR but a daytime nitrate-related factor (28% of OM) at LRK. NOx was correlated with BOA and LO-OOA for NOx concentrations higher than 1 ppb at both sites, producing 0.5 ± 0.1 μg m-3 for CTR-LO-OOA and 1.0 ± 0.3 μg m-3 for CTR-BOA above 1 ppb additional biogenic OM for each 1 ppb increase of NOx.

Observation of SOA tracers at a mountainous site in Hong Kong: Chemical characteristics, origins and implication on particle growth

Secondary organic aerosol (SOA) is an important constituent of airborne fine particles. PM_2.5 (particles with aerodynamic diameters≤2.5μm) samples were collected at a mountainous site in Hong Kong in autumn of 2010, and analyzed for SOA tracers. Results indicated that the concentrations of isoprene SOA tracers (54.7±22.7ng/m³) and aromatics SOA tracers (2.1±1.6ng/m³) were on relatively high levels in Hong Kong. Secondary organic carbon (SOC) derived from isoprene, monoterpenes, sesquiterpenes and aromatics was estimated with the SOA tracer based approach, which constituted 0.35±0.15μg/m³ (40.6±5.7%), 0.20±0.03μg/m³ (30.4±5.5%), 0.05±0.02μg/m³ (5.6±1.7%) and 0.26±0.20μg/m³ (21.3±8.2%) of the total estimated SOC. Biogenic SOC (0.60±0.18μg/m³) dominated over anthropogenic SOC (0.26±0.20μg/m³) at this site. In addition to the total estimated SOC (17.8±4.6% of organic carbon (OC) in PM_2.5), primary organic carbon (POC) emitted from biomass burning also accounted for a considerable proportion of OC (11.6±3.2%). Insight into the OC origins found that regional transport significantly (p<0.05) elevated SOC from 0.37±0.17 to 1.04±0.39μg/m³. Besides, SOC load could also increase significantly if there was influence from local ship emission. Biomass burning related POC in regional air masses (0.81±0.24μg/m³) was also higher (p<0.05) than that in samples affected by local air (0.29±0.35μg/m³). Evidences indicated that SOA formation was closely related to new particle formation and the growth of nucleation mode particles, while biomass burning was responsible for some particle burst events in Hong Kong. This is the first SOA study in afforested areas of Hong Kong.

Emulsified and Liquid-Liquid Phase-Separated States of α-Pinene Secondary Organic Aerosol Determined Using Aerosol Optical Tweezers

We demonstrate the first capture and analysis of secondary organic aerosol (SOA) on a droplet suspended in an aerosol optical tweezers (AOT). We examine three initial chemical systems of aqueous NaCl, aqueous glycerol, and squalane at ∼75% relative humidity. For each system we added α-pinene SOA-generated directly in the AOT chamber-to the trapped droplet. The resulting morphology was always observed to be a core of the original droplet phase surrounded by a shell of the added SOA. We also observed a stable emulsion of SOA particles when added to an aqueous NaCl core phase, in addition to the shell of SOA. The persistence of the emulsified SOA particles suspended in the aqueous core suggests that this metastable state may persist for a significant fraction of the aerosol lifecycle for mixed SOA/aqueous particle systems. We conclude that the α-pinene SOA shell creates no major diffusion limitations for water, glycerol, and squalane core phases under humid conditions. These experimental results support the current prompt-partitioning framework used to describe organic aerosol in most atmospheric chemical transport models and highlight the prominence of core-shell morphologies for SOA on a range of core chemical phases.

Direct Measurements of Gas/Particle Partitioning and Mass Accommodation Coefficients in Environmental Chambers

Secondary organic aerosols (SOA) are a major contributor to fine particulate mass and wield substantial influences on the Earth's climate and human health. Despite extensive research in recent years, many of the fundamental processes of SOA formation and evolution remain poorly understood. Most atmospheric aerosol models use gas/particle equilibrium partitioning theory as a default treatment of gas-aerosol transfer, despite questions about potentially large kinetic effects. We have conducted fundamental SOA formation experiments in a Teflon environmental chamber using a novel method. A simple chemical system produces a very fast burst of low-volatility gas-phase products, which are competitively taken up by liquid organic seed particles and Teflon chamber walls. Clear changes in the species time evolution with differing amounts of seed allow us to quantify the particle uptake processes. We reproduce gas- and aerosol-phase observations using a kinetic box model, from which we quantify the aerosol mass accommodation coefficient (α) as 0.7 on average, with values near unity especially for low volatility species. α appears to decrease as volatility increases. α has historically been a very difficult parameter to measure with reported values varying over 3 orders of magnitude. We use the experimentally constrained model to evaluate the correction factor (Φ) needed for chamber SOA mass yields due to losses of vapors to walls as a function of species volatility and particle condensational sink. Φ ranges from 1-4.

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.

Degradation of indoor limonene by outdoor ozone: A cascade of secondary organic aerosols

In indoor air, terpene-ozone reactions can form secondary organic aerosols (SOA) in a transient process. 'Real world' measurements conducted in a furnished room without air conditioning were modelled involving the indoor background of airborne particulate matter, outdoor ozone infiltrated by natural ventilation, repeated transient limonene evaporations, and different subsequent ventilation regimes. For the given setup, we disentangled the development of nucleated, coagulated, and condensed SOA fractions in the indoor air and calculated the time dependence of the aerosol mass fraction (AMF) by means of a process model. The AMF varied significantly between 0.3 and 5.0 and was influenced by the ozone limonene ratio and the background particles which existed prior to SOA formation. Both influencing factors determine whether nucleation or adsorption processes are preferred; condensation is strongly intensified by particulate background. The results provide evidence that SOA levels in natural indoor environments can surpass those known from chamber measurements. An indicator for the SOA forming potential of limonene was found to be limona ketone. Multiplying its concentration (in μg/m³) by 450(±100) provides an estimate of the concentration of the reacted limonene. This can be used to detect a high particle formation potential due to limonene pollution, e.g. in epidemiological studies considering adverse health effects of indoor air pollutants.

Feasibility of Photosensitized Reactions with Secondary Organic Aerosol Particles in the Presence of Volatile Organic Compounds

The ability of a complex mixture of organic compounds found in secondary organic aerosol (SOA) to act as a photosensitizer in the oxidation of volatile organic compounds (VOCs) was investigated. Different types of SOAs were produced in a smog chamber by oxidation of various biogenic and anthropogenic VOCs. The SOA particles were collected from the chamber onto an inert substrate, and the resulting material was exposed to 365 nm radiation in an air flow containing ∼200 ppbv of limonene vapor. The mixing ratio of limonene and other VOCs in the flow was observed with a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). The photosensitized uptake of limonene was observed for several SOA materials, with a lower limit for the reactive uptake coefficient on the scale of ∼10^-5. The lower limit for the uptake coefficient under conditions of Los Angeles, California on the summer solstice at noon was estimated to be on the order of ∼10^-6. Photoproduction of oxygenated VOCs (OVOCs) resulting from photodegradation of the SOA material also occurred in parallel with the photosensitized uptake of limonene. The estimated photosensitized limonene uptake rates by atmospheric SOA particles and vegetation surfaces appear to be too small to compete with the atmospheric oxidation of limonene by the hydroxyl radical or ozone. However, these processes could play a role in the leaf boundary layer where concentrations of oxidants are depleted and concentrations of VOCs are enhanced relative to the free atmosphere.

Isomerization of Second-Generation Isoprene Peroxy Radicals: Epoxide Formation and Implications for Secondary Organic Aerosol Yields

We report chamber measurements of secondary organic aerosol (SOA) formation from isoprene photochemical oxidation, in which radical concentrations were systematically varied and the molecular composition of semi- to low-volatility gases and SOA were measured online. Using a detailed chemical kinetics box model, we find that to explain the behavior of low-volatility products and SOA mass yields relative to input H₂O₂ concentrations, the second-generation dihydroxy hydroperoxy peroxy radical (C₅H₁₁O₆·) must undergo an intramolecular H-shift with a net forward rate constant of order 0.1 s^-1 or higher. This finding is consistent with quantum chemical calculations that suggest a net forward rate constant of 0.3-0.9 s^-1. Furthermore, these calculations suggest that the dominant product of this isomerization is a dihydroxy hydroperoxy epoxide (C₅H₁₀O₅), which is expected to have a saturation vapor pressure ∼2 orders of magnitude higher, as determined by group-contribution calculations, than the dihydroxy dihydroperoxide, ISOP(OOH)₂(C₅H₁₂O₆), a major product of the peroxy radical reacting with HO₂. These results provide strong constraints on the likely volatility distribution of isoprene oxidation products under atmospheric conditions and, thus, on the importance of nonreactive gas-particle partitioning of isoprene oxidation products as an SOA source.

Formation of Low-Volatility Organic Compounds in the Atmosphere: Recent Advancements and Insights

Secondary organic aerosol (SOA) formation proceeds by bimolecular gas-phase oxidation reactions generating species that are sufficiently low in volatility to partition into the condensed phase. Advances in instrumentation have revealed that atmospheric SOA is less volatile and more oxidized than can be explained solely by these well-studied gas-phase oxidation pathways, supporting the role of additional chemical processes. These processes-autoxidation, accretion, and organic salt formation-can lead to exceedingly low-volatility species that recently have been identified in laboratory and field studies. Despite these new insights, the identities of the condensing species at the molecular level and the relative importance of the various formation processes remain poorly constrained. The thermodynamics of autoxidation, accretion, and organic salt formation can be described by equilibrium partitioning theory; a framework for which is presented here. This framework will facilitate the inclusion of such processes in model representations of SOA formation.

Potential of Aerosol Liquid Water to Facilitate Organic Aerosol Formation: Assessing Knowledge Gaps about Precursors and Partitioning

Isoprene epoxydiol (IEPOX), glyoxal, and methylglyoxal are ubiquitous water-soluble organic gases (WSOGs) that partition to aerosol liquid water (ALW) and clouds to form aqueous secondary organic aerosol (aqSOA). Recent laboratory-derived Setschenow (or salting) coefficients suggest glyoxal's potential to form aqSOA is enhanced by high aerosol salt molality, or "salting-in". In the southeastern U.S., aqSOA is responsible for a significant fraction of ambient organic aerosol, and correlates with sulfate mass. However, the mechanistic explanation for this correlation remains elusive, and an assessment of the importance of different WSOGs to aqSOA is currently missing. We employ EPA's CMAQ model to the continental U.S. during the Southern Oxidant and Aerosol Study (SOAS) to compare the potential of glyoxal, methylglyoxal, and IEPOX to partition to ALW, as the initial step toward aqSOA formation. Among these three studied compounds, IEPOX is a dominant contributor, ∼72% on average in the continental U.S., to potential aqSOA mass due to Henry's Law constants and molecular weights. Glyoxal contributes significantly, and application of the Setschenow coefficient leads to a greater than 3-fold model domain average increase in glyoxal's aqSOA mass potential. Methylglyoxal is predicted to be a minor contributor. Acid or ammonium - catalyzed ring-opening IEPOX chemistry as well as sulfate-driven ALW and the associated molality may explain positive correlations between SOA and sulfate during SOAS and illustrate ways in which anthropogenic sulfate could regulate biogenic aqSOA formation, ways not presently included in atmospheric models but relevant to development of effective control strategies.

Experimental and Theoretical Investigations on the Reaction of 1,3-Butadiene with Cl Atom in the Gas Phase

Temperature dependent rate coefficients for the reaction of Cl atom with 1,3-butadiene were measured over the temperature range 269-363 K relative to its reaction with isoprene and 1-pentene. Theoretical calculations were performed for the title reaction using CVT/SCT in combination with CCSD(T)/6-31+G (d,p)//MP2/6-311+G(2df,2p) level of theory, to complement our experimental measurements. The test molecule would survive for 1 h in the atmosphere, and therefore, it can be considered as a very short-lived compound. 1,3-Butadience cannot contribute to global warming as it is very short-lived. However, 4 ppm of ozone is estimated to be formed by the test molecule, which can be considered to be reasonably significant.

Effect of Pellet Boiler Exhaust on Secondary Organic Aerosol Formation from α-Pinene

Interactions between anthropogenic and biogenic emissions, and implications for aerosol production, have raised particular scientific interest. Despite active research in this area, real anthropogenic emission sources have not been exploited for anthropogenic-biogenic interaction studies until now. This work examines these interactions using α-pinene and pellet boiler emissions as a model test system. The impact of pellet boiler emissions on secondary organic aerosol (SOA) formation from α-pinene photo-oxidation was studied under atmospherically relevant conditions in an environmental chamber. The aim of this study was to identify which of the major pellet exhaust components (including high nitrogen oxide (NO_x), primary particles, or a combination of the two) affected SOA formation from α-pinene. Results demonstrated that high NO_x concentrations emitted by the pellet boiler reduced SOA yields from α-pinene, whereas the chemical properties of the primary particles emitted by the pellet boiler had no effect on observed SOA yields. The maximum SOA yield of α-pinene in the presence of pellet boiler exhaust (under high-NO_x conditions) was 18.7% and in the absence of pellet boiler exhaust (under low-NO_x conditions) was 34.1%. The reduced SOA yield under high-NO_x conditions was caused by changes in gas-phase chemistry that led to the formation of organonitrate compounds.

Quantitative Infrared Absorption Spectra and Vibrational Assignments of Crotonaldehyde and Methyl Vinyl Ketone Using Gas-Phase Mid-Infrared, Far-Infrared, and Liquid Raman Spectra: s-cis vs s-trans Composition Confirmed via Temperature Studies and ab Initio Methods

Methyl vinyl ketone (MVK) and crotonaldehyde are chemical isomers; both are also important species in tropospheric chemistry. We report quantitative vapor-phase infrared spectra of crotonaldehyde and MVK vapors over the 540-6500 cm^-1 range. Vibrational assignments of all fundamental modes are made for both molecules on the basis of far- and mid-infrared vapor-phase spectra, liquid Raman spectra, along with density functional theory and ab initio MP2 and high energy-accuracy compound theoretical models (W1BD). Theoretical results indicate that at room temperature the crotonaldehyde equilibrium mixture is approximately 97% s-trans and only 3% s-cis conformer. Nearly all observed bands are thus associated with the s-trans conformer, but a few appear to be uniquely associated with the s-cis conformer, notably ν₁₆^c at 730.90 cm^-1, which displays a substantial intensity increase with temperature (70% upon going from 5 to 50 ^o C). The intensity of the corresponding mode of the s-trans conformer decreases with temperature. Under the same conditions, the MVK equilibrium mixture is approximately 69% s-trans conformer and 31% s-cis. W1BD calculations indicate that for MVK this is one of those (rare) cases where there are comparable populations of both conformers, approximately doubling the number of observed bands and exacerbating the vibrational assignments. We uniquely assign the bands associated with both the MVK s-cis conformer as well as those of the s-trans, thus completing the vibrational analyses of both conformers from the same set of experimental spectra. Integrated band intensities are reported for both molecules along with global warming potential values. Using the quantitative IR data, potential bands for atmospheric monitoring are also discussed.

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 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.

Photodegradation of Secondary Organic Aerosol Particles as a Source of Small, Oxygenated Volatile Organic Compounds

We investigated the photodegradation of secondary organic aerosol (SOA) particles by near-UV radiation and photoproduction of oxygenated volatile organic compounds (OVOCs) from various types of SOA. We used a smog chamber to generate SOA from α-pinene, guaiacol, isoprene, tetradecane, and 1,3,5-trimethylbenzene under high-NOx, low-NOx, or ozone oxidation conditions. The SOA particles were collected on a substrate, and the resulting material was exposed to several mW of near-UV radiation (λ ∼ 300 nm) from a light-emitting diode. Various OVOCs, including acetic acid, formic acid, acetaldehyde, and acetone were observed during photodegradation, and their SOA-mass-normalized fluxes were estimated with a Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-ToF-MS). All the SOA, with the exception of guaiacol SOA, emitted OVOCs upon irradiation. Based on the measured OVOC emission rates, we estimate that SOA particles would lose at least ∼1% of their mass over a 24 h period during summertime conditions in Los Angeles, California. This condensed-phase photochemical process may produce a few Tg/year of gaseous formic acid, the amount comparable to its primary sources. The condensed-phase SOA photodegradation processes could therefore measurably affect the budgets of both particulate and gaseous atmospheric organic compounds on a global scale.

Chemical Characterization of Secondary Organic Aerosol from Oxidation of Isoprene Hydroxyhydroperoxides

Atmospheric oxidation of isoprene under low-NOx conditions leads to the formation of isoprene hydroxyhydroperoxides (ISOPOOH). Subsequent oxidation of ISOPOOH largely produces isoprene epoxydiols (IEPOX), which are known secondary organic aerosol (SOA) precursors. Although SOA from IEPOX has been previously examined, systematic studies of SOA characterization through a non-IEPOX route from 1,2-ISOPOOH oxidation are lacking. In the present work, SOA formation from the oxidation of authentic 1,2-ISOPOOH under low-NOx conditions was systematically examined with varying aerosol compositions and relative humidity. High yields of highly oxidized compounds, including multifunctional organosulfates (OSs) and hydroperoxides, were chemically characterized in both laboratory-generated SOA and fine aerosol samples collected from the southeastern U.S. IEPOX-derived SOA constituents were observed in all experiments, but their concentrations were only enhanced in the presence of acidified sulfate aerosol, consistent with prior work. High-resolution aerosol mass spectrometry (HR-AMS) reveals that 1,2-ISOPOOH-derived SOA formed through non-IEPOX routes exhibits a notable mass spectrum with a characteristic fragment ion at m/z 91. This laboratory-generated mass spectrum is strongly correlated with a factor recently resolved by positive matrix factorization (PMF) of aerosol mass spectrometer data collected in areas dominated by isoprene emissions, suggesting that the non-IEPOX pathway could contribute to ambient SOA measured in the Southeastern United States.

Efficient Isoprene Secondary Organic Aerosol Formation from a Non-IEPOX Pathway

With a large global emission rate and high reactivity, isoprene has a profound effect upon atmospheric chemistry and composition. The atmospheric pathways by which isoprene converts to secondary organic aerosol (SOA) and how anthropogenic pollutants such as nitrogen oxides and sulfur affect this process are subjects of intense research because particles affect Earth's climate and local air quality. In the absence of both nitrogen oxides and reactive aqueous seed particles, we measure SOA mass yields from isoprene photochemical oxidation of up to 15%, which are factors of 2 or more higher than those typically used in coupled chemistry climate models. SOA yield is initially constant with the addition of increasing amounts of nitric oxide (NO) but then sharply decreases for input concentrations above 50 ppbv. Online measurements of aerosol molecular composition show that the fate of second-generation RO2 radicals is key to understanding the efficient SOA formation and the NOx-dependent yields described here and in the literature. These insights allow for improved quantitative estimates of SOA formation in the preindustrial atmosphere and in biogenic-rich regions with limited anthropogenic impacts and suggest that a more-complex representation of NOx-dependent SOA yields may be important in models.