Aqueous Photochemistry of Secondary Organic Aerosol of α-Pinene and α-Humulene Oxidized with Ozone, Hydroxyl Radical, and Nitrate Radical

Effect of nitrogen oxides and sulfur oxides to triphenyl phosphate degradation and cytotoxicity on surface of different transition metal salts

Nitrogen oxides and sulfur oxides, as the dominant toxic gases in the atmosphere, can induce severe human health problems under the composite pollutant conditions. Currently the effect of nitrogen or sulfur oxides in atmospheric environment to the degradation and cytotoxicity of triphenyl phosphate (TPhP) on atmospheric particle surfaces still remain poorly understood. Hence, laboratory simulation methods were used in this study to investigate the effect and related mechanism. First, particle samples were prepared with the TPhP coated on MnSO4, CuSO4, FeSO4 and Fe2(SO4)3 surface. The results showed that, when nitrogen or sulfur oxides were present, more significant TPhP degradation on all samples can be observed under both light and dark conditions. The results proved nitrogen oxides and sulfur oxides were the vital influence factors to the degradation of TPhP, which mainly promoted the OH generation in the polluted atmosphere. The mechanism study indicated that diphenyl hydrogen phosphate (DPhP) and OH-DPhP were two main stable degradation products. These degradation products originated from the phenoxy bond cleavage and hydroxylation of TPhP caused by hydroxyl radicals. In addition, no TPhP related organosulfates (OSs) or organic nitrates (ON) formation were observed. Regarding the cytotoxicity, all the particles can induce more significant cellular injury and apoptosis of A549 cells, which may be relevant to the adsorbed nitrogen oxides or sulfur oxides on particles surfaces. The superfluous reactive oxygen species (ROS) generation was the possible reason of cytotoxicity. This research can supply a comprehensive understanding of the promoting effect of nitrogen and sulfur oxides to TPhP degradation and the composite cytotoxicity of atmospheric particles.

Emerging investigator series: secondary organic aerosol formation from photooxidation of acyclic terpenes in an oxidation flow reactor

One major challenge in predicting secondary organic aerosol (SOA) formation in the atmosphere is incomplete representation of biogenic volatile organic compounds (BVOCs) emitted from plants, particularly those that are emitted as a result of stress - a condition that is becoming more frequent in a rapidly changing climate. One of the most common types of BVOCs emitted by plants in response to environmental stress are acyclic terpenes. In this work, SOA is generated from the photooxidation of acyclic terpenes in an oxidation flow reactor and compared to SOA production from a reference cyclic terpene - α-pinene. The acyclic terpenes used as SOA precursors included β-myrcene, β-ocimene, and linalool. Results showed that oxidation of all acyclic terpenes had lower SOA yields measured after 4 days photochemical age, in comparison to α-pinene. However, there was also evidence that the condensed organic products that formed, while a smaller amount overall, had a higher oligomeric content. In particular, β-ocimene SOA had higher oligomeric content than all the other chemical systems studied. SOA composition data from ultra-high performance liquid chromatography with electrospray ionization mass spectrometry (UHPLC-ESI-MS) was combined with mechanistic modeling using the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) to explore chemical mechanisms that could lead to this oligomer formation. Calculations based on composition data suggested that β-ocimene SOA was more viscous with a higher glass transition temperature than other SOA generated from acyclic terpene oxidation. This was attributed to a higher oligomeric content compared to other SOA systems studied. These results contribute to novel chemical insights about SOA formation from acyclic terpenes and relevant chemistry processes, highlighting the importance of improving underrepresented biogenic SOA formation in chemical transport models.

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

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

Aqueous-Phase Reactions of Anthropogenic Emissions Lead to the High Chemodiversity of Atmospheric Nitrogen-Containing Compounds during the Haze Event

Nitrogen-containing organic compounds (NOCs), a type of important reactive-nitrogen species, are abundant in organic aerosols in haze events observed in Northern China. However, due to the complex nature of NOCs, the sources, formation, and influencing factors are still ambiguous. Here, the molecular composition of organic matters (OMs) in hourly PM2.5 samples collected during a haze event in Northern China was characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). We found that CHON compounds (formulas containing C, H, O, and N atoms) dominated the OM fractions during the haze and showed high chemodiversity and transformability. Relying on the newly developed revised-workflow and oxidation-hydrolyzation knowledge for CHON compounds, 64% of the major aromatic CHON compounds (>80%) could be derived from the oxidization or hydrolyzation processes. Results from FT-ICR MS data analysis further showed that the aerosol liquid water (ALW)-involved aqueous-phase reactions are important for the molecular distribution of aromatic-CHON compounds besides the coal combustion, and the ALW-involved aromatic-CHON compound formation during daytime and nighttime was different. Our results improve the understanding of molecular composition, sources, and potential formation of CHON compounds, which can help to advance the understanding for the formation, evolution, and control of haze.

Sensitivity Constraints of Extractive Electrospray for a Model System and Secondary Organic Aerosol

The quantification of an aerosol chemical composition is complicated by the uncertainty in the sensitivity of each species detected. Soft-ionization response factors can vary widely from molecule to molecule. Here, we have employed a method to separate molecules by their volatility through systematic evaporation with a thermal denuder (TD). The fraction remaining after evaporation is compared between an extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF) and a scanning mobility particle sizer (SMPS), which provides a comparison between a quantified mass loss by the SMPS and the signal loss in the EESI-TOF. The sensitivity of the EESI-TOF is determined for both a simplified complex mixture (PEG-300) and also for a complex mixture of α-pinene secondary organic aerosol (SOA). For PEG-300, separation is possible on a molecule-by-molecule level with the TD and provides insights into the molecule-dependent sensitivity of the EESI-TOF, showing a higher sensitivity toward the most volatile molecule. For α-pinene SOA, sensitivity determination for specific classes is possible because of the number of molecular formula observed by the EESI-TOF. These classes are separated by their volatility and are broken down into monomers (O3-5,6-7,8+), dimers (O4-7,8+), and higher order oligomers (e.g., trimers and tetramers). Here, we show that the EESI-TOF initially measures 60.1% monomers, 32.7% dimers, and 7.2% trimers and tetramers in α-pinene SOA, but after sensitivity correction, the distribution of SOA is 37.4% monomers, 56.1% dimers, and 6.4% trimers and tetramers. These results provide a path forward for the quantification of aerosol components with the EESI-TOF in other applications and potentially for atmospheric measurements.

Aging of α-Pinene Secondary Organic Aerosol by Hydroxyl Radicals in the Aqueous Phase: Kinetics and Products

The reaction of hydroxyl radicals (OH) with a water-soluble fraction of the α-pinene secondary organic aerosol (SOA) was investigated using liquid chromatography coupled with negative electrospray ionization mass spectrometry. The SOA was generated by the dark ozonolysis of α-pinene, extracted into the water, and subjected to chemical aging by the OH. Bimolecular reaction rate coefficients (k_OH) for the oxidation of terpenoic acids by the OH were measured using the relative rate method. The unaged SOA was dominated by the cyclobutyl-ring-retaining compounds, primarily cis-pinonic, cis-pinic, and hydroxy-pinonic acids. Aqueous oxidation by the OH resulted in the removal of early-stage products and dimers, including well-known oligomers with MW = 358 and 368 Da. Furthermore, a 2- to 5-fold increase in the concentration of cyclobutyl-ring-opening products was observed, including terpenylic and diaterpenylic acids and diaterpenylic acid acetate as well as some of the newly identified OH aging markers. At the same time, results obtained from the kinetic box model showed a high degree of SOA fragmentation following the reaction with the OH, which indicates that non-radical reactions occurring during the evaporation of water likely contribute to the high yields of terpenoic _aqSOAs reported previously. The estimated atmospheric lifetimes showed that in clouds, terpenoic acids react with the OH exclusively in the aqueous phase. Aqueous OH aging of the α-pinene SOA results in a 10% increase of the average O/C ratio and a 3-fold decrease in the average k_OH value, which is likely to affect the cloud condensation nuclei activity of the _aqSOA formed after the evaporation of water.

Non-linear effects of secondary organic aerosol formation and properties in multi-precursor systems

Secondary organic aerosol (SOA) contributes significantly to ambient fine particulate matter that affects climate and human health. Monoterpenes represent an important class of biogenic volatile organic compounds (VOCs) and their oxidation by nitrate radicals poses a substantial source of SOA globally. Here, we investigate the formation and properties of SOA from nitrate radical oxidation of two common monoterpenes, α-pinene and limonene. When two monoterpenes are oxidized simultaneously, we observe a ~50% enhancement in the formation of SOA from α-pinene and a ~20% reduction in limonene SOA formation. The change in SOA yields is accompanied by pronounced changes in aerosol chemical composition and volatility. These non-linear effects are not observed in a sequential oxidation experiment. Our results highlight that unlike currently assumed in atmospheric models, the interaction of products formed from individual VOCs should be accounted for to accurately describe SOA formation and its climate and health impacts.

The Effect of Human Occupancy on Indoor Air Quality through Real-Time Measurements of Key Pollutants

The primarily emitted compounds by human presence, e.g., skin and volatile organic compounds (VOCs) in breath, can react with typical indoor air oxidants, ozone (O₃), and hydroxyl radicals (OH), leading to secondary organic compounds. Nevertheless, our understanding about the formation processes of the compounds through reactions of indoor air oxidants with primary emitted pollutants is still incomplete. In this study we performed real-time measurements of nitrous acid (HONO), nitrogen oxides (NO_x = NO + NO₂), O₃, and VOCs to investigate the contribution of human presence and human activity, e.g., mopping the floor, to secondary organic compounds. During human occupancy a significant increase was observed of 1-butene, isoprene, and d-limonene exhaled by the four adults in the room and an increase of methyl vinyl ketone/methacrolein, methylglyoxal, and 3-methylfuran, formed as secondary compounds through reactions of OH radicals with isoprene. Intriguingly, the level of some compounds (e.g., m/z 126, 6-methyl-5-hepten-2-one, m/z 152, dihydrocarvone, and m/z 194, geranyl acetone) formed through reactions of O₃ with the primary compounds was higher in the presence of four adults than during the period of mopping the floor with commercial detergent. These results indicate that human presence can additionally degrade the indoor air quality.

Sunlight can convert atmospheric aerosols into a glassy solid state and modify their environmental impacts

Secondary organic aerosol is well known to affect Earth's climate, regional weather, visibility, and public health. Once these aerosols are formed, they are transported throughout the atmosphere for days or even weeks. We show that exposure of secondary organic aerosols to UV solar radiation leads to a surprising and remarkable increase in viscosity by as much as five orders of magnitude. We also show that this UV exposure can lead to an increased abundance of aerosols that are in the glassy solid state in the troposphere, with important implications for climate predictions. Overall, our results clearly demonstrate that aging by exposure to solar radiation needs to be considered when predicting the environmental impacts of secondary organic aerosols.

Secondary organic aerosol (SOA) plays a critical, yet uncertain, role in air quality and climate. Once formed, SOA is transported throughout the atmosphere and is exposed to solar UV light. Information on the viscosity of SOA, and how it may change with solar UV exposure, is needed to accurately predict air quality and climate. However, the effect of solar UV radiation on the viscosity of SOA and the associated implications for air quality and climate predictions is largely unknown. Here, we report the viscosity of SOA after exposure to UV radiation, equivalent to a UV exposure of 6 to 14 d at midlatitudes in summer. Surprisingly, UV-aging led to as much as five orders of magnitude increase in viscosity compared to unirradiated SOA. This increase in viscosity can be rationalized in part by an increase in molecular mass and oxidation of organic molecules constituting the SOA material, as determined by high-resolution mass spectrometry. We demonstrate that UV-aging can lead to an increased abundance of aerosols in the atmosphere in a glassy solid state. Therefore, UV-aging could represent an unrecognized source of nuclei for ice clouds in the atmosphere, with important implications for Earth’s energy budget. We also show that UV-aging increases the mixing times within SOA particles by up to five orders of magnitude throughout the troposphere with important implications for predicting the growth, evaporation, and size distribution of SOA, and hence, air pollution and climate.

Effects of Nitrogen Oxides on the Production of Reactive Oxygen Species and Environmentally Persistent Free Radicals from α-Pinene and Naphthalene Secondary Organic Aerosols

Reactive oxygen species (ROS) and environmentally persistent free radicals (EPFR) play an important role in chemical transformation of atmospheric aerosols and adverse aerosol health effects. This study investigated the effects of nitrogen oxides (NO_x) during photooxidation of α-pinene and naphthalene on the EPFR content and ROS formation from secondary organic aerosols (SOA). Electron paramagnetic resonance (EPR) spectroscopy was applied to quantify EPFR content and ROS formation. While no EPFR were detected in α-pinene SOA, we found that naphthalene SOA contained about 0.7 pmol μg^-1 of EPFR, and NO_x has little influence on EPFR concentrations and oxidative potential. α-Pinene and naphthalene SOA generated under low NO_x conditions form OH radicals and superoxide in the aqueous phase, which was lowered substantially by 50-80% for SOA generated under high NO_x conditions. High-resolution mass spectrometry analysis showed the substantial formation of nitroaromatics and organic nitrates in a high NO_x environment. The modeling results using the GECKO-A model that simulates explicit gas-phase chemistry and the radical 2D-VBS model that treats autoxidation predicted reduced formation of hydroperoxides and enhanced formation of organic nitrates under high NO_x due to the reactions of peroxy radicals with NO_x instead of their reactions with HO₂. Consistently, the presence of NO_x resulted in the decrease of peroxide contents and oxidative potential of α-pinene SOA.

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

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

Surfactant-Assisted Ozonolysis of Alkenes in Water: Mitigation of Frothing Using Coolade as a Low-Foaming Surfactant

Aqueous-phase ozonolysis in the atmosphere is an important process during cloud and fog formation. Water in the atmosphere acts as both a reaction medium and a reductant during the ozonolysis. Inspired by the atmospheric aqueous-phase ozonolysis, we herein report the ozonolysis of alkenes in water assisted by surfactants. Several types of surfactants, including anionic, cationic, and nonionic surfactants, were investigated. Although most surfactants enhanced the solubility of alkenes in water, they also generated excessive foaming during the ozone bubbling, which led to the loss of products. Mitigation of the frothing was accomplished by using Coolade as a nonionic and low-foaming surfactant. Coolade-assisted ozonolysis of alkenes in water provided the desired carbonyl products in good yields and comparable to those achieved in organic solvents. During the ozonolysis reaction, water molecules trapped within the polyethylene glycol region of Coolade were proposed to intercept the Criegee intermediate to provide a hydroxy hydroperoxide intermediate. Decomposition of the hydroxy hydroperoxide led to formation of the carbonyl product without the need for a reductant typically required for the conventional ozonolysis using organic solvents. This study presents Coolade as an effective surfactant to improve the solubility of alkenes while mitigating frothing during the ozonolysis in water.

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.

O3 Concentration and Its Relation with BVOC Emissions in a Subtropical Plantation

An empirical model of O3 is developed using the measurements of emissions of biogenic volatile organic compounds (BVOCs), O3 concentration, global solar radiation, photosynthetically active radiation (PAR) and meteorological variables in a subtropical Pinus plantation, China, during 2013–2016. In view of the different structures of isoprene and monoterpenes, two empirical models of O3 concentration are developed, considering PAR absorption and scattering due to gases, liquids and particles (GLPs), as well as PAR attenuation caused by O3 and BVOCs. The estimated O3 is in agreement with the observations, and validation of the O3 empirical model is conducted. O3 concentrations are more sensitive to changes in PAR and water vapor than S/Q (horizontal diffuse to global solar radiation) and BVOC emissions. O3 is positive to changes in isoprene emission at low light and high GLPs, or negative at high light and low GLPs; O3 is negative to changes in monoterpene emissions. O3 are positive with the changes of PAR, water vapor and S/Q. It is suggested to control human-induced high BVOC emissions, regulate plant cutting, and reduce NOx and SO2 emissions more strictly than ever before. There are inverted U-shape interactions between O3 and its driving factors, and S/Q controls their turning points.

Important Oxidants and Their Impact on the Environmental Effects of Aerosols

Oxidants are central species in the atmosphere, where they not only determine secondary particle formation but also impact human health and climate change. In general, they are unstable, highly reactive, and recyclable and have been studied in field observations, laboratory studies, and model simulations. The most widely investigated oxidants, such as OH radicals, O₃, and Cl atom, HONO, NO₃, N₂O₅, and Criegee Intermediates (CIs) have attracted more attention recently. Furthermore, secondary particles formed in the oxidations processes impact the particle physicochemical properties, such as hygroscopicity and optical properties and therefore impact the atmospheric radiation balance. Therefore, the newest investigation results of important oxidants (HONO, NO₃, N₂O₅, and CIs) are reviewed in this manuscript, and the environmental effects of secondary particles formed through corresponding oxidation processes are also stated. Furthermore, some perspectives are further discussed in the article.

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.

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.

Influence of humidity and iron(iii) on photodegradation of atmospheric secondary organic aerosol particles

The absorption of solar actinic radiation by atmospheric secondary organic aerosol (SOA) particles drives condensed-phase photochemical processes, which lead to particle mass loss by the production of CO, CO2, hydrocarbons, and various oxygenated volatile organic compounds (OVOCs). We examined the influence of relative humidity (RH) and Fe(iii) content on the OVOC release and subsequent mass loss from secondary organic aerosol material (SOM) during UV irradiation. The samples were generated in a flow tube reactor from the oxidation of d-limonene by ozone. The SOM was collected with a Micro Orifice Uniform Deposit Impactor (MOUDI) on CaF2 windows. To selected samples, a variable amount of FeCl3 was added before irradiation. The resulting SOM samples, with or without added FeCl3, were irradiated with a 305 nm light-emitting diode and the release of several OVOCs, including acetic acid, acetone, formic acid and acetaldehyde, was measured with a Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-ToF-MS). The release of OVOCs from photodegradation of SOM at typical ambient mid-values of RH (30-70%) was 2-4 times higher than under dry conditions. The release of OVOCs was slightly enhanced in the presence of low concentrations of iron (0.04 Fe molar ratio) but it was suppressed at higher concentrations (0.50 Fe molar ratio) of iron indicating the existence of a complicated radical chemistry driving the photodegradation of SOM. Our findings suggest that the presence of iron in atmospheric aerosol particles will either increase or decrease release of OVOCs due to the photodegradation of SOM depending on whether the relative iron concentration is low or high, respectively. At atmospherically relevant RH conditions, the expected fractional mass loss induced by these photochemical processes from limonene SOA particles would be between 2 and 4% of particle mass per hour. Therefore, photodegradation is an important aging mechanism for this type of SOA.

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.

cis-Pinonic Acid Oxidation by Hydroxyl Radicals in the Aqueous Phase under Acidic and Basic Conditions: Kinetics and Mechanism

Aqueous-phase oxidation of cis-pinonic acid (CPA) by hydroxyl radicals (OH) was studied using a relative rate technique under acidic and basic conditions. Liquid chromatography (LC) coupled to the negative electrospray ionization (ESI) quadrupole tandem mass spectrometry (MS/MS) was used to monitor the concentrations of CPA and reference compounds. The measured second order reaction rate coefficients of CPA with OH were: 3.6 ± 0.3 × 10⁹ M^-1 s^-1 (pH 2) and 3.0 ± 0.3 × 10⁹ M^-1 s^-1 (pH 10) - combined uncertainties are 2σ. These results indicated that the lifetimes of CPA in the atmosphere are most likely independent from the aqueous-phase pH. LC-ESI/MS/MS was also used to tentatively identify the CPA oxidation products. Formation of carboxylic acids with molecular weight (MW) 216 Da (most likely C₁₀H₁₆O₅) and MW 214 Da (C₁₀H₁₄O₅) was confirmed with LC-ESI/MS/MS. When the initial CPA concentration was increased from 0.3 to 10 mM, formation of additional products was observed with MW 188, 200, 204, and 232 Da. Hydroperoxy, hydroxyl and carbonyl-substituted CPA derivatives were tentatively identified among the products. Similar products were formed by the CPA oxidation by OH in the gas-phase, at the air-water interface as well as in the solid phase (dry film). Formation of the stable adduct of CPA and H₂O₂ was also observed when the reaction mixture was evaporated to dryness and redissolved in water. Acquired mass spectrometric data argues against formation of oligomers.