Experimental product study of the OH-initiated oxidation of m-xylene

Characterization of products formed from the oxidation of toluene and m-xylene with varying NOx and OH exposure

Aromatic volatile organic compounds (VOCs) are an important precursor of secondary organic aerosol (SOA) in the urban environment. SOA formed from the oxidation of anthropogenic VOCs can be substantially more abundant than biogenic SOA and has been shown to account for a significant fraction of fine particulate matter in urban areas. A potential aerosol mass (PAM) chamber was used to investigate the oxidised products from the photo-oxidation of m-xylene and toluene. The experiments were carried out with OH radical as oxidant in both high- and low-NO_x conditions and the resultant aerosol samples were collected using quartz filters and analysed by GC × GC-TOFMS. Results show the oxidation products derived from both precursors included ring-retaining and -opening compounds (unsaturated aldehydes, unsaturated ketones and organic acids) with a high number of ring-opening compounds observed from toluene oxidation. Glyoxal and methyl glyoxal were the major ring-cleavage products from both oxidation systems, indicating that a bicyclic route plays an important role in their formation. SOA yields were higher for both precursors under high-NO_x (toluene: 0.111; m-xylene: 0.124) than at low-NO_x (toluene: 0.089; m-xylene: 0.052), likely linked to higher OH concentrations during low-NO_x experiments which may lead to higher degree of fragmentation. DHOPA (2,3-dihydroxy-4-oxo-pentanoic acid), a known tracer of toluene oxidation, was observed in both oxidation systems. The mass fraction of DHOPA in SOA from toluene oxidation was about double the value reported previously, but it should not be regarded as a tracer solely for oxidation of toluene as m-xylene oxidation gave a similar relative yield.

Bridged-ozonolysis of mixed aromatic hydrocarbons and organic amines: Inter-inhibited decay rate, altered product yield and synergistic-effect-enhanced secondary organic aerosol formation

Ozonolysis of aromatic hydrocarbons (AHs) or organic amines (OAs) occurs via different transformation processes, with varying rate constants and contributions to secondary organic aerosol (SOA) formation. However, to date no data is available on the ozonolysis of mixtures of AHs and OAs. This study investigated the kinetics, products and SOA yield from ozonolysis of mixture of trimethylamine with styrene, toluene or m-xylene. In the mixed system, the decay rates of styrene and trimethylamine were (1.32 ± 0.26) × 10^-4 s^-1 and (0.80 ± 0.02) × 10^-4 s^-1, decreasing up to 36.5 % and 54.4 % compared with their respective individual systems. This inter-inhibition of decay rates increased the yield of main products from styrene (i.e. benzaldehyde) by 23.5 % and trimethylamine (i.e. nitromethane) by 346.4 %. Ozonolysis of styrene or trimethylamine produced formaldehyde, which acted as a bridged product connecting the ozonolysis pathways of these two substrates, altering the yields of all products. Ozonolysis of styrene to benzaldehyde determined the increase of SOA particle number concentration (from 9.5 × 10⁵ to 1.9 × 10⁶ particles cm^-3), while trimethylamine ozonolysis to N, N-dimethylformamide contributed to synergistic-effect-enhanced SOA yield (from (64.3 ± 3.5)% to (68.1 ± 4.8)%). The findings provide a novel insight into the kinetics and mechanism of ozonolysis, as well as the resulting SOA formation from mixtures of AHs and OAs, helping to comprehensively understand the transformation and fate of organics in real atmospheric environments.

Sulfuric acid-dimethylamine particle formation enhanced by functional organic acids: an integrated experimental and theoretical study

Atmospheric new particle formation (NPF), which has been observed globally in clean and polluted environments, is an important source of boundary-layer aerosol particles and cloud condensation nuclei, but the fundamental mechanisms leading to multi-component aerosol formation have not been well understood. Here, we use experiments and quantum chemical calculations to better understand the involvement of carboxylic acids in initial NPF from gas phase mixtures of carboxylic acid, sulfuric acid (SA), dimethylamine, and water. A turbulent flow tube coupled to an ultrafine condensation particle counter with particle size magnifier has been set up to measure NPF. Experimental results show that pyruvic acid (PA), succinic acid (SUA), and malic acid (MA) can enhance sulfuric acid-dimethylamine nucleation in the order PA < SUA < MA with a greater enhancement observed at lower SA concentrations. Computational results indicate that the carboxylic and hydroxyl groups are related to the enhancement. This experiment-theory study shows the formation of multi-component aerosol particles and the role of the organic functional group, which may aid in understanding the role of organics in aerosol nucleation and growth in polluted areas, and help to choose organic molecules of specific structures for simulation.

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.

Unexpected Oligomerization of Small α-Dicarbonyls for Secondary Organic Aerosol and Brown Carbon Formation

Large amounts of small α-dicarbonyls (glyoxal and methylglyoxal) are produced in the atmosphere from photochemical oxidation of biogenic isoprene and anthropogenic aromatics, but the fundamental mechanisms leading to secondary organic aerosol (SOA) and brown carbon (BrC) formation remain elusive. Methylglyoxal is commonly believed to be less reactive than glyoxal because of unreactive methyl substitution, and available laboratory measurements showed negligible aerosol growth from methylglyoxal. Herein, we present experimental results to demonstrate striking oligomerization of small α-dicarbonyls leading to SOA and BrC formation on sub-micrometer aerosols. Significantly more efficient growth and browning of aerosols occur upon exposure to methylglyoxal than glyoxal under atmospherically relevant concentrations and in the absence/presence of gas-phase ammonia and formaldehyde, and nonvolatile oligomers and light-absorbing nitrogen-heterocycles are identified as the dominant particle-phase products. The distinct aerosol growth and light absorption are attributed to carbenium ion-mediated nucleophilic addition, interfacial electric field-induced attraction, and synergetic oligomerization involving organic/inorganic species, leading to surface- or volume-limited reactions that are dependent on the reactivity and gaseous concentrations. Our findings resolve an outstanding discrepancy concerning the multiphase chemistry of small α-dicarbonyls and unravel a new avenue for SOA and BrC formation from atmospherically abundant, ubiquitous carbonyls and ammonia/ammonium sulfate.

Explosive Secondary Aerosol Formation during Severe Haze in the North China Plain

Severe haze events with exceedingly high-levels of fine aerosols occur frequently over the past decades in the North China Plain (NCP), exerting profound impacts on human health, weather, and climate. The development of effective mitigation policies requires a comprehensive understanding of the haze formation mechanisms, including identification and quantification of the sources, formation, and transformation of the aerosol species. Haze evolution in this region exhibits distinct physical and chemical characteristics from clean to polluted periods, as evident from increasing stagnation and relative humidity, but decreasing solar radiation as well as explosive secondary aerosol formation. The latter is attributed to highly elevated concentrations of aerosol precursor gases and is reflected by rapid increases in the particle number and mass concentrations, both corresponding to nonequilibrium chemical processes. Considerable new knowledge has been acquired to understand the processes regulating haze formation, particularly in light of the progress in elucidating the aerosol formation mechanisms. This review synthesizes recent advances in understanding secondary aerosol formation, by highlighting several critical chemical/physical processes, that is, new particle formation and aerosol growth driven by photochemistry and aqueous chemistry as well as the interaction between aerosols and atmospheric stability. Current challenges and future research priorities are also discussed.

Remarkable nucleation and growth of ultrafine particles from vehicular exhaust

High concentrations of ultrafine particles (UFPs), approaching 1 million/cm³, are frequently produced from new particle formation under urban environments, but the fundamental mechanisms regulating nucleation and growth for UFPs are poorly understood. From simultaneous ambient and environmental chamber measurements, we demonstrate remarkable formation of UFPs from urban traffic emissions. By replicating ambient conditions using an environmental chamber method, we elucidate the roles of existing particles, photochemistry, and synergy of multipollutant photooxidation in nucleation and growth of UFPs. Our results reveal that synergetic oxidation of vehicular exhaust leads to efficient formation of UFPs under urban conditions. Recognition of this large urban source for UFPs is essential to accurately assessing their impacts and to effectively developing mitigation policies.

High levels of ultrafine particles (UFPs; diameter of less than 50 nm) are frequently produced from new particle formation under urban conditions, with profound implications on human health, weather, and climate. However, the fundamental mechanisms of new particle formation remain elusive, and few experimental studies have realistically replicated the relevant atmospheric conditions. Previous experimental studies simulated oxidation of one compound or a mixture of a few compounds, and extrapolation of the laboratory results to chemically complex air was uncertain. Here, we show striking formation of UFPs in urban air from combining ambient and chamber measurements. By capturing the ambient conditions (i.e., temperature, relative humidity, sunlight, and the types and abundances of chemical species), we elucidate the roles of existing particles, photochemistry, and synergy of multipollutants in new particle formation. Aerosol nucleation in urban air is limited by existing particles but negligibly by nitrogen oxides. Photooxidation of vehicular exhaust yields abundant precursors, and organics, rather than sulfuric acid or base species, dominate formation of UFPs under urban conditions. Recognition of this source of UFPs is essential to assessing their impacts and developing mitigation policies. Our results imply that reduction of primary particles or removal of existing particles without simultaneously limiting organics from automobile emissions is ineffective and can even exacerbate this problem.

Sensitive Detection of Gas-Phase Glyoxal by Electron Attachment Reaction Ionization Mass Spectrometry

Glyoxal (GLY) acts as a key contributor to tropospheric ozone production and secondary organic aerosol (SOA) formation on local to regional scales. The detection of GLY provides useful indicators of fast photochemistry occurring in the lower troposphere. The fast and sensitive detection of GLY is thus important, while traditional chemical ionization such as the proton-transfer reaction (PTR) is extremely limited by the poor detection limit and extensive fragmentation. To address these limitations, electron attachment reaction (EAR) ionization was applied to detect GLY. The generation of parent anions (GLY^-) without fragmentation was observed, and cryogenic photoelectron imaging spectroscopy further characterized the structure of GLY^-. The detection limit was estimated to be as low as (52 ± 1) pptv (parts per trillion by volume) with 1 min measurements. Other components in ambient air, such as water, carbon dioxide, and trace gases (acetone, propanal, etc.) have no effect on the detection of GLY. The EAR ionization is more promising than PTR ionization in detecting GLY. The detection of GLY in ambient air by the EAR ionization has been demonstrated.

Formation and Optical Properties of Brown Carbon from Small α-Dicarbonyls and Amines

Brown Carbon (BrC) aerosols scatter and absorb solar radiation, directly affecting the Earth's radiative budget. However, considerable uncertainty exists concerning the chemical mechanism leading to BrC formation and their optical properties. In this work, BrC particles were prepared from mixtures of small α-dicarbonyls (glyoxal and methylglyoxal) and amines (methylamine, dimethylamine, and trimethylamine). The absorption and scattering of BrC particles were measured using a photoacoustic extinctometer (405 and 532 nm), and the chemical composition of the α-dicarbonyl-amine mixtures was analyzed using orbitrap-mass spectrometry and thermal desorption-ion drift-chemical ionization mass spectrometry. The single scattering albedo for methylglyoxal-amine mixtures is smaller than that of glyoxal-amine mixtures and increases with the methyl substitution of amines. The mass absorption cross-section for methylglyoxal-amine mixtures is two times higher at 405 nm wavelength than that at 532 nm wavelength. The derived refractive indexes at the 405 nm wavelength are 1.40-1.64 for the real part and 0.002-0.195 for the imaginary part. Composition analysis in the α-dicarbonyl-amine mixtures reveals N-heterocycles as the dominant products, which are formed via multiple steps involving nucleophilic attack, steric hindrance, and dipole-dipole interaction between α-dicarbonyls and amines. BrC aerosols, if formed from the particle-phase reaction of methylglyoxal with methylamine, likely contribute to atmospheric warming.

Characterization of secondary organic aerosol from photo-oxidation of gasoline exhaust and specific sources of major components

To further explore the composition and distribution of secondary organic aerosol (SOA) components from the photo-oxidation of light aromatic precursors (toluene, m-xylene, and 1,3,5-trimethylbenzene (1,3,5-TMB)) and idling gasoline exhaust, a vacuum ultraviolet photoionization mass spectrometer (VUV-PIMS) was employed. Peaks of the molecular ions of the SOA components with minimum molecular fragmentation were clearly observed from the mass spectra of SOA, through the application of soft ionization methods in VUV-PIMS. The experiments comparing the exhaust-SOA and light aromatic mixture-SOA showed that the observed distributions of almost all the predominant cluster ions in the exhaust-SOA were similar to that of the mixture-SOA. Based on the characterization experiments of SOA formed from individual light aromatic precursors, the SOA components with molecular weights of 98 and 110 amu observed in the exhaust-SOA resulted from the photo-oxidation of toluene and m-xylene; the components with a molecular weight of 124 amu were derived mainly from m-xylene; and the components with molecular weights of 100, 112, 128, 138, and 156 amu were mainly derived from 1,3,5-TMB. These results suggest that C₇-C₉ light aromatic hydrocarbons are significant SOA precursors and that major SOA components originate from gasoline exhaust. Additionally, some new light aromatic hydrocarbon-SOA components were observed for the first time using VUV-PIMS. The corresponding reaction mechanisms were also proposed in this study to enrich the knowledge base of the formation mechanisms of light aromatic hydrocarbon-SOA compounds.

Reassessing the atmospheric oxidation mechanism of toluene

Photochemical oxidation of aromatic hydrocarbons leads to tropospheric ozone and secondary organic aerosol (SOA) formation, with profound implications for air quality, human health, and climate. Toluene is the most abundant aromatic compound under urban environments, but its detailed chemical oxidation mechanism remains uncertain. From combined laboratory experiments and quantum chemical calculations, we show a toluene oxidation mechanism that is different from the one adopted in current atmospheric models. Our experimental work indicates a larger-than-expected branching ratio for cresols, but a negligible formation of ring-opening products (e.g., methylglyoxal). Quantum chemical calculations also demonstrate that cresols are much more stable than their corresponding peroxy radicals, and, for the most favorable OH (ortho) addition, the pathway of H extraction by O₂ to form the cresol proceeds with a smaller barrier than O₂ addition to form the peroxy radical. Our results reveal that phenolic (rather than peroxy radical) formation represents the dominant pathway for toluene oxidation, highlighting the necessity to reassess its role in ozone and SOA formation in the atmosphere.

Persistent sulfate formation from London Fog to Chinese haze

Sulfate aerosols exert profound impacts on human and ecosystem health, weather, and climate, but their formation mechanism remains uncertain. Atmospheric models consistently underpredict sulfate levels under diverse environmental conditions. From atmospheric measurements in two Chinese megacities and complementary laboratory experiments, we show that the aqueous oxidation of SO₂ by NO₂ is key to efficient sulfate formation but is only feasible under two atmospheric conditions: on fine aerosols with high relative humidity and NH₃ neutralization or under cloud conditions. Under polluted environments, this SO₂ oxidation process leads to large sulfate production rates and promotes formation of nitrate and organic matter on aqueous particles, exacerbating severe haze development. Effective haze mitigation is achievable by intervening in the sulfate formation process with enforced NH₃ and NO₂ control measures. In addition to explaining the polluted episodes currently occurring in China and during the 1952 London Fog, this sulfate production mechanism is widespread, and our results suggest a way to tackle this growing problem in China and much of the developing world.

OH-Initiated Oxidation of m-Xylene on Black Carbon Aging

Laboratory experiments are conducted to investigate aging of size-classified black carbon (BC) particles from OH-initiated oxidation of m-xylene. The variations in the particle size, mass, effective density, morphology, optical properties, hygroscopicity, and activation as cloud condensation nuclei (CCN) are simultaneously measured by a suite of aerosol instruments, when BC particles are exposed to the oxidation products of the OH-m-xylene reactions. The BC aging is governed by the coating thickness (Δrve), which is correlated to the reaction time and initial concentrations of m-xylene and NOx. For an initial diameter of 100 nm and Δrve = 44 nm, the particle size and mass increase by a factor of 1.5 and 10.4, respectively, and the effective density increases from 0.43 to 1.45 g cm(-3) due to organic coating and collapsing of the BC core. The BC particles are fully converted from a highly fractal to nearly spherical morphology for Δrve = 30 nm. The scattering, absorption, and single scattering albedo of BC particles are enhanced accordingly with organic coating. The critical supersaturation for CCN activation is reduced to 0.1% with Δrve = 44 nm. The results imply that the oxidation of m-xylene exhibits larger impacts in modifying the BC particle properties than those for the OH-initiated oxidation of isoprene and toluene.

Markedly enhanced absorption and direct radiative forcing of black carbon under polluted urban environments

Black carbon (BC) exerts profound impacts on air quality and climate because of its high absorption cross-section over a broad range of electromagnetic spectra, but the current results on absorption enhancement of BC particles during atmospheric aging remain conflicting. Here, we quantified the aging and variation in the optical properties of BC particles under ambient conditions in Beijing, China, and Houston, United States, using a novel environmental chamber approach. BC aging exhibits two distinct stages, i.e., initial transformation from a fractal to spherical morphology with little absorption variation and subsequent growth of fully compact particles with a large absorption enhancement. The timescales to achieve complete morphology modification and an absorption amplification factor of 2.4 for BC particles are estimated to be 2.3 h and 4.6 h, respectively, in Beijing, compared with 9 h and 18 h, respectively, in Houston. Our findings indicate that BC under polluted urban environments could play an essential role in pollution development and contribute importantly to large positive radiative forcing. The variation in direct radiative forcing is dependent on the rate and timescale of BC aging, with a clear distinction between urban cities in developed and developing countries, i.e., a higher climatic impact in more polluted environments. We suggest that mediation in BC emissions achieves a cobenefit in simultaneously controlling air pollution and protecting climate, especially for developing countries.

Hygroscopic Characteristics of Alkylaminium Carboxylate Aerosols

The hygroscopic growth factor (HGF) and cloud condensation nuclei (CCN) activity for a series of alkylaminium carboxylate aerosols have been measured using a hygroscopicity tandem differential mobility analyzer coupled to a condensation particle counter and a CCN counter. The particles, consisting of the mixtures of mono- (acetic, propanoic, p-toluic, and cis-pinonic acid) and dicarboxylic (oxalic, succinic, malic, adipic, and azelaic acid) acid with alkylamine (mono-, di-, and trimethylamines), represent those commonly found under diverse environmental conditions. The hygroscopicity parameter (κ) of the alkylaminium carboxylate aerosols was derived from the HGF and CCN results and theoretically calculated. The HGF at 90% RH is in the range of 1.3 to 1.8 for alkylaminium monocarboxylates and 1.1 to 2.2 for alkylaminium dicarboxylates, dependent on the molecular functionality (i.e., the carboxylic or OH functional group in organic acids and methyl substitution in alkylamines). The κ value for all alkylaminium carboxylates is in the range of 0.06-1.37 derived from the HGF measurements at 90% RH, 0.05-0.49 derived from the CCN measurements, and 0.22-0.66 theoretically calculated. The measured hygroscopicity of the alkylaminium carboxylates increases with decreasing acid to base ratio. The deliquescence point is apparent for several of the alkylaminium dicarboxylates but not for the alkylaminium monocarboxylates. Our results reveal that alkylaminium carboxylate aerosols exhibit distinct hygroscopic and deliquescent characteristics that are dependent on their molecular functionality, hence regulating their impacts on human health, air quality, and direct and indirect radiative forcing on climate.

Interfacial Tensions of Aged Organic Aerosol Particle Mimics Using a Biphasic Microfluidic Platform

Secondary organic aerosol (SOA) particles are a major component of atmospheric particulate matter, yet their formation processes and ambient properties are not well understood. These complex particles often contain multiple interfaces due to internal aqueous- and organic-phase partitioning. Aerosol interfaces can profoundly affect the fate of condensable organic compounds emitted into the atmosphere by altering the way in which ambient organic vapors interact with suspended particles. To accurately predict the evolution of SOA in the atmosphere, we must improve our understanding of aerosol interfaces. In this work, biphasic microscale flows are used to measure interfacial tension of reacting methylglyoxal, formaldehyde, and ammonium sulfate aqueous mixtures with a surrounding oil phase. Our experiments show a suppression of interfacial tension as a function of organic content that remains constant with reaction time for methylglyoxal-ammonium sulfate systems. We also reveal an unexpected time dependence of interfacial tension over a period of 48 h for ternary solutions of both methylglyoxal and formaldehyde in aqueous ammonium sulfate, indicating a more complicated behavior of surface activity where there is competition among dissolved organics. From these interfacial tension measurements, the morphology of aged atmospheric aerosols with internal liquid-liquid phase separation is inferred.

Reaction Mechanism for m-Xylene Oxidation in the Claus Process by Sulfur Dioxide

In the Claus process, the presence of aromatic contaminants such benzene, toluene, and xylenes (BTX), in the H2S feed stream has a detrimental effect on catalytic reactors, where BTX form soot particles and clog and deactivate the catalysts. Among BTX, xylenes are proven to be most damaging contaminant for catalysts. BTX oxidation in the Claus furnace, before they enter catalyst beds, provides a solution to this problem. A reaction kinetics study on m-xylene oxidation by SO2, an oxidant present in Claus furnace, is presented. The density functional theory is used to study the formation of m-xylene radicals (3-methylbenzyl, 2,6-dimethylphenyl, 2,4-dimethylphenyl, and 3,5-dimethylphenyl) through H-abstraction and their oxidation by SO2. The mechanism begins with SO2 addition on the radicals through an O-atom rather than the S-atom with the release of 180.0-183.1 kJ/mol of reaction energies. This exothermic reaction involves energy barriers in the range 3.9-5.2 kJ/mol for several m-xylene radicals. Thereafter, O-S bond scission takes place to release SO, and the O-atom remaining on aromatics leads to CO formation. Among four m-xylene radicals, the resonantly stabilized 3-methylbenzyl exhibited the lowest SO2 addition and SO elimination rates. The reaction rate constants are provided to facilitate Claus process simulations to find conditions suitable for BTX oxidation.

Hydration of a sulfuric acid–oxalic acid complex: acid dissociation and its atmospheric implication

We have investigated structural characteristics and thermodynamics of the hydration of a sulfuric acid–oxalic acid complex using density functional theory to gain insight into the ternary nucleation and its atmospheric implication.

Elucidating severe urban haze formation in China

As the world's second largest economy, China has experienced severe haze pollution, with fine particulate matter (PM) recently reaching unprecedentedly high levels across many cities, and an understanding of the PM formation mechanism is critical in the development of efficient mediation policies to minimize its regional to global impacts. We demonstrate a periodic cycle of PM episodes in Beijing that is governed by meteorological conditions and characterized by two distinct aerosol formation processes of nucleation and growth, but with a small contribution from primary emissions and regional transport of particles. Nucleation consistently precedes a polluted period, producing a high number concentration of nano-sized particles under clean conditions. Accumulation of the particle mass concentration exceeding several hundred micrograms per cubic meter is accompanied by a continuous size growth from the nucleation-mode particles over multiple days to yield numerous larger particles, distinctive from the aerosol formation typically observed in other regions worldwide. The particle compositions in Beijing, on the other hand, exhibit a similarity to those commonly measured in many global areas, consistent with the chemical constituents dominated by secondary aerosol formation. Our results highlight that regulatory controls of gaseous emissions for volatile organic compounds and nitrogen oxides from local transportation and sulfur dioxide from regional industrial sources represent the key steps to reduce the urban PM level in China.

Heterogeneous chemistry of glyoxal on acidic solutions. An oligomerization pathway for secondary organic aerosol formation

The heterogeneous chemistry of glyoxal on sulfuric acid surfaces has been investigated at various acid concentrations and temperatures, utilizing a low-pressure fast flow laminar reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS). The uptake coefficient (γ) of glyoxal ranges from (1.2 ± 0.06) × 10(-2) to (2.5 ± 0.01) × 10(-3) for 60-93 wt % H2SO4 at 253-273 K. The effective Henry's Law constant (H*) ranges from (98.9 ± 4.9) × 10(5) to (1.6 ± 0.1) × 10(5) M atm(-1) for 60-93 wt % at 263-273 K. Both the uptake coefficient and Henry's Law constant increase with decreasing acid concentration and temperature. Our results reveal a reaction mechanism of hydration followed by oligomerization for glyoxal on acidic media, indicating an efficient aqueous reaction of glyoxal on hygroscopic particles leading to secondary organic aerosol formation.

Atmospheric oxidation mechanism of m-xylene initiated by OH radical

The atmospheric oxidation mechanism of m-xylene (mX) initiated by the OH radical is investigated at M06-2X and ROCBS-QB3 levels, coupled with reaction kinetics calculations by using transition state theory and unimolecular RRKM-ME theory. The calculations show that the reaction between OH and mX is dominated by OH addition to the C2 and C4 positions, forming adducts mX-2-OH (R2) and mX-4-OH (R4). In the atmosphere, R2 and R4 react with O2 by irreversible H-abstraction to dimethylphenols or by reversible additions to bicyclic radical intermediates, which would recombine again with O2 to form bicyclic peroxy radicals, to bicyclic alkoxyl radicals by reacting with NO or HO2, and eventually to final products such as glyoxal, methylglyoxal, and their coproducts. The effects of reaction pressure and temperature are explored by RRKM-ME calculations. A mechanism at 298 K is proposed on the basis of current predictions and previous experimental and modeling results. The predicted product yields support the values in the SAPRC mechanism, even though the predicted yield of 1.0% for glyoxal is lower than the value of ∼11% from the experimental measurements.

Acid-catalyzed reactions of epoxides for atmospheric nanoparticle growth

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

Assessing the effects of anthropogenic aerosols on Pacific storm track using a multiscale global climate model

Atmospheric aerosols affect weather and global general circulation by modifying cloud and precipitation processes, but the magnitude of cloud adjustment by aerosols remains poorly quantified and represents the largest uncertainty in estimated forcing of climate change. Here we assess the effects of anthropogenic aerosols on the Pacific storm track, using a multiscale global aerosol-climate model (GCM). Simulations of two aerosol scenarios corresponding to the present day and preindustrial conditions reveal long-range transport of anthropogenic aerosols across the north Pacific and large resulting changes in the aerosol optical depth, cloud droplet number concentration, and cloud and ice water paths. Shortwave and longwave cloud radiative forcing at the top of atmosphere are changed by -2.5 and +1.3 W m(-2), respectively, by emission changes from preindustrial to present day, and an increased cloud top height indicates invigorated midlatitude cyclones. The overall increased precipitation and poleward heat transport reflect intensification of the Pacific storm track by anthropogenic aerosols. Hence, this work provides, for the first time to the authors' knowledge, a global perspective of the effects of Asian pollution outflows from GCMs. Furthermore, our results suggest that the multiscale modeling framework is essential in producing the aerosol invigoration effect of deep convective clouds on a global scale.

Atmospheric chemistry of benzyl alcohol: kinetics and mechanism of reaction with OH radicals

The atmospheric oxidation of benzyl alcohol has been investigated using smog chambers at ICARE, FORD, and EUPHORE. The rate coefficient for reaction with OH radicals was measured and an upper limit for the reaction with ozone was established; kOH = (2.8 ± 0.4) × 10(-11) at 297 ± 3 K (averaged value including results from Harrison and Wells) and kO(3) < 2 × 10(-19) cm(3) molecule(-1) s(-1) at 299 K. The products of the OH radical initiated oxidation of benzyl alcohol in the presence of NOX were studied. Benzaldehyde, originating from H-abstraction from the -CH(2)OH group, was identified using in situ FTIR spectroscopy, HPLC-UV/FID, and GC-PID and quantified in a yield of (24 ± 5) %. Ring retaining products originating from OH-addition to the aromatic ring such as o-hydroxybenzylalcohol and o-dihydroxybenzene as well as ring-cleavage products such as glyoxal were also identified and quantified with molar yields of (22 ± 2)%, (10 ± 3)%, and (2.7 ± 0.7)%, respectively. Formaldehyde was observed with a molar yield of (27 ± 10)%. The results are discussed with respect to previous studies and the atmospheric oxidation mechanism of benzyl alcohol.

Multiphase chemistry of atmospheric amines

Heterogeneous reactions of amines have been recently shown to play an important role in the formation and transformation of atmospheric aerosols. This perspective summarizes the latest laboratory progress in the multiphase chemistry of amines. Particular emphasis is given to the contributions of amines to new particle formation, growth of submicron particles, and alteration in the physiochemical properties of pre-existing particles, including hygroscopicity, thermostability, density, phase, and optical properties, from exposure to gaseous amines. The atmospheric implications of the multiphase reactions of amines, including the potential impact on direct and indirect climate forcing of aerosols, and future research directions are discussed.

Soot aging from OH-initiated oxidation of toluene

We have conducted laboratory experiments to investigate the impacts of secondary organic aerosol formation on soot properties from OH-initiated oxidation of toluene. Monodisperse soot particles are exposed to the oxidation products of the OH-toluene reaction in an environmental chamber, and variations in particle size, mass, organic mass faction, morphology, effective density, hygroscopicity, and optical properties are simultaneously determined by an integrated aerosol analytical system. The thickness of the organic coating, correlated to reaction time and initial reactant concentrations, is shown to largely govern the particle properties. With the development of organic coating, the soot core is changed from a highly fractal to compact form, evident from the measured effective density and dynamic shape factor. The organic coating increases the particle hygroscopicity, and further exposure of coated soot to elevated relative humidity results in a more spherical particle. The single scattering albedo and scattering and absorption cross sections are also enhanced with the organic coating. Our results suggest that the oxidation products of anthropogenic pollutants alter the composition and properties of soot particles and lead to increased particle density, hygroscopicity, and optical properties, considerably enhancing their impacts on air quality, climate forcing, and human health.

Investigation of the role of bicyclic peroxy radicals in the oxidation mechanism of toluene

The products of the primary OH-initiated oxidation of toluene were investigated using the turbulent flow chemical ionization mass spectrometry technique under different oxygen, NO, and initial OH radical concentrations as well as a range of total pressures. The bicyclic peroxy radical intermediate, a key proposed intermediate species in the Master Chemical Mechanism (MCM) for the atmospheric oxidation of toluene, was detected for the first time. The toluene oxidation mechanism was shown to have a strong oxygen concentration dependence, presumably due to the central role of the bicyclic peroxy radical in determining the stable product distribution at atmospheric oxygen concentrations. The results also suggest a potential role for bicyclic peroxy radical + HO(2) reactions at high HO(2)/NO ratios. These reactions are postulated to be a source of the inconsistencies between environmental chamber results and predictions from the MCM.

Dicarbonyl products of the OH radical-initiated reaction of a series of aromatic hydrocarbons

Aromatic hydrocarbons are important constituents of vehicle exhaust and of nonmethane organic compounds in ambient urban air. We used a derivatization technique with methane positive chemical ionization gas chromatography/mass spectrometry to investigate the carbonyl products formed from the OH radical-initiated reactions of toluene, the xylenes, and the trimethylbenzenes. Characteristic diderivatized molecular ions of dicarbonyl products were obtained. Consistent with previous studies, the 1,2-dicarbonyls glyoxal, methylglyoxal, and biacetyl were observed, as were all but one of the possible unsaturated 1,4-dicarbonyl coproducts. Unsaturated 1,4-diketones had formation yields similar to their potential coproduct 1,2-dicarbonyls. However, apart from HC(O)CH=CHCHO, unsaturated 1,4-dialdehydes and keto-aldehydes were generally observed in lower yield than their potential 1,2-dicarbonyl coproducts.

Theoretical study of reaction mechanisms of OH radical with toluene 1,2-epoxide/2-methyloxepin

In this study, the reaction mechanism of toluene 1,2-epoxide/2-methyloxepin with OH radical was studied by means of quantum chemical computations performed using B3LYP/6-31G(d,p), B3LYP/6-311G(2df,2p), and BHandHLYP/6-31G(d,p) methods. Ground state, intermediate, and transition states were determined. The results indicated that the 2-methyloxepin, A, isomer is more stable, by 2.4 kcal/mol, than toluene 1,2-epoxide, B. Two reaction pathways were studied, RP-A and RP-B, corresponding to the reaction of OH with toluene 1,2-epoxide and 2-methyloxepin, respectively. The localization of a pre-reactive complex for RP-A is crucial for the accurate estimation of the rate constant, k=1.0x10(-10) cm3 molecule(-1) s(-1), which is in good agreement with that determined experimentally, whereas for RP-B the rate constant is 1.3x10(-14) cm3 molecule(-1) s(-1). Under atmospheric conditions, both pathways yield 6-oxohepta-2,4-dienal as a main product, and from the energetic and kinetic results it was found that RP-A is the preferred pathway. The study of the oxide/oxepin mechanism is relevant because, aside from its relatively high concentration in the troposphere, this compound has carcinogenic and mutagenic properties.

Atmospheric oxidation mechanism of p-xylene: a density functional theory study

We report an investigation of the mechanistic features of OH-initiated oxidation reactions of p-xylene using density function theory (DFT). Reaction energies for the formation of the aromatic intermediate radicals have been obtained to determine their relative stability and reversibility, and their activation barriers have been analyzed to assess the energetically favorable pathways to propagate the p-xylene oxidation. OH addition is predicted to occur dominantly at the ortho position, with branching ratios of 0.8 and 0.2 for ortho and ipso additions, respectively, and the calculated overall rate constant is in agreement with available experimental studies. Under atmospheric conditions, the p-xylene peroxy radicals arising from initial OH and subsequent O(2) additions to the ring are shown to cyclize to form bicyclic radicals, rather than to react with NO to lead to ozone formation. With relatively low barriers, isomerization of the p-xylene bicyclic radicals to more stable epoxide radicals likely occurs, competing with O(2) addition to form bicyclic peroxy radicals. The study provides thermochemical and kinetic data for assessment of the photochemical production potential of ozone and formation of toxic products and secondary organic aerosol from p-xylene oxidation.

Photolysis of 4-Oxo-2-pentenal in the 190−460 nm Region

We have studied the gas-phase photolysis of 4-oxo-2-pentenal by laser photolysis combined with cavity ring-down spectroscopy. Absorption cross sections of cis- and trans-4-oxo-2-pentenal have been measured in the 190-460 nm region. The product channel following 193, 248, 308, and 351 nm photolysis of 4-oxo-2-pentenal was investigated. The HCO radical is a photodissociation product of 4-oxo-2-pentenal only at 193 and 248 nm. The HCO quantum yields from the photolysis of a mainly trans-4-oxo-2-pentenal sample are 0.13 +/- 0.02 and 0.014 +/- 0.003 at 193 and 248 nm, where errors quoted (1sigma) represent experimental scatter. The HCO quantum yields from the photolysis of a mainly cis-4-oxo-2-pentenal sample are 0.078 +/- 0.012 and 0.018 +/- 0.007 at 193 and 248 nm, where errors quoted (1sigma) represent experimental scatter. The end-products from 193, 248, 308, and 351 nm photolysis of 4-oxo-2-pentenal (the 4-oxo-2-pentenal sample had a tran/cis ratio of 1.062:1) have been determined by FTIR. Ethane, methyl vinyl ketone, and 5-methyl-3H-furan-2-one have been observed, suggesting the occurrence of 4-oxo-2-pentenal photolysis pathways such as CH(3)COCH=CHCHO + hnu --> CH(3) + COCH=CHCHO, CH(3)COCH=CHCHO + hnu --> CH(3)COCH=CH(2) + CO, and CH(3)COCH=CHCHO + hnu --> 5-methyl-3H-furan-2-one. The estimated yields for the CH(3) + COCH=CHCHO channel are about 25%, 33%, 31%, and 23% at 193, 248, 308, and 351 nm, respectively. The absolute uncertainties in the determination of CH(3) + COCH=CHCHO yields are within 55% at 193 nm, and 65% at 248, 308, and 351 nm. The estimated yields for the CH(3)COCH=CH(2) + CO channel are about 25%, 23%, 40%, and 33% at 193, 248, 308, and 351 nm, respectively. The absolute uncertainties in the determination of CH(3)COCH=CH(2) yields are within 80% at 193 and 248 nm and 65% at 308 and 351 nm. The estimated yields for the 5-methyl-3H-furan-2-one channel are about 1.2%, 2.1%, 5.3%, and 5.5% at 193, 248, 308, and 351 nm, respectively. The absolute uncertainties in the determination of 5-methyl-3H-furan-2-one yields are about 23%, 86%, 40%, and 46% at 193, 248, 308, and 351 nm. Results from our investigation indicate that photolysis is a dominant removal pathway for 4-oxo-2-pentenal degradation in the atmosphere.