Oligomerization Mechanism of Methylglyoxal Regulated by the Methyl Groups in Reduced Nitrogen Species: Implications for Brown Carbon Formation.

Uncertain chemical mechanisms leading to brown carbon (BrC) formation affect the drivers of the radiative effects of aerosols in current climate predictions. Herein, the aqueous-phase reactions of methylglyoxal (MG) and typical reduced nitrogen species (RNSs) are systematically investigated by using combined quantum chemical calculations and laboratory experiments. Imines and diimines are identified from the mixtures of methylamine (MA) and ammonia (AM) with MG, but not from dimethylamine (DA) with the MG mixture under acidic conditions, because deprotonation of DA cationic intermediates is hindered by the amino groups occupied by two methyl groups. It leads to N-heterocycle (NHC) formation in the MG + MA (MGM) and MG + AM (MGA) reaction systems but to N-containing chain oligomer formation in the MG + DA (MGD) reaction system. Distinct product formation is attributed to electrostatic attraction and steric hindrance, which are regulated by the methyl groups of RNSs. The light absorption and adverse effects of NHCs are also strongly related to the methyl groups of RNSs. Our finding reveals that BrC formation is mainly contributed from MG reaction with RNSs with less methyl groups, which have more abundant and broad sources in the urban environments.

Mechanisms of isomerization and hydration reactions of typical ß-diketone at the air-droplet interface.

Acetylacetone (AcAc) is a typical class of ß-diketones with broad industrial applications due to the property of the keto-enol isomers, but its isomerization and chemical reactions at the air-droplet interface are still unclear. Hence, using combined molecular dynamics and quantum chemistry methods, the heterogeneous chemistry of AcAc at the air-droplet interface was investigated, including the attraction of AcAc isomers by the droplets, the distribution of isomers at the air-droplet interface, and the hydration reactions of isomers at the air-droplet interface. The results reveal that the preferential orientation of two AcAc isomers (keto- and enol-AcAc) to accumulate and accommodate at the acidic air-droplet interface. The isomerization of two AcAc isomers at the acidic air-droplet interface is more favorable than that at the neutral air-droplet interface because the "water bridge" structure is destroyed by H3O+, especially for the isomerization from keto-AcAc to enol-AcAc. At the acidic air-droplet interface, the carbonyl or hydroxyl O-atoms of two AcAc isomers display an energetical preference to hydration. Keto-diol is the dominant products to accumulate at the air-droplet interface, and excessive keto-diol can enter the droplet interior to engage in the oligomerization. The photooxidation reaction of AcAc will increase the acidity of the air-droplet interface, which indirectly facilitate the uptake and formation of more keto-diol. Our results provide an insight into the heterogeneous chemistry of ß-diketones and their influence on the environment.

Distinctive Heterogeneous Reaction Mechanism of ClNO2 on the Air-Water Surface Containing Cl.

The heterogeneous reaction of nitryl chloride (ClNO2) on the air-water surface plays a significant role in the chloride lifecycle. The air-water surface is ubiquitous on ice surfaces under supercooled conditions, affecting the uptake and heterogeneous reaction processes of trace gases. Previous studies suggest that ClNO2 is formed on Cl-doped ice surfaces following the N2O5 uptake. Herein, a distinctive heterogeneous reaction mechanism of ClNO2 is suggested on an air-water surface containing Cl under supercooled conditions using combined classic molecular dynamics (MD) and Born-Oppenheimer MD simulations. It is found that N2O5 dissociates into a NO2+ and NO3- ionic pair on the top air-water surface. In the top layer of the surface containing barely any Cl-, NO2+ proceeds through hydrolysis and produces H3O+ and HNO3. Thus, surface acidification appears because of H3O+ yields. With NO2+ diffusion to the deep layer of the surface, NO2+ reacts with Cl- and forms ClNO2. Note that ClNO2 formation competes with NO2+ hydrolysis, and the rate of ClNO2 formation is 27.7[Cl-] larger than that of NO2+ hydrolysis. Afterward, the reaction of ClNO2 with Cl- becomes barrierless with the catalysis by H3O+, which is not feasible on a neutral surface. Cl2 is thus generated and escapes into the atmosphere (low solubility of Cl2), contributing to the Cl radical. The proposed mechanism bolsters the current understanding of ClNO2's fate and its role in Cl chemistry in extremely cold environments like the Arctic and other high-latitude regions in wintertime.

Carbenium ion-mediated oligomerization of methylglyoxal for secondary organic aerosol formation.

Secondary organic aerosol (SOA) represents a major constituent of tropospheric fine particulate matter, with profound implications for human health and climate. However, the chemical mechanisms leading to SOA formation remain uncertain, and atmospheric models consistently underpredict the global SOA budget. Small α-dicarbonyls, such as methylglyoxal, are ubiquitous in the atmosphere because of their significant production from photooxidation of aromatic hydrocarbons from traffic and industrial sources as well as from biogenic isoprene. Current experimental and theoretical results on the roles of methylglyoxal in SOA formation are conflicting. Using quantum chemical calculations, we show cationic oligomerization of methylglyoxal in aqueous media. Initial protonation and hydration of methylglyoxal lead to formation of diols/tetrol, and subsequent protonation and dehydration of diols/tetrol yield carbenium ions, which represent the key intermediates for formation and propagation of oligomerization. On the other hand, our results reveal that the previously proposed oligomerization via hydration for methylglyoxal is kinetically and thermodynamically implausible. The carbenium ion-mediated mechanism occurs barrierlessly on weakly acidic aerosols and cloud/fog droplets and likely provides a key pathway for SOA formation from biogenic and anthropogenic emissions.

An unexpected catalyst dominates formation and radiative forcing of regional haze.

Although regional haze adversely affects human health and possibly counteracts global warming from increasing levels of greenhouse gases, the formation and radiative forcing of regional haze on climate remain uncertain. By combining field measurements, laboratory experiments, and model simulations, we show a remarkable role of black carbon (BC) particles in driving the formation and trend of regional haze. Our analysis of long-term measurements in China indicates declined frequency of heavy haze events along with significantly reduced SO2, but negligibly alleviated haze severity. Also, no improving trend exists for moderate haze events. Our complementary laboratory experiments demonstrate that SO2 oxidation is efficiently catalyzed on BC particles in the presence of NO2 and NH3, even at low SO2 and intermediate relative humidity levels. Inclusion of the BC reaction accounts for about 90-100% and 30-50% of the sulfate production during moderate and heavy haze events, respectively. Calculations using a radiative transfer model and accounting for the sulfate formation on BC yield an invariant radiative forcing of nearly zero W m-2 on the top of the atmosphere throughout haze development, indicating small net climatic cooling/warming but large surface cooling, atmospheric heating, and air stagnation. This BC catalytic chemistry facilitates haze development and explains the observed trends of regional haze in China. Our results imply that reduction of SO2 alone is insufficient in mitigating haze occurrence and highlight the necessity of accurate representation of the BC chemical and radiative properties in predicting the formation and assessing the impacts of regional haze.

Remarkable nucleation and growth of ultrafine particles from vehicular exhaust.

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.

Competing esterification and oligomerization reactions of typical long-chain alcohols to secondary organic aerosol formation.

Organosulfate (OSA) nanoparticles, as secondary organic aerosol (SOA) compositions, are ubiquitous in urban and rural environments. Hence, we systemically investigated the mechanisms and kinetics of aqueous-phase reactions of 1-butanol/1-decanol (BOL/DOL) and their roles in the formation of OSA nanoparticles by using quantum chemical and kinetic calculations. The mechanism results show that the aqueous-phase reactions of BOL/DOL start from initial protonation at alcoholic OH-groups to form carbenium ions (CBs), which engage in the subsequent esterification or oligomerization reactions to form OSAs/organosulfites (OSIs) or dimers. The kinetic results reveal that dehydration to form CBs for BOL and DOL reaction systems is the rate-limiting step. Subsequently, about 18% of CBs occur via oligomerization to dimers, which are difficult to further oligomerize because all reactive sites are occupied. The rate constant of BOL reaction system is one order of magnitude larger than that of DOL reaction system, implying that relative short-chain alcohols are more prone to contribute OSAs/OSIs than long-chain alcohols. Our results reveal that typical long-chain alcohols contribute SOA formation via esterification rather than oligomerization because OSA/OSI produced by esterification engages in nanoparticle growth through enhancing hygroscopicity.

Formation Mechanism of Iodinated Aromatic Disinfection Byproducts: Acid Catalysis with H2OI.

Iodinated aromatic disinfection byproducts (I-DBPs) are a group of nonregulated but highly toxic DBPs. The formation of I-DBPs is attributed mainly to HOI because it is the most abundant reactive iodine species in chloraminated water. In this study, we used computational modeling of thermodynamics to examine the mechanism of iodination of aromatic contaminants, e.g., dipeptides and phenols. Computational prediction of the energy barriers of the formation of iodinated tyrosylglycine (I-Tyr-Gly) (66.9 kcal mol-1) and hydroxylated Tyr-Gly (OH-Tyr-Gly) (46.0 kcal mol-1) via iodination with HOI favors the formation of OH-Tyr-Gly over I-Tyr-Gly. Unexpectedly, mass spectrometry experiments detected I-Tyr-Gly but not OH-Tyr-Gly, suggesting that I-Tyr-Gly formation cannot be attributed to HOI alone. To clarify this result, we examined the thermodynamic role of the most reactive iodine species H2OI+ in the formation of aromatic I-DBPs under chloramination. Computational modeling of thermodynamic results shows that the formation of a loosely bonded complex of aromatic compounds with H2OI+ is the key step to initiate the iodination process. When H2OI+ serves as an acid catalyst and an iodinating agent, with HOI or H2O acting as a proton acceptor, the energy barrier of I-DBP formation was significantly lower (10.8-13.1 kcal mol-1). Therefore, even with its low concentration, H2OI+ can be involved in the formation of I-DBPs. These results provide insight into the mechanisms of aromatic I-DBP formation and important information for guiding research toward controlling I-DBPs in drinking water.

Severe haze in northern China: A synergy of anthropogenic emissions and atmospheric processes.

Regional severe haze represents an enormous environmental problem in China, influencing air quality, human health, ecosystem, weather, and climate. These extremes are characterized by exceedingly high concentrations of fine particulate matter (smaller than 2.5 µm, or PM2.5) and occur with extensive temporal (on a daily, weekly, to monthly timescale) and spatial (over a million square kilometers) coverage. Although significant advances have been made in field measurements, model simulations, and laboratory experiments for fine PM over recent years, the causes for severe haze formation have not yet to be systematically/comprehensively evaluated. This review provides a synthetic synopsis of recent advances in understanding the fundamental mechanisms of severe haze formation in northern China, focusing on emission sources, chemical formation and transformation, and meteorological and climatic conditions. In particular, we highlight the synergetic effects from the interactions between anthropogenic emissions and atmospheric processes. Current challenges and future research directions to improve the understanding of severe haze pollution as well as plausible regulatory implications on a scientific basis are also discussed.

Competing pathways of cresol formation in toluene photooxidation: OH-toluene adducts react with NO2 or with O2?

Methyl-hydroxy-cyclohexadienyl radicals (OTAs) are the key products of the photooxidation of toluene, with implications for the fate of toluene. Hence, we investigated the photooxidation mechanisms and kinetics of three main OTAs (o-OTA, m-OTA, and p-OTA) with NO2 using quantum chemical calculations as well as the fate of OTAs under the different concentration ratios of NO2 and O2. The mechanism results show that the pathway of H-abstraction by NO2 to anti-HONO (anti-H-abstraction) is more favorable than the syn-H-abstraction pathway, because the strong interaction between OTAs and NO2 is formed in the transition states of the anti-H-abstraction pathways. The branching ratios of the anti-H-abstraction pathways are more than 99% in the temperature range of 216-298 K. The total rate constant of the OTA-NO2 reaction is 9.9 × 10-12 cm3/(molecule∙sec) at 298 K, which is contributed about 90% by o-OTA + NO2, and the main products are o-cresol and anti-HONO. The half-lives of the OTA-NO2 reaction in some polluted areas of China are 35 times longer than those of the OTA-O2 reaction. In the atmosphere, the NO2- and O2- initiated reactions of OTAs have the same ability to form cresols as [NO2] is up to 142.1 ppmV, which is impossible to achieve. It implies that under the experimental condition, the [NO2]/[O2] should be controlled to be less than 7.8 × 10-5 to simulate real atmospheric oxidation of toluene. Our results reveal that for the photooxidation of toluene, the yield of cresol is not affected by the concentration of NO2 under the atmospheric environment.

Remediation of preservative ethylparaben in water using natural sphalerite: Kinetics and mechanisms.

As a typical class of emerging organic contaminants (EOCs), the environmental transformation and abatement of preservative parabens have raised certain environmental concerns. However, the remediation of parabens-contaminated water using natural matrixes (such as, naturally abundant minerals) is not reported extensively in literature. In this study, the transformation kinetics and the mechanism of ethylparaben using natural sphalerite (NS) were investigated. The results show that around 63% of ethylparaben could be absorbed onto NS within 38 hr, whereas the maximum adsorption capacity was 0.45 mg/g under room temperature. High temperature could improve the adsorption performance of ethylparaben using NS. In particular, for the temperature of 313 K, the adsorption turned spontaneous. The well-fitted adsorption kinetics indicated that both the surface adsorption and intra-particle diffusion contribute to the overall adsorption process. The monolayer adsorption on the surface of NS was primarily responsible for the elimination of ethylparaben. The adsorption mechanism showed that hydrophobic partitioning into organic matter could largely govern the adsorption process, rather than the ZnS that was the main component of NS. Furthermore, the ethylparaben adsorbed on the surface of NS was stable, as only less than 2% was desorbed and photochemically degraded under irradiation of simulated sunlight for 5 days. This study revealed that NS might serve as a potential natural remediation agent for some hydrophobic EOCs including parabens, and emphasized the significant role of naturally abundant minerals on the remediation of EOCs-contaminated water bodies.

Mechanism for Rapid Conversion of Amines to Ammonium Salts at the Air-Particle Interface.

The effect of sulfuric acid (SA) concentrations on heterogeneous reactions of amines such as methylamine (MA), dimethylamine (DMA), and trimethylamine (TMA) at the air-particle interface is investigated using combined classical molecular dynamics, Born-Oppenheimer molecular dynamics, and quantum chemical calculations. The results show that the mixtures of these amine vapors can accumulate at the air-particle interface and then participate in two types of heterogeneous reactions depending on the SA concentrations in the aqueous particles. At high SA concentrations, amines are neutralized by H3O+ and form ammonium salts within only a few picoseconds. At low SA concentrations, amines mainly proceed by hydrolysis reactions and produce ionic pairs of ammonium and OH-. However, the formed ionic pair is extremely unstable, and the reverse reaction takes place. Considering that the salt conversion time scales of amines at high SA concentrations are 2.5-15 times faster than those at low SA concentration, amine accumulation at high acidity particles is more favored.

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.

Superoxide radical enhanced photocatalytic performance of styrene alters its degradation mechanism and intermediate health risk on TiO2/graphene surface.

Enhancement of reactive oxygen species (ROS) on semiconductor coupled by carbon material promotes photocatalytic performance toward aromatic hydrocarbons, while the contribution to their degradation mechanism and health risk is not well understood. Herein, photocatalytic degradation of styrene on TiO2 and TiO2/reduced graphene oxide (TiO2/rGO) surface is compared under dry air condition to investigate the role of ·O2- in styrene degradation. TiO2/rGO shows 4.8 times higher degradation efficiency than that of TiO2, resulting in 16% reduced production of intermediates with identical composition. The improved formation of ·O2- on TiO2/rGO is confirmed responsible for these variations. Theoretical calculation further reveals the enhancement of ·O2- thermodynamically favoring conversion of styrene to acetophenone, turning the most dominant intermediate from benzoic acid on TiO2 to acetophenone on TiO2/rGO. The accumulated formation of acetophenone on TiO2/rGO poses increased acute threat to human beings. Our findings proclaim that ROS promoted photocatalytic performance of semiconductor after carbon material composition ultimately changes the priority order of degradation pathways to form by-product with higher threat toward human beings. And more attentions are advised focusing on the relevance with degradation efficiency, intermediate and toxicity of aromatic hydrocarbons on carbon material based photocatalyst.

Assessing the role of mineral particles in the atmospheric photooxidation of typical carbonyl compound.

Mineral particles are ubiquitous in the atmosphere and exhibit an important effect on the photooxidation of volatile organic compounds (VOCs). However, the role of mineral particles in the photochemical oxidation mechanism of VOCs remains unclear. Hence, the photooxidation reactions of acrolein (ARL) with OH radical (OH) in the presence and absence of SiO2 were investigated by theoretical approach. The gas-phase reaction without SiO2 has two distinct pathways (H-abstraction and OH-addition pathways), and carbonyl-H-abstraction is the dominant pathway. In the presence of SiO2, the reaction mechanism is changed, i.e., the dominant pathway from carbonyl-H-abstraction to OH-addition to carbonyl C-atom. The energy barrier of OH-addition to carbonyl C-atom deceases 21.33 kcal/mol when SiO2 is added. Carbonyl H-atom of ARL is occupied by SiO2 via hydrogen bond, and carbonyl C-atom is activated by SiO2. Hence, the main product changes from H-abstraction product to OH-adduct in the presence of SiO2. The OH-adduct exhibits a thermodynamic feasibility to yield HO2 radical and carboxylic acid via the subsequent reactions with O2, with implications for O3 formation and surface acidity of mineral particles.

Enhanced H-abstraction contribution for oxidation of xylenes via mineral particles: Implications for particulate matter formation and human health.

Xylenes are important aromatic hydrocarbons having broad industrial emissions and profound implication to air quality and human health. Generally, homogeneous atmospheric oxidation of xylenes is initiated by hydroxyl radical (OH) resulting in minor H-abstraction and major OH-addition pathways. However, the effect of mineral particles on the homogeneous atmospheric oxidation mechanism of xylenes is still not well understood. In the present study, the heterogeneous atmospheric oxidation of xylenes on mineral particles (TiO2) is examined in detail. Both the experimental data and theoretical calculations are combined to achieve the feast. The experimental results detected a major H-abstraction (≥87.18%) and minor OH-addition (≤12.82%) pathways for the OH-initiated heterogeneous oxidation of three xylenes on TiO2 under ultraviolet (UV) irradiation. Theoretical calculations demonstrated favorable H-abstraction on methyl group of xylenes by surface OH with large exothermic energies, because of the reason that their methyl group rather than the phenyl ring is more occupied by TiO2 via hydrogen bonding. Furthermore, the particle monitor and acute risk assessment results indicated that the H-abstraction products significantly enhance the formation of particulate matter and health risk to human beings. Taken together, these results indicate that the atmospheric oxidation mechanism of xylenes is altered in the presence of mineral particles, highlighting the necessity to re-evaluate its implication in the environment and human health.

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.

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.

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 Acetylacetone: Implications for Ozone and Secondary Organic Aerosol Formation.

Acetylacetone (AcAc) is a common atmospheric oxygenated volatile organic compound due to broad industrial applications, but its atmospheric oxidation mechanism is not fully understood. We investigate the mechanism, kinetics, and atmospheric fate of the OH-initiated oxidation for the enolic and ketonic isomers of AcAc using quantum chemical and kinetic rate calculations. OH addition to enol-AcAc is more favorable than addition to keto-AcAc, with the total rate constant of 1.69 × 10-13 exp(1935/T) cm3 molecule-1 s-1 over the temperature range of 200-310 K. For the reaction of the enol-AcAc with OH, the activation energies of H-abstraction are at least 4 kcal mol-1 higher than those of OH-addition, and the rate constants for OH-addition are by 2-3 orders of magnitude higher than those for H-abstraction. Oxidation of AcAc is predicted to yield significant amounts of acetic acid and methylglyoxal, larger than those are currently recognized. A lifetime of less than a few hours for AcAc is estimated throughout the tropospheric conditions. In addition, we present field measurements in Beijing and Nanjing, China, showing significant concentrations of AcAc in the two urban locations. Our results reveal that the OH-initiated oxidation of AcAc contributes importantly to ozone and SOA formation under polluted environments.