Atmospheric Fate Research Articles

Atmospheric chemistry of ketones: Reaction of OH radicals with 2-methyl-3-pentanone, 3-methyl-2-pentanone and 4-methyl-2-pentanone.

This work reports new kinetic and mechanistic information on the atmospheric chemistry of ketones. Both absolute and relative rate methods were used to determine the rate constants for OH reactions with 2-methyl-3-pentanone (2M3P), 3-methyl-2-pentanone (3M2P) and 4-methyl-2-pentanone (4M2P), three widely used compounds in the industry. This work constitutes the first temperature dependence study of the reactions of OH with 2M3P and 3M2P. The following rate constants values are recommended at 298 K (in 10-12 cm3 molecule-1 s-1): kOH+2M3P = 3.49 ± 0.5; kOH+3M2P = 6.02 ± 0.14 and kOH+4M2P = 11.02 ± 0.42. The following Arrhenius expressions (in units of cm3 molecule-1 s-1) adequately describe the measured rate constants for OH reactions with 2M3P and 3M2P in the temperature range 263-373 K: k2M3P = (2.33 ± 0.06) × 10-12 exp((127.4 ± 18.6)/T) and k3M2P = (1.05 ± 0.14) × 10-12 exp((537 ± 41)/T). Products studies from the reactions of OH with the investigated ketones were conducted in a 7.3 m3 simulation chamber using PTR-ToF-MS, UHPLC-MS and GC-MS. A series of short chain carbonyl compounds including formaldehyde, acetone, acetaldehyde, 2-butanone and 2-methypropanal were observed as products. Combining the yields of carbonyls measured with those estimated from the SAR method, we propose various mechanistic degradation schemes of the investigated ketones initiated by reaction with OH radicals.

Atmospheric Fate and Risk Investigation of Typical Liquid Crystal Monomers

Liquid crystal monomers (LCMs), widely used in liquid crystal displays, have been recognized as a class of potential emerging pollutants in the environment. However, their atmospheric fate and toxi...

Optical Properties of Secondary Organic Aerosol Produced by Nitrate Radical Oxidation of Biogenic Volatile Organic Compounds.

Nighttime oxidation of biogenic volatile organic compounds (BVOCs) by nitrate radicals (NO3·) represents one of the most important interactions between anthropogenic and natural emissions, leading to substantial secondary organic aerosol (SOA) formation. The direct climatic effect of such SOA cannot be quantified because its optical properties and atmospheric fate are poorly understood. In this study, we generated SOA from the NO3· oxidation of a series BVOCs including isoprene, monoterpenes, and sesquiterpenes. The SOA were subjected to comprehensive online and offline chemical composition analysis using high-resolution mass spectrometry and optical properties measurements using a novel broadband (315–650 nm) cavity-enhanced spectrometer, which covers the wavelength range needed to understand the potential contribution of the SOA to direct radiative forcing. The SOA contained a significant fraction of oxygenated organic nitrates (ONs), consisting of monomers and oligomers that are responsible for the detected light absorption in the 315–400 nm range. The SOA created from β-pinene and α-humulene was further photochemically aged in an oxidation flow reactor. The SOA has an atmospheric photochemical bleaching lifetime of >6.2 h, indicating that some of the ONs in the SOA may serve as atmosphere-stable nitrogen oxide sinks or reservoirs and will absorb and scatter incoming solar radiation during the daytime.

Concentration Depth Profile-Based Multilayer Sorption Surface Tension Model for Aqueous Solutions.

Surface tension of chemically complex aqueous droplets is significant to atmospheric aerosol particle dynamics and fate. Isotherm-based predictive surface tension models are available which consider one layer of solute molecules sorbed at the liquid-vapor interface. However, the concentration depth profile (CDP) of solute molecules near the surface is continuous, making the single monolayer assumption inappropriate. Here, this work extends the isotherm framework by dividing the surface region into multiple layers to capture the continuity of the spatial distribution of solute molecules for binary solutions. Partition functions are established based on the displacement of water molecules by solute molecules. The number of displaced water molecules and energy of solute molecules at the surface and in the bulk are key model parameters relating surface tension and solute activity. Number densities of surface molecules from molecular dynamic (MD) simulations available in the literature are applied to determine model parameters. Finally, the model is extended to predict surface tension for mixture solutions, considering both independent and dependent adsorptions of different solute species to the liquid-vapor interface. The proposed model works well for both electrolyte and nonelectrolyte solutions and their mixtures from pure solvent to pure solute.

Assessing the atmospheric fate of pesticides used to control mosquito populations in Houston, TX

During the summer months, urban areas are literal hot spots of mosquito-borne disease transmission and air pollution. Public health authorities release aerosolized pesticides directly into the atmosphere to help control adult mosquito populations and thereby reduce the threat of diseases, such as Zika Virus. The primary adulticides (i.e. pesticides used to control adult mosquito populations) in Houston, TX are permethrin and malathion. These adulticides are typically sprayed at night using ultra-low volume sprayers. Particulate matter (PM) samples including total suspended and fine PM (PM < 2.5 μm in aerodynamic diameter) were collected at four ground-based sites across Houston in 2013 and include daytime, nighttime, and 24 h samples. Malathion is initially sprayed as coarse aerosol (5–25 μm), but is measured in fine aerosol (<2.5 μm) and coarse aerosol in the urban atmosphere. Particle size is relevant both for deposition velocities and for human exposure. Atmospheric permethrin concentrations measured in nighttime samples peak at 60 ng m−3, while malathion nighttime concentrations peak near 40 ng m−3. Malaoxon, an oxidation product of malathion, was also frequently detected at concentrations >10 ng m−3, indicating significant nighttime oxidation. Based on the loss of malathion and the increase in malaoxon, the atmospheric half-life of malathion in Houston was estimated at <12 h, which was significantly shorter than previous half-life estimates (∼days). Importantly, malaoxon is estimated to be 22–33 times more toxic to humans than malathion. Both the aerosol size and the half-life are critical for mosquito control, human exposure, and risk assessment of these routine pesticides.

A WRF-CMAQ modeling of atmospheric PAH cycling and health risks in the heavy petrochemical industrialized Lanzhou valley, Northwest China

Polycyclic aromatic hydrocarbons (PAHs) are omnipresent air pollutants with mutagenic and carcinogenic characteristics. Lanzhou city in Northwest China, where the first photochemical smog incident occurred in China, is located in an area characterized by mountain-valley topography. The city has been known as a heavy and petrochemical industry base and heavily contaminated by PAHs. There are large knowledge gaps in the understanding of atmospheric environmental fate of PAHs in Lanzhou city due to sparse observation data. We performed extensive numerical investigations of PAHs for August (summertime) and December (wintertime) of 2014 in Lanzhou city, using the Community Multiscale Air Quality (CMAQ) model, aiming to fill knowledge gaps in the spatial-temporal distribution and health consequences across the city. Strong seasonality in the atmospheric concentrations of PAHs was found, with high concentrations in winter and low concentrations in summer. The modeling results show high days in which daily air concentrations of BaP exceeding the level Ⅱ of Chinese National Standard (CNS) (2.5 ng/m3) in several August days in industrial suburb and urban area. In December, 77% and 80% of simulated BaP daily air concentrations were well above this national standard in the two areas. The modeled NAP and PHE in the atmosphere were predominantly in gas phase, whereas BaP was mainly in particulate phase. It was also found that the dry deposition dominated total PAHs deposition in Lanzhou. The incremental lifetime cancer risk (ILCR) ranged from 4.0 × 10−6 to 16.0 × 10−6, which were higher than the satisfactory risk level of 1.0 × 10−6, suggesting that local residents in Lanzhou city have a potential carcinogenic risk through exposure to ambient BaP. Especially, children had a higher exposure risk than adults. We suggest that the different actions in different regions of the city should be taken by the local government and those industries with high PAH emission, so as to reduce PAH contamination and risk.

Characterizing the Air Emissions, Transport, and Deposition of Per- and Polyfluoroalkyl Substances from a Fluoropolymer Manufacturing Facility.

Per- and polyfluoroalkyl substances (PFASs) have been released into the environment for decades, yet contributions of air emissions to total human exposure, from inhalation and drinking water contamination via deposition, are poorly constrained. The atmospheric transport and fate of a PFAS mixture from a fluoropolymer manufacturing facility in North Carolina were investigated with the Community Multiscale Air Quality (CMAQ) model applied at high resolution (1 km) and extending ∼150 km from the facility. Twenty-six explicit PFAS compounds, including GenX, were added to CMAQ using current best estimates of air emissions and relevant physicochemical properties. The new model, CMAQ-PFAS, predicts that 5% by mass of total emitted PFAS and 2.5% of total GenX are deposited within ∼150 km of the facility, with the remainder transported out. Modeled air concentrations of total GenX and total PFAS around the facility can reach 24.6 and 8500 ng m-3 but decrease to ∼0.1 and ∼10 ng m-3 at 35 km downwind, respectively. We find that compounds with acid functionality have higher deposition due to enhanced water solubility and pH-driven partitioning to aqueous media. To our knowledge, this is the first modeling study of the fate of a comprehensive, chemically resolved suite of PFAS air emissions from a major manufacturing source.

Superoxide Formation from Aqueous Reactions of Biogenic Secondary Organic Aerosols.

Reactive oxygen species (ROS) play a central role in aqueous-phase processing and health effects of atmospheric aerosols. Although hydroxyl radical (•OH) and hydrogen peroxide (H2O2) are regarded as major oxidants associated with secondary organic aerosols (SOA), the kinetics and reaction mechanisms of superoxide (O2•-) formation are rarely quantified and poorly understood. Here, we demonstrate a dominant formation of O2•- with molar yields of 0.01-0.03% from aqueous reactions of biogenic SOA generated by •OH photooxidation of isoprene, β-pinene, α-terpineol, and d-limonene. The temporal evolution of •OH and O2•- formation is elucidated by kinetic modeling with a cascade of aqueous reactions including the decomposition of organic hydroperoxides, •OH oxidation of primary or secondary alcohols, and unimolecular decomposition of α-hydroxyperoxyl radicals. Relative yields of various types of ROS reflect a relative abundance of organic hydroperoxides and alcohols contained in SOA. These findings and mechanistic understanding have important implications on the atmospheric fate of SOA and particle-phase reactions of highly oxygenated organic molecules as well as oxidative stress upon respiratory deposition.

Autoxidation mechanism for atmospheric oxidation of tertiary amines: Implications for secondary organic aerosol formation

Tertiary amines are one kind of identified amines in the atmosphere. Here, the atmospheric oxidation mechanism and kinetics of tertiary amines were investigated by using computational methods. As proxies of these amines, trimethylamine (TMA) and triethylamine (TEA) have been selected. Results indicate that N-containing peroxy radicals (NRO2⋅), which are key intermediates in ⋅OH initiated oxidation of TMA and TEA, can follow a so-called autoxidation mechanism (a chain reaction of H-shift followed by O2 addition) even on the condition of high NO/HO2⋅ concentration. Such unique mechanism can be ascribed to the ability of N-atom in facilitating the unimolecular H-shift of NRO2⋅ and the absence of H-atoms on N-atom. However, different from TMA reaction system, the pathway dissociating into fragmental products can compete with the autoxidation pathway for TEA system. More importantly, TEA reaction system cannot lead to the formation of products with high O/C ratio due to the autoxidation pathway terminated by the release of fragmental molecules. Such difference can be corroborated by previously observing lower secondary organic aerosol yield of TEA oxidation than that of TMA oxidation. The unveiled mechanism enhances current understanding on atmospheric fate of amines and autoxidation mechanism.

Multipollutant impacts to U.S. receptors of regional on-road freight in Ontario, Canada

ABSTRACT On-road freight is a significant source of air pollutant and greenhouse gas emissions. The resulting economic damages can cross borders through processes of atmospheric fate and transport, regardless of whether that freight serves local or regional demand. Understanding patterns of freight demand and atmospheric processes can thus inform inter-jurisdictional efforts to mitigate multipollutant damages. We quantify how different freight trips across 49 census divisions in the Province of Ontario, Canada create an economic burden on downwind US receptors. We apply an integrated modeling approach combining a travel demand model, a mobile emissions simulator, and marginal damages from emissions. Economic damages include the increased risk of premature death from PM2.5 related to primary PM2.5 (represented by damages from inert primary PM2.5), NOX, SO2, and NH3, and the global effects of climate change from greenhouse gases (CO2, CH4, N2O). Over 90% of the $1.4 billion (2010USD) in transboundary air pollutant damages at US receptors result from regional freight demand across Ontario in 2012. A single major freight corridor, the ON-401 expressway, contributes more than half of all damages. Most of these damages impact the states situated to the south and east of the province. Mean estimates of annual damages range from millions to tens of millions (2010USD) across major eastern metropolitan areas including New York, Boston, Philadelphia, and D.C. Most of these damages result from NOX, which constitutes 95% of inorganic PM2.5-related pollutant emissions by mass. Thus, targeting NOx from freight movements along the ON-401 expressway could avoid millions to tens of millions of damages annually in eastern US cities. These results indicate that local green freight policies may be unable to address the environmental burden at cross-border receptors. Cooperation is needed among local, provincial, and federal governments to encourage policies targeting the most harmful emissions along routes servicing regional freight demands. Implications: On-road freight movement in Ontario can yield billions of dollars in annual economic damages to US residents through its effects on air pollution and climate change. We use an integrated modeling approach combining an on-road freight travel demand, mobile emissions, and marginal damages of emissions to quantify and study these economic damages. Regional freight contributes approximately 90% of damages, with one major freight corridor, the ON-401 expressway, contributing 59%. Most damages derive from emissions of NOx and amount to millions to tens of millions of dollars in annual damages across major Eastern US cities. Thus, targeting NOx from freight movements along the ON-401 expressway could avoid millions of damages annually in eastern US cities.

Diol it up: The influence of NaCl on methylglyoxal surface adsorption and hydration state at the air-water interface.

Methylglyoxal (MG)-an atmospherically important α-dicarbonyl implicated in aqueous-phase secondary organic aerosol formation-is known to be surface-active. Due to the presence of carbonyl moieties, MG can hydrate to form geminal diols in solution. Recently, it has been shown that MG exists predominantly as a monohydrate at the neat air-water interface. However, inorganic aerosol constituents have the potential to "salt-out" MG to the interface, shift its hydration equilibria, and catalyze self- and cross-oligomerization reactions. Here, we study the influence of the non-reactive salt, sodium chloride (NaCl), on the MG's surface adsorption and hydration state using vibrational sum frequency spectroscopy. The presence of NaCl is found to enhance MG's surface activity but not to the extent that water is fully excluded from the interface. Perturbations in the interfacial water structure are attributed to shifts in MG's hydration equilibrium at higher ionic strengths. Evidence of surface-active MG oligomer species is presented, but such oligomers are not thought to contribute significantly to the interfacial population. This work builds on the published studies on MG in pure water and gives insight into the interface's perturbation by NaCl, which has important implications for understanding MG's atmospheric fate.

Atmospheric degradation of chrysene initiated by OH radical: A quantum chemical investigation

Chrysene, a four-ring polycyclic aromatic hydrocarbon (PAH), is recalcitrant to biodegradation and persistent in the environment due to its low water solubility. Here, we investigated the atmospheric degradation process of chrysene initiated by OH radical in the presence of O2 and NOX using quantum chemical calculations. The reaction mechanisms were elucidated by density functional theory (DFT) at M06–2X/6–311++G(3df,2p)//M06–2X/6-311+G(d,p) level, and the kinetics calculations were conducted with Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The results show that the oxidation products of atmospheric chrysene are oxygenated PAHs (OPAHs) and nitro-PAHs (NPAHs), including nitro-chrysene, hydroxychrysene, hydroxychrysenone, 11-benzo[a]fluorenone and dialdehydes. Most of the products have deleterious effects on the environment and human beings due to their acute toxicity, carcinogenicity and mutagenicity. The overall rate constant for the reaction of chrysene with OH radical is 4.48 × 10−11 cm3 molecule−1 s−1 and the atmospheric lifetime of chrysene determined by OH radical is 6.4 h. The present work provided a comprehensive understanding on the degradation mechanisms and kinetics of chrysene, which could help to clarify its atmospheric fate and environmental risks.

Atmospheric chemistry of CHF2CF2OCH2CF3: Reactions with Cl atoms, fate of CHF2CF2OC•HCF3 radical, formation of OH radical and Criegee Intermediate

Abstract The kinetics and mechanism for hydrogen abstraction reactions of CHF2CF2OCH2CF3 with Cl atoms were studied using the DFT-based M06-2X/6-311++G(d,p) quantum chemical method. The thermal rate coefficients for H-abstraction reactions were evaluated by canonical variational transition state theory along with the small curvature tunneling correction over the temperature range of 200–1000 K. A three-parameter model equation has been proposed to describe the temperature variation of rate coefficient as k(CHF2CF2OCH2CF3+Cl) = 5.24 × 10−22T3.2exp(-580/T)(in cm3 molecule−1s−1). Atmospheric lifetime of CHF2CF2OCH2CF3 is estimated to be 3.10 years and global warming potentials at the 100-year time horizon is calculated as 601. The atmospheric fate of generated CHF2CF2OC•HCF3 radical are reported. The rate coefficient values for the reaction of important degradation products like CHF2CF2OCHO and CHF2CF2OOH with OH/Cl are also reported for the first time. Production of OH radicals from the unimolecular decomposition of CHF2CF2OC(OO•)HCF3 peroxy radical and the possible formation of Fluorinated Criegee Intermediate from self-reaction of RO2 radicals are discussed.

Atmospheric Fate of Methyl Isocyanate, CH3NCO: OH and Cl Reaction Kinetics and Identification of Formyl Isocyanate, HC(O)NCO

CH3NCO, methyl isocyanate, is a toxic compound that is formed in the atmosphere following the use of agricultural and industrial precursors as well as emission from biomass burning. The atmospheric abundance and chemistry of CH3NCO, however, is presently not sufficiently well characterized to evaluate its overall environmental and health impacts. In this study, rate coefficients for the gas-phase reaction of OH radicals (k1) and Cl-atoms (k2) with CH3NCO were measured using pulsed laser photolysis–laser-induced fluorescence and relative rate methods at 295–375 and 296 K, respectively. We obtained k1(295 K) = 1.36 × 10–13 cm3 molecule–1 s–1 with the temperature dependence given by the Arrhenius expression k1(295–375 K) = 6.11 × 10–12 exp(−(1119 ± 62)/T) cm3 molecule–1 s–1. The Cl atom rate coefficient was determined to be k2(296 K) = 5.58 × 10–12 cm3 molecule–1 s–1. k1(295 K) from this study is compared with a previous room temperature measurement and an explanation for the significantly greater literature value, a factor of 26.6, is discussed. As part of this work, the stable end products following the OH radical- and Cl atom-initiated oxidation of CH3NCO were obtained in an environmental chamber reactor with infrared absorption detection. Formyl isocyanate, HC(O)NCO, was identified by comparison with its theoretically calculated infrared absorption spectrum as a major stable end product. A CH3NCO atmospheric degradation mechanism is proposed. In addition, quantitative UV and infrared absorption spectra of CH3NCO are reported.

Atmospheric Chemistry of Methyl Isocyanide-An Experimental and Theoretical Study.

The reaction of CH3NC with OH radicals was studied in smog chamber experiments employing PTR-ToF-MS and long-path FTIR detection. The rate coefficient was determined to be kCH3NC+OH = (7.9 ± 0.6) × 10–11 cm3 molecule–1 s–1 at 298 ± 3 K and 1013 ± 10 hPa; methyl isocyanate was the sole observed product of the reaction. The experimental results are supported by CCSD(T*)-F12a/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ quantum chemistry calculations showing the reaction to proceed primarily via electrophilic addition to the isocyanide carbon atom. On the basis of the quantum chemical data, the kinetics of the OH reaction was simulated using a master equation model revealing the rate coefficient to be nearly independent of pressure at tropospheric conditions and having a negative temperature dependence with kOH = 4.2 × 10–11 cm3 molecule–1 s–1 at 298 K. Additional quantum chemistry calculations on the CH3NC reactions with O3 and NO3 show that these reactions are of little importance under atmospheric conditions. The atmospheric fate of methyl isocyanide is discussed.

Legacy and emerging flame retardants (FRs) in the urban atmosphere of Pakistan: Diurnal variations, gas-particle partitioning and human health exposure

Atmospheric concentration of legacy (LFRs) and emerging flame retardants (EFRs) including 8 polybrominated diphenyl ethers (PBDEs), 6 novel brominated flame retardants (NBFRs), 2 dechlorane plus isomers (DP), and 8 chlorinated organophosphate flame retardants (OPFRs) were consecutively measured in eight major cities across Pakistan. A total of 96 samples (48 PM2.5 & 48 PUFs) were analyzed and the concentrations of ∑8PBDEs (gaseous+particulate) ranged between 40.8 and 288 pg/m3 with an average value of 172 pg/m3. ∑6NBFRs ranged between 12.0 and 35.0 pg/m3 with an average value of 22.5 pg/m3 while ∑8OPFRs ranged between 12,900–40,800 pg/m3 with an average of 24,700 pg/m3. Among the studied sites, Faisalabad city exhibited the higher concentrations of FRs among all cities which might be a consequence of textile mills and garment manufacturing industries. While analyzing the diurnal patterns, OPFRs depicted higher concentrations during night-time. The estimated risks of all groups of FRs from inhalation of ambient air were negligible for all the cities, according to USEPA guidelines. Nonetheless, our study is the first to report gaseous and particulate concentrations of FRs in air on a diurnal basis across major cities in Pakistan, offering insights into the atmospheric fate of these substances in urban areas in a sub-tropical region.

Understanding the Interfacial Behavior of Typical Perfluorocarboxylic Acids at Surfactant‐Coated Aqueous Interfaces

AbstractSea surface microlayers (SMLs) play an important role in the transport and fate of surfactants from both anthropogenic and biogenic sources. The formation of sea spray aerosols (SSAs) at the SML has been recognized as a critical pathway for the entry of surfactants into the atmosphere. Perfluorocarboxylic acids (PFCAs) have become increasingly concerning as a class of emerging organic pollutants, having high enrichment in surface seawaters due to their remarkable surface activity. Using artificial seawater coated by extensively existed surfactants, including stearyl alcohol (C18OH) and stearic acid (SA) as simplified models of the SML and fresh SSAs, the interfacial behaviors of soluble and insoluble PFCAs were examined. Information about lateral packing and chain conformation of films was obtained by means of Langmuir trough and infrared reflection‐absorption spectroscopy (IRRAS). Perfluorooctanoic acid (PFOA) dissolved in aqueous subphases was found to be incorporated into lipid monolayers via alterations in surface pressure‐area (π‐A) isotherms. With negligible solubility, perfluorotetradecanoic acid (PFTA) formed condensed mixed monolayers with other lipids at the air‐water interface. The mixed monolayers of C18OH/PFTA and SA/PFTA were expanded by the addition of sea salts into the pure water subphase. Compared with the π‐A isotherms of SA/PFTA, decreased molecular areas were observed for those of C18OH/PFTA. This research suggests that PFCAs can be incorporated into air‐water interfaces by acting as film‐forming materials. The introduction of PFCAs will alter the surface properties and thus the atmospheric fate and behavior of SSAs and SMLs.

Volatility Dependence of the Aerosol Size Distributions of Nonpolar Organic Compounds: A Case Study in Shanghai

AbstractThe aerosol size distribution of organic compounds is one of the fundamental characteristics that regulates their atmospheric deposition and environmental fate. In this study, we characterized the organic components (mostly nonpolar organic compounds) in size‐resolved particles, with a focus on polycyclic aromatic hydrocarbons (PAHs), oxygenated PAHs (O‐PAHs), n‐alkanes, and polybrominated diphenyl ethers (PBDEs), in the urban Shanghai atmosphere during a 1‐year sampling period using gas chromatography (GC) coupled to tandem mass spectrometry (GC‐MS/MS) and two‐dimensional GC with a flame ionization detector (GC × GC‐FID). These compounds ranged from volatile organic compounds (VOCs) to extremely low volatility organic compounds (ELVOCs). We related their aerosol size distributions with their volatilities. On average, high positive correlations (p &lt; 0.01) were observed between the size distribution characteristics (e.g., geometric mean diameter [GMD] and mass fraction in coarse particles) and volatility (logarithmic value of the saturation mass concentration, log C*) for the compounds that we examined. A higher ambient temperature tends to increase the slope of the regression line between size distribution and volatility (log C*). Further dry deposition modeling of size‐resolved particles showed a positive correlation (p &lt; 0.01) between the bulk dry deposition velocity and GMD and between the bulk dry deposition velocity and volatility of the target compounds, indicating that low‐volatility organic compounds have a lower dry deposition velocity and are more likely to be transported to remote areas if they do not have a short half‐life in the atmosphere.

Database for the kinetics of the gas-phase atmospheric reactions of organic compounds

Abstract. We present a digital, freely available, searchable, and evaluated compilation of rate coefficients for the gas-phase reactions of organic compounds with OH, Cl, and NO3 radicals and with O3. Although other compilations of many of these data exist, many are out of date, most have limited scope, and all are difficult to search and to load completely into a digitized form. This compilation uses results of previous reviews, though many recommendations are updated to incorporate new or omitted data or address errors, and includes recommendations on many reactions that have not been reviewed previously. The database, which incorporates over 50 years of measurements, consists of a total of 2765 recommended bimolecular rate coefficients for the reactions of 1357 organic substances with OH, 709 with Cl, 310 with O3, and 389 with NO3, and is much larger than previous compilations. Many compound types are present in this database, including naturally occurring chemicals formed in or emitted to the atmosphere and anthropogenic compounds such as halocarbons and their degradation products. Recommendations are made for rate coefficients at 298 K and, where possible, the temperature dependences over the entire range of the available data. The primary motivation behind this project has been to provide a large and thoroughly evaluated training dataset for the development of structure–activity relationships (SARs), whose reliability depends fundamentally upon the availability of high-quality experimental data. However, there are other potential applications of this work, such as research related to atmospheric lifetimes and fates of organic compounds, or modelling gas-phase reactions of organics in various environments. This database is freely accessible at https://doi.org/10.25326/36 (McGillen et al., 2019).

Aqueous-Phase Decomposition of Isoprene Hydroxy Hydroperoxide and Hydroxyl Radical Formation by Fenton-like Reactions with Iron Ions.

Isoprene hydroxy hydroperoxides (ISOPOOH) formed by the photooxidation of isoprene under low-NO conditions play an important role in the formation and evolution of secondary organic aerosols, yet multiphase processes of ISOPOOH are poorly understood. By applying electron paramagnetic resonance spectroscopy, we observe that ISOPOOH undergoes aqueous-phase decomposition upon interacting with Fe(II) ions to form OH and organic radicals at room temperature. To reproduce the measured dependence of OH formation on the Fe concentrations by kinetic modeling, we postulate that Fe(II) ions react with ISOPOOH via Fenton-like reactions to form OH radicals with a rate constant of 7.3 × 10-18 cm3 s-1. At low concentrations, oxalate forms monocomplexes with Fe(II) ions, which can promote OH formation by ISOPOOH. However, at high concentrations, oxalate scavenges OH radicals, thereby lowering aqueous OH concentrations. These findings provide new insight for the atmospheric fate of ISOPOOH and reactive oxygen species generation in the aqueous phase.