OH-initiated Oxidation Research Articles

Formation of acetonitrile (CH3CN) under cold interstellar, tropospheric and combustion mediums

The knowledge of the formation of gas-phase carcinogenic acetonitrile (CH3CN) is limited in interstellar, troposphere, and combustion mediums; thus, its formation and fate are of great importance for all these gas-phase environments when accessing its toxicity and its wide range of applications. In this work, we propose a mechanism for the formation of CH3CN from the reaction of OH/O2 on ethanimine (CH3CH=NH) using ab initio/Density Functional Theory (DFT) potential energy surface in combination with microcanonical variational transition state theory (µVTST) and Ramsperger–Kassel–Marcus (RRKM)/master equation (ME) simulation to predict the rate constants and branching fraction in the temperature range of 100 K to 1000 K and pressure range of 0.0001 bar to 100 bar. The reaction starts with cis (Z) and trans (E) CH3CH=NH isomer with OH radical followed by spontaneous formation via pre-reactive complex, forming the carbon and nitrogen-centered radicals. The O2 radical then attacks the carbon and nitrogen-centered radicals to form acetonitrile (CH3CN) and HO2 radicals. The results show that N–H and C–H dominate the H-atoms abstraction by OH radicals is similar to its isoelectronic analogous reaction system, i.e., CH3CHO + OH/O2 and CH3CHCH2 + OH/O2 and similar to methanimine (CH2NH) systems. The calculated rate constants for OH-initiated oxidation of CH3CH=NH are in the range of ~ 10–11 cm3 molecule−1 s−1 (at 300 K) and are in very good agreement with previous experimental values of its isoelectronic reaction system. The atmospheric lifetime due to the loss of CH3CHNH by OH radical (10 to 11 h) is in very good agreement with the similar pollutants in the troposphere temperature range between 200 and 320 K. The results indicate that its contribution to global warming is negligible. However, the formation of products such as CH3CN may interact with other atmospheric species, which could lead to the production of potentially hazardous compounds such as cyanogen (N2C2) and hydrogen cyanide (HCN).

Biogenic and Anthropogenic Secondary Organic Aerosols Become Fluorescent after Highly Acidic Aging.

Primary biological aerosol particles (PBAPs) and secondary organic aerosol (SOA) both contain organic compounds that share similar chemical and optical properties. Fluorescence is often used to characterize PBAPs; however, this may be hindered due to interferences from fluorophores in SOA. Despite extensive efforts to understand the aging of SOA under elevated particle acidity conditions, little is known about how these processes affect the fluorescence of SOA and thereby their interference with the measurements of PBAPs. The objective of this study is to investigate the fluorescence of SOA and understand the influence of acidity on the optical properties of organic aerosols and potential interference for the analysis of bioaerosols. The SOA was generated by O3- or OH-initiated oxidation of d-limonene or α-pinene, as well as by OH-initiated oxidation of toluene or xylene. The SOA compounds were then aged by exposure to varying concentrations of aqueous H2SO4 for 2 days. Absorption and fluorescence spectrophotometry were used to examine the changes in the optical properties before and after aging. The key observation was the appearance of strongly light-absorbing and fluorescent compounds at pH = ∼-1, suggesting that acidity is a major driver of SOA aging. The aged SOA from biogenic precursors (d-limonene and α-pinene) resulted in stronger fluorescence than the aged SOA from toluene and xylene. The absorption spectra of the aged SOA changed drastically in shape upon dilution, whereas the shapes of the fluorescence spectra remained the same, suggesting that the fluorophores and chromophores in SOA are separate sets of species. The fluorescence spectra of aged SOA overlapped with the fluorescence spectra of PBAPs, suggesting that SOA exposed to highly acidic conditions can be confused with PBAPs detected by fluorescence-based methods. These processes are likely to play a role in the atmospheric regions where high concentrations of H2SO4 persist, such as the upper troposphere and lower stratosphere.

Secondary Organic Aerosol from OH-Initiated Oxidation of Mixtures of d-Limonene and β-Myrcene.

The chemical composition and physical properties of secondary organic aerosol (SOA) generated through OH-initiated oxidation of mixtures containing β-myrcene, an acyclic monoterpene, and d-limonene, a cyclic monoterpene, were investigated to assess the extent of the chemical interactions between their oxidation products. The SOA samples were prepared in an environmental smog chamber, and their composition was analyzed offline using ultraperformance liquid chromatography coupled with electrospray ionization high-resolution mass spectrometry (UPLC-ESI-HRMS). Our results suggested that SOA containing β-myrcene showed a higher proportion of oligomeric compounds with low volatility compared to that of SOA from d-limonene. The formula distribution and signal intensities of the mixed SOA could be accurately predicted by a linear combination of the mass spectra of the SOA from individual precursors. Effects of cross-reactions were observed in the distribution of isomeric oxidation products within the mixed SOA, as made evident by chromatographic analysis. On the whole, β-myrcene and d-limonene appear to undergo oxidation by OH largely independently from each other, with only subtle effects from cross-reactions influencing the yields of specific oxidation products.

Spectroscopic Characterization and Photochemistry of the Atmospherically Relevant Methanesulfenic Acid.

Methanesulfenic acid, CH3SOH, is a fleeting intermediate in the ·OH-initiated oxidation reactions of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) in the atmosphere. Herein, we report the characterization and photochemistry of CH3SOH in Ar- and N2-matrices at 10 K. The characterization of CH3SOH with matrix-isolation IR and UV-vis spectroscopy is supported by D and 13C isotope labeling experiments and quantum chemical calculations. In line with the observed absorption at 260 nm for CH3SOH, its photolysis at 254 nm leads to dissociation by yielding the novel water complex H2CS···H2O, which exhibits a five-membered ring structure with intermolecular S···HO and CH···O hydrogen bonding interactions. Upon further irradiation at 193 nm, the H2CS···H2O complex undergoes dehydrogenation to form CS···H2O, which can further convert to HC(O)SH under irradiation at 254 nm. When the photolysis of CH3SOH was performed in an O2-doped Ar-matrix, methanesulfonic acid (MSA, CH3SO3H) was obtained as the oxidation product.

Gas-Phase Kinetic Investigation of the OH-Initiated Oxidation of a Series of Methyl-Butenols under Simulated Atmospheric Conditions.

Five biogenic unsaturated alcohols have been investigated under simulated atmospheric conditions regarding their gas-phase OH reactivity. The gas-phase rate coefficients of OH radicals with 2-methyl-3-buten-2-ol (k1), 3-methyl-2-buten-1-ol (k2), 3-methyl-3-buten-1-ol (k3), 2-methyl-3-buten-1-ol (k4), and 3-methyl-3-buten-2-ol (k5) at 298 ± 2 K and 1000 ± 10 mbar total pressure of synthetic air were determined under low- and high-NOx conditions using the relative kinetic technique. The present work provides for the first time the rate coefficients of gas-phase reactions of hydroxyl radicals with 2-methyl-3-buten-1-ol and 3-methyl-3-buten-2-ol. The following rate constants were measured (in 10-11 cm3 molecule-1 s-1): k1 = 6.32 ± 0.49, k2 = 14.55 ± 0.93, k3 = 10.04 ± 0.78, k4 = 5.31 ± 0.37, and k5 = 11.71 ± 1.29. No significant differences in the measured rate coefficients were obtained when either 365 nm photolysis of CH3ONO in the presence of NO or 254 nm photolysis of H2O2 was used as a source of OH radicals. Reactivity toward other classes of related compounds such as alkenes and saturated alcohols is discussed. A comparison of the structure-activity relationship (SAR) estimates derived from the available accepted methodologies with experimental data available for unsaturated alcohols is provided. Atmospheric lifetimes for the investigated series of alkenols with respect to the main atmospheric oxidants are given and discussed.

Volatility and chemical composition of secondary organic aerosol derived from acenaphthylene and acenaphthene under various oxidant conditions

Acenaphthene (ACE) and acenaphthylene (ACY), distinguished by their unique structures, exhibit high reactivity among polycyclic aromatic hydrocarbons (PAHs). Particularly, the rapid ozonolysis of the carbon-carbon double bond in ACY and the swift oxidation of the fused cyclopenta ring in both ACE and ACY by NO3 underscore their distinct atmospheric fate. Through the utilization of a thermo denuder and high-resolution mass spectrometry, the chemical composition and volatility of four types of secondary organic aerosols (SOA) derived from ACE and ACY are investigated. The chemical composition of SOA formed by ACY ozonolysis is characterized by low O: C ratios and high unsaturation, suggesting the retention of aromatic structures. This leads to measured high SOA volatility and is consistent with the low reactivity of O3 with aromatic compounds. In contrast to ozonolysis, the reaction of NO3 with ACY could yield SOA with very low volatility, characterized by abundant dimers and highly oxidized products. Furthermore, the prevalence of compounds with 11 carbon atoms suggests the presence of complex mechanisms in the ACY oxidation process initiated by NO3, where dealkylation reactions and cleavage reactions may be pivotal. The diverse chemical composition and volatility of SOA from ACE and ACY reacting with NO3 radicals highlight the substantial impact of precursor structure on SOA chemical composition, particularly concerning the presence of active carbon-carbon double bonds. The higher O: C ratio of SOA from OH-initiated oxidation of ACE indicates the significant role of OH in the oxidation of PAHs. These findings offer valuable insights into the atmospheric fate of these PAHs, underscoring the necessity for a comprehensive understanding of their degradation processes and the formation of SOA.

Secondary Organic Aerosol Formation and Chemistry from the OH-Initiated Oxidation of Monofunctional C10 Species

Secondary Organic Aerosol Formation and Chemistry from the OH-Initiated Oxidation of Monofunctional C₁₀ Species

Role of Hydroperoxyl Radicals in Heterogeneous Oxidation of Oxygenated Organic Aerosols.

Heterogeneous oxidative aging of organic aerosols (OA) occurs ubiquitously in the atmosphere, initiated by oxidants, such as the hydroxyl radicals (•OH). Hydroperoxyl radicals (HO2•) are also an important oxidant in the troposphere, and its gas-phase chemistry has been well studied. However, the role of HO2• in heterogeneous OA oxidation remains elusive. Here, we carry out •OH-initiated heterogeneous oxidation of several OA model systems under different HO2• conditions in a flow tube reactor and characterize the molecular oxidation products using a suite of mass spectrometry instrumentation. By using hydrogen-deuterium exchange (HDX) with thermal desorption iodide-adduct chemical ionization mass spectrometry, we provide direct observation of organic hydroperoxide (ROOH) formation from heterogeneous HO2• and peroxy radicals (RO2•) reactions for the first time. The ROOH may contribute substantially to the oxidation products, varied with the parent OA chemical structure. Furthermore, by regulating RO2• reaction pathways, HO2• also greatly influence the overall composition of the oxidized OA. Last, we suggest that the RO2• + HO2• reactions readily occur at the OA particle interface rather than in the particle bulk. These findings provide new mechanistic insights into the heterogeneous OA oxidation chemistry and help fill the critical knowledge gap in understanding atmospheric OA oxidative aging.

Ab initio kinetics of OH-initiated oxidation of naphthalene: A comprehensive revisited study

Naphthalene, the most prevalent and smallest member of the Polycyclic Aromatic Hydrocarbons (PAHs) family, has garnered significant scientific attention for its harmful environmental impacts and those of its degradation products. In this work, we comprehensively recharacterized the gas-phase reaction of naphthalene with OH radicals in a wide range of conditions (T = 200–2000 K &P = 0.76–7600 Torr), covering both atmospheric and combustion chemistry. Within the Rice–Ramsperger–Kassel–Marcus (RRKM)-based master equation framework, the detailed kinetic model was constructed on the potential energy surface explored at the ROCBS-QB3//M06–2X/aug-cc-pVTZ level of theory. Our model not only helps to resolve the existing large discrepancy (up to ∼1000 times) among the previously reported rate constants but also reveals mechanistic insights, including time-resolved species profiles and product distribution, for naphthalene transformation. It is shown that although naphthalene cannot be considered a persistent organic compound (POP) due to its rapid reaction with OH radicals, its chemical conversion can generate harmful substances such as O3 and carcinogenic derivatives. Also, the detailed conversion of naphthalene in water was investigated for the first time, revealing that naphthalene and its important degradation products pose a significant danger to aquatic life as hazardous pollutants.

Theoretical analysis of the OH-initiated atmospheric oxidation reactions of imidazole.

Imidazoles are present in Earth's atmosphere in both the gas-phase and in aerosol particles, and have been implicated in the formation of brown carbon aerosols. The gas-phase oxidation of imidazole (C3N2H4) by hydroxyl radicals has been shown to be preferentially initiated via OH-addition to position C5, producing the 5-hydroxyimidazolyl radical adduct. However, the fate of this adduct upon reaction with O2 in the atmospheric gas-phase is currently unknown. We employed an automated approach to investigate the reaction mechanism and kinetics of imidazole's OH-initiated gas-phase oxidation, in the presence of O2 and NOx. The explored mechanism included reactions available to first-generation RO2 radicals, as well as alkoxyl radicals produced from RO2 + NO reactions. Product distributions were obtained by assembling and solving a master equation, under conditions relevant to the Earth's atmosphere. Our calculations show a complex, branched reaction mechanism, which nevertheless converges to yield two major closed-shell products: 4H-imidazol-4-ol (4H-4ol) and N,N'-diformylformamidine (FMF). At 298 K and 1 atm, we estimate the yields of 4H-4ol and FMF from imidazole oxidation initiated via OH-addition to position C5 to be 34 : 66, 12 : 85 and 2 : 95 under 10 ppt, 100 ppt and 1 ppb of NO respectively. This work also revealed O2-migration pathways between the α-N-imino peroxyl radical isomers. This reaction channel is fast for the first-generation RO2 radicals, and may be important during the atmospheric oxidation of other unsaturated organic nitrogen compounds as well.

Reaction pathways leading to HPALD intermediates in the OH-initiated oxidation of isoprene.

In this study, we revisited the mechanism of isoprene oxidation by OH radicals, focusing on the formation of hydroperoxyaldehydes (HPALDs) in the reactions following O2-addition at the α-position to Z,Z'-OH-allyl radical products of the 1,6-H shift of the 1st-generation Z-δ-OH-isoprenylperoxy radicals. Utilizing high-level ab initio quantum chemical calculations and a master equation approach, we provide theoretical confirmation that the formation of δ-HPALDs dominates by far and show that production of β-HPALDs by the mechanism proposed by Wennberg et al. (Chem. Rev., 2018, 118, 3337-3390) is negligible. Besides the dominance of the δ-HPALD formation channel, our investigation also reveals a novel though minor reaction channel resulting in the formation of an allylic δ-hydroperoxy acid and OH radical. Of primary importance for the assessment of the respective channels is the identification of a chemically activated mechanism driving the δ-HPALD formation process under atmospheric conditions. Different from traditional thermally activated pathways, we found that the rovibrationally hot peroxy radicals resulting from O2 addition to Z,Z'-OH-allyl radicals undergo prompt rearrangement and decomposition at a rate faster than their collisional relaxation, predominantly yielding δ-HPALDs in a chemically activated manner with high efficiency under atmospheric conditions.

Atmospheric Oxidation of Hydroperoxy Amides.

Recently, hydroperoxy amides were identified as major products of OH-initiated autoxidation of tertiary amines in the atmosphere. The formation mechanism is analogous to that found for ethers and sulfides but substantially faster. However, the atmospheric fate of the hydroperoxy amides remains unknown. Using high-level theoretical methods, we study the most likely OH-initiated oxidation pathways of the hydroperoxy and dihydroperoxy amides derived from trimethylamine autoxidation. Overall, we find that the OH-initiated oxidation of the hydroperoxy amides predominantly leads to the formation of imides under NO-dominated conditions and more highly oxidized hydroperoxy amides under HO2-dominated conditions. Unimolecular reactions are found to be surprisingly slow, likely due to the restricting, planar structure of the amide moiety.

Mechanistic and Kinetic Insights into OH-Initiated Atmospheric Oxidation of Hymexazol: A Computational Study.

Hymexazol is a volatile fungicide widely used in agriculture, causing its abundance in the atmosphere; thus, its atmospheric fate and conversion are of great importance when assessing its environmental impacts. Herein, we report a theoretical kinetic mechanism for the oxidation of hymexazol by OH radicals, as well as the subsequent reactions of its main products with O2 and then with NO by using the Rice-Ramsperger-Kassel-Marcus-based Master equation kinetic model on the potential energy surface explored at the ROCBS-QB3//M06-2X/aug-cc-pVTZ level. The predicted total rate constants ktotal(T, P) for the reaction between hymexazol and OH radicals show excellent agreement with scarcely available experimental values (e.g., 3.6 × 10-12 vs (4.4 ± 0.8) × 10-12 cm3/molecule/s at T = 300 K and P = 760 Torr); thus, the calculated kinetic parameters can be confidently used for modeling/simulation of N-heterocycle-related applications under atmospheric and even combustion conditions. The model shows that 3,4-dihydroxy-5-methyl-4,5-dihydro-1,2-oxazol-5-yl (IM2), 3,5-dihydroxy-5-methyl-4,5-dihydro-1,2-oxazol-4-yl (IM3), and (3-hydroxy-1,2-oxazol-5-yl)methyl (P8) are the main primary intermediates, which form the main secondary species of (3,4-dihydroxy-5-methyl-4,5-dihydro-1,2-oxazol-5-yl)dioxidanyl (IM4), (3,5-dihydroxy-5-methyl-4,5-dihydro-1,2-oxazol-4-yl)dioxidanyl (IM7), and ([(3-hydroxy-1,2-oxazol-5-yl)methyl]dioxidanyl (IM11), respectively, through the reactions with O2. The main secondary species then can react with NO to form the main tertiary species, namely, (3,4-dihydroxy-5-methyl-4,5-dihydro-1,2-oxazol-5-yl)oxidanyl (P19), (3,5-dihydroxy-5-methyl-4,5-dihydro-1,2-oxazol-4-yl)oxidanyl (P21), and [(3-hydroxy-1,2-oxazol-5-yl)methyl]oxidanyl (P23), respectively, together with NO2. Besides, hymexazol could be a persistent organic pollutant in the troposphere due to its calculated half-life τ1/2 of 13.7-68.1 h, depending on the altitude.

Atmospheric Chemistry of HOHg(II)O• Mimics That of a Hydroxyl Radical.

HOHg(II)O•, formed from HOHg(I)• + O3, is a key intermediate in the OH-initiated oxidation of Hg(0) in the atmosphere. As no experimental data are available for HOHg(II)O•, we use computational chemistry (CCSD(T)//M06-2X/AVTZ) to characterize its reactions with atmospheric trace gases (NO, NO2, CH4, C2H4, CH2O and CO). In summary, HOHg(II)O•, like the analogous BrHg(II)O• radical, largely mimics the reactivity of •OH in reactions with NOx, alkanes, alkenes, and aldehydes. The rate constant for its reaction with methane (HOHg(II)O• + CH4 → Hg(II)(OH)2 + •CH3) is about four times higher than that of •OH at 298 K. All of these reactions maintain mercury as Hg(II), except for HOHg(II)O• + CO → HOHg(I)• + CO2. Considering only the six reactions studied here, we find that reduction by CO dominates the fate of HOHg(II)O• (79-93%) in many air masses (in the stratosphere and at ground level in rural, marine, and polluted urban regions) with only modest competition from HOHg(II)O• + CH4 (<15%). We expect that this work will help global modeling of atmospheric mercury chemistry.

An aldehyde as a rapid source of secondary aerosol precursors: theoretical and experimental study of hexanal autoxidation

Abstract. Aldehydes are common constituents of natural and polluted atmospheres, and their gas-phase oxidation has recently been reported to yield highly oxygenated organic molecules (HOMs) that are key players in the formation of atmospheric aerosol. However, insights into the molecular-level mechanism of this oxidation reaction have been scarce. While OH initiated oxidation of small aldehydes, with two to five carbon atoms, under high-NOx conditions generally leads to fragmentation products, longer-chain aldehydes involving an initial non-aldehydic hydrogen abstraction can be a path to molecular functionalization and growth. In this work, we conduct a joint theoretical–experimental analysis of the autoxidation chain reaction of a common aldehyde, hexanal. We computationally study the initial steps of OH oxidation at the RHF-RCCSD(T)-F12a/VDZ-F12//ωB97X-D/aug-cc-pVTZ level and show that both aldehydic (on C1) and non-aldehydic (on C4) H-abstraction channels contribute to HOMs via autoxidation. The oxidation products predominantly form through the H abstraction from C1 and C4, followed by fast unimolecular 1,6 H-shifts with rate coefficients of 1.7×10-1 and 8.6×10-1 s−1, respectively. Experimental flow reactor measurements at variable reaction times show that hexanal oxidation products including HOM monomers up to C6H11O7 and accretion products C12H22O9−10 form within 3 s reaction time. Kinetic modeling simulations including atmospherically relevant precursor concentrations agree with the experimental results and the expected timescales. Finally, we estimate the hexanal HOM yields up to seven O atoms with mechanistic details through both C1 and C4 channels.

Molecular rearrangement of bicyclic peroxy radicals is a key route to aerosol from aromatics

The oxidation of aromatics contributes significantly to the formation of atmospheric aerosol. Using toluene as an example, we demonstrate the existence of a molecular rearrangement channel in the oxidation mechanism. Based on both flow reactor experiments and quantum chemical calculations, we show that the bicyclic peroxy radicals (BPRs) formed in OH-initiated aromatic oxidation are much less stable than previously thought, and in the case of the toluene derived ipso-BPRs, lead to aerosol-forming low-volatility products with up to 9 oxygen atoms on sub-second timescales. Similar results are predicted for ipso-BPRs formed from many other aromatic compounds. This reaction class is likely a key route for atmospheric aerosol formation, and including the molecular rearrangement of BPRs may be vital for accurate chemical modeling of the atmosphere.

Daytime isoprene nitrates under changing NOx and O3

Abstract. Organonitrates are important species in the atmosphere due to their impacts on NOx, HOx, and O3 budgets, and their potential to contribute to secondary organic aerosol (SOA) mass. This work presents a steady-state modelling approach to assess the impacts of changes in NOx and O3 concentrations on the organonitrates produced from isoprene oxidation. The diverse formation pathways to isoprene organonitrates dictate the responses of different groups of organonitrates to changes in O3 and NOx. For example, organonitrates predominantly formed from the OH-initiated oxidation of isoprene favour formation under lower-ozone and moderate-NOx concentrations, whereas organonitrates formed via daytime NO3 oxidation show the highest formation under high-O3 concentrations with little dependence on NOx concentrations. Investigating the response of total organonitrates reveals complex and nonlinear behaviour with implications that could inform expectations of changes to organonitrate concentrations as efforts are made to reduce NOx and O3 concentrations, including a region of NOx–O3 space where total organonitrate concentration is relatively insensitive to changes in NOx and O3. These conclusions are further contextualised by estimating the volatility of the isoprene organonitrates revealing the potential for high concentrations of low-volatility species under high-ozone conditions.

A Multiconformational Transition State Theory Approach to OH Tropospheric Degradation of Fluorotelomer Aldehydes.

Experimental work on the OH-initiated oxidation reactionsof fluorotelomer aldehydes (FTALs) strongly suggests thatthe respective rate coefficients do not depend on the sizeof the CxF2x+1 fluoroalkyl chain. FTALs hence representa challenging test to our multiconformer transition statetheory (MC-TST) protocol based on constrained transition state randomization (CTSR), since the calculated ratecoefficients should not show significant variations with increasing values of x. In this work we apply the MC-TST/CTSR protocol to the x = 2, 3 cases and calculate both rate coefficients at 298.15 K with a value ofk = (2.4±1.4)×10-12 cm3 molecule-1 s-1 , practicallycoincident with the recommended experimental value ofkexp = (2.8±1.4)×10-12 cm3 molecule-1 s-1. We alsoshow that the use of tunneling corrections based on improved semiclassical TST is critical in obtaining Arrhenius-Kooij curves with a correct behavior at lower temperatures.

Theoretical Study on the Mechanisms, Kinetics, and Toxicity Evaluation of OH-Initiated Atmospheric Oxidation Reactions of Coniferyl Alcohol

In this paper, we investigated the mechanisms, kinetics, and toxicity evaluation of the OH-initiated reaction of coniferyl alcohol (4-(3-hydroxy-1-propenyl)-2-methoxyphenol) in the atmosphere using theoretical calculations. The initial reaction of coniferyl alcohol with OH radicals had two pathways, H-abstraction and OH-addition reactions. The total reaction rate constants were 2.32 × 10−9 cm3 molecule−1 s−1 (in gas-phase) and 9.44 × 109 s−1 M−1 (in liquid-phase) for the preliminary reactions of coniferyl alcohol with OH radicals at 298 K, respectively, and the half-lives of the total reaction (including all initial H-abstraction and OH-addition reactions) of coniferyl alcohol with OH radical in the atmosphere, urban and remote clouds were 8.3 × 10−2 h, 5.83 × 103 h and 9.27 × 102 h, respectively. The temperature had a strong and positive influence on the initial reaction rate constant. The branching ratios of H-abstraction and OH-addition reactions were 3.68% and 97.69%, respectively, making the OH-addition reactions become dominant reactions. The ecotoxicity evaluation revealed that the toxicity levels of coniferyl alcohol and its products were similar and non-toxic. However, all these products have developmental toxicity, with most of them having no mutagenicity. Therefore, further attention should be paid to the oxidation process and product toxicity evaluation of coniferyl alcohol in the atmosphere.

Heterogeneous and Photosensitized Oxidative Degradation Kinetics of the Plastic Additive Bisphenol-A in Sea Spray Aerosol Mimics.

Plastics have become ubiquitous in the world's oceans, and recent work indicates that they can transfer from the ocean to the atmosphere in sea spray aerosol (SSA). Hazardous chemical residues in plastics, including bisphenol-A (BPA), represent a sizable fraction of consumer plastics and have been measured consistently in air over both terrestrial and marine environments. However, the chemical lifetimes of BPA and mechanisms by which plastic residues degrade with respect to photochemical and heterogeneous oxidation processes in aerosols are unknown. Here, we present the photosensitized and OH-initiated heterogeneous oxidation kinetics of BPA in the aerosol phase consisting of pure-component BPA and internal mixtures of BPA, NaCl, and dissolved photosensitizing organic matter. We found that photosensitizers enhanced BPA degradation in binary-component BPA + photosensitizer aerosol mixtures when irradiated in the absence of OH. OH-initiated degradation of BPA was enhanced in the presence of NaCl with and without photosensitizing species. We attribute this enhanced degradation to greater mobility and thus reaction probability between BPA, OH, and reactive chlorine species (RCS) formed through reaction between OH and dissolved Cl- in the more liquid-like aerosol matrix in the presence of NaCl. Addition of the photosensitizers in the ternary-component BPA + NaCl + photosensitizer aerosol led to no enhancement in the degradation of BPA following light exposure compared to the binary-component BPA + NaCl aerosol. This was attributed to quenching of triplet state formation by dissolved Cl- in the less viscous aqueous aerosol mixtures containing NaCl. Based upon measured second-order heterogeneous reaction rates, the estimated lifetime of BPA with respect to heterogeneous oxidation by OH is one week in the presence of NaCl compared to 20 days in the absence of NaCl. This work highlights the important heterogeneous and photosensitized reactions and the role of phase state, which affect the lifetimes of hazardous plastic pollutants in SSA with implications for understanding pollutant transport and exposure risks in coastal marine environments.