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Chemical mechanisms form the core of atmospheric models to describe degradation pathways of pollutants and ultimately inform air quality and climate policymakers and other stakeholders. The accuracy of chemical mechanisms relies on the quality of their input data, which originate from experimental (laboratory, field, chamber) and theoretical (quantum chemistry, theoretical kinetics, machine learning) studies. The development of robust mechanisms requires rigorous and transparent procedures for data collection, mechanism construction and evaluation and the creation of reduced or operationally defined mechanisms. Developments in analytical techniques have led to a large number of identified chemical species in the atmospheric multiphase system that have proved invaluable for our understanding of atmospheric chemistry. At the same time, advances in software and machine learning tools have enabled automated mechanism generation. We discuss strategies for mechanism development, applying empirical or mechanistic approaches. We show the general workflows, how either approach can lead to robust mechanisms and that the two approaches complement each other, resulting in reliable predictions. Current challenges are discussed related to global change, including shifts in emission scenarios that result in new chemical regimes (e.g., low-NO scenarios, wildfires, mega- and gigacities) and that require the development of new or expanded gas- and aqueous-phase mechanisms. In addition, new mechanisms should be developed to also target oxidation capacity and aerosol chemistry impacting climate, human and ecosystem health.

The term open-air factor (OAF) was coined following microbiological research in the 1960s and 1970s which established that rural air had powerful germicidal properties and attributed this to Criegee intermediates formed in the reaction of ozone with alkenes. We have re-evaluated those early experiments applying the current state of knowledge of ozone–alkene reactions. Contrary to previous speculation, neither Criegee intermediates nor the HO radicals formed in their decomposition are directly responsible for the germicidal activity attributed to the OAF. We identify other potential candidates, which are formed in ozone–alkene reactions and have known (and likely) germicidal properties, but the compounds responsible for the OAF remain a mystery. There has been very little research into the OAF since the 1970s, and this effect seems to have been largely forgotten. In this opinion piece we remind the community of the germicidal open-air factor. Given the current global pandemic spread by an airborne pathogen, understanding the natural germicidal effects of ambient air, solving the mystery of the open-air factor and determining how this effect can be used to improve human welfare should be a high priority for the atmospheric science community.

The Indo-Gangetic Plain (IGP) is home to 9 % of the global population and is responsible for a large fraction of agricultural crop production in Pakistan, India, and Bangladesh. Levels of fine particulate matter (mean diameter µm, PM2.5) across the IGP often exceed human health recommendations, making cities across the IGP among the most polluted in the world. Seasonal changes in the physical environment over the IGP are dominated by the large-scale south Asian monsoon system that dictates the timing of agricultural planting and harvesting. We use the WRF-Chem model to study the seasonal anthropogenic, pyrogenic, and biogenic influences on fine particulate matter and its constituent organic aerosol (OA) over the IGP that straddles Pakistan, India, and Bangladesh during 2017–2018. We find that surface air quality during pre-monsoon (March–May) and monsoon (June–September) seasons is better than during post-monsoon (October–December) and winter (January–February) seasons, but all seasonal mean values of PM2.5 still exceed the recommended levels, so that air pollution is a year-round problem. Anthropogenic emissions influence the magnitude and distribution of PM2.5 and OA throughout the year, especially over urban sites, while pyrogenic emissions result in localised contributions over the central and upper parts of IGP in all non-monsoonal seasons, with the highest impact during post-monsoon seasons that correspond to the post-harvest season in the agricultural calendar. Biogenic emissions play an important role in the magnitude and distribution of PM2.5 and OA during the monsoon season, and they show a substantial contribution to secondary OA (SOA), particularly over the lower IGP. We find that the OA contribution to PM2.5 is significant in all four seasons (17 %–30 %), with primary OA generally representing the larger fractional contribution. We find that the volatility distribution of SOA is driven mainly by the mean total OA loading and the washout of aerosols and gas-phase aerosol precursors that result in SOA being less volatile during the pre-monsoon and monsoon season than during the post-monsoon and winter seasons.

This article, the eighth in the series, presents kinetic and photochemical data sheets evaluated by the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. It covers the gas-phase thermal and photochemical reactions of organic species with four, or more, carbon atoms (≥ C4) available on the IUPAC website in 2021, including thermal reactions of closed-shell organic species with HO and NO3 radicals and their photolysis. The present work is a continuation of volume II (Atkinson et al., 2006), with new reactions and updated data sheets for reactions of HO (77 reactions) and NO3 (36 reactions) with ≥ C4 organics, including alkanes, alkenes, dienes, aromatics, oxygenated, organic nitrates and nitro compounds in addition to photochemical processes for nine species. The article consists of a summary table (Table 1), containing the recommended kinetic parameters for the evaluated reactions, and a supplement containing the data sheets, which provide information upon which recommendations are made.

This article, the seventh in the series, presents kinetic and photochemical data sheets evaluated by the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. It covers an extension of the gas-phase and photochemical reactions related to Criegee intermediates previously published in Atmospheric Chemistry and Physics (ACP) in 2006 and implemented on the IUPAC website up to 2020. The article consists of an introduction, description of laboratory measurements, a discussion of rate coefficients for reactions of O3 with alkenes producing Criegee intermediates, rate coefficients of unimolecular and bimolecular reactions and photochemical data for reactions of Criegee intermediates, and an overview of the atmospheric chemistry of Criegee intermediates. Summary tables of the recommended kinetic and mechanistic parameters for the evaluated reactions are provided. Data sheets summarizing information upon which the recommendations are based are given in two files, provided as a Supplement to this article.

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).

Reaction with the hydroxyl (OH) radical is the dominant removal process for volatile organic compounds (VOCs) in the atmosphere. Rate coefficients for reactions of OH with VOCs are therefore essential parameters for chemical mechanisms used in chemistry transport models, and are required more generally for impact assessments involving the estimation of atmospheric lifetimes or oxidation rates for VOCs. Updated and extended structure–activity relationship (SAR) methods are presented for the reactions of OH with aliphatic organic compounds, with the reactions of aromatic organic compounds considered in a companion paper. The methods are optimized using a preferred set of data including reactions of OH with 489 aliphatic hydrocarbons and oxygenated organic compounds. In each case, the rate coefficient is defined in terms of a summation of partial rate coefficients for H abstraction or OH addition at each relevant site in the given organic compound, so that the attack distribution is defined. The information can therefore guide the representation of the OH reactions in the next generation of explicit detailed chemical mechanisms. Rules governing the representation of the subsequent reactions of the product radicals under tropospheric conditions are also summarized, specifically their reactions with O2 and competing processes.

The North China Plain (NCP) frequently experiences heavy haze pollution, particularly during wintertime. In winter 2015–2016, the NCP region suffered several extremely severe haze episodes with air pollution red alerts issued in many cities. We have investigated the sources and aerosol evolution processes of the severe pollution episodes in Handan, a typical industrialized city in the NCP region, using real-time measurements from an intensive field campaign during the winter of 2015–2016. The average (±1σ) concentration of submicron aerosol (PM1) during 3 December 2015–5 February 2016 was 187.6 (±137.5) µg m−3, with the hourly maximum reaching 700.8 µg m−3. Organic was the most abundant component, on average accounting for 45 % of total PM1 mass, followed by sulfate (15 %), nitrate (14 %), ammonium (12 %), chloride (9 %) and black carbon (BC, 5 %). Positive matrix factorization (PMF) with the multilinear engine (ME-2) algorithm identified four major organic aerosol (OA) sources, including traffic emissions represented by a hydrocarbon-like OA (HOA, 7 % of total OA), industrial and residential burning of coal represented by a coal combustion OA (CCOA, 29 % of total OA), open and domestic combustion of wood and crop residuals represented by a biomass burning OA (BBOA, 25 % of total OA), and formation of secondary OA (SOA) in the atmosphere represented by an oxygenated OA (OOA, 39 % of total OA). Emissions of primary OA (POA), which together accounted for 61 % of total OA and 27 % of PM1, are a major cause of air pollution during the winter. Our analysis further uncovered that primary emissions from coal combustion and biomass burning together with secondary formation of sulfate (mainly from SO2 emitted by coal combustion) are important driving factors for haze evolution. However, the bulk composition of PM1 showed comparatively small variations between less polluted periods (daily PM2. 5 ≤ 75 µg m−3) and severely polluted periods (daily PM2. 5 > 75 µg m−3), indicating relatively synchronous increases of all aerosol species during haze formation. The case study of a severe haze episode, which lasted 8 days starting with a steady buildup of aerosol pollution followed by a persistently high level of PM1 (326.7–700.8 µg m−3), revealed the significant influence of stagnant meteorological conditions which acerbate air pollution in the Handan region. The haze episode ended with a shift of wind which brought in cleaner air masses from the northwest of Handan and gradually reduced PM1 concentration to −3 after 12 h. Aqueous-phase reactions under higher relative humidity (RH) were found to significantly promote the production of secondary inorganic species (especially sulfate) but showed little influence on SOA.

This article, the sixth in the ACP journal series, presents data evaluated by the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. It covers the heterogeneous processes involving liquid particles present in the atmosphere with an emphasis on those relevant for the upper troposphere/lower stratosphere and the marine boundary layer, for which uptake coefficients and adsorption parameters have been presented on the IUPAC website since 2009. The article consists of an introduction and guide to the evaluation, giving a unifying framework for parameterisation of atmospheric heterogeneous processes. We provide summary sheets containing the recommended uptake parameters for the evaluated processes. The experimental data on which the recommendations are based are provided in data sheets in separate appendices for the four surfaces considered: liquid water, deliquesced halide salts, other aqueous electrolytes and sulfuric acid.

This article, the fifth in the ACP journal series, presents data evaluated by the IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry. It covers the heterogeneous processes on surfaces of solid particles present in the atmosphere, for which uptake coefficients and adsorption parameters have been presented on the IUPAC website in 2010. The article consists of an introduction and guide to the evaluation, giving a unifying framework for parameterisation of atmospheric heterogeneous processes. We provide summary sheets containing the recommended uptake parameters for the evaluated processes. Four substantial appendices contain detailed data sheets for each process considered for ice, mineral dust, sulfuric acid hydrate and nitric acid hydrate surfaces, which provide information upon which the recommendations are made.

This article, the fourth in the series, presents kinetic and photochemical data sheets evaluated by the IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry. It covers the gas phase and photochemical reactions of organic halogen species, which were last published in 1997, and were updated on the IUPAC website in 2006/07. The article consists of a summary sheet, containing the recommended kinetic parameters for the evaluated reactions, and four appendices containing the data sheets, which provide information upon which the recommendations are made.

Smog chamber/FTIR techniques were used to study the products and mechanisms of OH radical and Cl atom initiated oxidation of trans-CF3CH=CHF in 700 Torr of N2/O2 diluent at 295±1 K. Hydroxyl radical initiated oxidation leads to the formation of CF3CHO and HC(O)F in yields which were indistinguishable from 100% and were not dependent on the O2 partial pressure. Chlorine atom initiated oxidation gives HC(O)F, CF3CHO, CF3C(O)Cl, and CF3C(O)CHFCl. The yields of CF3C(O)Cl and CF3C(O)CHFCl increased at the expense of HC(O)F and CF3CHO as the O2 partial pressure was increased over the range 5–700 Torr. The results are discussed with respect to the atmospheric chemistry and environmental impact of trans-CF3CH=CHF.

The formation of formaldehyde via hydrogen atom transfer from the methoxy radical to molecular oxygen is a key step in the atmospheric photochemical oxidation of methane, and in the propagation of deuterium from methane to molecular hydrogen. We report the results of the first investigation of the branching ratio for HCHO and HCDO formation in the CH2DO + O2 reaction. Labeled methoxy radicals (CH2DO) were generated in a photochemical reactor by photolysis of CH2DONO. HCHO and HCDO concentrations were measured using FTIR spectroscopy. Significant deuterium enrichment was seen in the formaldehyde product, from which we derive a branching ratio of 88.2±1.1% for HCDO and 11.8±1.1% for HCHO. The implications of this fractionation on the propagation of deuterium in the atmosphere are discussed.

No abstract available.

The products, kinetics, and mechanism of the reaction Cl + 3-pentanone have been measured by UV irradiation of Cl2/3-pentanone/N2(O2) mixtures using primarily GC analysis with selected cross checks by FTIR. In the absence of O2, the products are 1- and 2-chloro-3-pentanone with yields of 21 and 78%, respectively. As the temperature is increased, the yield of 1-chloro-3-pentanone increases modestly relative to the 2-chloro-3-pentanone yield. On the basis of this increase, the activation energy for hydrogen abstraction at the 1 position is determined to be 500 (±500) cal mole−1relative to abstraction at the 2 position. In the presence of 500 ppm of O2with 900−1000 ppm of Cl2at 297 K, the yield of 2-chloro-3-pentanone decreases dramatically from 78 to 2.5%, while the 1-chloro-3-pentanone decreases only modestly from 21 to 17%. The observed oxygenated species are acetaldehyde (59%), 2,3-pentanedione (9%), and propionyl chloride (56%). Increasing the temperature to 420 K (O2= 500 ppm) suppresses these oxygenated products, and 2-chloro-3-pentanone again becomes the primary product, indicating that the O2addition reaction to the 2-pentanonyl radical has become reversible. At 500 K and 10 000 ppm O2, a new product channel opens which forms a small yield (∼4%) of ethylvinylketone. Computer modeling of the product yields has been performed to gain an understanding of the overall reaction mechanism in the presence and absence of O2. The reaction of chlorine atoms with 3-pentanone proceeds with a rate constant of 8.1 (±0.8) × 10−11cm3molecule−1s−1independent of temperature over the range of 297−490 K (Ea= 0 ± 200 cal mole−1). Rate constant ratios of k(C2H5C(O)CHCH3+ Cl2)/k(C2H5C(O)CHCH3+ O2) = 0.0185 ± 0.0037 and k(C2H5C(O)CH2CH2+ Cl2)/k(C2H5CH2C(O)CH2CH2+ O2) = 2.7 ± 0.4 were determined at 297 K in 800−950 Torr of N2/O2diluent. In 800−950 Torr of N2/O2diluent, the major fate of the alkoxy radical CH3CH(O)C(O)C2H5is decomposition to give C2H5C(O) radicals and CH3CHO. These results show that the chemical mechanisms of the 3-pentanone reactions are very similar to those observed for butanone. In addition, the rate constants of the reactions of chlorine atoms with 1-chloro-3-pentanone [3 (±0.6) × 10−11over the range of 297−460 K], 2,3-pentanedione [1.4 (±0.3) × 10−11at 297 K], and ethylvinylketone [1.9 (±0.4) × 10−10over the range of 297−400 K, decreasing rapidly above 400 K] were measured at ambient pressure. [ABSTRACT FROM AUTHOR]

Smog chamber/FTIR techniques were used to determine rate constants of k(Cl+n-butanol) = (2.21 ± 0.38) × 10−10and k(OH+n-butanol) = (8.86 ± 0.85) × 10−12cm3molecule−1s−1in 700 Torr of N2/O2diluent at 296 ± 2K. The sole primary product identified from the Cl atom initiated oxidation of n-butanol in the absence of NO was butyraldehyde (38 ± 2%, molar yield). The primary products of the Cl atom initiated oxidation of n-butanol in the presence of NO were (molar yield) butyraldehyde (38 ± 2%), propionaldehyde (23 ± 3%), acetaldehyde (12 ± 4%), and formaldehyde (33 ± 3%). The substantially lower yields of propionaldehyde, acetaldehyde, and formaldehyde as primary products in experiments conducted in the absence of NO suggests that chemical activation is important in the atmospheric chemistry of CH3CH2CH(O)CH2OH and CH3CH(O)CH2CH2OH alkoxy radicals. The primary products of the OH radical initiated oxidation of n-butanol in the presence of NO were (molar yields) butyraldehyde (44 ± 4%), propionaldehyde (19 ± 2%), and acetaldehyde (12 ± 3%). In all cases, the product yields were independent of oxygen concentration over the partial pressure range of 10−600 Torr. The yields of propionaldehyde, acetaldehyde, and formaldehyde quoted above were not corrected for secondary formation via oxidation of higher aldehydes and should be treated as upper limits. The reactions of Cl atoms and OH radicals with n-butanol proceed 38 ± 2 and 44 ± 4%, respectively, via attack on the α-position to give an α-hydroxy alkyl radical which reacts with O2to give butyraldehyde. The results are discussed with respect to the atmospheric chemistry of n-butanol. [ABSTRACT FROM AUTHOR]

The products, kinetics, and mechanism of the reaction Cl + butanone have been measured by UV irradiation of Cl2/butanone/N2(O2) mixtures using either GC or FTIR analysis. In the absence of O2, the products are 1-, 3-, and 4-chlorobutanone with yields of 3.1%, 76%, and 22.5%, respectively. As the temperature is increased, the yields of 1- and 4-chlorobutanone increase relative to the 3-chlorobutanone yield. On the basis of these increases, the activation energies for hydrogen abstraction at the 1 and 4 positions are determined to be 1800 (±300) and 470 (+300, −150) cal mol−1relative to abstraction at the 3 position. In the presence of 400 ppm of O2with 700−900 ppm of Cl2at 297 K, the yields of 1- and 3-chlorobutanone decrease dramatically from 3.1% to 0.25% and from 76% to 2%, respectively, while the 4-chlorobutanone decreases only slightly from 22.5% to 18.5%. The observed oxygenated species are acetaldehyde (52%), butanedione (11%), and propionyl chloride (2.5%). Increasing the temperature to 400 K (O2= 500 ppm) suppresses these oxygenated products and 1- and 3-chlorobutanone again become the primary products, indicating that the O2addition reaction to the 1- and 3-butanonyl radicals is becoming reversible. At 500 K and very high O2mole fraction (170 000 ppm), a new product channel opens which forms a substantial yield (∼20%) of methylvinylketone. Computer modeling of the product yields has been performed to gain an understanding of the overall reaction mechanism in the presence and absence of O2. The reaction of chlorine atoms with butanone proceeds with a rate constant of 4.0 (±0.4) × 10−11cm3molecule−1s−1independent of temperature over the range 297−475 K (Ea= 0 ± 200 cal mol−1). Rate constant ratios of k(CH2C(O)C2H5+ Cl2)/k(CH2C(O)C2H5+ O2) = 0.027 ± 0.008, k(CH3C(O)CHCH3+ Cl2)/ k(CH3C(O)CHCH3+ O2) = 0.0113 ± 0.0011, and k(CH3C(O)CH2CH2+ Cl2)/k(CH3C(O)CH2CH2+ O2) = 1.52 ± 0.32 were determined at 297 K in 800−950 Torr of N2diluent. In 700−900 Torr of N2/O2diluent, the major fate of the alkoxy radicals CH3C(O)CH(O)CH3and OCH2C(O)C2H5is decomposition to give CH3C(O) radicals and CH3CHO and HCHO and C(O)C2H5radicals, respectively. [ABSTRACT FROM AUTHOR]

Smog chamber/FTIR techniques were used to study the atmospheric chemistry of 3-pentanol and determine rate constants of k(Cl+3-pentanol) = (2.03 ± 0.23) × 10 −10and k(OH+3-pentanol) = (1.32 ± 0.15) × 10 −11cm 3molecule −1s −1in 700 Torr of N 2/O 2diluent at 296 ± 2 K. The primary products of the Cl atom initiated oxidation of 3-pentanol in the absenceof NO were (with molar yields) 3-pentanone (26 ± 2%), propionaldehyde (12 ± 2%), acetaldehyde (13 ± 2%) and formaldehyde (2 ± 1%). The primary products of the Cl atom initiated oxidation of 3-pentanol in the presenceof NO were (with molar yields) 3-pentanone (51 ± 4%), propionaldehyde (39 ± 2%), acetaldehyde (44 ± 4%) and formaldehyde (4 ± 1%). The primary products of the OH radical initiated oxidation of 3-pentanol in the presenceof NO were (with molar yields) 3-pentanone (58 ± 3%), propionaldehyde (28 ± 2%), and acetaldehyde (37 ± 2%). In all cases the product yields were independent of oxygen concentration over the partial pressure range 10−700 Torr. The reactions of Cl atoms and OH radicals with 3-pentanol proceed 26 ± 2 and 58 ± 3%, respectively, via attack on the 3-position to give an α-hydroxyalkyl radical, which reacts with O 2to give 3-pentanone. The results are discussed with respect to the literature data and atmospheric chemistry of 3-pentanol. [ABSTRACT FROM AUTHOR]

The reaction of CH3C(O)CH2O2 with HO2 has been studied at 296 K and 700 Torr using long path FTIR spectroscopy, during photolysis of Cl2/acetone/methanol/air mixtures. The branching ratio for the reaction channel forming CH3C(O)CH2O, OH and O2 (reaction (3b)) was investigated in experiments in which OH radicals were scavenged by addition of benzene to the system, with subsequent formation of phenol used as the primary diagnostic for OH radical formation. The observed prompt formation of phenol under conditions when CH3C(O)CH2O2 reacts mainly with HO2 indicates that this reaction proceeds partially by channel (3b), which forms OH both directly and indirectly, by virtue of secondary generation of CH3C(O)O2 (from CH3C(O)CH2O) and its reaction with HO2 (reaction (6c)). The secondary generation of OH radicals was confirmed by the observed formation of CH3C(O)OOH, a well-established product of the CH3C(O)O2 HO2 reaction (via channel (6a)). A number of delayed sources of OH also contribute to the observed phenol formation, such that full characterisation of the system required simulations using a detailed chemical mechanism. The dependence of the phenol and CH3C(O)OOH yields on the initial peroxy radical precursor reagent concentration ratio, [methanol]0/[acetone]0, were well described by the mechanism, consistent with a small but significant fraction of the reaction of CH3C(O)CH2O2 with HO2 proceeding via channel (3b). This allowed a branching ratio of k3b/k3 = 0.15 ± 0.08 to be determined. The results therefore provide strong indirect evidence for the participation of the radical-forming channel of the title reaction. [ABSTRACT FROM AUTHOR]