Acetone photolysis is a potentially important source of hydroperoxyl and hydroxyl radicals (HOx) to the upper troposphere. The extent to which acetone photolysis is a significant source of HOx in the upper troposphere is unclear in part because of scarce measurements of the acetone photolysis quantum yield (Phi(acetone)) in the actinic region (i.e., lambda > 300 nm). Past measurements of the Phi(acetone) have derived temperature- and pressure-dependent parameterizations that lead to significantly different conclusions about the importance of acetone to HOx formation in the upper troposphere. Here, we focus on previously published data to derive the recommended Phi(acetone) for atmospheric chemical modeling. Using Stern-Volmer analyses, we determine temperature- and pressure-dependent Phi(acetone) using updated measurements of fundamental acetone photolysis parameters. We also use simulations of the Phi(acetone) produced from recent photophysical modeling to derive temperature- and pressure-dependent parameterizations of the Phi(acetone). In contrast to the current Phi(acetone) parameterization used in atmospheric chemical modeling, our parameterization reflects a predicted nonzero Phi(acetone) in the wavelength region above 320 nm. Despite the increased Phi(acetone) values in the higher wavelength region of the acetone photolysis action spectrum, the modeled effect on HOx production is not significantly different from HOx produced using the current recommended Phi(acetone) parameterization. Uncertainties in the acetone photolysis mechanism remain; thus, more direct temperature- and pressure-dependent measurements of Phi(acetone) are warranted.

Ozone and stratospheric ozone tracer files obtained from full-chemistry and tagged ozone simulations in GEOS-Chem model are uploaded here. These files have been used to estimate the upper tropospheric ozone concentrations during the North Indian Ocean cyclones.

Multiphase chemistry occurs between chemicals in different atmospheric phases, typically involving gas-solid and gas-liquid interactions. The importance of atmospheric multiphase chemistry has long been recognized. Its central role extends from acid precipitation and stratospheric ozone depletion to its impact on the oxidizing capacity of the troposphere and to the roles that aerosol particles play in driving chemistry-climate interactions and affecting human health. This opinion article briefly introduces the subject of multiphase chemistry and tracks its development before and after the start of Atmospheric Chemistry and Physics. Most of the article focuses on research opportunities and challenges in the field. Central themes are that a fundamental understanding of the chemistry at the molecular level underpins the ability of atmospheric chemistry to accurately predict environmental change and that the discipline of multiphase chemistry is strongest when tightly connected to atmospheric modeling and field observations.

Nitrous oxide (N2O) is the fourth most important greenhouse gas in the atmosphere and is considered the most important current source gas emission for global stratospheric ozone depletion (O3). It has natural and anthropogenic sources, mainly as an unintended by-product of food production activities. This work examines the identification and quantification of trends in the N2O concentration from the middle troposphere to the middle stratosphere (MTMS) by in situ and remote sensing observations. The temporal variability of N2O is addressed using a comprehensive dataset of in situ and remote sensing N2O concentrations based on aircraft and balloon measurements in the MTMS from 1987 to 2018. We determine N2O trends in the MTMS, based on observations. This consistent dataset was also used to study the N2O seasonal cycle to investigate the relationship between abundances and its emission sources through zonal means. The results show a long-term increase in global N2O concentration in the MTMS with an average of 0.89 ± 0.07 ppb/yr in the troposphere and 0.96 ± 0.15 ppb/yr in the stratosphere, consistent with 0.80 ppb/yr derived from ground-based measurements and 0.799 ± 0.024 ppb/yr ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer) satellite measurements.

The transition away from the production and consumption of high global warming potential (GWP) hydrofluorocarbons (HFCs) under the 2016 Kigali Amendment to the Montreal Protocol on Substances that Deplete the Ozone Layer (Montreal Protocol) has prompted air conditioning, refrigeration, and heat pump equipment manufacturers to seek alternative refrigerants with lower direct climate impacts. Additional factors affecting alternative refrigerant choice include safety (i.e., flammability and toxicity), environmental, and thermodynamic constraints. At the same time, manufacturers are incentivized to seek refrigerants with higher energy efficiency, which saves on electricity costs and reduces indirect greenhouse gas emissions from electricity generation. The life cycle climate performance (LCCP) metric is commonly used to assess the combined direct and indirect climate impacts of refrigerant-use equipment. Here, we consider an additional impact on climate performance: the degradation of refrigerant in equipment, i.e., the direct climate impacts of high-GWP byproducts that can form as the result of adding trifluoroiodomethane (CF3I) to refrigerant blends to reduce flammability. Such a production of high-GWP gases could change the acceptability of CF3I-containing refrigerants. Further, it highlights the need to understand refrigerant degradation within equipment in calculations of the environmental acceptability of new cooling technology.

Remarkable perturbations in the stratospheric abundances of chlorine species and ozone were observed over Southern Hemisphere mid-latitudes following the 2020 Australian wildfires1,2. These changes in atmospheric chemical composition suggest that wildfire aerosols affect stratospheric chlorine and ozone depletion chemistry. Here we propose that wildfire aerosol containing a mixture of oxidized organics and sulfate3-7 increases hydrochloric acid solubility8-11 and associated heterogeneous reaction rates, activating reactive chlorine species and enhancing ozone loss rates at relatively warm stratospheric temperatures. We test our hypothesis by comparing atmospheric observations to model simulations that include the proposed mechanism. Modelled changes in 2020 hydrochloric acid, chlorine nitrate and hypochlorous acid abundances are in good agreement with observations1,2. Our results indicate that wildfire aerosol chemistry, although not accounting for the record duration of the 2020 Antarctic ozone hole, does yield an increase in its area and a 3-5% depletion of southern mid-latitude total column ozone. These findings increase concern2,12,13 that more frequent and intense wildfires could delay ozone recovery in a warming world.

Hydrofluoroolefins (HFOs) and related haloalkenes are a family of compounds, primarily man-made, with many industrial applications. Gas-phase electrophilic addition of OH to the olefinic bond represents the primary sink for these chemicals in the atmosphere. The degree and type of halogenation strongly affect their chemical reactivity, leading to differing reactivities with the OH radical that have presented a challenge for structure-activity relationships (SARs). Here, we investigate and extend the SARs to estimate temperature-dependent OH reaction rate coefficients, k(T), at tropospheric temperatures. We considered two techniques: the group-additivity approach of Atkinson and coworkers and the recent method of Tokuhashi and co-workers; we found the latter to make superior predictions for halogenated olefins. We extended Tokuhashi et al.'s SAR to include more olefins and to predict temperature dependencies. We compared SAR predictions against new absolute k(T) for two HFOs (3,3,3-trifluoropropene and 1,1,3,3-tetrafluoropropene) measured using the pulsed-laser photolysis-laser-induced fluorescence technique from 212 to 373 K. The Arrhenius expressions were determined as k(OH+CF3CH=CH2)(T) = (8.86 +/- 0.82) x 10(-13) exp[(159 +/- 26)/T] and k(OH+CF2HCH=CF2)(T) = (7.46 +/- 0.34) x 10(-13) exp[(365 +/- 12)/T]. The measured k(T) was predicted accurately between 200 and 400 K using the modified Tokuhashi approach. Our new measurements of the OH reaction rate coefficient with 3,3,3-trifluoropropene were in excellent agreement with literature determinations. We represent the first reported k(T) for 1,1,3,3-tetrafluoropropene. Given that new HFOs enter the market frequently, such estimation techniques may be a helpful screening tool for assessing their environmental impact before they are examined further and reach the mass-production phase.

Abstract Extremes in stratospheric abundances of HCl (record low), ClONO2 (record high), and ozone (record low) were observed over southern hemisphere mid-latitudes following the 2020 Australian wildfires by satellite records spanning 15–20 years. These unprecedented changes suggest that wildfire aerosols pose a challenge to stratospheric chlorine and ozone depletion chemistry. Here we propose a wildfire smoke chemical mechanism linking data from the field and the laboratory, and test it by comparison of these extraordinary observations to model simulations. Aged wildfire aerosols likely contain a mix of oxidized organics. Laboratory data indicate that HCl is extremely soluble in such species at relatively warm stratospheric temperatures in mid-latitudes and Antarctic autumn. We find that the HCl solubility in oxidized organics, and associated enhancements in heterogeneous reaction rates in/on the aerosols, yield remarkable modelled changes in 2020 HCl, ClONO2, and HOCl abundances, in good agreement with observations. Our results indicate that wildfire smoke does not explain the record duration of the 2020 Antarctic ozone hole, but does yield an increase in its area, as well as 3-5% depletion of southern mid-latitude total column ozone. These findings increase concern that more frequent and intense wildfires could delay ozone recovery in a warming world.

Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases. Despite extensive research on molecular chlorine (Cl 2 ), a Cl precursor, in the polar atmosphere, its sources in other regions are still poorly understood. Here we report the daytime Cl 2 concentrations of up to 1 ppbv observed in a coastal area of Hong Kong, revealing a large daytime source of Cl 2 (2.7 pptv s −1 at noon). Field and laboratory experiments indicate that photodissociation of particulate nitrate by sunlight under acidic conditions (pH < 3.0) can activate chloride and account for the observed daytime Cl 2 production. The high Cl 2 concentrations significantly increased atmospheric oxidation. Given the ubiquitous existence of chloride, nitrate, and acidic aerosols, we propose that nitrate photolysis is a significant daytime chlorine source globally. This so far unaccounted for source of chlorine can have substantial impacts on atmospheric chemistry.

Measurements of photolysis quantum yields are challenging because of the difficulties in measuring the first-generation photodissociation products, interference from other products or contaminants, sufficient photon fluxes and/or low absorption cross sections of the photolyte to make detectable amounts of products, and quantification of the photon flux. In the case of acetone (and other atmospherically relevant ketones) the uncertainty in the photolysis quantum yield creates uncertainty in the calculated OH radical and acyl peroxy nitrate production in the atmosphere. We present a new method for determining photodissociation product quantum yields by measuring acyl peroxy radicals (RC(O)O2) produced in the photolysis of ketones in air using chemical ionization mass spectrometry (CIMS). We show good agreement of our CIMS method with previously published quantum yields of the acyl radical from photolysis of biacetyl and methyl ethyl ketone (MEK) at 254 nm. Additionally, we highlight the capabilities of this CIMS method through the measurement of photolysis branching ratios for MEK. We suggest future applications of CIMS (in the laboratory and field) to measure RC(O)O2 and associated photolysis processes.

Abstract. Rate coefficients for the reaction of NO3 radicals with a series of aromatic aldehydes were measured in a 7300 liter simulation chamber at ambient temperature and pressure by relative and absolute methods. The rate coefficients for benzaldehyde (BA), ortho-tolualdehyde (O-TA), meta-tolualdehyde (M-TA), para-tolualdehyde (P-TA), 2,4-dimethyl benzaldehyde (2,4-DMBA), 2,5-dimethyl benzaldehyde (2,5-DMBA) and 3,5-dimethyl benzaldehyde (3,5-DMBA) were: k1 = 2.6 ± 0.3, k2 = 8.8 ± 0.8, k3 = 4.8 ± 0.5, k4 = 4.9 ± 0.5, k5 = 15.1 ± 1.4, k6 = 12.7 ± 1.2 and k7 = 6.2 ± 0.6, respectively, in the units of 10−15 cm3 molecule−1 s−1 at 298 ± 2 K. The rate coefficient k13 for the reaction of the NO3 radical with deuterated benzaldehyde (benzaldehyde-d1) was found to be half that of k1. The end product of the reaction with an excess of NOx was measured to be C6H5C(O)O2NO2. Theoretical calculations of aldehydic bond energies and reaction pathways indicate that NO3 radical reacts with aromatic aldehydes through the abstraction of aldehydic hydrogen atom. The atmospheric implications of the measured rate coefficients are briefly discussed.

We have investigated trifluoroacetic acid (TFA) formation from emissions of HFO-1234yf (CF3CFH2), its dry and wet deposition, and rainwater concentration over India, China, and the Middle East with GEOS-Chem and WRF-Chem models. We estimated the TFA deposition and rainwater concentrations between 2020 and 2040 for four previously published HFO-1234yf emission scenarios to bound the possible levels of TFA. We evaluated the capability of GEOS-Chem to capture the wet deposition process by comparing calculated sulfate in rainwater with observations. Our calculated TFA amounts over the USA, Europe, and China were comparable to those previously reported when normalized to the same emission. A significant proportion of TFA was found to be deposited outside the emission regions. The mean and the extremes of TFA rainwater concentrations calculated for the four emission scenarios from GEOS-Chem and WRF-Chem were orders of magnitude below the no observable effect concentration. The ecological and human health impacts now, and the continued use of HFO-1234yf in India, China, and the Middle East, are estimated to be insignificant based on the current understanding, as summarized by Neale et al. (2021).

Which published papers have transformed our understanding of the chemical processes in the troposphere and shaped the field of atmospheric chemistry? By way of expert solicitation and interactive peer review, this paper explores the influence of the ideas in peer-reviewed articles based on input from our community of atmospheric scientists. We explore how these papers have shaped the development of the field of atmospheric chemistry and identify the major landmarks in the field of atmospheric chemistry through the lens of those papers' impact on science, legislation and environmental events. We also explore the ways in which one can identify the papers that have most impacted the field and discuss the advantages and disadvantages of the various approaches. Our work highlights the difficulty of creating a simple list, and we explore the reasons for this difficulty. The paper also provides a history of the development of our understanding of tropospheric chemistry and points some ways for the future.

Plug gaps to measure ozone-destroying chemicals and greenhouse gases and verify compliance with Paris and Montreal treaties. Plug gaps to measure ozone-destroying chemicals and greenhouse gases and verify compliance with Paris and Montreal treaties.

In 1974, Mario J. Molina and F. Sherwood Rowland warned that chlorofluorocarbons (CFCs) could destroy the stratospheric ozone layer, which protects Earth against the harmful effects of ultraviolet radiation [Molina and Rowland Nature 1974, 249, 810]. In 1975, Ramanathan warned that CFCs are powerful greenhouse gases (GHGs) and would rival carbon dioxide (CO2) in causing climate change if left unabated [Ramanathan Science 1975, 190, 50]. The 1987 Montreal Protocol on Substances that Deplete the Ozone Layer (Protocol), arguably the most successful global environmental treaty in history, was enacted in response to these warnings. This Protocol has phased out almost 99% of the production and consumption of ozone depleting substances (ODSs). Other papers have explored the "world avoided" by actions under the Protocol [Prather et al. Nature 1996, 381, 551; Newman et al. Atmos. Chem. Phys. 2009, 9, 2113; Morgenstern et al. Geophys. Res. Lett. 2008, 35, 1]. They concluded that the ozone layer would have been highly depleted across the globe by the mid-21st century without the Protocol and that the Protocol contributed significantly to reduce climate change. This paper explores what could have been achieved if the world had acted against the continued use of ODSs, which were both ozone-depleting and greenhouse gases, immediately after Molina and Rowland warned of stratospheric ozone depletion and Ramanathan warned of climate forcing using chemicals and technology that were already globally available in the mid-1970s. We show that such "precautionary principle" actions would have reduced global ozone layer depletion, reduced the extent of the ozone hole, brought forward the dates for ozone layer recovery, and helped minimize climate change.