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Sulfurous acid (H2SO3) is known to be thermodynamically instable decomposing into SO2 and H2O. All attempts to detect this elusive acid in solution failed up to now. Reported H2SO3 formation from an experiment carried out in a mass spectrometer as well as results from theoretical calculations, however, indicated a possible kinetic stability in the gas phase. Here, it is shown experimentally that H2SO3 is formed in the OH radical‐initiated gas‐phase oxidation of methanesulfinic acid (CH3S(O)OH) at 295±0.5 K and 1 bar of air with a molar yield of 53-17+7 ${{53}_{-17}^{+\ 7}}$ %. Further main products are SO2, SO3 and methanesulfonic acid. CH3S(O)OH represents an important intermediate product of dimethyl sulfide oxidation in the atmosphere. Global modeling predicts an annual H2SO3 production of ∼8 million metric tons from the OH+CH3S(O)OH reaction. The investigated H2SO3 depletion in the presence of water vapor results in k(H2O+H2SO3) <3×10−18 cm3 molecule−1 s−1, which indicates a lifetime of at least one second for atmospheric humidity. This work provides experimental evidence that H2SO3, once formed in the gas phase, is kinetically stable enough to allow its characterization and subsequent reactions. [ABSTRACT FROM AUTHOR]

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Chemical processing of reactive nitrogen species, especially of NOx (= NO + NO2) and nitrous acid (HONO), determines the photochemical ozone production and oxidation capacity in the troposphere. However, sources of HONO and NOx in the remote marine atmosphere are still poorly understood. In this work, the multiphase chemistry mechanism CAPRAM in the model framework SPACCIM was used to study HONO formation at Cape Verde (CVAO) in October 2017, adopted with the input of current parameterizations for various HONO sources. Three simulations were performed that adequately reproduced ambient HONO levels and its diurnal pattern. The model performance for NOx and O3 improves significantly when considering dust‐surface‐photocatalytic conversions of reactive nitrogen compounds with high correlation coefficients up to 0.93, 0.56, and 0.89 for NO, NO2, and O3, respectively. Photocatalytic conversion of the adsorbed HNO3 on dust is modeled to be the predominant contributor for daytime HONO at CVAO, that is, accounting for about 62% of the chemical formation rate at noontime. In contrast, the ocean‐surface‐mediated conversion of NO2 to HONO and other discussed pathways are less important. The average OH levels at midday (9:00–16:00) modeled for cluster trajectory 1, 2, and 3 are 5.2, 5.1, and 5.2 × 106 molecules cm−3, respectively. Main OH formation is driven by O3 photolysis with a contribution of 74.6% to the total source rate, while HONO photolysis is negligible (∼1.8%). In summary, this study highlights the key role of dust aerosols for HONO formation and NOx cycling at CVAO and possibly in other dust‐affected regions, urgently calling for further investigations using field and model studies. Plain Language Summary: Chemical processing of NOx (= NO + NO2) and nitrous acid (HONO) is important for the tropospheric O3 budget and oxidation capacity. However, the sources of HONO and cycling of NOx in the remote marine atmosphere are still poorly explored. A detailed multiphase chemistry model simulation showed a better performance of HONO, NOx and O3 when considering dust‐surface‐photocatalytic conversions of reactive nitrogen compounds, especially the photocatalytic conversion of the adsorbed HNO3 on dust. The simulations demonstrated that OH formation is mainly driven by the O3 photolysis, while HONO photolysis is a negligible OH radical source due to its low concentration levels at Cape Verde. The study highlights the key role of dust aerosols for HONO and NOx chemistry in the remote marine boundary layer. Key Points: The sources of HONO and NOx at Cape Verde are well modeled with CAPRAMPhotocatalytic conversion of adsorbed HNO3 on dust is the predominant contributor for daytime HONOPhotolysis of O3 is the prevailing source of OH radical at Cape Verde, while HONO photolysis is a negligible OH radical source [ABSTRACT FROM AUTHOR]

Chemical processes in clouds and fogs can substantially alter atmospheric oxidant budgets and lead to aerosol mass formation. However, many regional and global models do not include detailed aqueous‐phase chemical mechanisms due to the (a) lack of complete understanding of the chemical processes and (b) computational burden of adding constituents. Current gas‐aqueous chemistry 0‐dimensional models were evaluated in a cloud‐chemistry box model intercomparison based on a mid‐September 2016 cloud chemistry event at Whiteface Mountain, New York. Multiphase mechanisms in the five participating models ranged from those appropriate for 3‐d models to highly complex with thousands of reactions. This study focused on oxidant levels in both phases and aqueous‐phase sulfate and organic acid formation. Comparison of gas‐phase‐only chemistry gives very similar oxidant predictions at night but shows significant differences during daytime with the hydroxyl radical (OH) variability of about an order of magnitude. The variability in the model results increases substantially with aqueous chemistry due to different Henry's Law constants, aqueous‐phase reaction rate constants, and chemical mechanisms. Using a prescribed liquid water content and pH value of 4.5, modeled aqueous OH, aldehyde, and organic acid concentrations differ by over an order of magnitude in daytime. Simulations were also conducted at a pH = 5.1, predicted variable pH, and with added transition metal ion chemistry. While we compare predicted and measured inorganic anions and water‐soluble organic carbon, we cannot do so for aqueous‐phase oxidant concentrations due to the lack of measurements. We highlight a need for recommended equilibrium and aqueous‐phase rate constants. Plain Language Summary: Accurate description of cloud chemistry is needed in models used to predict air quality and climate. A comprehensive international cloud box model intercomparison with physical and chemical observational constraints from mountain top sampling highlights the need for a concerted effort to develop robust aqueous‐phase chemical mechanisms. Prediction of some key species differ among the models by orders of magnitude. We investigate the underlying, fundamental chemical explanations for the discrepancy as a diagnosis to understand the chemical parameters most in need of further investigation efforts. Key Points: Five box models participated in a cloud chemistry intercomparison representing a case at Whiteface Mountain ObservatoryModel results were often quite different indicating a need for recommended equilibria and aqueous‐phase reaction rate constantsComparisons of simple to detailed chemistry schemes show need for development of effective simplified organic aqueous chemistry schemes [ABSTRACT FROM AUTHOR]