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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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Fine particulate matter (PM2.5) samples collected at Mount (Mt.) Tai in the North China Plain during summer 2014 were analyzed for dicarboxylic acids and related compounds (oxocarboxylic acids and α-dicarbonyls) (DCRCs). The total concentration of DCRCs was 1050±580 and 1040±490ng m-3 during the day and night, respectively. Although these concentrations were about 2 times lower than similar measurements in 2006, the concentrations reported here were about 1-13 times higher than previous measurements in other major cities in the world. Molecular distributions of DCRCs revealed that oxalic acid (C2) was the dominant species (50%), followed by succinic acid (C4) (12%) and malonic acid (C3) (8%). WRF modeling revealed that Mt. Tai was mostly in the free troposphere during the campaign and long-range transport was a major factor governing the distributions of the measured compounds at Mt. Tai. A majority of the samples (79%) had comparable concentrations during the day and night, with their day-night concentration ratios between 0.9 and 1.1. Multi-day transport was considered an important reason for the similar concentrations. Correlation analyses of DCRCs and their gas precursors and between C2 and sulfate indicated precursor emissions and aqueous-phase oxidations during long-range transport also likely play an important role, especially during the night. Source identification indicated that anthropogenic activities followed by photochemical aging accounted for about 60% of the total variance and were the dominant source at Mt. Tai. However, biomass burning was only important during the first half of the measurement period. Measurements of potassium (K+) and DCRCs were about 2 times higher than those from the second half of the measurement period. The concentration of levoglucosan, a biomass burning tracer, decreased by about 80% between 2006 and 2014, indicating that biomass burning may have decreased between 2006 and 2014.

Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO3) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO3-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO3-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO3 radical, the difficulty of characterizing the spatial distributions of BVOC and NO3 within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry-climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO3-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO3-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

In this work, an optimized protocol to generate an expansion-type liquid clouds with and without UV-visible light irradiation conditions for simulation chamber studies is presented. Sensitivity of the process to key parameters such as initial relative humidities, temperature inhomogeneities, droplets lifetime or seed particle number is illustrated. The obtained clouds have shown that not all seeds particles were always activated and so an iterative numerical method has been re-designed to separate cloud droplets from non-activated seed particles during data analysis allowing the characterization of the cloud droplet formation properties without CCN counter data. Two types of experiments, clouds without irradiation (N-IC) and under UV-visible light irradiation conditions (IC), have been conducted in the CESAM multiphase atmospheric chamber. Measured cloud droplet lifetimes were in good agreement with atmospheric droplet lifetimes. The achieved supersaturation in the cloud was mostly sensitive to the initial relative humidity in the chamber. The comparison between the cloud formation pattern of N-IC and LC was also investigated. Under illumination conditions, the generated clouds clearly showed a gradual activation of seed particles into droplets and thus of the microphysical properties like LWC and droplet concentration, while under dark conditions, clouds faced a flash activation of seed particles. Because this phenomenon may also impact the air/water partitioning of semi-volatile compounds, and it should be considered for further studies, especially in further multiphase photochemical studies implying water-soluble volatile organic compounds in the CESAM chamber. [ABSTRACT FROM AUTHOR]

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]