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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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The hydroxyl radical (OH) dominates the removal of atmospheric pollutants, with nitrous acid (HONO) recognised as a major OH source. For remote regions HONO production through the action of sunlight at aerosol surfaces can provide a source of nitrogen oxides. In this study, HONO production rates at illuminated aerosol surfaces are measured under atmospheric conditions, and a model consistent with the data is developed, and aerosol production of HONO in the atmosphere is shown to be significant.
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https://doi.org/10.5194/acp-2020-1216
https://doi.org/10.5194/acp-2020-1216

  01 Dec 2020

01 Dec 2020

Review status: this preprint is currently under review for the journal ACP.

Production of HONO from heterogeneous uptake of NO2 on illuminated TiO2 aerosols measured by Photo-Fragmentation Laser Induced Fluorescence

Joanna E. Dyson1, Graham A. Boustead1, Lauren T. Fleming1, Mark Blitz1,2, Daniel Stone1, Stephen R. Arnold3, Lisa K. Whalley1,2, and Dwayne E. Heard1 Joanna E. Dyson et al.
  • 1School of Chemistry, University of Leeds, LS2 9JT, UK
  • 2National Centre of Atmospheric Science, University of Leeds, LS2 9JT, UK
  • 3School of Earth and Environment, University of Leeds, LS2 9JT, UK

Abstract. The rate of production of HONO from illuminated TiO2 aerosols in the presence of NO2 was measured using an aerosol flow tube coupled to a photo-fragmentation laser induced fluorescence detection apparatus. The reactive uptake coefficient of NO2 to form HONO, γNO2→HONO, was determined for NO2 mixing ratios in the range 34–400 ppb, with γNO2→HONO spanning the range (9.97 ± 3.52) × 10−6 to (1.26 ± 0.17) × 10−4 at a relative humidity of 15 ± 1 % and for a lamp photon flux of (1.63 ± 0.09) × 1016 photons cm−2 s −1 (integrated between 290 and 400 nm), which is similar to values of ambient actinic flux at midday. γNO2→HONO increased as a function of NO2 mixing ratio at low NO2 before peaking at (1.26 ± 0.17) × 10−4 at 51 ppb NO2 and then sharply decreasing at higher NO2 mixing ratios, rather than levelling off which would be indicative of surface saturation. The dependence of HONO production on relative humidity was also investigated, with a peak in production of HONO from TiO2 aerosol surfaces found at ~25 % RH. Possible mechanisms consistent with the observed trends in both the HONO production and reactive uptake coefficient were investigated using a zero-dimensional kinetic box model. The modelling studies supported a mechanism for HONO production on the aerosol surface involving two molecules of NO2, as well as a surface HONO loss mechanism which is dependent upon NO2. In a separate experiment, significant production of HONO was observed from illumination of mixed nitrate/TiO2 aerosols in the absence of NO2. However, no statistically significant production of HONO was seen from the illumination of pure nitrate aerosols. The rate of production of HONO observed from mixed nitrate/TiO2 aerosols was scaled to ambient conditions found at the Cape Verde Atmospheric Observatory (CVAO) in the remote tropical marine boundary layer. The rate of HONO production from aerosol particulate nitrate photolysis containing a photocatalyst was found to be similar to the missing HONO production rate necessary to reproduce observed concentrations of HONO at CVAO. These results provide evidence that particulate nitrate photolysis may have a significant impact on the production of HONO and hence NOx in the marine boundary layer where mixed aerosols containing nitrate and a photocatalytic species such as TiO2, as found in dust, are present.

Joanna E. Dyson et al.

 
Status: open (until 26 Jan 2021)
Status: open (until 26 Jan 2021)
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Joanna E. Dyson et al.

Joanna E. Dyson et al.

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Short summary
The hydroxyl radical (OH) dominates the removal of atmospheric pollutants, with nitrous acid (HONO) recognised as a major OH source. For remote regions HONO production through the action of sunlight at aerosol surfaces can provide a source of nitrogen oxides. In this study, HONO production rates at illuminated aerosol surfaces are measured under atmospheric conditions, and a model consistent with the data is developed, and aerosol production of HONO in the atmosphere is shown to be significant.
Citation
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