# Production of HONO from NO 2 uptake on illuminated TiO 2 aerosol particles and following the illumination of mixed TiO 2∕ammonium nitrate particles

The rate of production of HONO from illuminated TiOinline-formula2 aerosols in the presence of NOinline-formula2 was measured using an aerosol flow tube system coupled to a photo-fragmentation laser-induced fluorescence detection apparatus. The reactive uptake coefficient of NOinline-formula2 to form HONO, inline-formula $M7inlinescrollmathml{\mathrm{italic \gamma }}_{chem{\mathrm{normal NO}}_{normal 2}\to \mathrm{normal HONO}}$ 56pt12ptsvg-formulamathimge965a9ac0719984a98bd7c86944994b2 acp-21-5755-2021-ie00003.svg56pt12ptacp-21-5755-2021-ie00003.png , was determined for NOinline-formula2 mixing ratios in the range 34–400 ppb, with inline-formula $M9inlinescrollmathml{\mathrm{italic \gamma }}_{chem{\mathrm{normal NO}}_{normal 2}\to \mathrm{normal HONO}}$ 56pt12ptsvg-formulamathimgfd26b5e871b886faf9e57384d30def4e acp-21-5755-2021-ie00004.svg56pt12ptacp-21-5755-2021-ie00004.png spanning the range (9.97 inline-formula± 3.52) inline-formula× 10inline-formula−6 to (1.26 inline-formula± 0.17) inline-formula× 10inline-formula−4 at a relative humidity of 15 inline-formula± 1 % and for a lamp photon flux of (1.63 inline-formula± 0.09) inline-formula×1016 photons cminline-formula−2 sinline-formula−1 (integrated between 290 and 400 nm), which is similar to midday ambient actinic flux values. inline-formula $M21inlinescrollmathml{\mathrm{italic \gamma }}_{chem{\mathrm{normal NO}}_{normal 2}\to \mathrm{normal HONO}}$ 56pt12ptsvg-formulamathimge4944b59f202921cae73615fc76d94d6 acp-21-5755-2021-ie00005.svg56pt12ptacp-21-5755-2021-ie00005.png increased as a function of NOinline-formula2 mixing ratio at low NOinline-formula2 before peaking at (1.26 inline-formula± 0.17) inline-formula $M25inlinescrollmathml×{normal 10}^{-normal 4}$ 32pt14ptsvg-formulamathimgae2ff01d6ba96c5714e9e8f527947715 acp-21-5755-2021-ie00006.svg32pt14ptacp-21-5755-2021-ie00006.png at inline-formula∼ 51 ppb NOinline-formula2 and then sharply decreasing at higher NOinline-formula2 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 TiOinline-formula2 aerosol surfaces found at inline-formula∼ 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 NOinline-formula2, as well as a surface HONO loss mechanism which is dependent upon NOinline-formula2. In a separate experiment, significant production of HONO was observed from illumination of mixed inline-formula $M33inlinescrollmathmlchem\mathrm{normal nitrate}/{\mathrm{normal TiO}}_{normal 2}$ 61pt14ptsvg-formulamathimge4d3fcd02f9990fcc9c76af506564a9c acp-21-5755-2021-ie00007.svg61pt14ptacp-21-5755-2021-ie00007.png aerosols in the absence of NOinline-formula2. However, no production of HONO was seen from the illumination of nitrate aerosols alone. The rate of production of HONO observed from mixed inline-formula $M35inlinescrollmathmlchem\mathrm{normal nitrate}/{\mathrm{normal TiO}}_{normal 2}$ 61pt14ptsvg-formulamathimg90e10df1e58a7f6aa9187f016c0f5d64 acp-21-5755-2021-ie00008.svg61pt14ptacp-21-5755-2021-ie00008.png 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 NOinline-formulax in the marine boundary layer where mixed aerosols containing nitrate and a photocatalytic species such as TiOinline-formula2, as found in dust, are present.

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Dyson, Joanna E. / Boustead, Graham A. / Fleming, Lauren T. / et al: Production of HONO from NO2 uptake on illuminated TiO2 aerosol particles and following the illumination of mixed TiO2∕ammonium nitrate particles. 2021. Copernicus Publications.

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