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. The rate of production of HONO from illuminated TiO2 aerosols in thepresence of NO2 was measured using an aerosol flow tube system coupledto a photo-fragmentation laser-induced fluorescence detection apparatus. Thereactive 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 arelative humidity of 15 ± 1 % and for a lamp photon flux of (1.63 ± 0.09) ×1016 photons cm−2 s−1 (integratedbetween 290 and 400 nm), which is similar to midday ambient actinic fluxvalues. γNO2→HONO increased as a function of NO2mixing ratio at low NO2 before peaking at (1.26 ± 0.17) ×10-4 at ∼ 51 ppb NO2 and then sharply decreasingat higher NO2 mixing ratios rather than levelling off, which would beindicative of surface saturation. The dependence of HONO production onrelative humidity was also investigated, with a peak in production of HONOfrom TiO2 aerosol surfaces found at ∼ 25 % RH.Possible mechanisms consistent with the observed trends in both the HONOproduction and reactive uptake coefficient were investigated using azero-dimensional kinetic box model. The modelling studies supported amechanism for HONO production on the aerosol surface involving two moleculesof NO2, as well as a surface HONO loss mechanism which is dependentupon NO2. In a separate experiment, significant production of HONO wasobserved from illumination of mixed nitrate/TiO2 aerosols in theabsence of NO2. However, no production of HONO was seen from theillumination of nitrate aerosols alone. The rate of production of HONOobserved from mixed nitrate/TiO2 aerosols was scaled to ambientconditions found at the Cape Verde Atmospheric Observatory (CVAO) in theremote tropical marine boundary layer. The rate of HONO production fromaerosol particulate nitrate photolysis containing a photocatalyst was foundto be similar to the missing HONO production rate necessary to reproduceobserved concentrations of HONO at CVAO. These results provide evidence thatparticulate nitrate photolysis may have a significant impact on theproduction of HONO and hence NOx in the marine boundary layer wheremixed aerosols containing nitrate and a photocatalytic species such asTiO2, as found in dust, are present.

atmospheric chemistry and physics

Production of HONO from NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; uptake on illuminated TiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; aerosol particles and following the illumination of mixed TiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;∕ammonium nitrate particles