The rate of production of HONO from illuminated TiO.sub.2 aerosols in the presence of NO.sub.2 was measured using an aerosol flow tube system coupled to a photo-fragmentation laser-induced fluorescence detection apparatus. The reactive uptake coefficient of NO.sub.2 to form HONO, Î³NO2âHONO, was determined for NO.sub.2 mixing ratios in the range 34-400 ppb, with Î³NO2âHONO spanning the range (9.97 Â± 3.52) x 10.sup.-6 to (1.26 Â± 0.17) x 10.sup.-4 at a relative humidity of 15 Â± 1 % and for a lamp photon flux of (1.63 Â± 0.09) x10.sup.16 photons cm.sup.-2 s.sup.-1 (integrated between 290 and 400 nm), which is similar to midday ambient actinic flux values. Î³NO2âHONO increased as a function of NO.sub.2 mixing ratio at low NO.sub.2 before peaking at (1.26 Â± 0.17) x10-4 at â¼ 51 ppb NO.sub.2 and then sharply decreasing at higher NO.sub.2 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 TiO.sub.2 aerosol surfaces found at â¼ 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 NO.sub.2, as well as a surface HONO loss mechanism which is dependent upon NO.sub.2 . In a separate experiment, significant production of HONO was observed from illumination of mixed nitrate/TiO2 aerosols in the absence of NO.sub.2 . However, no production of HONO was seen from the illumination of nitrate aerosols alone. 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 NO.sub.x in the marine boundary layer where mixed aerosols containing nitrate and a photocatalytic species such as TiO.sub.2, as found in dust, are present.