Effect of inorganic salts on N'containing organic compounds formed by heterogeneous reaction of NO2 with oleic acid

Fatty acids are ubiquitous constituents of grime on urban and indoor surfaces and they represent important surfactants on organic aerosol particles in the atmosphere. Here, we assess the heterogeneous processing of NO2 on films consisting of pure oleic acid (OA) or a mixture of OA and representative salts for urban grime and aerosol particles, namely Na2SO4 and NaNO3. The uptake coefficients of NO2 on OA under light irradiation (300 nm < λ < 400 nm) decreased with increasing relative humidity (RH), from (1.4 ± 0.1) × 10-6 at 0% RH to (7.1 ± 1.6) × 10-7 at 90% RH. The uptake process of NO2 on OA gives HONO as a reaction product, and the highest HONO production was observed upon the heterogeneous reaction of NO2 with OA in the presence of nitrate (NO3-) ions. The formation of gaseous nitroaromatic compounds was also enhanced in the presence of NO3- ions upon light-induced heterogeneous processing of NO2 with OA, as revealed by membrane inlet single-photon ionization time-of-flight mass spectrometry (MI-SPI-TOFMS). These results suggest that inorganic salts can affect the heterogeneous conversion of gaseous NO2 on fatty acids and enhance the formation of HONO and other N-containing organic compounds in the atmosphere.