Photochemical formation of nitrite/nitrous acid (HONO) upon irradiation of nitrophenols in aqueous solution and in viscous secondary organic aerosol proxy

Irradiated nitrophenols can produce nitrite and HONO in bulk aqueous solutions and in viscous aqueous films, simulating the conditions of a high solute strength aqueous aerosol, with comparable quantum yields in solution and viscous films (10-5-10-4 in the case of 4-nitrophenol), and overall reaction yields up to 0.3 in solution. The process is particularly important for the para-nitrophenols, possibly because their less sterically hindered nitro groups can be released more easily as nitrite/HONO. The nitrophenols giving the highest photoproduction rates of nitrite/HONO (most notably, 4-nitrophenol and 2-methyl-4-nitrophenol) could significantly contribute to the occurrence of nitrite in aqueous phases in contact with the atmosphere. Interestingly, dew water evaporation has shown potential to contribute to the gas-phase HONO levels during the morning, which accounts for the possible importance of the studied process.