The UV photolysis at lambda >= 290 nm in air of a mixture of NaNO2/NaCl coated with 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC) was followed in real time in the absence and presence of water vapor by using diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) at 23 degrees C. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) was used to confirm the identification of the products. Photolysis of NO2- is known to generate O-, which in the presence of water forms OH + OH-. Irradiation of the OPPC/NaNO2/NaCl mixture led to a loss of nitrite and the formation of organic nitrates and carbonyl compounds. In the absence of added water vapor, carboxylate ions were also formed. These products are due to oxidation of OPPC by O- and OH radicals. The organic products formed per calculated O-/OH generated by photolysis increased with relative humidity, consistent with a competition between OPPC and NO2- for OH. This suggests a new mechanism of oxidation of organics on particles and on surfaces in air that have nitrite ions available for photolysis. Similar chemistry is likely to occur for nitrate ions, which also photolyze to generate O-.