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The photodegradation of nitrous acid in the troposphere is an initiation step in the formation of photochemical smog. NOx emissions from internal combustion engines react with atmospheric water vapor during sundown hours to form nitrous acid. Daytime sunlight cleaves nitrous acid into OB and NO radicals, which attack hydrocarbons emitted by industry to form the constituents of photochemical smog. In order to increase our understanding of these reactions, we have studied them in the liquid phase. Aqueous nitrous acid was extracted into a liquid benzene matrix to form a clear solution of approximately 0.075 M nitrous acid in benzene, which was irradiated at 366 nm. The photolysis generated a yellow liquid phase and a dark red precipitate. The liquid phase products identified by GC-MS are: p-benzoquinone; phenol; nitrobenzene; 0-, m -, and p-nitrophenol; 1,3-and l,4-dinitrobenzene; 2,4dinitrophenol; biphenyl; and 3-and 4-nitrobiphenyl. Both photochemical and thermal reactions appear to be responsible for the formation of these products. Changes in the product abundances are observed over several hours of irradiation with the sharpest changes occurring in the first hour. Reaction pathways are proposed for all products, and potential mechanisms are discussed. The red precipitate is insoluble in benzene and ether but soluble in water, methanol and acetone. 13C-NMR spectroscopy has been inconclusive in identification of the red precipitate.



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