The wall as a source of hydroxyl radicals in smog chambers

Abstract The role of the wall of a smog chamber as a radical source has been investigated in several ways. From data in the literature evidence is obtained that HNO 3 , present on the reactor wall, may react with NO in the gas phase according to the reaction HNO 3 ( wall ) + 2 NO + H 2 O → 3 HNO 2 to give nitrous acid. Nitrous acid may subsequently photolyze to give hydroxyl radicals. Experimental evidence about the occurrence of this reaction was obtained by u.v.-irradiation of propane-NO x mixtures with and without NH 3 . The presence of NH 3 resulted in a drastically reduced photochemical reactivity, suggesting that neutralization of nitric acid prevented the reaction of HNO 3 with NO. Inclusion of the reaction mentioned above into a computer-model gave a good agreement between experimental and calculated concentration profiles of NO, NO 2 and O 3 in experiments with CO-NO x and propane-NO x , mixtures. The results of our findings and those from others are discussed.