Nitrous acid in the atmosphere and laboratory experiments on its photolysis

Low concentrations of nitrous acid in air were estimated by a differential absorption method. Over a three-month period, atmospheric concentrations at one location in southern England varied between 0.4 and 11 ppb while those of nitrogen dioxide varied between 4 and 21 ppb. The highest proportion of acid was found in air which had passed over the industrial regions of W. Europe. In the laboratory, concentrations of about 1 ppm of nitrous acid with less than 0.03 ppm of nitrogen dioxide were produced by blowing air over a shallow layer of very dilute acidified nitrite solution. Exposure of the vapour to the light from a pyrex-shielded mercury arc caused decomposition, with production of nitrogen dioxide. This decomposition was apparently prevented by some olefins. The experiments did not support the view that nitrous acid is a source of hydroxyl radicals in the atmosphere.