Ammonia and ammonium sulfate in the troposphere over the United Kingdom

Ammonia occurs in the troposphere chiefly as ammonia gas or as ammonium sulfate. The principal source of the ammonia, at least in the UK, appears to be the urea in animal urine. The ammonia thus released is converted on a time scale of hours into ammonium sulfate, the conversion process being quite rapid in the dark, as well as in the light, and over a range of relative humidity (R.H.) values. The most significant mechanisms probably are reaction with sulfur dioxide and oxygen in mist or cloud droplets and reactions with sulfuric acid droplets from power-station plumes. There is no positive evidence for appreciable reaction on solid particulates, but the possibility cannot at present be excluded. Photochemical reactions probably make only a small contribution. A direct gasphase reaction between ammonia and sulfur dioxide does not seem possible at the concentrations and temperatures concerned. It is important to elucidate the mechanisms so that ammonium sulfate levels in the troposphere and their effects, e.g., on visibility, can be predicted.