Mass balances of sulphur and nitrogen oxides over Great Britain

From aircraft measurements in air entering and leaving Great Britain conclusions about the cycles of nitrogen and sulphur oxides on a regional scale are drawn. Using a tracer it is possible to follow the dispersion of plumes over distances of hundreds of km. For sulphur oxides, in the absence of precipitation, overall removal is shown not to be very efficient with 70 % of the emitted SO₂ still airborne over the coast. Estimates are made of the deposition velocity of SO₂ and its rate of oxidation to sulphate. For nitrogen oxides the removal mechanisms in dry conditions appear to be of similar magnitude, though individual loss mechanisms are less certain. The importance of background levels in air entering the country is assessed. The roles of other airborne materials, such as O₃ and ammonium (of which observations were made) are related to the S and NO_x cycles in the atmosphere. It is argued that they and their degree of mixing with polluted air may have a strong influence on removal in rain.