The sulfur cycle.

Even granting our uncertainties about parts of our model of the sulfur cycle, we can draw some conclusions from it: 1) Man is now contributing about one half as much as nature to the total atmospheric burden of sulfur compounds, but by A.D. 2000 he will be contributing about as much, and in the Northern Hemisphere alone he will be more than matching nature. 2) In industrialized regions he is overwhelming natural processes, and the removal processes are slow enough (several days, at least) so that the increased concentration is marked for hundreds to thousands of kilometers downwind. 3) Our main areas of uncertainty, and ones that demand immediate attention because of their importance to the regional air pollution question, are: (i) the rates of conversion of H(2)S and SO(2) to sulfate particles in polluted as well as unpolluted atmospheres; (ii) the efficiency of removal of sulfur compounds by precipitation in polluted air. And for a better understanding of the global model we need to know: (i) the amount of biogenic H(2)S that enters the atmosphere over the continents and coastal areas; (ii) means of distinguishing man-made and biogenic contributions to excess sulfate in air and precipitation; (iii) the volcanic production of sulfur compounds, and their influence on the particle concentration in the stratosphere; (iv) the large-scale atmospheric circulation patterns that exchange air between stratosphere and troposphere (although absolute amounts of sulfate particles involved are small relative to the lower tropospheric burden); (v) the role of the oceans as sources or sinks for SO(2).