A global overview of atmospheric acid deposition fluxes

This paper presents a summary of global acid deposition flux data taken from a global assessment report on acid deposition prepared for UNEP/WMO (Whelpdale and Kaiser, 1996). There is a large variation in the spacial coverage and reliability of monitoring around the world. Many more stations measure wet deposition than collect appropriate data for estimating dry deposition. The widespread regions with highest precipitation concentrations and deposition fluxes of sulphate and nitrate coincide closely with the regions of highest density of SO2 and NOx precursor emissions occurring primarily in the mid-latitude, northern hemisphere belt where a large fraction of the world's fossil fuels is consumed. Organic acids in precipitation make a minor contribution to acidity (<20%) in industrial regions, but in the rest of the world they are of same order, or even exceed, inorganic acids. Less is known about dry deposition, but it appears to predominate near strong emission sources with wet deposition predominating farther downwind. The molar ratio of the N/S contribution to acidic deposition is close to 1.0 over large areas of Europe and North America, but is highly variable elsewhere, being highest in equatorial regions due to biomass burning and lowest near smelters and other large sources of SO2.