ON THE LARGE-SCALE IMPACT OF ARID DUST ON PRECIPITATION CHEMISTRY OF THE CONTINENTAL NORTHERN-HEMISPHERE

Airborne calcium is an indicator of a major alkaline component, calcit e, when associated with soil-derived dust. Hence, a study of the distr ibution of calcium in aerosol and precipitation is as important as sul phate, and ammonium nitrate for a balanced understanding of the distri bution of background acidity in precipitation. The large-scale distrib ution of the concentration of dissolved continental calcium in north h emispheric precipitation has been examined critically in relation to t he geographical distribution of the arid zones and loessial belts-insi de and outside the desert margins. Some unfamiliar but potential aeoli an impacts are also discussed. The two distributions are closely tied, once long-distance transports are explained. Available data point to the vast arid region to the east and north of the Caspian Sea in the s outhwestern part of the Asian C.I.S. (Commonwealth of Independent Stat es) to be housing the most energetic and frequent dust-producers in th e Northern Hemisphere. Some of the weakest and less frequent sources s eem to lie in the region of southwestern U.S.A. Intermediate positions are occupied by the central Asian deserts and the vast Saharan belt. Some large-scale implications of these data to neutralization of preci pitation acidity are suggested. It is apparent that the usual distribu tion of calcium concentration is bracketed by the range 0.5-8 mg l-1 w ith anomalous maxima in individual samples exceeding 10 mg l-1, someti mes reaching a little over 40 mg l-1. Such high maxima are attributed mainly to calcareous silt particles of loessial origin. The sudden tem poral decrease in the annual average calcium concentration from 1.4 pp m to approximately 0.5 ppm between the mid-1950s to the early 1980s is almost identical for the eastern U.S.A. and northern Europe. Temporal decrease in soil aridity and/or identical recent changes in precipita tion sampling strategy in the U.S.A. and Scandinavia may have caused t his change.