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, calcite, when associated with soil-derived dust. Hence, a study of the distribution of calcium in aerosol and precipitation is as important as sulphate, and ammonium nitrate for a balanced understanding of the distribution of background acidity in precipitation. The large-scale distribution of the concentration of dissolved continental calcium in north hemispheric precipitation has been examined critically in relation to the geographical distribution of the arid zones and loessial belts-inside and outside the desert margins. Some unfamiliar but potential aeolian 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 southwestern part of the Asian C.I.S. (Commonwealth of Independent States) to be housing the most energetic and frequent dust-producers in the Northern Hemisphere. Some of the weakest and less frequent sources seem 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 precipitation acidity are suggested. It is apparent that the usual distribution of calcium concentration is bracketed by the range 0.5-8 mg l-1 with anomalous maxima in individual samples exceeding 10 mg l-1, sometimes reaching a little over 40 mg l-1. Such high maxima are attributed mainly to calcareous silt particles of loessial origin. The sudden temporal decrease in the annual average calcium concentration from 1.4 ppm 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 precipitation sampling strategy in the U.S.A. and Scandinavia may have caused this change.