Relationship between the acidity and chemical composition of rainwater and climatological conditions along a transition zone between large deserts and Mediterranean climate, Israel

During five winters (1981/1982, 1982/1983, 1983/1984, 1988/1989 and 1989/1990) we collected 569 rainwater samples for chemical analyses. The geographical and meteorological (classified by the wind direction as an indicator of long-range transport) variations of rain chemistry were studied in relation to natural and anthropogenic sources and transport of the constituents. The regional effect on rain chemical composition is a latitudinal function in a transition zone between large deserts in the south and a Mediterranean climate in the centre and northern Israel, and a longitudinal function in the land-sea configuration. The average rainwater salinity, mainly contributed by non-sea-salt fraction (NSSF), varies by more than one order of magnitude from south to north. These variations were attributed to higher input of continental components at the southern regions and to differences of annual precipitation between the regions. In an W E cross-section through Israel, sea-salt fraction (SSF) is influenced mainly by the distance from the Mediterranean Sea. There is a progressive change from basic (with a mean pH of 7.45) and non-acid rains in the south to more acid (a mean pH of 5.49 and about 65% acid rains) in the north. The NSSF is mainly comprised of carbonate dust particles which contribute to neutralize the acidity of precipitation in which H2SO4 is the main H+ donor. The latitudinal acidity distribution and the regional pH variations of contemporaneous rain events, lead to the conclusion that the main effect on the pH is not a local anthropogenic source, rather a regional neutralizing capacity for each rain event. (Sr/Ca)(nss) ratios indicated the dominance of chalky dust transported from the southern deserts (North Africa, Sinai and the Negev). Higher shares of local limestone/dolomitic dust in central and northern Israel due to a lower input of dust coming in from the remote southern deserts, is marked by lower (Sr/Ca)(nss) ratios. Wind direction effects display higher continental fraction and lower acidity in rains related to the southwest wind component, especially in regions of northern Israel. The combined regional and wind direction effects are presented in a linear multi-variant model equations and show the dominance of the region over the wind direction component. (C) 2000 Elsevier Science Ltd. All rights reserved.