Chemical reaction during the coagulation of ammonium sulphate and mineral particles in the atmosphere

Until now the climatic effects of mineral dust in the atmosphere has been l imited to a consideration of physical processes. Here we present evidence f or a chemical reaction occurring upon the coagulation of an Asian dust (kos a) aerosol and ammonium sulphate particles in the atmosphere in which the c alcium carbonate in the kosa particles reacted with the ammonium sulphate, at a rate measured in days, to yield koktaite [(NH4)(2)Ca(SO4)(2). H2O] as an intermediate product and, finally, gypsum (CaSO4. 2H(2)O). Koktaite was present in an aerosol sample collected in Beijing, China during a kosa even t, but it is absent from the soils from which the kosa aerosol originates. This reaction may have significant climatic consequences. (C) 1998 Elsevier Science B.V. All rights reserved.