THE ACIDIFICATION PROCESS UNDER THE CLOUD IN SOUTHWEST CHINA - OBSERVATION RESULTS AND SIMULATION

Mean ionic concentrations in rain water (RW) and cloud water (CW) for urban, suburban and rural locations in southwest (SW) and eastern (E) China, from sampling periods between 1985 and 1989 are reported. In SW China the ammonium, calcium and hydrogen cations, and the sulphate an ion are present at elevated levels in urban RW. The mean concentration s of these ions are all lower in CW, so that washout is the predominan t process leading to acidification. Washout is also important over sub urban regions but rainout provides most of the acidity of precipitatio n in rural areas. Simultaneous observations of CW and RW concentration s over short time intervals have confirmed the dominance of washout pr ocesses in the large cities Chongqing and Guiyang. The chemical compos itions of CW and RW exhibit changes with the weather system behaviour as well as with the sampling location. In E China the acidity of rainf all is largely neutralized by alkaline particulates. The mean ionic co ncentrations in RW show an increasing trend from 1985 to 1988. The amb ient air quality in China has deteriorated over the same period, with concentrations of sulphur dioxide and suspended particulates at high l evels. The below-cloud acidification process has been simulated by a m odel which includes scavenging of both gas and aerosol species. The im portance of rainout, gas washout or aerosol washout processes is found to vary with the location and with the different ion species consider ed. Gas washout,always leads to overall acidification. Aerosol alkaliz ation has been identified in some suburban areas. The effects on the r ainwater acidification predicted from the reduction of sulphur dioxide gas emissions have also been simulated, and are expected to be neglig ible at Chongqing and Guiyang. The concentration of the hydrogen perox ide oxidant, not that of the pollutant sulphur dioxide, is the control ling factor for the pH of the rainfall in these regions. Copyright (C) 1996 Elsevier Science Ltd