REGIONAL BACKGROUND ACIDITY AND CHEMICAL-COMPOSITION OF PRECIPITATIONIN THAILAND

In a joint Thai-Swedish project, within the framework of the IGAC/DEBI TS project, precipitation has been collected for chemical analysis on a daily basis since mid-1991 at two sites in the countryside of Thaila nd. The quality assurance plan includes investigation of the effect of different sampling equipment (including wet-only collectors), samplin g time and stability of samples collected in tropical conditions. Cons istent results, unbiased by local conditions, have been obtained. The data show striking day-to-day fluctuations, reflecting changes in tran sport direction and an inhomogeneous distribution of sources. For days with little precipitation the concentration of the components ranges from low to high values while the range is smaller and concentration g enerally lower during days of high precipitation amount. Mean concentr ations are similar at the two stations but the correlation between dai ly samples is low as can be expected for a distance between the statio ns of 500 km. A seasonal variation could not be significantly determin ed. The best correlation between components is found for H+ and SO42-, mutually between Na+, Cl-, Mg2+ and Ca2+ and between NH4+ and NO3-. A bsence of correlation between Ca2+ and SO42- suggests that SO42- is no t soil derived. The samples are generally slightly acidic which, stoch iometrically, can be explained as an imbalance between acidifying comp onents in modest to low concentration and neutralizing components in s omewhat lower concentration. The concentrations are lower than those p reviously reported from stations in eastern and southern Asia, except for some very remote locations.