Regional background acidity and chemical composition of precipitation in Thailand

In a joint Thai-Swedish project, within the framework of the IGAC/DEBITS project, precipitation has been collected for chemical analysis on a daily basis since mid-1991 at two sites in the countryside of Thailand. The quality assurance plan includes investigation of the effect of different sampling equipment (including wet-only collectors), sampling time and stability of samples collected in tropical conditions. Consistent results, unbiased by local conditions, have been obtained. The data show striking day-to-day fluctuations, reflecting changes in transport 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 generally lower during days of high precipitation amount. Mean concentrations are similar at the two stations but the correlation between daily samples is low as can be expected for a distance between the stations of 500 km. A seasonal variation could not be significantly determined. The best correlation between components is found for H+ and SO42− , mutually between Na+ Cl−, Mg2+ and Ca2+ and between NH4+ and NO3−. Absence of correlation between Ca2+ and SO42− suggests that SO42− is not soil derived. The samples are generally slightly acidic which, stochiometrically, can be explained as an imbalance between acidifying components in modest to low concentration and neutralizing components in somewhat lower concentration. The concentrations are lower than those previously reported from stations in eastern and southern Asia, except for some very remote locations.