Chemical composition and acidity of precipitation: A monitoring program innortheastern Vietnam

Rainwater has been sampled weekly from five sites (Dos. 1-5) in northeaster n Vietnam in the period of May 1997 to Apr. 2000 (except Hoabinh site, from Aug. 1999 to Apr. 2000). Since Aug. 1999, weekly dry deposition samples in cluding acidic gas and aerosol have been additionally collected at Hanoi (n o. 4) and Hoabinh (no. 5) using filter pack system. In general, the pH in r ainwater was frequently higher than 5.0. However, the trend of lower pH was observed during the winter and the beginning of autumn (from Nov. to Apr.) . Interestingly, the highest frequency of the acidifying rainwater (32 %) a nd the lowest pH value (min. pH = 4.0) were observed in Hoabinh site. Acidi c pH of rain water was also observed in Viettri (no. 3) and Hanoi (no. 4), indicating the local effects of human and industrial activities. Ca (2+) an d SO42- were generally found as predominant in both rainwater and aerosol. SO2 and NH3 in Hanoi and Hoabinh were monitored out of corresponding enviro nmental features.