Rainwater characterization at Global Atmospheric Watch in Danum Valley, Sabah

Air quality and pollution may be partly assessed by examination of rainwater compositions. Long term monitoring is essential in detecting signs of changing trend and hence any plausible air pollution and atmospheric acidification. Weekly samples of rainwater were collected by using a wet-only sampler at Global Atmospheric Watch (GAW) station in Danum Valley, Sabah in East Malaysia for a period between January 2005 and December 2006. Composition of wet deposition was determined for major cations (H⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, and NH4⁺) and major anions (Cl⁻, NO3⁻ SO4²⁻, CH3COO⁻ and HCOO⁻). In addition, the pH, electroconductivity and volume of rainwater were also determined. The results reveal that there was a steady decrease in pH of rainwater from annual average of 5.37 in 2005 to 5.14 in 2006. These pH values were below 5.6, the limiting value for clean rainwater. The lowest value recorded was 4.79 in October 2006 while the highest was 5.80 in January 2005. This is a worrying indication that atmospheric acidification may has made its presence felt in this remote area which was destined to be pristine environment. The volume-weighted mean concentrations of major ions followed the order: H⁺ > SO4²⁻ > Cl⁻ > Na⁺ > Ca²⁺ > NO3⁻ > NH4⁺ > K⁺ > Mg²⁺ > acetate > oxalate > formate. Both acidic and basic species concentrations were much lower compared to typical values in urban areas. The acid rain precursors; SO4²⁻ and NO3⁻ were well correlated and so were the sea-salt species, Mg- - ²⁺, Na⁺ and Cl⁻. The buffering systems to potential acidity are mainly due to crustal origins (Ca²⁺) and anthropogenic activities (NH4⁺) with contribution from sea salts (Na⁺, Mg²⁺ and K⁺). Biogenic emissions producing weak organic acids was negligible.