The contribution of sulfate to rainfall pH around Kilauea Volcano,Hawaii

Between December 1986 and June 1987, the mean pH of rainfall downwind of the Kilauea main vent was found to be 4.5 (range 4.0 to 5.6), 1.2 units higher than the year before (1985-86), although 84% of the 12 sequential samples fell below pH 5.0. The SO₄ content, however, was 34% higher, averaging 18.5 mg L^?1. Upwind, in open forest the mean pH 4.7 was little changed from that measured before. Mean SO₄, however, has fallen to a low of 2.5 mg L^?1, but, more significantly, in 9 out of 12 sequential samples S04 was not detectable at all (i.e. < 0.5 mg L^?1). The calculated pH, assuming 100% H₂SO₄ would be 5.3 yet 58% of these samples fell below pH 5.0, the lowest being 4.0. Disparities between pH measured and calculated on the basis of SO₄ content indicated that other acid species were present in the precipitation. Oxidation of rain samples with H₂O₂ greatly increased SO₄ content and lowered pH downwind, but failed in most samples to alter either parameter in the upwind collections. These observations, together with the elimination of HCl and N03 by others, suggested that SO₂ contributes significantly to acidity downwind, but that in most upwind samples a source of H⁺ other than mineral acids, presumably organic compounds, must be of major importance.