Mid-latitude northern hemisphere background sulfate concentration in rainwater

The pH is not sufficient to characterize the acidity of precipitation, but rather its acid-base components must be described. The chemistry of natural emission sources as well as the mechanism of precipitation formation determine the chemistry of precipitation at mid-latitude, Northern Hemisphere locations. With the ocean biota as a source of atmospheric aerosol SO₄²⁻, it is expected that this “background” chemistry will be dominated by SO₄²⁻. For the purpose of this study, background was defined as a remote site generally upwind of urban areas, with the additional requirement that samples with evidence of contamination by anthropogenic sources be excluded. Canadian and U.S. data from long term precipitation monitoring sites along the coasts of British Columbia, Oregon, and Washington were evaluated to estimate a background SO₄²⁻ concentration in rainwater. In addition to screening the data for charge balance, collection efficiency, and anthropogenic influence, the data were corrected for SO₄²⁻ associated with sea salt. The results of this analysis suggest that the mid-latitude, Northern Hemisphere background excess SO₄²⁻ concentration in rainfall occurs most frequently in the range of 2–16 μeqℓ⁻¹ with a mean of 5.5 μeqℓ⁻¹ and an average measured pH of 5.3.