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 SO(2/4), it is expected that this 'background' chemistry will be dominated by SO(2/4). For the purpose of the 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 SO(2/4) concentration in rainwater.