The changing face of lower tropospheric sulfur oxides in the United States

Sulfur oxides, sulfur dioxide and airborne sulfate, SOx, are short-lived species in the troposphere whose concentrations in air and precipitation have changed dramatically in association with fossil fuel combustion. The historic rise in concentration is coincident with the era of the so-called Anthropocene. Unlike concentrations of long-lived species such as carbon dioxide, atmospheric SOx in the United States (US) peaked between 1970 and 2005 then declined. The rise and fall of SOx is traced by comparing national data on emission changes, ambient concentrations, and precipitation sulfate from prior to World War II to the present. Surface SOx concentrations and precipitation sulfate have decreased with emissions in most parts of the US after the late 1970s. Continued reduction toward a natural background condition has depended on aggressive management of anthropogenic emission sources. Annual average ambient concentrations of SO2 and SO4 have become more uniform across the US at levels of 1-3 ppbv and 0.3-3 mu g/m(3), respectively. Precipitation SO4 has a nominal concentration generally less than 0.5 mg/L. The effective lifetime of SOx in the troposphere is a few days. This duration limits the spatial extent of emission source influence of SOx to regional scales, wherein spatial gradients in species concentrations lead to variations in human exposure and impacts on vulnerable terrestrial and aquatic ecosystems. The effects of domestic emission reductions on SOx levels are moderated by intra-and intercontinental transport of SOx from Canada, Mexico, Asia and elsewhere. The trends in tropospheric SOx concentrations illustrate the results of more than a century of rising public awareness and action to progressively reduce a US environmental risk, accomplished with advances in energy production technology that have maintained economic well-being.