Trends in eastern US sulfur air quality from the Atmospheric Integrated Research Monitoring Network

[1] Surface air quality data obtained at long-term stations operated by the Atmospheric Integrated Research Monitoring Network of the National Oceanic and Atmospheric Administration are combined with precipitation chemistry data from the National Atmospheric Deposition Program to construct indicators of air chemistry in the lower atmosphere starting in 1985/ 1986 and extending through 1997. The analysis is based on data from seven stations east of the Mississippi River. On the whole, levels of airborne sulfur reached a maximum in the late 1980s; since then, levels have been declining steadily at a rate of about 2.8% per annum, but with an increase to about 5% per annum for a period of the 1990s. There is no clear-cut step function that could be attributed to the initiation of emissions trading and resulting SO2 emissions reductions in about 1995. Instead, the data appear to indicate that the new regulatory trading philosophies have continued the long-term improvement in overall air quality that was already under way. The data from individual sites indicate that the benefits of emissions decreases are not necessarily evident at all locations. Improvements appear to be moderated to a considerable extent by local factors that may dominate in some situations. Overall, the air quality data correlate well with changes in emissions of sulfur as reported by the Environmental Protection Agency. The analysis shows that the percentage improvement in sulfur concentrations in the planetary boundary layer east of the Mississippi corresponds one to one to the percentage change in emissions from electric utilities (within the statistical uncertainties involved). Overall, the results support the hypothesis that regional air quality is governed by the largest point-source emitters, rather than by the integral of all upwind sources.