Pollution Aerosol in the Northeast: Northeastern-Midwestern Contributions

In recent years, atmospheric scientists have been endeavoring to determine the relative contributions of local and distant sources to pollution aerosol in eastern North America. Elemental analysis of aerosol from various sites in the northeastern United States has revealed a persistent northeastern ``foreground'' upon which pulses of midwestern aerosol are superimposed every few days, in response to large-scale meteorological features. Regional apportionment of tracer elements and sulfate in summer and winter samples from Narragansett, Rhode Island, and Underhill, Vermont, shows that most of the elements come predominantly from northeastern sources. Notable exceptions include arsenic and indium, for which Canadian nonferrous smelters are important sources, and sulfur and selenium, for which the Midwest is an important source. During 1982 and 1983, the Northeast and the Midwest contributed comparably to aerosol sulfate at Narragansett and Underhill, in spite of the fact that the emissions of sulfur dioxide in the Midwest were ten times those in the Northeast; Canadian smelters accounted for less than 10 percent of the total sulfate. During a major pollution episode in July 1982, northeastern and midwestern sources produced comparable sulfate concentrations in Rhode Island, whereas midwestern sources dominated northeastern sources in Vermont. Thus, although distant midwestern sources affect the quantity of pollution aerosol in the Northeast and may dominate episodically, nearer northeastern sources are comparably important on the long term. The regional elemental tracer system has shown that pollution aerosol in the Northeast is composed of a northeastern ``foreground'' plus superimposed pulses from the Midwest. When these pulses are strong enough, they form the classical episodes of midwestern aerosol. During periods of stagnation in the Northeast, however, aerosol produced locally can reach concentrations that rival or exceed those coming from the Midwest, even for sulfate. These two types of episodes can be distinguished clearly with elemental tracers. Over the long term, the Northeast is its most important source for elements that are distributed broadly over eastern North America or enriched in northeastern emissions (vanadium, antimony, zinc, manganese, and probably many other elements not part of our tracer system). For elements enriched in distant sources, such as arsenic and indium from the Canadian nonferrous smelters and selenium and sulfur from t