Levels of many pollutants drop off rapidly as one moves away from urban industrial, residential, and automotive sources. Figure 2 shows 1994 ratios of urban to rural air concentrations of selected Vermont pollutants. This formation takes time, such that the maximum effects of Vermont sulfate and ozone precursors may occur tens to hundreds of miles downwind of sources. At the same time, Vermont concentrations of sulfate and ozone are strongly influenced by transport from sources hundreds of miles upwind of our borders. From a human health perspective, we are fortunate that sulfate and ozone are eventually cleansed from the air by chemical reactions and physical deposition processes.
Sulfates dissolve in cloud droplets and are efficiently removed by wet deposition.
Ozone is removed from the air by chemical reactions, and by physical interactions with the leaves of trees and agricultural crops. From an ecological perspective, these atmospheric cleansing processes for sulfate, ozone, and trace metals such as lead and mercury pose a serious concern for our forest and aquatic ecosystems. Wet sulfate deposition is a major component of acid rain, which has acidified a number of Vermont's lakes and streams and may contribute to red spruce decline at higher elevations. Ozone pollution causes visible damage to the leaves of ozone indicator plants such as blackberry and milkweed at locations throughout the state. Less obvious, but more important effects may occur as ozone can reduce photosynthesis, growth, and frost-hardiness in a variety of plant species.