Factors Influencing Air Pollution

Forested and Agricultural Lands

The volatile organic compounds and nitrogen oxides that form ozone come from both natural and human activities. Industrial development and motor vehicles are not the only source of air pollution. Natural VOC emissions are a much larger part of total VOC emissions than natural NOx emissions are of total NOx emissions. Additionally, isoprene, which accounts for the largest portion of biogenic VOCs, is highly reactive and one of the most effective VOCs in producing ozone.

Natural volatile organic compounds are emitted primarily from forested lands. Natural VOC emissions increase with higher temperatures and intense sunlight. Deciduous trees cause higher emissions than evergreens, but large evergreen forests can have signi ficant emissions.

Farming areas are relatively minor sources of emissions of nitrogen oxides in the Mid-Atlantic region. Soil temperature and land use influence the amount of nitrogen emissions produced by organisms in the soil. Natural emissions of nitrogen oxides are hi gher in agricultural areas than in urbanized areas. As noted on page 25, below, however, natural sources account for only about 3% of the total emissions of NOx in the mid-Atlantic region.

Because of natural emissions of nitrogen oxides and volatile organic compounds, some ozone is present even in the absence of human activities. Scientists estimate that an average natural background ozone concentration is 25 to 45 parts per billion. This is much lower than the current ozone standard of 125 parts per billion. Thus, natural emissions by themselves would not cause unhealthful ozone levels.

Sources: EPA (1996), Air Quality Criteria for Ozone and Related Photochemical Oxidants.
National Research Council (1992) Rethinking the Ozone Problem in Urban and Regional Air Pollution, National Academy of Sciences Press, Washington, D.C.

Percent Forested Area by County

Map: Percent Agricultural Area by County

Population Density in the Mid-Atlantic Region

Normal, every day human activities cause emissions that form ozone. These activities including transportation, electricity use, the use of solvents, adhesives, paints, etc., and home heating and cooling. All of these activities c ontribute to ozone formation in the summertime.

Population centers in the Mid-Atlantic Region are generally the same areas that are most affected by ozone air pollution.

Map: County Level Population Density

Growth in Travel and Population

While population has grown slowly over the past two decades, the amount of auto travel has grown much faster. One measure of auto travel is "Vehicle Miles Traveled."

Vehicle Miles Traveled, or VMT, is the product of the number of vehicles times the number of miles traveled. Thus, one vehicle traveling 10 miles would generate 10 VMT. Two vehicles, each traveling 10 miles, would generate 20 VMT.

Because newer cars pollute less per mile than older vehicles, pollution does not increase as fast as VMT. In fact, advances in new car technology and the use of cleaner fuel mean that emissions from motor vehicles actually have declined even as VMT has i ncreased. This trend is predicted to continue until shortly after the turn of the century, when growth is predicted to catch up and cause increases in air pollution.

Sources: US Department of Transportation (1996), "Transportation Air Quality, Selected Facts and Figures."

State Transportation Statistics

Chart: Population of Mid-Atlantic States

Chart: Population of Mid-Atlantic Metro Areas

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