Characteristics of oxidant precursor emissions from anthropogenic sources in the United States

Abstract

Hydrocarbons (HC) and nitrogen oxides (NO x ) have been identified as the most important precursor pollutants for oxidant formation in the atmosphere. These pollutants are emitted both from natural and anthropogenic sources; however, these two types of sources are generally geographically separated. Anthropogenic emissions are dominant in and around urban centers, where the majority of severe oxidant problems occur. Based on data gathered by the MAP3S/RAINE emissions inventory project, anthropogenic emissions of HC in the conterminous United States were 24.8 million metric tons in 1979. HC emissions were predominantly from area sources, the transportation sector being the largest contributor with 39.8% of the total. State-by-state breakdowns are also included. Based on analyses by other emissions inventory projects, the nonreactive fraction of the emissions from the transportation sector is less than 20% by weight. The highest proportion of HC emissions occur at low altitudes (0–50 m range) in high population density areas. Anthropogenic emissions of NO x were 23.7 million metric tons in 1979; 50.8% were from point sources. The transportation sector and the electric utilities sector account for 37.1% and 30.9% of the NO x emissions, respectively. The NO 2 fraction of the emissions from the transportation and electric utilities sectors is less than 10% by weight, based on NO/NO 2 speciation data from two other emissions inventories. Highest rates of NO x emissions occur in high population density areas and are released at low altitude (0–50 m); three areas of high electric generating capacity were found to have high release altitudes.