Nonmethane Organic Compound to Nitrogen Oxide Ratios and Organic Composition in Cities and Rural Areas

Abstract

The observed ranges in nonmethane organic compound (NMOC) concentrations, NMOC composition and nitrogen oxides (NOX) concentrations have been evaluated for urban and nonurban areas at ground level and aloft of the contiguous United States. The ranges in NMOC to NOX ratios also are considered. The NMOC composition consistently shifts towards less reactive compounds, especially the alkanes, in air parcels over nonurban areas compared to the NMOC composition near ground level within urban areas. The values for the NMOC to NOX ratios, 1.2 to 4.2, in air aloft over nonurban areas are lower than in air at ground level urban sites, ≥8, and much lower than in air at ground level nonurban sites, ≥20. The layers of air aloft over a number of nonurban areas of the United States tend to accumulate NOX emissions from the tall stacks of large fossil fuel power plants located at nonurban sites. During the night into the morning hours, the air aloft is isolated from any fresh NMOC emissions predominately coming from near surface sources. Conversely, during this extended period of restricted vertical mixing, air near the surface accumulates NMOC emissions while this air is isolated from the major NOX sources emitting aloft. These differences in the distribution of NMOC and NOX sources appear to account for the much larger NMOC to NOX ratios reported near ground level compared to aloft over nonurban areas. Two types of experimental results are consistent with these conclusions: (1) observed increases in surface rural NOX concentrations during the morning hours during which the mixing depth increases to reach the altitude at which NOX from the stacks of fossil fuel power plants is being transported downwind; (2) high correlations of total nitrate at rural locations with Se, which is a tracer for coal-fired power plant NOX emissions. The implications of these conclusions from the standpoint of air quality strategies are suggested by use of appropriate scenarios applied to both urban and regional scale photochemical air quality models. The predictions from urban model scenarios with NMOC to NOX ratios up to 20 are that NOX control will result in the need for the control of more NMOC emissions than necessary in the absence of NOX control, in order to meet the O3 standard. On a regional scale, control of NOX emissions from fossil fuel power plants has little overall effect regionally but does result on a more local scale in both small decreases and increases in O3 concentrations compared to the baseline scenario without NOX control. The regional modeling results obtained to date suggest that NOX control may be effective in reducing O3 concentrations only for a very limited set of conditions in rural areas.