Relative production of ozone and nitrates in urban and rural power plant plumes 1. Composite results based on data from 10 field measurement days

A rather limited number of large power plants are responsible for about 2/3 and 1/3 of the U.S. anthropogenic emissions of SO2 and NOx, respectively. Considerable uncertainty continues to prevail about the local and regional impact of their potentially harmful secondary products (e.g., ozone, sulfates, nitrates). We have analyzed state-of-the-art data of the Southern Oxidant Study (SOS)-Nashville Field Study (1994, 1995) for 10 days of summer daytime field measurements by instrumented aircraft in the plumes of three large, tall-stack, base-load, Tennessee Valley Authority (TVA) coal-fired power plants in northwestern Tennessee: Gallatin (G), located within the Nashville urban ozone nonattainment area, and Cumberland (C) and Johnsonville (JV) in rural isoprene-rich forested areas about 100 km to the west of Nashville. The average 1995 emissions of NOx from these three sources ranged over more than an order of magnitude. In this paper, we have explored plume chemical evolution and the magnitude, efficiency, and yield of ozone and NOz (NOz oxidation products, mostly inorganic and organic nitrates) production in a broad variety of plume transport and chemistry scenarios within the convective boundary layer (CBL) in rural and urban settings. The results show that(I) plume chemical maturity and peak production capacities of ozone and NOz were realized quite close to the sources, within 30-40 km and 4 hours of daytime transport for Gallatin (smallest NOx emission rate, Q(NOx), and suburban environment) and typically within 100 km and 6 hours of CBL transport for Cumberland (highest Q(NOx) and rural environment rich in isoprene); (2) the ozone impact of Gallatin on Nashville can exceed that of Cumberland, and under favorable transport and chemical conditions, both power plants can contribute as much as 50 ppb of excess ozone to the urban area, raising local peak levels well in excess of 100 ppb; (3) an estimated 3.1 +/- 0.7 molecules of ozone and more than 0.6 molecules of NOz may be produced in large isolated rural power plant plumes (PPPs) per molecule of NOz release, and the corresponding peak yields of ozone and NOz may be significantly greater in urban PPPs; (4) the rate of NOz production approximate to 10-15% h(-1) in isolated rural PPPs, and higher in urban PPPs; (5) NOz production is favored in all PPPs at first when the chemistry is VOC-limited: later, with increasing VOC ingestion from the background, the chemistry increasingly favors NOx-limited ozone production, starting at plume edges, and ultimately throughout the diluted plume. These results have major implications on outstanding issues related to the environmental impact and regulatory control of electric utility industry NOx emissions.