Major factors influencing gas-phase chemistry in power-plant plumes during long-range transport—I. Release time and dispersion rate for dispersion into a ‘rural’ ambient atmosphere

Abstract

A gas-phase chemical kinetic scheme combined with a simple dispersion model has been used to examine the influence of season, time of release and dispersion rate on the chemical behaviour of a powerplant plume emitted into an ambient atmosphere defined by rural emissions. Simulations were carried out over 24 h for a plume trajectory primarily over the sea at a typical Northern European latitude. The temporal behaviour of in-plume hydroxyl radical concentrations is a complex function of the parameters studied. For daytime plume releases, mean OH concentrations over 24 h are predicted to be lower than the ambient values and to decrease with dispersion rate. For evening and night releases, mean OH concentrations are calculated to be greater than the corresponding ambient values and also the plume concentrations for a daytime release, with little dependence on dispersion rate. The effect of the variations of parameters studied on mean OH concentrations is much smaller than the maximum effect during the simulation. Mean effective first-order rate constants for the gas-phase oxidation of plume SO 2 are estimated to be ca 0.5% h −1, 0.2% h −1 and 0.03% h −1, for summer, autumn/spring and winter, respectively. Most of the plume and ambient NO x is predicted to be converted to HNO 3 in summer and autumn/spring within 24 h and concentrations of nitric acid are predicted to greatly exceed those of H 2SO 4. Ozone, H 2O 2 and PAN concentrations in power-plant plumes are normally predicted to be less than the corresponding ambient values. A significant O 3 excess in plumes is only expected for slowly dispersing plumes under summer conditions after 24 h, and even in these cases, the total O 3 produced over the simulation time is less than that in the corresponding ambient air. The differences between reaction rates in atmospheres defined by diffuse sources and those produced by large point sources may have significance in determining control strategies.