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

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 power-plant plume emitted into an ambient atmosphere defined by urban 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 OH radical concentrations is a complex function of the parameters studied. For daytime releases, mean OH concentrations over 24 h are predicted to be lower than the ambient values. For evening and night releases, mean OH concentrations are calculated to be greater than the corresponding ambient values for rapidly dispersing plumes.

Mean effective first-order rate constants for the gas-phase oxidation of plume SO₂ are estimated to be ca 0.4% h⁻¹, 0.2% h⁻¹ and 0.02% h⁻¹, for summer, autumn/spring, and winter, respectively, and are lower than the mean values calculated for the ambient troposphere.

Ozone, H₂O₂ and PAN concentrations in power plant plumes are normally predicted to be less than the corresponding ambient values over most of the simulation period. A significant ozone excess in plumes is only expected for slowly dispersing plumes under summer conditions after 24 h.

Most of the plume and ambient NO_x is predicted to be converted to HNO₃ in summer and autumn/spring within 24 h and concentrations of HNO₃ are predicted to greatly exceed those of H₂SO₄.

These results are compared with those obtained previously for simulations of dispersion into a ‘rural’ ambient atmosphere.