Mixing of ambient air in a plume and its effects on the oxidation of NO

Abstract

Formation of NO2 in the exhaust plumes of Dutch power plants is studied by both field measurement and modelling. It is argued that models that do not take inhomogeneous mixing into account cannot adequately describe the measuring results, since NO to NO2 oxidation rates are determined by actual local concentrations. A simple way to introduce inhomogeneous mixing is described and incorporated in a simulation model roughly following that of Carmichael and Peters (1981, Atmospheric Environment 15, 1069-1074). Model calculations are applied to airborne and ground measurements which were carried out under widely varying atmospheric conditions. The results show that, at least at plume height (some hundreds of metres), a significant part of the oxidation process occurs in an inhomogeneous mixed plume. NO2 NOx ratios are calculated and the contribution of oxygen to the oxidation of NO and the large deviations of photostationary equilibrium are analyzed. Deviations of photo-stationary equilibrium are explained by two causes, i.e. the various time rates at which the physical processes of dispersion and mixing and the chemical processes of molecular reactions in the atmosphere occur; and also by an extra contribution of ozone in unmixed ambient air parcels in the plume. © 1986.