Formation of N02 in Power-Plant Plumes: Measurements and Modelling

摘要

Nitrogen oxides are air pollutants involved in both acid deposition of e.g. HNO3 and formation of photochemical oxidants, e.g. ozone and peroxy acetyl nitrate (PAN). Conventional power plants are important sources of NO. After emission, NO is oxidized by ozone into NO2 during dispersion of the flue-gas plume in the atmosphere. Modelling of this fairly rapid conversion process is hampered by the interaction of the mixing process and the chemical kinetics. A box-type model which incorporates inhomogeneous mixing was used to model the measured deviations of photostationary equilibrium in the plume. Measured deviations of this equilibrium can be explained by two causes, i.e. the various time rates at which the physical equilibrium can be 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 occur, and an extra contribution of ozone in parcels of unmixed ambient air in the plume. An empirical model is presented in order to avoid the difficulties encountered in modelling inhomogeneous mixing. This model is based on sixty measuring flights in the plumes of power plants, carried out under widely varying atmospheric conditions. The NO-oxidation rate can be described accurately by the phenomenological relation NO2/NOx = A(1-exp(-ax)), x being the distance from the source, the parameter A depending on ozone background concentration and solar radiation intensity and the parameter x depending on the wind speed.