Lagrangian modelling of plume chemistry for secondary pollutants in large industrial plumes

Industrial sources of nitrogen oxides range from boilers and metal furnaces to fossil fuel power stations. Current UK regulatory practice for these plumes adopts a fixed proportion of nitrogen dioxide, as a simple and conservative method of assessment. The method of Janssen is another possible approach. The aim of the present paper is to use a fundamental description of plume chemistry to investigate the sensitivity of nitrogen dioxide and ozone to changes in the background pollutants. The newly developed Met Office's NAME III Lagrangian dispersion model was used to simulate the inhomogeneous mixing of an idealised plume with background air. Concentrations of nitrogen oxides in the plume and of ozone and hydrocarbons in the background were carried on Lagrangian particles and mixed together. Titration of background ozone and formation of secondary pollutants were studied as background concentrations were varied systematically. We present a range of model results depicting the plume chemistry. Raised concentrations of nitrogen dioxide appeared on the plume edges, especially at night. Ozone was reduced in the plume region by day and by night, but in daylight, with sufficient hydrocarbons, increased ozone is formed further downwind. We compared the model results with the limiting case from the Janssen method; the limiting values for the NO2:NOx ratio were of similar magnitude. The rate of increase of the ratio to the Janssen photochemical limit depended on the atmospheric stability, as this affects the turbulent mixing in the plume. We discuss possible application of the work as an alternative to empirical regulatory approaches for managing NO2.