The impact of local emissions on the formation of secondary pollutants in urban plumes.

The results of modelling studies are reported together with a further analysis of an existing field measurement dataset. Emphasis is on the maximum ozone concentrations occurring in the urban plume from London, UK, on days of photochemical activity. Nitrogen oxide and volatile organic compound emissions from automotive exhausts are expected to be a large proportion of the total emission of these species, and the effect of their distinct diurnal variation is apparent in the field data. Factorial sensitivity studies on an expanding box model of the urban plume also show the dominance of urban emissions in the determination of plume maxima 100km downwind of the upwind urban boundary. These effects are not additive and depend strongly on the level of other variables.