THE OZONE INCREMENTS IN URBAN PLUMES

The role of urban plumes in the exposure of suburban/rural areas to pollution by ozone is discussed, and literature on direct measurement of ozone within plumes reviewed. There is a virtual absence of reports of ozone destruction within a NOx-rich urban plume and reasons are given for this gap in the current published data. These negative ozone increments are important because the particular air quality problems encountered downwind of a city will depend on its character as a source or sink of ozone. Simple plume reconstruction methods are used to verify and extend data from measurements in the London plume. There is an optimum time for air-parcel release with respect to increased plume ozone, and a change in the typical diurnal variation of ground-level ozone concentrations when the receptor site is situated within an urban plume. On a surface constructed from air parcel emission times and times travelled downwind of London, a major peak in plume ozone increment is observed 6-8 h downwind of a post rush-hour release in the urban centre. A secondary maximum in the surface is also apparent for air parcels 2-3 h after release in the early afternoon. These are not the emission and travel times usually adopted in modelling studies of the urban plume from London. Sites in the urban plume are also found to have a statistically different diurnal variation compared to rural/suburban sites which are not influenced by the plume. This is due to the titration of ozone mixing into the plume from aloft by plume NO.