Photochemical trajectory modelling of ozone during the summer PUMA campaign in the UK West Midlands.

A Photochemical Trajectory Model (PTM), coupled with the Master Chemical Mechanism, is set up for summertime conditions and used to simulate pollutant levels measured in Birmingham, UK during the summer Pollution in the Urban Midlands Atmosphere (PUMA) campaign. In general, ozone is modelled well by the PTM, with reasonably good correlation and approximately 90% of afternoon and evening concentrations within a factor of two of measurements. The contribution of local biogenic emissions in the West Midlands to ozone formation during this period was also assessed and found to be of minor importance. Initially, the ozone episode of 26th June 1999, where levels of up to 87 ppb were observed was not reproduced by the UK PTM model. Sensitivity studies showed that the major cause for this was transport uncertainty in the origin and pathway of the air during conditions of slow moving anticyclonic conditions. The results indicate that the ozone episode was caused by recirculation of air polluted within the UK, with additional precursor emissions over the Netherlands, Belgium and France.