Modelling surface ozone during the 2003 heat wave in the UK

The EMEP Unified model, normally applied to the European domain at 50 km horizontal resolution, has been applied to the UK at a finer resolution of 5 km. This new application is called EMEP4UK. The EMEP4UK model is driven by meteorology from the Weather Research and Forecast model (WRF) and emissions from the National Atmospheric Emissions Inventory (NAEI). The WRF model has been nudged every six hours using NCEP/NCAR GFS reanalysis in order to properly represent the ‘real’ meteorology observed during a particular time period. The present paper focuses on the simulation of surface ozone concentrations during August 2003, when large parts of Europe, including the UK, experienced extremely high temperatures and surface ozone concentrations. The evaluation in this paper focuses on comparison of model results with measurements taken during the TORCH campaign in August 2003, based in Writtle, SE England. EMEP4UK was able to accurately simulate most of the ozone peaks which occurred during 2003 and in particular those during August. Measured maximum hourly means reached 130 pbb, while modelled values reached 120 ppb. We conducted a series of model sensitivity runs, varying individual model parameters (e.g., temperature, isoprene, precursor emissions, and ozone deposition) in order to isolate the sensitivity of ozone concentrations during the heat wave to each of these. The two factors which control surface ozone concentration at Writtle have been found to be ozone dry deposition and NOx emissions.