Model evaluation and photochemical regime assessment over the UK with an atmospheric chemistry model.

Ozone is a potent air pollutant, which has historically been controlled by reductions in either NOx or VOC emissions. The UK has reduced these emissions for the last decades and a re-evaluation of the current state of the O3 emissions sensitivity is now necessary. We evaluate the photochemical regime over the UK within a regional (0.25 0.3125) GEOS-Chem v13.1.2 simulation of atmospheric composition for 2017. We update the model to use emissions from the European Monitoring and Evaluation Programme and we evaluate the model predictions using observations of the surface concentration of CO, Non-Methane Volatile Organic Compounds, NOx, NH3, SO2, Particulate Matter mass, Sulfate, Nitrate, Ammonia, Chloride, and O3 from the Department of Environment, Food and Rural Affairs monitoring networks. From this simulation we diagnose the radical termination step (OH+NO2 or RO2+RO2) in order to diagnose whether reductions in NOx or VOC emissions would lead to lower O3 concentrations. We conclude that on average the vast majority of the country is in a NOx limited regime but there is significant seasonal and meteorological variability which leads to a more complex picture.