A risk based application of the regional model CMAQ to policy decisions

A risk based approach to assessing compliance with EU limit values is described, using the advanced chemical transport model CMAQ to determine the regional component of NO2 and particulate matter under various conditions over the UK. A new air quality data analysis retrieval tool AirDART is used to extract concentrations for selected areas of the country. Roadside concentrations in a street canyon are then calculated using a dispersion model. The two model calculations need to be combined to obtain concentrations in future years assuming appropriate changes in emission. To merge the local and regional contributions exactly requires assumptions regarding parameterisations and is computationally expensive. From a risk based viewpoint not every possible condition need be considered in order to make policy decisions. Instead future trends under typical conditions are estimated, allowing the direct effect of local action plans and of national measures to be assessed. The approach is applied to London, this being a worst case UK example, to demonstrate the procedure.