The use of air quality models to estimate the local and transboundary influence during smog episodes

Air pollution episodes occur regularly in Eastern Canada. Ozone and PM_2.5, the main components of smog, are produced from precursors that can be emitted locally or transported over long distances by atmospheric circulation. In most cases, both local and remote sources are responsible for the ozone or PM exceedances over southern Quebec. Policy makers need tools in order to estimate the effectiveness of proposed local regulation versus more general emission reduction programs at the national and international level. By using air quality models, one is able to estimate both the local and the transboundary influence and determine if the pollution during a particular event is mainly due to local sources or to remote sources. The following assumptions are made. The concentrations of ozone and PM_2.5 at a particular location and time may be split into three components, the background, the local and the transboundary (or long range transport) components, the sum of which giving the total concentration. In order to estimate each of the three terms, three model simulations for the pollution episode being analyzed are needed. The first one is the simulation with the usual input data, including all anthropogenic emissions. The second and third simulations are done by turning off successively anthropogenic emissions within the jurisdiction and outside the jurisdiction. Then the three components are deduced from the three model results. A few examples done with the AURAMS model will be analyzed and the results will be compared with surface measurements and computed backtrajectories. AURAMS is a comprehensive, source-based, eulerian air quality model developed by the Meteorological Service of Canada. It includes a full gas phase, aqueous phase and heterogeneous phase chemistry in order to properly simulate ozone and PM_2.5 on a regional scale.