Abstract

Most modeling studies that examine background ozone concentrations used global chemical transport models to estimate the magnitude of hemispheric scale air pollutant transport. It is unclear, however, to what degree sub-grid scale processes not captured by the coarse resolution of such models might alter regional estimates of foreign air pollution impact. This study uses a regional atmospheric chemistry model (WRF/Chem) to determine which fine-scale processes have the largest impact on long-range ozone and ozone precursor transport over the Western United States. We conducted a 36 km x 36 km WRF/Chem simulation for all of April, 2006 using dynamic boundary conditions supplied by the MOZART global chemical transport model. We then compare the resultant down-scaled forecasts of ozone and other atmospheric constituents to the corresponding coarse resolution MOZART forecasts. We validate both models against the North American Regional Reanalysis for meteorological accuracy, EPA Air Quality System surface ozone observations, multiple ozonesondes, and high resolution/multi-constituent observations made at the Mount Bachelor Observatory in Bend, OR. Our results indicate that WRF/Chem overestimates the surface impact of stratospheric intrusions over orographically complex regions of the inter-mountain West, but that WRF/Chem accuracy is slightly better than MOZART accuracy where such intrusions do not impact surface ozone. We conclude by discussing the implications of our findinigs for future applications of global and regional atmospheric chemistry models.