Policy Relevant Pollutant Background Simulations for the US using a Multi-scale Regional Climate Modeling System

Global change will clearly have a significant impact on the environment. Among the concerns for future air quality in the United States, intercontinental transport of pollution has become increasingly important. In this study, we examined the effect of the changes in chemical boundary conditions, including the upper troposphere, and emissions from lightning and wildfires to produce ozone background concentrations within the continental US as the basis for developing an understanding of policy relevant background levels and how they may change in the future. Meteorological fields were downscaled from the results of the ECHAM5 global climate model using the Weather Research Forecast (WRF) model. Two nested domains were employed, one covering most of the Northern Hemisphere from eastern Asia to North America using 200k km grid cells (semi-hemispheric domain) and one covering the continental US using 36 km grid cells (CONUS). Meteorological results from WRF were used to drive the MEGAN biogenic emissions model, the SMOKE emissions processing tool, and the CMAQ chemical transport model to predict ozone concentrations for current (1995-2004) and future summertime conditions (2045-2054). The MEGAN model was used to calculate biogenic emissions for all simulations and the BlueSky framework, including a stochastic Fire Scenario Builder (FSB), to estimate total area burned and wildfire emissions. For the semi-hemispheric domain, year 2000 global emissions of gases (ozone precursors) from anthropogenic (outside of North America), natural, and biomass burning sources from the POET and EDGAR emission inventories were used. For the future decade, the current emissions were projected to the year 2050 following the Intergovernmental Panel for Climate Change (IPCC) A1B emission scenario. WRF and CMAQ results from the semi-hemispheric domain simulations provided the boundary conditions for CONUS simulations. For the CONUS simulations, anthropogenic emissions from the US, Canada, and Mexico were omitted so that only global background concentrations, and local biogenic, wildfire, and lightning emissions were treated. We present results showing the changes in ozone background concentrations during summertime conditions due to changes in future climate and global emissions.