Influence of global changes on modeled ozone response to changes in local emissions and the policy implications for ozone abatement strategies in the US

As part of an ongoing analysis on the effects of global changes upon US air quality, we report results based upon the WRF/CMAQ and MM5/CMAQ modeling frameworks in terms of the effect that future climate (including associated sensitivities with land use and biogenic emissions) and future global emissions have on the sensitivity of modeled ozone response to US anthropogenic emissions changes. Results are presented in a manner that is consistent with the current use of models in the development of State Implementation Plans (SIP) to highlight the importance for regulators of evaluating the impact that global changes may have on the successfulness of regulatory policies. Results are based on representative years within a current decade (1990-1999 for MM5/CMAQ and 1995-2004 for WRF/CMAQ) and a future decade (2045-2054). Meteorological fields for the CMAQ simulations were obtained by downscaling results from the ECHAM5 and PCM global climate models using WRF and MM5, respectively. The future climate is based on the Intergovernmental Panel for Climate Change (IPCC) A1B scenario (ECHAM5) and A2 scenario (PCM). The WRF and MM5 meteorology were used to drive the MEGAN biogenic emissions model, the SMOKE emissions processing system, and the CMAQ chemical transport model. WRF/CMAQ simulations were conducted on two nested domains covering most of the Northern Hemisphere from eastern Asia to North America (220-km horizontal resolution; hemispheric domain) and covering the continental US (36-km horizontal resolution; CONUS domain), while MM5/CMAQ simulations were conducted for the continental US only. Chemical boundary conditions for CONUS are modified to study the impact of global emissions changes. Future chemical boundary conditions for the WRF/CMAQ CONUS simulations were taken from results of the hemispheric simulations with global emissions based on IPCC A1B scenario; for the MM5/CMAQ simulations they were derived from results of the global chemical transport model MOZART with global emissions based on IPCC A2 scenario Current decade US anthropogenic emissions for the WRF/CMAQ are based on the US EPA’s 2002 National Emissions Inventory (NEI) and for the MM5/CMAQ based on the 1999 NEI. Future decade US anthropogenic emissions for the WRF/CMAQ and MM5/CMAQ simulations were developed using growth factors from the US EPA MARKAL nine-region database and energy system and the US EPA Economic Growth Analysis System, respectively. Disclaimer: The content and conclusions of this work does not necessarily reflect the views and policies of the California Air Resources Board.