Source Attribution of Surface Ozone in the Western United States Using an Adjoint Method

Abstract:

We use a global chemical transport model (GEOS-Chem) and its adjoint to quantify source contributions to surface O₃ in the Western United States, with a focus on biomass burning impact in the Pacific Northwest region and lightning impact in the southwestern U.S. We conduct model simulations at both 2°×2.5° (globally) and 0.55°×0.66° (one-way nested over North America) horizontal resolutions for year 2006-2008, three of the stronger fire years and North American Monsoon (NAM) years in the past decade. Model simulated maximum daily 8 h averaged (MAD8) O₃ are within ± 2 ppb of the observations at 12 sites from the Clean Air Status and Trend Network (CASTNet) and the Mt. Bachelor Observatory (MBO). Sensitivity simulations show maximum surface O₃ enhancement of ~9 ppb on average due to biomass burning during July-September and ~2 ppb on average caused by lightning NOx during NAM. The adjoint model computes the sensitivity of surface O₃ concentrations to both O₃ production rates and different emission sources over the history of the air parcel reaching the observation sites. We use the adjoint model to identify the source regions and transport paths for the five fire plumes measured at MBO during the summer of 2008 and the results are consistent with the HYSPLIT back trajectories. We find during these fire plumes at MBO, contributions to surface O₃ from biomass burning emitted NOx are more than three times higher than those from anthropogenic emitted NOx, indicating significant enhancement of O₃ due to fire emissions. At CASTNet sites in the southwestern U.S., contributions to surface O₃ from lightning emitted NOx are about one fifth of those from anthropogenic emitted NOx during NAM.