The Impact of Biomass Burning and North American Monsoon On the Surface Ozone in the Western U.S. mountain ranges

We analyze the surface O3 observations from the Clean Air Status and Trend Network (CASTNet) using a global chemical transport model (GEOS-Chem) to investigate the impact of biomass burning on surface O3 in the western U.S. (WUS) mountain ranges during the June-October fire season of 2007, one of the stronger fire years in the WUS in the past decade. GEOS-Chem O3 captures the observed seasonal, synoptic and daily variations. Model daily afternoon average surface O3 concentrations at the CASTNet sites are within 2 ppb of the observations, with correlation coefficients of 0.51-0.83 and Taylor scores of 0.64-0.92. Model results show July-September maximum surface O3 enhancement of ~9 ppb on average because of biomass burning. Peaks in fire-contributed surface O3 correspond broadly with high levels of potassium (K), reaffirming a strong fire influence. We find a policy relevant background (PRB) O3 of 45.6 ppb on average during July-September. Fire-contributed O3 accounts for up to 30% of the PRB O3, highest in the intense fire region (Montana, Idaho, and Wyoming) with maxima in August and September. At most of the CASTNet sites in the Southwestern (SW) U.S. including the state of Utah, Nevada, Arizona, Colorado and New Mexico, we find sudden decreases in surface O3 from late July through early August, when North American (NA) summer monsoon is at its peak strength. This corresponds to the period when model results are biased high by up to 30 ppb these sites in the SW U.S. Sensitivity simulation indicates that the overestimation is primarily due to the excessive lightning NOx emission in the model. After redistributing the lightning flash rates with that from National Lightning Data Network, the model better agrees with observation. We use CMAP and GPCP precipitation as a proxy for NA monsoon, and find there is a strong anti-correlation between surface O3 and precipitation (-0.5~-0.7 averaged over 2003~2012 for CASTNet sites in SW U.S.). We use GEOS-Chem simulation to interpret the impact of the two competing factors associated with NA monsoon that affect surface O3: the reduced photochemistry due to increase in cloud coverage and the enhanced lightning activities which generate O3 precursors. Fraction of GEOS-Chem simulated daily fire-contributed O3 to PRB O3 during summer fire season (July to September) at three CASTNet sites.