Space-based constraints on lightning NOx production in the GEOS-Chem model over Southwest U.S. during North American summer monsoon

Nitrogen oxides (NOx≡ NO + NO2) produced by lightning make a major contribution to the production of tropospheric ozone. We conduct GEOS-Chem simulations at at 2°×2.5° (globally) and 0.55°×0.66° (one-way nested over North America) horizontal resolutions for 2006 and 2007, and compare simulation results with surface ozone observations from the Clean Air Status and Trend Network (CASTNet). In both years, largest discrepancies are seen during the late July and early August, when model results are baised high by up to 30 ppbv at elevated mountainous sites in the southern Rockies. Our sensitivity simulation shows that these biases are primarily caused by excessive lighting NOx emissions in the model, corresponding with deep convective activities associated with the North American summer monsoon that casts influences throughout much of the Southwest U.S. We compare tropospheric NO2 columns produced from the model with that from space-based observations of OMI and SCIAMACHY. Large discrepancies are found during late July and early August in the Southwest U.S., and model results are biased up to three times of that from satellite observations. We examined three lightning parameterization methods used in the model, which are based on cloud top height (CTH), convective mass flux (MFLUX) and convective precipitation (PRECON), respectively. The CTH scheme yields better agreement with space-based observations. We then use lightning flash rates observed by LIS/OTD satellite to constrain the parameterization of the lightning source of NOx in the model during the North American summer monsoon season. The resulting model simulated surface ozone concentrations are in better consistency with CASTNet observations.