Utilization of satellite-derived canopy heights in dry deposition computations to improve surface O3 simulations

Simulation results from the NOAA National Air Quality Forecasting Capability (NAQFC, based on CMAQ4.6) for August 2009 are being analyzed to investigate the causes of recurrent summer daytime ozone (O3) overpredictions. Daytime peak O3 concentrations as predicted by the NAQFC are routinely biased 5-10 ppbv high as compared with surface AIRNow observations during summer over the eastern United States. For August 2009, model sensitivity studies are used in conjunction with comparisons of simulation results with surface measurements (AIRNow O3) to investigate the underlying cause for the frequent high O3 biases. Initial results suggest that overprediction of O3 in the nighttime boundary layer may lead to next-day afternoon high biases. The large nighttime O3 overpredictions are observed over the forested regions of the eastern US. Satellite-derived canopy heights (from NASA ICESat, Terra and Aqua satellites) are used to evaluate the impact of more realistic canopy height data on model parameterization of dry deposition velocity. Use of the satellite-derived canopy height (<40 meter) significantly reduces aerodynamic resistance and thereby increases surface O3 deposition, leading to reduced surface O3 concentration over forested regions in the model simulation. Reduction of surface O3 over the forested regions also results in surface O3 reductions over neighboring continental and oceanic areas, thereby reducing observed O3 biases.