Interpretive analysis of observed and modeled mesoscale ozone photochemistry in areas with numerous point sources

Abstract

Abstract The Regional Acid Deposition Model (RADM) has been applied to several of the field experiments that were part of the Acid Model Operational and Diagnostic Evaluation Study (AcidMODES) to assess the model's ability to simulate photochemical production of ozone in regions dominated by point source emissions. The comparison of model simulations at different grid resolutions suggests that increased resolution improves the simulation of ozone photochemistry in such regions. Further analysis of NO x and NO x concentrations and photochemical production rates of ozone, however, show that the model's response to large point source emissions is very unsystematic both spatially and temporally. This is due to the model's inability to simulate realistically the small-scale (subgrid) gradients in precursor concentrations in and around large point source plumes. Because of the inherently nonlinear nature of ozone photochemistry with respect to concentrations of NO x and VOC, ozone formation rates in model grid cells depend enormously on grid resolution, dispersion rates (primarily wind speed and mixed layer height), chemical background (VOCs and radicals) and NO x emission rates. Thus, the notion that increased grid resolution leads to better simulations of ozone photochemistry is not necessarily true. This analysis points to the needed to incorporate some kind of subgrid parameterization of plume photochemistry into mesoscale Eulerian grid models.