Diagnostic analyses of a regional air quality model:: Changes in modeled processes affecting ozone and chemical-transport indicators from NOx point source emission reductions

The impact of nitrogen oxide (NOx) emission reductions from major point sources on the key physical and chemical processes contributing to ozone formation and accumulation is studied, and the extent of change in the chemical regime is examined using selected photochemical indicators in the eastern United States. The Community Multiscale Air Quality (CMAQ) chemical-transport model, equipped with the process analysis technique, was applied in modeling scenarios involving 2002 base case emissions and an emissions scenario containing real-world point source NOx reductions implemented before the summer ozone season of 2004. Spatial patterns and temporal variations in process rates and changes in chemical-transport indicators are highlighted from results of summer 2002 days, representative of generally southwesterly wind flows across the Midwestern source region with ozone transport toward the northeastern states. Substantial decreases exceeding 50% in O-3 chemical production rates were associated with the largest NOx point source emission reductions, causing declines in ozone concentrations at the surface and aloft in downwind areas. The decreases in the various physical processes and their spatial difference patterns closely resembled the change in maximum O-3 concentrations. The net ozone production efficiency was found to increase, since the decline in O-3 concentrations was less than the decrease in reactive nitrogen products (NOz). The O-3/NOx ratio also increased between the base case and NOx reduction scenario results, indicating a noticeable shift in the chemical regime toward more NOx-limited conditions in plume-impacted areas downwind of the sources. The drop in surface NOx concentrations in modeled and observed results at a location just downwind of the Ohio River Valley source region is attributable to the point source NOx emission reductions.