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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, D19303, doi:10.1029/2007JD009537, 2008

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

J. M. Godowitch

Atmospheric Sciences Modeling Division, Air Resources Laboratory, National Oceanic and Atmospheric Administration, Research Triangle Park, North Carolina, USA


C. Hogrefe

Atmospheric Sciences Research Center, University at Albany, State University of New York, Albany, New York, USA


S. T. Rao

Atmospheric Sciences Modeling Division, Air Resources Laboratory, National Oceanic and Atmospheric Administration, Research Triangle Park, North Carolina, USA


Abstract

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 O3 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 O3 concentrations. The net ozone production efficiency was found to increase, since the decline in O3 concentrations was less than the decrease in reactive nitrogen products (NOz). The O3/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.

Received 24 October 2007; accepted 27 June 2008; published 4 October 2008.

Keywords: process analysis rates; photochemical ozone; emission reductions.

Index Terms: 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0345 Atmospheric Composition and Structure: Pollution: urban and regional (0305, 0478, 4251); 0368 Atmospheric Composition and Structure: Troposphere: constituent transport and chemistry; 3307 Atmospheric Processes: Boundary layer processes; 3355 Atmospheric Processes: Regional modeling.

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Citation: Godowitch, J. M., C. Hogrefe, and S. T. Rao (2008), Diagnostic analyses of a regional air quality model: Changes in modeled processes affecting ozone and chemical-transport indicators from NOx point source emission reductions, J. Geophys. Res., 113, D19303, doi:10.1029/2007JD009537.