Dispersion of an urban photochemical plume in Phoenix metropolitan area

Air quality simulations were conducted using MM5/CMAQ modeling platform to study the intricacies introduced by photochemical reactions during the dispersion of urban pollution plume of Phoenix metropolis. The simulation days included the sole ozone episode recorded during 1996–2005, which violated the previous 1-h ozone standard (0.12 ppm). The modeling results suggest that the Phoenix urban plume can be described in terms of “inert passive dispersion” and “chemically active dispersion”. The former is exemplified by the CO distribution and takes the form of a Gaussian-like plume, for which the source is located at the ground level of the urban core or a freeway. The passive dispersion, nevertheless, is directly subjected to heterogeneities of topography and flow patterns, and hence cannot be strictly Gaussian. The case of active dispersion is much more complicated, and leads to a different plume shape, depending on the chemical reactivity of pollutant species. Secondary pollutants such as ozone and its precursors cause the plume core to have its maximum concentration far downwind of the urban area. Chemical species such as VOCs, which are directly emitted from a source as well as transformed by other primary pollutants, form a plume that qualitatively resembles a transition from an inert plume (CO) to a highly reactive plume (NOx).