Regional-scale ozone transport in the vicinity of Phoenix during a spring field campaign

A field campaign was conducted in the vicinity of Phoenix during a four-week period in May and June of 1998 to examine the meteorological processes associated with ozone and particulates. The U.S. Department of Energy’s G-1 aircraft obtained air chemistry aloft on 16 days during the field campaign, usually between 08 and 11 LST and 14 and 17 LST. High ozone concentrations ( > 80 ppb) were frequently observed in the morning above 2 km MSL. The moderate to strong winds speeds that were observed throughout the period suggest that regional-scale transport, rather than multi-day accumulation of pollutants over Phoenix, was responsible for the high ozone concentrations aloft.

In this study, a meteorological model is used to elucidate the interactions of synoptic, mesoscale, and boundary-layer processes responsible for the regional-scale transport of ozone over Phoenix during a 28-day simulation period. To simulate the multi-scale meteorological processes, a nested grid approach is used in which the first grid encompasses North America with a horizontal grid spacing of 90 km, the second grid encompasses the southwestern U.S and western Mexico with a grid spacing of 30 km, and a third grid encompasses southern California, Arizona, and northwestern Mexico with a grid spacing of 15 km. A four-dimensional data assimilation technique that incorporates rawinsonde observations and radar wind profiler data is employed by the mesoscale model to limit the forecast errors in the meteorological fields throughout the 28-day simulation period. Results from a Lagrangian particle dispersion model indicate that transport from southern California was possible during May 21-23 and June 2-7. During these two periods, the highest background ozone concentrations were observed. Air parcel trajectories showed that the synoptic winds and a thermally-driven mountain-plain circulation over Arizona was responsible for advecting particles from Los Angeles and San Diego over Phoenix within a 2-day period.

To evaluate the relative contribution of stratospheric and tropospheric sources of ozone over Phoenix during the field campaign period, the mesoscale models’ meteorological fields are used as input to a photochemical model. The photochemical model is run with and without chemistry to isolate the processes associated with tropopause folds and the downward transport of ozone. The simulation without chemistry produces increases in near-surface ozone during the May 21-23 and June 2-7 periods, suggesting that stratospheric ozone may have also contributed to the high near-surface ozone concentrations over Phoenix. A photochemical model simulation that also includes emissions, photochemical production, and deposition is currently being performed and the results will be presented at the conference and in the extended abstract.