Evolution of the boundary layer and thermally-driven circulations associated with the transport and mixing of ozone in Phoenix

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. Surface ozone concentrations were typically the highest, and occurred later in the day, for northern and northeastern monitoring stations. The largest values occurred at a mountain site located well away from the city. Analysis of the radar wind profiler data show that the spatial ozone pattern may be attributed to the development of thermally-driven upslope flows during the day. Air chemistry measurements made by the U.S. Department of Energy’s G-1 aircraft indicate that the urban ozone plume was advected downwind of the city over the foothills during several of the afternoon flights.

In this study, a mesoscale meteorological model and a photochemical model is used to examine the boundary-layer processes responsible for the transport and mixing of ozone in the vicinity of Phoenix. Several 24-h case studies are examined. To simulate the terrain-induced circulations associated with the ridges and valleys along the foothills north and east of Phoenix, four nested grids are used with a 2.5 km horizontal grid spacing on the inner nested grid. 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 simulation period. Southerly upslope, thermally-driven winds are produced by the model around noon. Ozone and ozone precursors are initially advected by these upslope winds to the north. As the mixed layer grows during the afternoon, the near-surface winds become southwesterly as they are coupled to a plain-to-mountain circulation that develops in response to the larger-scale terrain along the Mogollon Rim. The shift in the wind direction produces a spatial ozone distribution with a local maximum value in the northeast part of the city, in agreement with observations. The highest concentrations, however, are produced over the foothills to the east where no surface measurements are routinely made. Aircraft flights over this region indicates that the model also captured the spatial and temporal variations of ozone downwind of the city. Days with higher surface ozone concentrations appear to be due to increases in the background values, rather than changes in photochemical or emission rates.