Extended Abstract (28K)2.1
A Study on Vertical Distribution of Ozone in PBL of the Phoenix Valley
Sang-Mi Lee, Arizona State University, Tempe, AZ; and M. Sinesi, M. Princevac, D. Zajic, J. L. McCulley, H. J. S. Fernando, and J. Anderson
Meteorological processes occurring in the planetary boundary layer as well as chemical and physical processes associated with vertical and temporal variations of ozone concentration in the lower troposphere were examined using measurements taken during the 2001 Phoenix ozone field experiment of the Department of Energy. An air quality model, Models-3/CMAQ, in conjunction with a mesoscale meteorological model, MM5 were used as supporting tools.
The layered structure of ozone concentration in the stable boundary layer and vertical mixing phenomena between the layers were well captured by measurements. The formation and breaking up of multiple ozone layers were explained by considering local mean circulation patterns and turbulent processes. For example, a low-level jet originated from the surrounding mountains transported an air mass with little ozone toward downtown Phoenix. Simultaneously, strong vertical wind shear in the vicinity of the low-level jet caused vertical mixing of ozone. Thermally driven turbulence during morning transition, which is the period between nighttime cooling and daytime heating, caused ground based vertical mixing, thus bringing in high ozone air from upper levels to the surface while fluxing ozone produced near the surface to upper levels. The phenomenon of nocturnal high ozone concentration was explained using the sudden increase of heat flux associated with the urban heat island.
The multi-scale photochemical model, Models-3/CMAQ, was implemented with enhanced vertical and horizontal resolutions. In general, this model computed the diurnal variation of ozone and other oxidants, and the predicted vertical profiles of ozone concentration showed a reasonable agreement with the measurements. The role of physical processes such as vertical diffusion, horizontal advection, chemical reactions, and deposition that cause the spatial and temporal variation of ozone was examined in light of measurements and computations. The concentrations of other air pollutants, however, were somewhat underestimated compared to measurements. This drawback of air quality predictions can be attributed to poor understanding of nighttime oxidant chemistry as well as due to poor performance of the mesoscale meteorological model in the surface layer.
Session 2, flow and dispersion in coastal and complex terrain
Monday, 20 May 2002, 11:00 AM-1:30 PM
Next paper