Spatial and Temporal Observations of the Planetary Boundary Layer During Ozone Episodes in the South Coast Air Basin

The South Coast Air Basin (the greater Los Angeles Area) in California has some of the worst air quality in the United States. Ozone concentrations in the basin often exceed the National Ambient Air Quality Standard (NAAQS) for ozone and other criteria pollutants. To better understand the chemical and meteorological processes that influence ozone, a consortium of agencies conducted the Southern California Ozone Study (SCOS97) during the summer of 1997. In order to meet the study's goals of understanding the horizontal dispersion of pollution by wind, transport of pollution within different aloft layers, and the vertical dilution of pollution in the mixed layer, the SCOS97 field study incorporated 26 radar wind profilers and RASS (Radio Acoustic Sounding Systems). This was the largest and highest density network ever deployed.

Radar wind profilers measure hourly wind profiles to about 4000 m. The RASS system measures hourly vertical profiles of virtual temperature up to about 1500 m. Reflectivity data collected by the radar wind profiler and the RASS virtual temperature data can be used to estimate the height and characteristics of the boundary layers that exist at different locations. Such layers include the surface-based mixed layer, the marine boundary layer, the convective boundary layer, and the nocturnal boundary layer.

We used data from all 26 radar wind profilers and RASS systems to estimate hourly surface-based mixing depths for three high-ozone episodes in 1997. The results gave insight into the diurnal and spatial characteristics of mixing depths over the South Coast Air Basin. Mixing depth data coupled with hourly vertical profiles of wind and temperature data, were used to understand the planetary boundary layer processes during ozone episodes.