Tracers of oppurtunity and pollutant transport in Southern California

Abstract

The Greater Los Angeles Distant Impact Study is an ongoing program to investigate the fate of urban pollutants in the southwestern United States. As an initial phase of this program the source strengths of halocarbon emissions in the Los Angeles Basin were investigated. These inert compounds, emitted in large quantities by various industrial and commercial activities, offer an oppurtunity to routinely trace polluted air leaving the Basin. Results show widespread emissions of halocarbons with two areas of peak concentrations found in the western part of the Basin; however, the distribution of pollutants leaving the Basin indicates that considerable mixing has taken place. Diurnal halocarbon concentrations peak during the late evening at the eastern end of the Basin due to the prevailing westerly winds. Pollutants are transported out of the Basin in discrete episodes of about 14 h duration with the highest concentrations lasting for about 6 h. Surface and upper air meteorological measurements were used to determine the dynamic and thermodynamic structure of air leaving the Basin. Under the prevailing conditions of strong onshore flow, pollutants were transported out of the Basin in a shallow surface layer; above this, the flow reversed in a deep well-mixed layer containing elevated pollutant concentrations. This layer was capped by the planetary boundary layer inversion. Halocarbons appear to be very useful tracers of urban air in the southwestern United States.