Air pollutant transport in a coastal environment—II. Three-dimensional simulations over Los Angeles basin

Abstract

The: air quality problems in the Los Angeles basin are related to the local climate. A strong temperature inversion and stagnant synoptic conditions associated with the Pacific high-pressure system favor the generation of severe air pollution episodes. Sea-breezes and mountain-induced flows play an important role: in controlling pollutant transport over the basin. The distributions of pollutants over the basin display complicated three-dimensional patterns, including distinct elevated layers. A three-dimensional meteorological model is used to simulate air flow patterns and pollutant transport in the Los Angeles basin under such conditions. The complex simulated flows are analyzed to explain the origin of the observed pollution layers. Pollutants found in the afternoon mixed layer may be dispersed to three different regions: the high desert to the east and north of the Los Angeles basin; the free troposphere above the temperature inversion; and within the temperature inversion layer itself. Several mechanisms transfer pollution into the inversion layer. Convergence zones in the San Fernando Valley and near Lake Elsinore lift polluted air into the inversion. A coupled sea-bree~eymountain-flow circulation along the southwest flank of the Santa Ana Mts creates pollution layers that extend westward over the coastal plain and offshore. The afternoon pollution layers frequently observed in the western Los Angeles basin are caused by the sea-breeze circulation or the coupled sea-breeze/mountain-flow circulation over the south slope of the San Gabriel Nlts. As the boundary layer stabilizes during the early evening, layers of polluted air are left suspended over the eastern basin. Pollutants trapped in the inversion layer act as a reservoir of poor-quality air for the basin, and may contribute to extreme pollutant concentrations seen after several days of stagnant air.