Anatomy of a Los Angeles smog episode: Pollutant transport in the daytime sea breeze regime

Abstract

The three-dimensional distribution of aerosols and trace gases in the Los Angeles air basin was mapped out during a smoggy day by instrumented aircraft. Strong vertical and horizontal gradients were observed in the concentrations of both primary and secondary pollutants.

The day began with much of the basin occupied by polluted air carried over from the day before. New emissions accumulated in the stagnant air until late morning, when a well-organized sea breeze developed at the surface. Onshore flow during the afternoon carried heavily polluted air into inland receptor areas, ahead of a shallow, well-defined, advancing layer of cleaner marine air. The highest ozone concentrations of the day were observed just above the marine layer in stagnant air decoupled from the surface and just ahead of the marine front in photochemically aged air transported into low emission density areas.

The data show that air pollution in the basin is a regional problem and that emissions in the western portion of the basin can result in high concentrations of secondary pollutants (e.g. > 0.25 ppm ozone) in areas over 50 km downwind. Layers of well aged pollutants are also shown to occur aloft. These layers can remain overnight and be re-entrained the next day by a deepening mixing layer, contributing to surface concentrations.