Airborne analysis of the Los Angeles aerosol

Abstract

As part of the Southern California ozone study (SCOS), a research aircraft was employed during August and September of 1997 to characterize the physical and chemical properties of the aerosol present over the Los Angeles Basin. Aerosol size distributions measured using a differential mobility analyzer and two optical particle counters were combined with filter-based composition measurements to derive a physicochemical description of the aerosol sampled. The accuracy of this description was evaluated through comparison of derived and directly measured aerosol properties including mass, absorption coefficient, hemispherical backscattering coefficient, and total scattering coefficient at two different relative humidities. The sampled aerosol exhibited a complex vertical structure possessing multiple elevated aerosol layers. The most pronounced of these layers were observed to form by injection of aerosol above the ground-level mixed layer along the southern edge of the San Gabriel Mountains, which form the northern boundary of much of the Los Angeles Basin. Over multiple inland areas, additional layers were observed at about 2500 m above sea level (asl), while off the coast of Santa Monica, thin but concentrated layers were detected about 500 m asl. In addition to the sharp vertical gradients in aerosol concentration observed, horizontal gradients at multiple locations were found to be sufficient to result in more than 50% variability within a 5×5 km computational grid cell commonly used in atmospheric models. Vertically resolved aerosol measurements made over one location during several flights, as well as over several locations during a morning and afternoon flight on the same day, were used to investigate the temporally and spatially resolved impact the aerosol had on gas-phase photolysis rates. These calculations predict that for a 10° zenith angle the sampled aerosol enhanced photolysis rates by up to about 5%, although a slight decrease was often observed near ground level.