Project at a Glance
Title: Aircraft sampling to determine atmospheric concentrations and size distributions of particulate matter and other pollutants over the South Coast Air Basin.
Principal Investigator / Author(s): Seinfeld, John H
Contractor: California Institute of Technology
Contract Number: 96-315
Research Program Area: Atmospheric Processes
Topic Areas: Field Studies, Monitoring
As part of the 1997 Southern California Ozone Study - North American Research Strategy for Tropospheric Ozone (SCOS97-NARSTO), 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. Over multiple inland areas, additional layers were observed at about 2,500 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 percent variability within a 5 x 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 the sampled aerosol enhanced photolysis rates by up to about five percent, although a slight decrease was often observed near ground level.
For questions regarding research reports, contact: Heather Choi at (916) 322-3893
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