Aircraft-measured ozone deposition in the San Joaquin Valley of California

Abstract In the summer of 1991 the San Joaquin Valley Air Pollution Study Agency organized a four-week international field experiment to measure ozone concentrations and fluxes in the San Joaquin Valley of California. The field data are being used to improve the understanding of the exchange processes occurring between the atmosphere and various types of vegetation, and to develop a comprehensive computer model for ozone transport and deposition in the valley. The National Research Council of Canada (NRC) Twin Otter atmospheric research aircraft flew 24 flights in this program, measuring fluxes and deposition velocities adjacent to three highly instrumented tower facilities, each situated over a vegetation type significant to the area, i.e. cotton, grapes, and natural grassland. Having established a correlation between aircraft- and tower-measured fluxes at these reference locations, the aircraft was then used to measure fluxes at regional scales over several additional types of vegetation throughout the San Joaquin Valley. The paper will describe the instrumentation and flight procedures used to make these airborne flux measurements, with emphasis on ozone. Summary data will be presented on the correlation between ozone: deposition velocity and the greenness index, the ratio of reflected infrared to red radiation which is an indication of the density of green vegetation beneath the aircraft. Sample results from several special studies will also be presented; these include repeated runs on a regional scale, a grid study, an overflight of the City of Fresno, runs made adjacent to Interstate-5, and repeated passes over the cotton site during the solar eclipse on 11 July 1991. More detailed results from the Twin Otter operations in this experiment are presented in a series of companion papers.