Project at a Glance

Title: Regional estimates of acid deposition fluxes in California for 1985-1994.

Principal Investigator / Author(s): Blanchard, Charles L

Contractor: Envair

Contract Number: 93-332

Research Program Area: Ecosystem & Multimedia Effects, Atmospheric Processes

Topic Areas: Acid Deposition, Monitoring


Acidic deposition occurs via precipitation, fog, cloud water, and dry deposition. Each of these processes is potentially important in California. The specific objectives of this project were to (1) evaluate the quality of the available deposition data; (2) compute estimates of the deposition of each species of interest, by mode of deposition, at each monitoring location in California having sufficient data available; (3) generalize the estimated deposition amounts to larger regions of interest, to the extent possible; (4) compare the magnitudes of wet and dry deposition; and (5) identify measurement and methodological requirements for improving the results. A previous report (Blanchard and Michaels, 1994) covered the periods July 1984 through June 1990 (wet deposition) and early 1988 through September 1991 (dry deposition). This report extends the previous work to cover the period from July 1984 through June 1994.

The California Acid Deposition Monitoring Program (CADMP) operated 25 to 35 wet-deposition and ten dry-deposition monitoring sites during the periods 1984 through 1994 and 1988 through 1994, respectively. In addition to the CADMP, the California Air Resources Board (CARB) funded a ten-site alpine wet-deposition network, which was operated by the University of California at Santa Barbara from 1m through 1994. The National Atmospheric Deposition Program / National Trends Network (NAPD / NTN) also operated eight wet-deposition monitoring sites in California. Precipitation-chemistry data from these three networks were combined and used for generating annual regional estimates of wet-deposition fluxes throughout California. Because so much fewer data were available from the dry-deposition network, estimates of dry-deposition fluxes were generated only for the monitoring locations and were not generalized to broader regions. Estimates of the magnitudes of wet plus dry deposition were also generated for the ten locations having both wet- and dry-deposition data available.

In all years (1985 through 1994) and at all monitoring sites, the magnitudes of deposition of sulfate and nitrate by precipitation were each no greater than 12 kilograms per hectare per year (kg ha-l yr-l), and were usually less than 6 kg ha-l yr-l. Excess sulfate (i.e., excluding the estimated sea-salt contribution) was no greater than 10 kg ha-l yr-l in all years at all sites and was usually less than 4 kg ha-l yr-l. Ammonium deposition was less than 5 kg ha-l yr-l in all years at all sites.

Regionally-averaged wet.deposition fluxes were lower than the site maxima listed above. During all years and for all regions, (1) mean nitrate deposition via precipitation was less than 6 kg ha-l yr-l; (2) excess sulfate (i.e., excluding sea-salt sulfate) and sulfate deposition were less than 4 kg ha-l yr-l; and (3) ammonium deposition was less than 2 kg ha-l yr-l. The interpolation uncertainties for wet deposition were generally less than 20 percent in the South Coast Air Basin, which has a large number of monitors. Elsewhere in California, wet deposition interpolation uncertainties were no more than 40 percent (Southern California) and 60 percent (northern California) for sulfate, excess sulfate and nitrate. Interpolation uncertainties were up to 100 percent for acidity, because acidity deposition rates were typically close to zero g ha-l yr-l.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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