Project at a Glance
Title: Regional estimates of acid deposition fluxes in California
Principal Investigator / Author(s): Blanchard, Charles
Contract Number: a132-149
Topic Areas: Acid Deposition, Monitoring, Stationary Sources
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 wet with dry deposition; and (5) identify measurement and methodological requirements for improving the results.
Sulfate and nitrate deposition via precipitation were each less than 8 kilograms per hectare per year (kg ha-1 yr-1); excess sulfate (i.e., excluding sea-salt sulfate), ammonium, and calcium deposition were less than 3 kg ha-1 yr-1. Wet deposition uncertainties were less than 20 percent in the South Coast Air Basin, which has a large number of monitors; uncertainties can be up to 100 percent in portions of northeastern and southeastern California, where little monitoring has been done.
The dry-deposition flux estimates are subject to uncertainties on the order of 50 percent. Estimated dry deposition of nitric acid (HNO3) at the 10 monitoring sites ranged from 1 to 87 kg ha-1 yr-1. At the 7 urban sites, HNO3 deposition accounted for 50 to 80 percent of the deposition of oxidized nitrogen species and 40 to 70 percent of the total nitrogen deposition.
At the three nonurban sites, wet nitrate and sulfate deposition approximately equalled or slightly exceeded dry deposition of oxidized nitrogen and sulfur species. In contrast, dry sulfur deposition at the urban sites was approximately 1 to 3 times the magnitude of wet sulfur deposition; dry deposition of oxidized nitrogen species at the urban sites ranged from about 5 to 30 times the magnitude of wet nitrate deposition. At all sites, dry deposition of reduced nitrogen species (ammonia and particulate ammonium) was about a factor of 2 greater than wet ammonium deposition.
For questions regarding this research project, including available data and progress status, contact: Heather Choi at (916) 322-3893
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