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Project at a Glance

Project Status: complete

Report Published 1987:

Title: Characterization of reactants, mechanisms and species in South Coast Air Basin cloudwater.

Principal Investigator / Author(s): Richards, L Willard

Contractor: Sonoma Technology, Inc

Contract Number: A4-032-32

Research Program Area: Atmospheric Processes

Topic Areas: Acid Deposition, Chemistry & Reactivity


Sonoma Technology, Inc. (STI) conducted a study in May and June 1985 for the California Air Resources Board in which an aircraft was used to collect cloud water, aerosol, and gas samples and to make other air quality and meteorological measurements in stratus clouds in the Los Angeles Basin. This study continued the research on Los Angeles Basin stratus begun in 1981. The average composition of the cloud water was in the range found in previous years, and was approximately 42% by equivalents nitric acid, 27% ammonium sulfate, 18% sodium chloride and 13% other metal nitrates and sulfates. The minimum, mean, and maximum values for the concentration of several cloud water species were: nitrate, 5, 22 and 40 µg/m3; sulfate, 4, 10.6 and 16 µg/m3; H202, 12, 57 and 167 µM; and pH, 2.9, 3.35 and 3.8. If sufficient SO2 were added to the clouds, the measured H202 would cause the sulfate concentrations to rapidly increase by an average of 5.2 µg/m3 or 65%. Five flights were designed to approximately follow air parcels to determine chemical conversion rates in the clouds. The ambient concentrations (µg/m3) of most non-volatile dissolved species in the cloud water usually decreased with time, showing that removal processes were important. One flight was made during a drizzle and trace metal concentrations in the cloud water decreased by about a factor of two each hour due to droplet settling. Increases with time in lead and ammonia concentrations in the cloud water showed that vertical mixing was important. There was a slight tendency for nitrate concentrations to increase with time, but sulfate concentrations were relatively constant because of the very small sulfur (IV) concentrations in the clouds. The settling of droplets out of stratus clouds through air below the clouds containing ammonia and then the evaporation of these relatively large droplets is suggested as a significant mechanism for the formation of large, nitrate-containing aerosol particles below the clouds.

The statements and conclusions in this report are those of the contractor and not necessarily those of the California Air Resources Board. The mention of commercial products, their source or their use in connection with material reported herein is not to be construed as either an actual or implied
endorsement of such products.


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

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