Research Projects

Project at a Glance

Project Status: complete

Title: Characterization of reactants, reaction mechanisms, and reaction products leading to extreme acid rain and acid aerosol conditions in Southern California.

Principal Investigator / Author(s): Richards, L. W.

Contractor: Sonoma Technology, Inc

Contract Number: A0-140-32


Research Program Area: Atmospheric Processes

Topic Areas: Acid Deposition


Abstract:

One daytime and four nighttime aircraft flights were conducted in November 1981 and May 1982 to collect samples of water and aerosol and to measure trace gas concentrations and other air quality parameters in stratus clouds both in the coastal and inland portions of the Los Angeles Basin. Total concentrations of sulfate and nitrate in clouds each exceeded about 100 Fg/m3. The average cloud water pH was 3.2 and the minimum was 2.4. Data for the relative ionic concentrations in equivalents typically fell in the range: 40-50% nitric acid, 20-30% ammonium sulfate and 10-20% sodium chloride. It was found that the reaction between sulfur (IV) and hydrogen peroxide was inhibited in the collected water samples, that sulfur (IV) concentrations in the water were hundreds of times those expected from the measured sulfur dioxide gas concentration, Henry's law, water pH, and acid-base equilibria. Sulfur (IV) accounted for an average of 14% of the cloud water sulfur. It is believed that sulfur (IV)-carbonyl compound adducts were largely responsible for these results. The one sampling night in the eastern part of the Basin, sulfate concentrations were much higher in a point source plume, and nitrate concentrations were similar in and out of the plume and averaged about 90 Fg/m3. These high nitrate concentrations could be due to the prior day's photochemistry, nighttime reaction of nitrogen dioxide and ozone in clouds to produce the nitrate radical and nitrogen pentoxide and the hydrolysis of PAN. On the average, the dissolved species in the cloud water in May were 50% by weight nitrate and 23% sulfate (after oxidation). It was calculated that the aerosol resulting from the evaporation of the cloud water averaged 24% nitrate and 38% sulfate and had a geometric mean diameter between 0.3 and 0.5 Fm. It is suggested that adducts between sulfur (IV) and carbonyl compounds persist in the dry aerosol. Prior data suggesting that sulfur (IV) may account for about 10% of the sulfur (reported as sulfate) in the Los Angeles aerosol are discussed.


 

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