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Project Status: complete

Report Published February 1989:

Title: Short-term trends and spatial variability in precipitation chemistry in the South Coast Air Basin 1989

Principal Investigator / Author(s): Hoffman, Michael R

Contractor: W. M. Keck Laboratories, California Institute of Technology

Contract Number: A4-142-32


Research Program Area: Atmospheric Processes

Topic Areas: Acid Deposition


Abstract:

Automated sub-event sequential rain samplers were used to sample rain during the winter and spring of 1987 in the South Coast Air Basin. The five sites used in the study were located at West Los Angeles, Pasadena, Henninger Flats, Mt. Wilson and Riverside. These sites were specifically chosen to represent a "vertical" profile in the basin (Pasadena to Henninger Flats to Mt. Wilson) and an east-west profile (Riverside to Pasadena to West Los Angeles). Measurements of aerosol and gas phase species also were made at the five sites using filter pack methods and an automated sampler.

The highest rainwater concentrations of NO3- and SO42- were observed at Pasadena; highest NH4+ levels were measured at Riverside; highest average levels of rainwater Na+ and Cl- were found at West Los Angeles. Rainwater pH levels varied from 4 to 6 at West Los Angeles and Henninger Flats, from 4 to 5.5 at Pasadena, from 4.3 to 5.7 at Mt. Wilson and from 4.5 to 6.8 at Riverside. Rainwater concentrations at all five sites generally were dominated by NH4+, SO42-, NO3- and H+. Na+ and Cl- were important contributors in some samples, particularly those collected near the coast. The highest deposition rates of some species were often observed at the site with the lowest rainwater concentrations, Mt. Wilson, due to heavier rainfall there.

Rainfall concentration data from the "vertical" profile indicate that much of the ionic loading in the rain at Henninger Flats (elevation 700 m) was picked up between there and Mt.Wilson (1700 m), while much of the ionic loading in the rain at Pasadena (200 m) was picked up between there and Henninger Flats. Measurements of the atmospheric burdens of aerosol and gas phase species prior to a storm indicated that considerably more N(V) and S(VI) were deposited in rainfall than could be accounted for by pre-storm atmospheric burdens of these species. Pre-storm burdens of Ca2+, Mg2+, Na+, and N(-III) were closer to the measured deposition of these species in the rainfall. In-cloud production of SO42- may account for at least a portion of the additional SO42- deposition, no evidence was seen for in-cloud production of NO3-.

The aerosol at elevated sites in the South Coast air basin is a mixture of sea salt and pollution derived secondary aerosol. The influence of sea salt declines with increasing distance from the coast. Nitric acid appears to react with the NaCl in sea salt aerosol to release HCl(g) and form NaNO3, in the aerosol. At inland sites aerosol concentrations differ during onshore and offshore flow. The highest concentrations are observed during the day when the onshore flow transports pollutants to the sites, while lower concentrations were observed at night when drainage flows from nearby mountains influenced the sites. Variations in liquid water content are a major influence on cloud water concentration.

Comparisons of the ionic concentrations in two size-segregated fractions collected during each sampling interval suggest that there is a large difference between the average composition of the smaller droplets and that of the larger droplets. For each time interval, the concentration of Na+, Ca2+ and Mg2+ in the large droplet fraction was observed to be higher than in the small droplet fraction. The concentrations of SO42-, NO3-, NH4+, and H+ were higher in the small droplet fraction. Chloride concentrations were nearly equal in both fractions. Differences in the composition of size-fractionated cloudwater samples suggest that large droplets are formed from sea salt and soil dust, which are large aerosol, and small droplets are formed on small secondary aerosol composed of primarily ammonium sulfate and ammonium nitrate. The concentrations of several components that exist partly in the gas phase (e.g. Cl-, HCOOH and CH3COOH) appear to be independent of droplet size.


 

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