Measurement and interpretation of acid rainfall in the Los Angeles Basin

Abstract

The purpose of the work was to define the extent, degree and pertinent chemical characteristics of acid precipitation in the Los Angeles Basin of Southern California. Precipitation samplers were placed at nine locations: Pasadena, Azusa, Big Bear Lake, Central Los Angeles, Long Beach, Mt. Wilson, Riverside, Westwood and Wrightwood. A total of 533 individual samples were analyzed from the nine locations, and 38 different storms were sampled at one or more of the locations. Increments of precipitation collected during a storm were analyzed for pH, titration acidity, chloride, nitrate, nitrite, sulfate, fluoride, bromide, orthophosphate, total phosphate, bicarbonate, sodium, potassium, calcium, magnesium, ammonium, organic carbon and suspended solids. The mean acidity in the Fall-Spring 1978-79 period ranged from a high of 38.4 μequiva1ents/1iter at Pasadena to a low of 2.45 μequiva1ents/liter at Big Bear Lake, with corresponding mean pH's of 4.41 at Pasadena and 5.42 at Big Bear Lake. At Pasadena, individual sample (0.25 inch increments of precipitation) acidities ranged from 1600 μequiva1ents/1iter to -8.1 μequivalents/liter, and individual sample pH's ranged from 2.89 to 6.24. Incremental sampling during storms revealed significant changes in pH and chemical composition with time, with early stages of precipitation generally showing low pH and high nitrate and sulfate concentrations. For the Fall-Spring 1978-79 period the mean ratio of nitrate to non-sea salt sulfate in precipitation varied from 0.4 at Long Beach to 2.8 at Big Bear Lake. The mean ratio at Pasadena was 0.9. Data on chemical composition of precipitation indicate that the observed net acidity is the result of partial neutralization of the strong acids H_2SO_4 and HNO_3 by basic NH_3 and metal carbonates and oxides from soil dust. The relative extent of mixing of acids and bases varies considerably with location in the Basin. At Pasadena, mean Fall-Spring 1978-79 concentrations suggest that the net acidity, 38.5 μeq/l, results from mixing of 31.4 μeq/l of HNO_3, 35.6 μeq/l, of H_2SO_4, 21.1 μeq/l, of NH3, and 7.4 μeq/l, of alkalinity from soil dust. Nitrate and nitrite in rainfall at Pasadena are correlated significantly with rainfall intensity, atmospheric ozone concentration and atmospheric nitric oxide concentrations. The same correlation is found for sulfate in rainfall. Inverse correlation of nitrates and sulfate with rainfall intensity is taken to reflect a rainfall dilution effect. For both nitrate and sulfate in rainfall a correlation is observed with Pb aerosol and total aerosol particulate matter.

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