Atmospheric transport of acidity in southern California by wet and dry mechanisms

• Main Author: Liljestrand, Howard Michael
• Format: Thesis
• Language: English
• Published: California Institute of Technology 1980
• Subjects: Atmospheric transport of acidity; Environmental Science and Engineering; Carbonic acid
• Online Access: https://dx.doi.org/10.7907/pgev-yh86; https://resolver.caltech.edu/CaltechETD:etd-10102006-110816

Acid precipitation samples collected at Pasadena, California, from February 1976 to April 1979 and at eight other southern California sites for shorter periods were analyzed to determine acid and base composition. The concentrations of major cations (H+, NH4+, Na+, K+, Ca2+, Mg2+) and anions (Cl-, NO2-, Br-, NO3-, SO42) as well as trace weak acids (Fe, Al, Mn, Si(OH)4, RCOOH) were determined. Titrations with base showed acidity predominantly to be due to strong acids (nitric and sulfuric) and weak acids (ammonium ion and carbonic acid). The pH was controlled by strong acidity at the urban sites in the Los Angeles Basin. The chemical composition of precipitation samples collected in ~0.25 in increments is modeled in several ways. Chemical balances are used to determine the contributions of sea salt, soil dust, stationary sources, mobile sources and non-point sources of ammonia. Multiple regression analysis is used to relate ground-level measurements of air quality and atmospheric conditions with rainwater nitrite plus nitrate and sulfate concentrations. Precipitation intensity, ozone, nitric oxide and nitrogen dioxide concentrations are most strongly correlated with rainwater nitrite plus nitrate. Precipitation intensity, ozone, and nitric oxide are most strongly correlated with rainwater sulfate. Gas-liquid equilibrium models yield the following predictions: Low partial pressures of ammonia (average of 0.001 - 0.006 ppbv within the basin) during precipitation scavenging; total sulfite amounting to less than 25% of the non-sea salt sulfur; and nitrite concentrations from NO and NO2 dissolution which are slightly larger than observed values. Kinetic models of the formation of nitrate and sulfate underestimate the observed concentrations. Spatial distributions of acids and base correspond with local sources. The mountain sites and the more rural eastern sites have significantly less net acidity than the western urban sites. Estimates of the dry deposition of atmospheric acids indicate the dry flux is ~6600 equivalents/HA-YR in the Los Angeles Basin compared to ~380 equivalents/HA-YR for the wet flux of strong acidity. The semi-arid climate and high ambient pollutant concentrations cause the large dry flux. Advection of pollutants is the most important mechanism for the removal of acidity from the Los Angeles airshed.