Measurements of H2O2, aldehydes and organic acids in Los Angeles rainwater: Their sources and deposition rates

Rainwater samples were collected in Los Angeles, during 1985–1991 to determine concentration levels, sources and deposition rates of atmospheric H₂O₂, aldehydes and organic acids, in addition to major cations, anions and pH. Volume-weighted mean concentrations of H₂O₂, aldehydes (formaldehyde + acetaldehyde + glyoxal + methylglyoxal) and organic acids (formic acid + acetic acid) in rain collected at Westwood were 4.4., 3.9 and 16.5 μM, respectively, during the 6-year study period. Monocarboxylic organic acids were estimated to account for 27% (2–80%) of total free acidity (as on overall average) in rain collected at Westwood, whereas sulfuric acid and nitric acid accounted for 39% and 34% of the total acidity, respectively. Concentrations of aldehydes were strongly dependent on precipitation volume and decreased with increasing precipitation volume, whereas H₂O₂ and organic acids were only weakly dependent on precipitation volume. These results indicate that concentrations of aldehydes in rain are mainly controlled by dilution, whereas H₂O₂ and organic acid concentrations are controlled by other factors, such as decomposition of H₂O₂ by reacting with S(IV) and continuous aqueous formation/decomposition of organic acids by reactions involving aldehydes, dissolved OH radicals and H₂O₂. Principal component analyses indicate that aldehydes in rainwater mainly originate from gases and aerosols derived from anthropogenic sources, whereas the sources of H₂O₂ and organic acids in rain do not correlate with anthropogenic sources or marine and continental sources. There is good agreement between reported gas-phase concentrations of H₂O₂, aldehydes and organic acids in Los Angeles and calculated equilibrium concentrations of these chemical species from their rainwater concentrations and Henry's law constants. Temporal variations of concentrations of chemical species indicate that H₂O₂, aldehydes and organic acids were highest in the early afternoon. Summer rains contained the highest concentration of these chemical species, suggesting the photochemical activities during rain storms significantly affect their concentration levels. Estimation of annual rate of wet and dry depositions of H₂O₂, aldehydes and organic acids for the period studied, indicates that 84% of H₂O₂, 97% of aldehydes and 94% of organic acids, respectively, are annually scavenged from the atmosphere, by dry deposition, which is the dominant process for removal of these atmospheric pollutants in Los Angeles.