Homologous series of C 1–C 10 monocarboxylic acids and C 1–C 6 carbonyls in Los Angeles air and motor vehicle exhausts

Abstract

Molecular distributions of monocarboxylic acids (C 1–C 10) and aldehydes (C 1–C 6) have been studied in the atmosphere of Los Angeles, as well as in motor exhaust and city dust samples. Acetic and formic acids followed by propionic acid were found to be the dominant organic acid in the gaseous phase. Formaldehyde and acetaldehyde are two dominant carbonyls, followed by propionaldehyde and glyoxal. The concentrations of organic acids and aldehydes in Los Angeles air during autumn 1984 was 0.52–5.2 and 0.94–5.8 ppb, respectively. In contrast, concentrations of the organic acids and aldehydes in motor exhaust collected under running modes were 140–410 and 340–2500 ppb, respectively. This suggests that internal combustion engines are important primary sources of both volatile organic acids and aldehydes in the urban atmosphere. By contrast, diurnal variations in concentrations of organic acids were observed during clear hot days (maximum temperature: up to 40°C), whereas such trends were largely absent during low overcast days. A positive correlation ( r=0.8) was observed between the acid concentrations and maximum ambient temperatures, suggesting that secondary photochemical production of organic acids is much more important in Los Angeles than primary emissions from automobiles during daytime under strong solar radiation.