FORMIC AND ACETIC-ACIDS IN COASTAL NORTH-CAROLINA RAINWATER

Formic and acetic acids contributed 23% of the acidity in rainwater received in Wilmington, NC, between October 1, 1987, and September 30, 1990. The rainwater concentrations, deposition, and percent contribution to acidity from these acids varied with season and storm origin; however, concentrations of formic and acetic acids were correlated with each other in rain from all types of storms and in both seasons. The concentrations of formic and acetic acids were highest in rain from local thunderstorms and least in maritime storm rain. Although formic and acetic acids are thought to come primarily from natural sources, their concentrations in rain of continental origin were highly correlated with several rainwater components that are dominated by anthropogenic sources (hydrogen ion, nitrate, and non-sea-salt sulfate). Maritime storm rainwater components had a different correlation pattern, which suggests that maritime storm rain is not simply more dilute continental rain. These data allow estimation of a background pH of 4.90 for rain in this geographical region. Contamination of formic and acetic acids occurred in samples preserved with chloroform and stored in high-density polyethylene bottles exposed to sunlight.