Decadal variations of rainwater formic and acetic acid concentrations in Wilmington, NC, USA

Concentrations of formic and acetic acid from January 2008 through March 2009 were compared to two previous studies at this location (conducted in 1987–1990 and 1996–1998) in order to quantify the extent to which temporal changes in DOC and pH can be explained by changes in these organic acids. The volume weighted 2008 formic and acetic acid concentrations (5.6 and 2.6 μM respectively) have decreased dramatically compared with those observed during the 1996–1998 study (9.9 and 7.3 μM) and are also lower than concentrations observed in the 1987–1990 study (7.4 and 3.6 μM). Changes in formic and acetic acids between 1996–97 and 2008 can account for approximately 50% of the DOC change and 40% of the H+ change in rainwater over this same time period. These changes are most pronounced during the growing season, which is also the tourist and high traffic season at this location. Determining causation of these changes is difficult due to multiple biogenic and anthropogenic sources. However, the ratio of formic to acetic acid has also reverted back to a value consistent with reduced vehicular emissions, possibly related to the introduction of improved emission control technology including the use of reformulated gasoline in the late 1990’s. Long term monitoring of seasonal, annual, and decadal trends will be of critical importance for evaluating the effects of future changes to atmospheric inputs such as the increased use of ethanol and other alternative fuels.

Research highlights
► Formic and acetic acid concentrations in coastal NC rain decreased over 10 years.
► The ratio of formic to acetic acid has increased.
► Changes are most pronounced during the growing season.
► Contribution to acidity from these two acids (25%) remained constant for 20 years.
► Formic and acetic acid contribute less to DOC now than in the past.