A PRELIMINARY-STUDY OF THE CARBON-ISOTOPIC CONTENT OF AMBIENT FORMIC-ACID AND 2 SELECTED SOURCES - AUTOMOBILE EXHAUST AND FORMICINE ANTS

Relatively large quantities (almost-equal-to 1 mg) of formic acid have been collected from the atmosphere and subjected to carbon-isotopic analysis, as a means of source discrimination. Ambient formic acid was captured on Ca(OH)2-treated filters using a high-volume sampler. The collection method was not only efficient (>96%), but also appears to have low artifact production. Most of the samples (36 out of 52) were collected over a two-year period at the summit of Mount Lemmon, Arizona, where a strong seasonality in HCOOH mixing ratio was observed (almost-equal-to 0.2 ppb during winter months to 1.5 ppb in the summer). Other collection sites included the Oregon coast, Colorado Rockies, urban Tucson, and the North Dakota prairie. The carbon-13 content of atmospheric HCOOH was found to be have little variation (-18 to -25 parts-per-thousand), regardless of location or season. This is consistent with a single dominant source of formic acid. The carbon-14 measurements of 6 Mount Lemmon samples showed high levels of modem carbon (93-113% modem). The emissions from formicine ants and automobile combustion were selected as two other potential sources for isotopic analysis. The HCOOH collected from auto exhaust was much more depleted in C-13 than the atmospheric samples, with a deltaC-13 of -28.0 and -48.6 parts per thousand from a leaded and unleaded automobile, respectively. Formicine ants, on the other hand, ranged from -17.2 to -20.6 parts per thousand.