Bj. Johnson et Ga. Dawson, A PRELIMINARY-STUDY OF THE CARBON-ISOTOPIC CONTENT OF AMBIENT FORMIC-ACID AND 2 SELECTED SOURCES - AUTOMOBILE EXHAUST AND FORMICINE ANTS, Journal of atmospheric chemistry, 17(2), 1993, pp. 123-140
Relatively large quantities (almost-equal-to 1 mg) of formic acid have
been collected from the atmosphere and subjected to carbon-isotopic a
nalysis, as a means of source discrimination. Ambient formic acid was
captured on Ca(OH)2-treated filters using a high-volume sampler. The c
ollection method was not only efficient (>96%), but also appears to ha
ve low artifact production. Most of the samples (36 out of 52) were co
llected 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). Othe
r collection sites included the Oregon coast, Colorado Rockies, urban
Tucson, and the North Dakota prairie. The carbon-13 content of atmosph
eric 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% mo
dem). The emissions from formicine ants and automobile combustion were
selected as two other potential sources for isotopic analysis. The HC
OOH collected from auto exhaust was much more depleted in C-13 than th
e atmospheric samples, with a deltaC-13 of -28.0 and -48.6 parts per t
housand from a leaded and unleaded automobile, respectively. Formicine
ants, on the other hand, ranged from -17.2 to -20.6 parts per thousan
d.