The carbon isotopic content and concentration of ambient formic and acetic acid.

Abstract

A direct method for source determination of atmospheric formic and acetic acid, through carbon isotopic analysis of the ambient acids and their potential sources, has been successfully developed and tested. These first carbon isotopic measurements of formic acid in the atmosphere were found to be fairly constant, regardless of location. This is consistent with a single dominating source of formic acid, with vegetation emissions being the most likely controlling source. Collection of relatively large quantities (0.3$-$3.0 mg) of the organic acids, which was necessary for carbon isotopic measurements, was effectively accomplished by a new method using calcium hydroxide-treated filters with a high-volume sampler. Samples were collected on a regular basis at Mount Lemmon, Arizona (elevation = 9200 feet A.S.L.). Atmospheric concentrations showed a well-defined seasonal pattern, with the lowest concentrations (about 0.2 ppbv) occurring in the middle of the winter, which steadily increased to a maximum of nearly 2 ppbv in the summer. The ¹³C content (δ ¹³C) of HCOOH averaged -20.9 ± 2.5 ‰ during the growing season (April-September) and -23.2 ± 3.5 ‰ during the non-growing season at Mount Lemmon. Isotopic measurements of formic acid from several other locations included two west coast marine sites (δ ¹³C range of -19.1 to -24.6 ‰), three Colorado Rocky Mountain samples averaging -23.2 ± 1.0 ‰, two from the prairie of North Dakota (-23.5 ± 1.0 ‰) and three samples collected in the urban Tucson, Arizona area (-20.8 ± 3.4 ‰). Source measurements included HCOOH emissions from two species of formicine ants (-18.8 ± 1.7 ‰), and HCOOH in automobile exhaust (-28 ‰ from leaded gasoline, and -48.6 ‰ from unleaded). Further support for a biogenic source of atmospheric HCOOH came from the carbon-14 analysis of six Mount Lemmon HCOOH samples (93-113% modern carbon), using accelerator mass spectrometry. Carbon isotopic fractionation during exchange of HCOOH between the liquid and gas phases was investigated through a gas/liquid partitioning experiment. A first experimental measurement of the Henry's Law coefficient (Kᴴ) averaged 7400 ± 800 M/atm. Fractionation between HCOOH(g) and HCOOH(aq) was found to be negligible.