Abstract
Abstract. An in situ method for studying gas-phase C2–C7 monocarboxylic volatile organic acids (VOAs) in ambient air was developed and evaluated. Samples were collected directly into the cold trap of the thermal desorption unit (TD) and analysed in situ using a gas chromatograph (GC) coupled to a mass spectrometer (MS). A polyethylene glycol column was used for separating the acids. The method was validated in the laboratory and tested on the ambient air of a boreal forest in June 2015. Recoveries of VOAs from fluorinated ethylene propylene (FEP) and heated stainless steel inlets ranged from 83 to 123 %. Different VOAs were fully desorbed from the cold trap and well separated in the chromatograms. Detection limits varied between 1 and 130 pptv and total uncertainty of the method at mean ambient mixing ratios was between 16 and 76 %. All straight chain VOAs except heptanoic acid in the ambient air measurements were found with mixing ratios above the detection limits. The highest mixing ratios were found for acetic acid and the highest relative variations for hexanoic acid. In addition, mixing ratios of acetic and propanoic acids measured by the novel GC-MS method were compared with proton-mass-transfer time-of-flight mass spectrometer (PTR-TOFMS) data. Both instruments showed similar variations, but differences in the mixing ratio levels were significant.
Highlights
Organic acids comprise a large fraction of the non-methane hydrocarbons in the atmosphere (Khare et al, 1999)
In the present study we developed an in situ gas chromatograph–mass spectrometer (GC-MS) measurement method for measuring C2–C7 monocarboxylic volatile organic acids (VOAs) in gas phase with 2 h time resolution at ambient air concentration levels, which we used to measure ambient air concentrations in a boreal forest site
Reaction rates of VOAs with hydroxyl radicals increased with increasing carbon number (Mellouki et al, 2015) and trees and other vegetation are known to produce stress-induced emissions of green leave volatile organic compounds which are aldehydes, esters and alcohols with 6carbon atoms (Hakola et al, 2001; Scala et al, 2013)
Summary
Organic acids comprise a large fraction of the non-methane hydrocarbons in the atmosphere (Khare et al, 1999). In the present study we developed an in situ gas chromatograph–mass spectrometer (GC-MS) measurement method for measuring C2–C7 monocarboxylic VOAs in gas phase with 2 h time resolution at ambient air concentration levels, which we used to measure ambient air concentrations in a boreal forest site. Earlier these types of in situ GC-MS methods have been used e.g. for measurements of aromatic hydrocarbons and monoterpenes (Hakola et al, 2012; Hellén et al, 2012a)
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