Ambient measurements of aromatic and oxidized VOCs by PTR-MS and GC-MS: intercomparison between four instruments in a boreal forest in Finland

摘要

Proton transfer reaction mass spectrometry (PTR-MS) and gas chromatography mass spectrometry GC-MS) are commonly used methods for automated in situmeasurements of various volatile organic compounds (VOCs) in the atmosphere.In order to investigate the reliability of such measurements, we operatedfour automated analyzers using their normal field measurement protocol sideby side at a boreal forest site. We measured methanol, acetaldehyde, acetone,benzene and toluene by two PTR-MS and two GC-MS instruments. The measurementswere conducted in southern Finland between 13 April and 14 May 2012. Thispaper presents correlations and biases between the concentrations measuredusing the four instruments. A very good correlation was found for benzene andacetone measurements between all instruments (the mean R value was 0.88 forboth compounds), while for acetaldehyde and toluene the correlation wasweaker (with a mean R value of 0.50 and 0.62, respectively). For somecompounds, notably for methanol, there were considerable systematicdifferences in the mixing ratios measured by the different instruments,despite the very good correlation between the instruments (mean R = 0.90).The systematic difference manifests as a difference in the linear regressionslope between measurements conducted between instruments, rather than as anoffset. This mismatch indicates that the systematic uncertainty in thesensitivity of a given instrument can lead to an uncertainty of 50–100 %in the methanol emissions measured by commonly used methods.