ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-8-6681-2008 Technical Note: Quantitative long-term measurements of VOC concentrations by PTR-MS – measurement, calibration, and volume mixing ratio calculation methods Taipale R. 1 Ruuskanen T. M. 1 Rinne J. 1 Kajos M. K. 1 Hakola H. 2 Pohja T. 3 Kulmala M. 1 University of Helsinki, Department of Physics, P.O. Box 68, 00014 University of Helsinki, Finland Finnish Meteorological Institute, Air Quality Research, P.O. Box 503, 00101 Helsinki, Finland University of Helsinki, Hyytiälä Forestry Field Station, Hyytiäläntie 124, 35500 Korkeakoski, Finland 19 11 2008 8 22 6681 6698 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/8/6681/2008/acp-8-6681-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/6681/2008/acp-8-6681-2008.pdf

Proton transfer reaction mass spectrometry (PTR-MS) is a technique for online measurements of atmospheric concentrations, or volume mixing ratios, of volatile organic compounds (VOCs). This paper gives a detailed description of our measurement, calibration, and volume mixing ratio calculation methods, which have been designed for long-term stand-alone field measurements by PTR-MS. The PTR-MS instrument has to be calibrated regularly with a gas standard to ensure the accuracy needed in atmospheric VOC measurements. We introduce a novel method for determining an instrument specific relative transmission curve using information obtained from a calibration. This curve enables consistent mixing ratio calculation for VOCs not present in a calibration gas standard. Our method proved to be practical, systematic, and sensitive enough to capture changes in the transmission over time. We also propose a new approach to considering the abundance of H<sub>3</sub>O<sup>+</sup>H<sub>2</sub>O ions in mixing ratio calculation. The approach takes into account the difference in the transmission efficiencies for H<sub>3</sub>O<sup>+</sup> and H<sub>3</sub>O<sup>+</sup>H<sub>2</sub>O ions. To illustrate the functionality of our measurement, calibration, and calculation methods, we present a one-month period of ambient mixing ratio data measured in a boreal forest ecosystem at the SMEAR II station in southern Finland. During the measurement period 27 March–26 April 2007, the hourly averages of the mixing ratios were 0.051–0.57 ppbv for formaldehyde, 0.19–3.1 ppbv for methanol, 0.038–0.39 ppbv for benzene, and 0.020–1.3 ppbv for monoterpenes. The detection limits for the hourly averages were 0.020, 0.060, 0.0036, and 0.0092 ppbv, respectively.

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