Technical Note: Quantitative long-term measurements of VOC concentrations by PTR-MS – measurement, calibration, and volume mixing ratio calculation methods
1University of Helsinki, Department of Physics, P.O. Box 68, 00014 University of Helsinki, Finland
2Finnish Meteorological Institute, Air Quality Research, P.O. Box 503, 00101 Helsinki, Finland
3University of Helsinki, Hyytiälä Forestry Field Station, Hyytiäläntie 124, 35500 Korkeakoski, Finland
Abstract. 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). The aim of this paper is to give 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. We also show how the information obtained from a calibration can be used to determine the instrument specific relative transmission curve, which enables quantitative mixing ratio calculation for VOCs which are not present in a calibration gas standard. 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 mixing ratios were 0.1–0.5 ppbv for formaldehyde, 0.2–3.0 ppbv for methanol, 0.04–0.39 ppbv for benzene, and 0.03–1.25 ppbv for monoterpenes.