Technical Note: Quantitative long-term measurements of VOC concentrations by PTR-MS – measurement, calibration, and volume mixing ratio calculation methods

Proton transfer reaction mass spectrometry (PTRMS) 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 longterm stand-alone field measurements by PTR-MS. The PTRMS 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 H3OH2O ions in mixing ratio calculation. The approach takes into account the difference in the transmission efficiencies for H3O and H3OH2O 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 Correspondence to: R. Taipale ([email protected]) for monoterpenes. The detection limits for the hourly averages were 0.020, 0.060, 0.0036, and 0.0092 ppbv, respectively.