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

R Taipale,M Kulmala,M K Kajos,J Rinne,H Hakola,T M Ruuskanen,T Pohja

doi:10.5194/acp-8-6681-2008

Abstract

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). 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 H3O+H2O ions in mixing ratio calculation. The approach takes into account the difference in the transmission efficiencies for H3O+ and H3O+H2O 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.

Highlights

Volatile organic compounds (VOCs) are emitted into the atmosphere from a wide variety of natural and anthropogenic sources
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 Proton transfer reaction mass spectrometry (PTR-MS)
The PTR-MS instrument has to be calibrated regularly to achieve and maintain the accuracy needed in atmospheric volatile organic compounds (VOCs) measurements

Summary

Introduction

Volatile organic compounds (VOCs) are emitted into the atmosphere from a wide variety of natural and anthropogenic sources. The largest VOC sources are tropical and extratropical forests, which emit especially isoprene, monoterpenes, and methanol. Biomass burning is a large source on a global scale and leads to emissions of numerous VOCs, including aromatic hydrocarbons, nitriles, and oxygenated compounds (Andreae and Merlet, 2001). VOCs play a significant role in tropospheric chemistry in urban, rural, and remote areas. They affect concentrations of the hydroxyl radical (OH), the nitrate radical (NO3), and ozone (O3), and the oxidative capacity of the lower atmosphere (Atkinson and Arey, 2003). VOCs are involved in the formation and growth of atmospheric aerosol particles (Kulmala et al, 2004; Allan et al, 2006; Tunved et al, 2006), which are an important factor in the climate system, either directly through the absorption and scattering of solar radiation, or indirectly by acting as cloud condensation

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