Measurements of volatile organic compounds in urban air using proton-transfer-reaction mass spectrometer

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The purpose of this project is to establish a technique to measure volatile organic compounds (VOCs) in the atmosphere using a proton-transfer-reaction mass spectrometer (PTR-MS) with a high time resolution and high precision. The second purpose is to characterize behaviors of VOCs in urban air based on VOC measurements using the PTR-MS.The PTR-MS technique is based on proton-transfer-reaction between target species (VOCs in this case) and proton hydrate (H3O+), which is produced in the instrument.H_3O^+ + VOC → H_2O + VOC H^+Because of this soft ionization of the target species, they can generally be quantified by mass spectrometer without making fragmentation. Using this method, VOCs can be measured with a time resolution of seconds, which is much faster than the conventional gas-chromatography (GC) technique, which require more than an hour to identify VOC species. To establish the PTR-MS technique to measure VOCs in ambient air in this study, methods to determine the zero level of … More the signal and calibrate the detection sensitivity have been developed, and dependence of sensitivity on the humidity of ambient air and other parameters have been carefully calibrated. Simultaneous measurements of various VOCs have also been made with GC-FID and diffusion scrubber techniques, such as benzene, toluene, xylene, and tri-methyl benzene (GC-FID) and formaldehyde and acetaldehyde (diffusion scrubber), respectively, and generally a good agreement was found.In this study, to characterize behaviors of VOCs in mega-city Tokyo, a series of measurements of VOCs (benzene, toluene, xylene, etc.) and oxygenated VOCs (formaldehyde, acetaldehyde, acetone, methanol, etc.) was made using the PTR-MS. A diurnal and seasonal variations have been studied using these data. Among various VOCs, formaldehyde and acetaldehyde show a clear diurnal variation with a maximum around noon in summer. By removing a contribution from primary emissions using a correlation with CO, a contribution from photochemical production was estimated to be 70-80% in summer. A good correlation between the secondary production amounts and O3+NO2 concentrations confirmed these enhancements were due to photochemistry. Possible processes of secondary production were studied using a box model.These new findings have been reported in various domestic and international conferences, such as Gordon Research Conference in September 2005. Less