Abstract
Proton transfer reaction mass spectrometry (PTR-MS) is a technique that has been developed recently. It has several advantages for the measurement of volatile organic compounds (VOCs) in the atmosphere. The PTR-MS instrument can detect OVOCs with high time resolution, as well as VOCs without the pre-condensation procedure. Although the PTR-MS is available as a commercial instrument from the IONICON company, the instrument requires calibrations for the determination of absolute concentration. In this study, calibrations of the PTR-MS instrument were carried out for OVOCs with diffusion tubes and several kinds of premixed standard gas cylinders. Substances used for calibration with the diffusion tube were as follows; acetonitrile (CH3CN), methanol (CH3OH), acetone (CH3COCH3), and acetaldehyde (CH3CHO). The results of these substances showed good linearity. The time variations of the calibration factors were almost constant for two months.
The calibration factor of formaldehyde (HCHO) was found to depend on water vapor. This phenomenon is explained by the reverse reaction between HCHOH+ and H2O. Signals of HCHO in wet conditions decreased 60% less than those in dry conditions. The back reaction rate constant (k2) was estimated at 3×10-11 [cm3 molecule-1 s-1] using the box model calculation.
Measurements in the ambient air in Tokyo were carried out. About 30 species containing OVOCs, aromatics and biogenic volatile organic compounds were measured. These showed characteristic diurnal variations. Aldehydes, acetone and isoprene concentrations had a daytime peak. On the contrary, aromatics had peaks in the evening. Methanol was almost constant. Thus, OVOCs concentrations can be measured by PTR-MS.
