Abstract
Abstract. A significant improvement in the PTR-MS instrument sensitivity to formaldehyde was obtained by drying the air sample to a dew point of −30 °C using a cold trap to condense and freeze water vapour. At warmer trap temperatures there was significant uptake of formaldehyde and other water soluble organics, suggesting the presence of a quasi-liquid layer on the ice surface. By removing water vapour to a low constant dew point, the PTR-MS can be operated at low E/N ratios, significantly increasing normalized sensitivities for all organics and removing their humidity dependence due to reactions with H+(H2O)2. At an E/N ratio of 80 Td, the formaldehyde normalized sensitivity was 25 Hz/ppbv per MHz H3O+ with an estimated detection limit of 78 pptv. Field testing demonstrated good agreement between HCHO measurements made at ambient humidity and corrected for water vapour effects compared to dehumidified sampling at −30 °C. Field testing also revealed that at an E/N ratio of 100 Td or lower there was a significant ion signal at m/z=49, likely CH3OOH. Sampling drying and operation at low E/N ratios enables sensitive measurements of HCHO and potentially CH3OOH, both important tropospheric photoproducts.
Highlights
The proton transfer reaction mass spectrometer (PTR-MS) is a commercial instrument sold by Ionicon Analytik (Austria)
The concern with the trap was that HCHO would be lost to the ice accumulating in the water trap and that other volatile organic compounds (VOCs), such as larger aromatics with lower vapour pressures, would be removed to the cold ice surface
The humidity dependence of the normalized sensitivity (Hz/ppbv per MHz H3O+) for formaldehyde and other VOC was investigated in both laboratory experiments and in a field study
Summary
The proton transfer reaction mass spectrometer (PTR-MS) is a commercial instrument sold by Ionicon Analytik (Austria). The rate of the back reaction is dependent on the water vapor concentration, the electric field E (V/cm), and the number density N (molecules/cm3) of the PTR-MS drift tube (Hansel et al, 1997). Midey et al (2000) have measured the rate constants for formaldehyde and acetaldehyde reacting with H3O+ and H+(H2O) using a SIFT They observed that both species reacted with H+(H2O) in ligand switching reactions, with the HCHO+H+(H2O) reaction rate being 75% of the calculated collision rate. We will show that the PTR-MS sensitivity to HCHO can be dramatically improved by removing water vapour from the sample stream This has the advantages of decreasing the rate of the back reaction and keeping the water vapour concentration in the drift tube constant so that HCHO and other VOC sensitivities are independent of ambient water vapour concentration. The reduction in the E/N ratio reduces the degree of fragmentation from dissociative protonation reactions and may allow for the determination of CH3OOH
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