Abstract. Proton transfer reactions between hydronium ions (H3O+) and volatile organic compounds (VOCs) provide a fast and high sensitive measurement technique for VOCs, leading to extensive use of proton-transfer-reaction mass spectrometry (PTR-MS) in atmospheric research. Based on the same ionization approach, we describe the development of a high-resolution (HR) time of flight chemical ionization mass spectrometer (ToF-CIMS) utilizing H3O+ as the reagent ions. The new H3O+ ToF-CIMS has sensitivities of 100–1000 cps/ppb (ion counts per second per part-per-billion mixing ratio of VOC) and detection limits of 20–600 ppt at 3σ for a 1-second integration time for simultaneous measurements of many VOC species of atmospheric relevance. Compared with similar instruments with quadrupole mass spectrometer, e.g. proton-transfer-reaction mass spectrometers, the ToF analyzer with mass resolution (m/Δm) of up to 6000 not only increases measurement frequency of the instrument, but also expands the number of measurable species. The humidity dependence of the instrument was characterized for various VOC species and the behaviors for different species can be explained by compound-specific properties that affect the ion chemistry. The new H3O+ ToF-CIMS was successfully deployed on the NOAA WP-3D research aircraft for the SONGNEX campaign in spring of 2015. The measured mixing ratios of several aromatics from the H3O+ ToF-CIMS agreed within ±10 % with independent gas chromatography (GC) measurements from whole air samples. Initial results from the SONGNEX measurements demonstrate that the H3O+ ToF-CIMS dataset will be valuable for the identification and characterization of emissions from various sources, investigation of secondary formation of many photochemical organic products and therefore the chemical evolution of gas-phase organic carbon in the atmosphere.
Theoretical chemical ionization rate constants of the concurrent reactions of hydronium ions (H3
O+
) and oxygen ions (O
2+) with selected organic iodides
