Characterisation of the multi-scheme chemical ionisation inlet-2 and the detection of gaseous iodine species
Authors: He Xu-Cheng , Shen Jiali , Iyer Siddharth , Juuti Paxton , Zhang Jiangyi , Koirala Mrisha , Kytökari Mikko M. , Worsnop Douglas R. , Rissanen Matti , Kulmala Markku , Maier Norbert M. , Mikkilä Jyri , Sipilä Mikko , Kangasluoma Juha 
Published: February 2023

Abstract. The multi-scheme chemical ionisation inlet 1 (MION1) allows fast switching between measuring atmospheric ions without chemical ionisation and neutral molecules by multiple chemical ionisation methods. In this study, the upgraded multi-scheme chemical ionisation inlet 2 (MION2) is presented. The new design features improved ion optics that increase the reagent ion concentration, a generally more robust operation and the possibility to run multiple chemical ionisation methods with the same ionisation time. To simplify the regular calibration of MION2, we developed an open-source flow reactor chemistry model (MARFORCE) to quantify the chemical production of sulfuric acid (H2SO4), hypoiodous acid (HOI) and hydroperoxyl radical (HO2). MARFORCE simulates convection-diffusion-reaction processes inside typical cylindrical flow reactors with uniform inner diameters. The model also provides options to simulate the chemical processes 1) when two flow reactors with different inner diameters are connected together and 2) when two flows are merged into one (connected by a Y-shape tee), but with reduced accuracy. Additionally, the chemical mechanism files in the model are compatible with the widely-used Master Chemical Mechanism, thus allowing future adaptation to simulate other chemical processes in flow reactors. We further carried out detailed characterisation of the bromide (Br−) and nitrate (NO3−) chemical ionisation methods with different ionisation times. We calibrated H2SO4, HOI and HO2 by combining gas kinetic experiments with the MARFORCE model. Sulfur dioxide (SO2), water (H2O) and molecular iodine (I2) were evaluated using dilution experiments from a gas cylinder (SO2), dew point mirror measurements (H2O), and a derivatization approach in combination with high-performance liquid chromatography quantification (I2), respectively. We find that the detection limit is negatively correlated with the fragmentation enthalpy of the analyte-reagent ion (Br−) cluster, i.e., a stronger binding (larger fragmentation enthalpy) leads to a lower detection limit. Additionally, a moderately longer reaction time enhances the detection sensitivity thus decreasing the detection limit. For example, the detection limit for H2SO4 is estimated to be 2.9 × 104 molec. cm−3 with a 300 ms ionisation time. A direct comparison suggests that this is even better than the widely-used Eisele-type chemical ionisation inlet. While the NO3− chemical ionisation method is generally more robust, we find that the Br− chemical ionisation method (Br−-MION2) is significantly affected by air water content. Higher air water content results in lower sensitivity for HO2 and SO2 within the examined conditions. On the other hand, a steep sensitivity drop of H2SO4, HOI and I2 is only observed when the dew point is greater than 0.5–10.5 ℃ (equivalent to 20–40 % RH; calculated at 25 ℃ hereafter). Future studies utilising atmospheric pressure Br− chemical ionisation method, including Br−-MION2, should carefully address the humidity effect on a molecular basis. By combining methods such as water-insensitive NO3−-MION2 with Br−-MION2, MION2 should be able to provide greater details of air composition than either of these methods alone. Combining instrument voltage-scanning, chemical kinetic experiments and quantum chemical calculations, we find that the HIO3 detection is not interfered with by iodine oxides under atmospherically relevant conditions. The IO3−, HIO3NO3− and HIO3Br− ions measured using the Br− and NO3− chemical ionisation methods are primarily, if not exclusively, produced from gaseous HIO3 molecules.