Measurements of iodine species and sulfuric acid using bromide chemical ionization

During the iodine oxidation experiments in CLOUD chamber, gas-phase iodine species and sulfuric acid were measured using Br-MION-CIMS and FIGAERO-CIMS .

Iodine chemistry and iodine-driven new particle formation play important roles in marine regions; however, these processes are still poorly understood. For the proper quantification of compounds, high time resolution, high sensitivity, and simultaneous measurements of several iodine species are needed.

Chemical Ionization Mass Spectrometers (CIMS) have been found helpful in analysing compounds in the atmosphere, providing fast response time and low detection limits. The bromine ion has already been used in CIMS to detect species such as chlorine, organic vapors, and sulphuric acid. Moreover, iodine species can form strong bonds with Br-; therefore, this study explores the possibility of using Br- as a reagent ion for measuring iodine-containing species.

Instrument set-up:

An intensive measurement campaign simulating high-latitude marine boundary layer was held at the CLOUD (Cosmics Leaving Outdoor Droplets) chamber at CERN. Two CIMS were used to detect gas-phase iodine species and sulfuric acid simultaneously:one was coupled to a Multi-scheme chemical ionization inlet (MION-CIMS) and the other one to a Filter Inlet for Gasses and AEROsols (FIGAERO-CIMS).

Highlighted results:

During a typical experiment, iodine atoms (I) were produced from I2 by illuminating the chamber with green light. Then, a reaction of I with ozone led to the formation of various iodine species within minutes. Species with the most significant levels, detected by MION-CIMS, are summarized in Fig. 1 with IO, HOI, and HIO3 being the most abundant.

Figure 1: Gas-phase inorganic species measured with the Br-MION-CIMS. (a) Mass defect versus of gas-phase halogen species and sulfuric acid during an iodine oxidation experiment at 69 % relative humidity and −10 ∘C. Ions shown here are either clustered with or formed via proton transfer to Br-. The size of the markers is proportional to the logarithm of the signal. (b) The high-resolution single peak fits for species in the mass defect plot.

The relative humidity dependence of bromine ion was investigated in the experiment, with the relative humidity varying from 40% to 80% and constant temperature at -10⁰C. A method to correct for the humidity effect at higher relative humidity was found: normalizing the signal with the sum of the two reagent ions: Br- and H2O•Br-. This finding validates the applicability of Br- as a regent ion to measurements in the ambient marine environment.

The detection limits of HOI, HIO3, I2 and H2SO4 were estimated for MION-CIMS to be 5.8x106, 1.2x105, 3,8x105 and 2,0x105 molec. cm-3, respectively. These detection limits were found to be one order of magnitude lower than those achieved by the FIGAERO-CIMS, which uses reduced pressure.

To sum up, simultaneous measurements of various iodine species and sulfuric acid with low detection limits were found to be unprecedented for online techniques.   

Reference: Wang, M., He, X.-C., Finkenzeller,H., Iyer, S., Chen, D., Shen, J., Simon, M., Hofbauer, V., Kirkby, J., Curtius,J., Maier, N., Kurtén, T., Worsnop, D. R., Kulmala, M., Rissanen, M., Volkamer,R., Tham, Y. J., Donahue, N. M., and Sipilä, M.: Measurement of iodine speciesand sulfuric acid using bromide chemical ionization mass spectrometers, Atmos.Meas. Tech., 14, 4187–4202,2021.

This work is distributed under the Creative Commons Attribution 4.0 License.

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Measurements of iodine species and sulfuric acid with Br-MION-MS

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