Measurement of iodine species and sulfuric acid using bromide chemical ionization mass spectrometers

Abstract

Abstract. Iodine species are important in the marine atmosphere for oxidation and new-particle formation. Understanding iodine chemistry and iodine new-particle formation requires high time resolution, high sensitivity, and simultaneous measurements of many iodine species. Here, we describe the application of a bromide chemical ionization mass spectrometer (Br-CIMS) to this task. During the iodine oxidation experiments in the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber, we have measured gas-phase iodine species and sulfuric acid using two Br-CIMS, one coupled to a Multi-scheme chemical IONization inlet (Br-MION-CIMS) and the other to a Filter Inlet for Gasses and AEROsols inlet (Br-FIGAERO-CIMS). From offline calibrations and intercomparisons with other instruments, we have quantified the sensitivities of the Br-MION-CIMS to HOI, I2, and H2SO4 and obtained detection limits of 5.8 × 106, 3.8 × 105, and 2.0 × 105 molec. cm−3, respectively, for a 2 min integration time. From binding energy calculations, we estimate the detection limit for HIO3 to be 1.2 × 105 molec. cm−3, based on an assumption of maximum sensitivity. Detection limits in the Br-FIGAERO-CIMS are around 1 order of magnitude higher than those in the Br-MION-CIMS; for example, the detection limits for HOI and HIO3 are 3.3 × 107 and 5.1 × 106 molec. cm−3, respectively. Our comparisons of the performance of the MION inlet and the FIGAERO inlet show that bromide chemical ionization mass spectrometers using either atmospheric pressure or reduced pressure interfaces are well-matched to measuring iodine species and sulfuric acid in marine environments.

Insights into the Chemistry of Iodine New Particle Formation: The Role of Iodine Oxides and the Source of Iodic Acid

“…22,30,31 Hydrolysis of I 2 O y by the water dimer has only been explored theoretically for I 2 O 5 , 32 although to date it is unclear whether this species actually forms in the gas phase to play a role in IOP formation. 4,19,22,28 Moreover, in our previous work, we were unable to detect gas-phase HIO 3 by near-threshold PIMS at 11.6 eV, while we did detect it in the particle phase after pyrolysis of IOPs formed in a flow tube in the presence of water vapor. 22 Gas-phase reactions between iodine species and H 2 O are, according to these experiments, slower than ∼10 −19 cm 3 molecule −1 s −1 .…”

“…There is however a major unknown about gas-phase HIO 3 : how does it form? The CIMS IO 3 – signal has been observed in the absence of HO x in laboratory flow tube experiments 23 and in CLOUD, 4 , 28 although the only known thermochemically feasible route from I 2 photooxidation to iodic acid is the recombination reaction 29 …”

“…There could even be a situation where both mechanisms could be competitive, and interestingly, CIMS also detects atmospheric I x O y in the form of I x O y ·NO 3 – or I x O y ·Br – , although these signals are uncalibrated. 4 , 28 …”

“…, although these signals are uncalibrated. 4,28 However, another possible explanation for the apparent contradiction between CIMS and PIMS gas-phase measurements is that the ions observed by CIMS may be generated, at least in part, by ion−molecule reactions between the reagent ion and iodine oxides. Our ab initio calculations indicated that different reactions between I x O y with x = 2 and NO 3 − , Br − , CH 3 COO − , and H 3 O + are exothermic and can potentially generate some of the ions and cluster ions that have been attributed to HIO 3 , in particular IO 3 − and HIO 3 •NO 3 − in the nitrate anion CIMS.…”

“…This mechanism has been amended recently considering the strong influence of instrumental settings on the observed mass spectra and currently also invokes iodous acid (HOIO, hereafter HIO 2 ) to explain the observed mass peaks . Concurrent CIMS measurements with different ionization sources appear to support the existence of gas-phase HIO 2 and HIO 3 in the CLOUD experiments ,, and by extension in the atmosphere.…”