Development of a chemical ionization mass spectrometry technique for the direct measurement of HO2
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Hydroperoxy radicals (HO2) play important roles in tropospheric photochemistry. Despite significant efforts in constraining HO2 abundance, notable and variable discrepancies exist between atmospheric models and observations. Disagreements may be explained by uncharacterized chemical pathways or deficiencies in current measurement techniques, likely attributable to the indirect nature of such techniques. A direct method for measuring HO2 would aid in accurately constraining HO2 chemistry in the atmosphere. In this work, the feasibility of several reagent ions for the direct detection of HO2 using chemical ionization mass spectrometry (CIMS) was explored. A direct HO2 detection scheme using bromide as a reagent ion is proposed. Laboratory characterizations suggest that the method is selective and applicable to laboratory and ambient measurements. Ambient observations were made with a high resolution time-of-flight chemical ionization mass spectrometer in Atlanta over the month of June 2015 to demonstrate the capabilities of this direct measurement technique. Observations displayed expected diurnal profiles, reaching daytime median values of ~5 ppt between 2 p.m. and 3 p.m. local time, and displayed no obvious artifacts. Measurement sensitivities of approximately 5.11.0 cps/ppt for a bromide ion (79Br−) count rate of 106 cps were observed. The relatively low instrument background allowed for a 3 lower detection limit of 0.7 ppt for 1 minute time resolution. Mass spectra of ambient measurements further support the selectivity of the Br− ionization technique. High resolution mass spectra showed that the 79Br−(HO2) peak was the major component of the signal at nominal mass-to-charge 112, suggesting high selectivity for HO2 at this mass-to-charge. Important measurement considerations and future improvements are discussed.