A new technique for the direct detection of HO₂ radicals using bromide chemical ionization mass spectrometry (Br-CIMS): initial characterization

Abstract. Hydroperoxy radicals (HO₂) play an important part in tropospheric photochemistry, yet photochemical models do not capture ambient HO₂ mixing ratios consistently. This is likely due to a combination of uncharacterized chemical pathways and measurement limitations. The indirect nature of current HO₂ measurements introduces challenges in accurately measuring HO₂; therefore a direct technique would help constrain HO_x chemistry in the atmosphere. In this work we evaluate the feasibility of using chemical ionization mass spectrometry (CIMS) and propose a direct HO₂ detection scheme using bromide as a reagent ion. Ambient observations were made with a high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) in Atlanta over the month of June 2015 to demonstrate the capability of this direct measurement technique. Observations displayed expected diurnal profiles, reaching daytime median values of ∼ 5 ppt between 2 and 3 p.m. local time. The HO₂ diurnal profile was found to be influenced by morning-time vehicular NO_x emissions and shows a slow decrease into the evening, likely from non-photolytic production, among other factors. Measurement sensitivities of approximately 5.1 ± 1.0 cps ppt⁻¹ for a bromide ion (⁷⁹Br⁻) count rate of 10⁶ cps were observed. The relatively low instrument background allowed for a 3σ lower detection limit of 0.7 ppt for a 1 min integration time. Mass spectra of ambient measurements showed the ⁷⁹BrHO₂⁻ peak was the major component of the signal at nominal mass-to-charge 112, suggesting high selectivity for HO₂ at this mass-to-charge. More importantly, this demonstrates that these measurements can be achieved using instruments with only unit mass resolution capability.