A new technique for the direct detection of HO 2 radicals using bromide chemical ionization mass spectrometry (Br-CIMS): initial characterization
Hydroperoxy radicals (HO 2) play an important part in tropospheric photochemistry, yet photochemical models do not capture ambient HO 2 mixing ratios consistently. This is likely due to a combination of uncharacterized chemical pathways and measurement limitations. The indirect nature of current HO 2 measurements introduces challenges in accurately measuring HO 2; 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 HO 2 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 HO 2 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 10 6 cps were observed. The relatively low instrument background allowed for a 3 σ lower detection limit of 0.7 ppt for a 1 min integration time. Mass spectra of ambient measurements showed the 79BrHO 2− peak was the major component of the signal at nominal mass-to-charge 112, suggesting high selectivity for HO 2 at this mass-to-charge. More importantly, this demonstrates that these measurements can be achieved using instruments with only unit mass resolution capability.