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Hydroperoxy radicals (HO2) play an important part in troposphericphotochemistry, yet photochemical models do not capture ambient HO2mixing ratios consistently. This is likely due to a combination ofuncharacterized chemical pathways and measurement limitations. The indirectnature of current HO2 measurements introduces challenges in accuratelymeasuring HO2; therefore a direct technique would help constrainHOx chemistry in the atmosphere. In this work we evaluate thefeasibility of using chemical ionization mass spectrometry (CIMS) andpropose a direct HO2 detection scheme using bromide as a reagent ion.Ambient observations were made with a high-resolution time-of-flightchemical ionization mass spectrometer (HR-ToF-CIMS) in Atlanta over themonth of June 2015 to demonstrate the capability of this direct measurementtechnique. Observations displayed expected diurnal profiles, reachingdaytime median values of ∼ 5 ppt between 2 and 3 p.m.local time. The HO2 diurnal profile was found to be influenced bymorning-time vehicular NOx emissions and shows a slow decrease into theevening, likely from non-photolytic production, among other factors.Measurement sensitivities of approximately 5.1 ± 1.0 cps ppt−1 for abromide ion (79Br−) count rate of 106 cps were observed. Therelatively low instrument background allowed for a 3σ lower detectionlimit of 0.7 ppt for a 1 min integration time. Mass spectra of ambientmeasurements showed the 79BrHO2− peak was the major componentof the signal at nominal mass-to-charge 112, suggesting high selectivity forHO2 at this mass-to-charge. More importantly, this demonstrates thatthese measurements can be achieved using instruments with only unit massresolution capability

atmospheric measurement techniques

A new technique for the direct detection of HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; radicals using bromide chemical ionization mass spectrometry (Br-CIMS): initial characterization