Articles | Volume 9, issue 8
Research article
19 Aug 2016
Research article |  | 19 Aug 2016

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

Javier Sanchez, David J. Tanner, Dexian Chen, L. Gregory Huey, and Nga L. Ng

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 HOx 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 NOx 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.

Short summary
HO2 radicals play an important role in tropospheric chemistry. Here we propose a new direct method for measuring HO2 radicals in the atmosphere using bromide anion chemical ionization mass spectrometry. Ambient measurements in Atlanta are presented. Instrument performance parameters: sensitivity, lower detection limit, and time resolution are discussed. We demonstrate that the technique provides excellent selectivity and is suitable for in situ ground-based HO2 measurements.