A new technique for the direct detection of HO<sub>2</sub> radicals using bromide chemical ionization mass spectrometry (Br-CIMS): initial characterization

Abstract

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.

Characterization of Ozone Production in San Antonio, Texas Using Observations of Total Peroxy Radicals

“…A 297 steady increase in [XO2] began at 9:00, with a peak of 50 pptv at 15:00 and then a decline to the 298 overnight value by 20:00. The shape of this profile is in agreement with other observations of 299 peroxy radicals from a variety of chemical environments (Sanchez et al, 2016;Mao et al, 300 2010;Whalley et al, 2018). Noise in the nighttime data is a result of higher RH and thus 301 degraded precision of the ECHAMP measurement technique and is not an indication of 302 significant nighttime variability.…”

Measurements of hydroperoxy radicals (HO<sub>2</sub>) at atmospheric concentrations using bromide chemical ionisation mass spectrometry

“…The LIF instrument uses two detection channels to simultaneously detect OH and HO 2 . The LIF instrument has been described in detail by Holland et al (2003), Fuchs et al (2011), and Tan et al (2017).…”

“…The accuracy of the LIF HO 2 measurement is ±10 % due to the uncertainty of the calibration. The typical precision of measurements gives an limit of detection of 1×10 7 mol cm −3 (2σ ) for an 80 s measurement (Tan et al, 2017).…”

Controlled nitric oxide production via O(<sup>1</sup>D) + N<sub>2</sub>O reactions for use in oxidation flow reactor studies

“…For ratios of model outputs with independent ±60 % uncertainties (e.g., NO : HO 2 ), propagated uncertainties of ±85 % were assumed. Addition of N 2 O at the highest mixing ratios that were used suppressed [OH] ±60 % uncertainty in model outputs The model also constrains mixing ratios of radical species such as HO 2 that are difficult to measure directly or require additional measurement techniques (Mauldin et al, 1999;Sanchez et al, 2016 The relative importance of these operating conditions is situationally dependent on the relative OH, O 3 , and NO 3 rate constants of the target species and photochemical age. To demonstrate proof of principle, we present NO − 3 -CIMS spectra of isoprene and α-pinene oxidation products in the following sections.…”