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.
Highlights
Hydroperoxy radicals (HO2) play an important role in the photochemistry of the troposphere. They are primarily formed from the OH initiated oxidation of CO and other volatile organic compounds (VOCs), with contributions from ozonolysis of alkenes, nitrate radical oxidation of VOCs, and photolysis of aldehydes (e.g., HCHO) (Geyer et al, 2003; Cooke et al, 2010; Volkamer et al, 2010; Alam et al, 2013; Stone et al, 2014)
We evaluated the potential of various chemical ionization schemes and propose the Br− ionization of HO2 to form a Br−(HO2) adduct as a direct method for measuring HO2 using chemical ionization mass spectrometry
A high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-chemical ionization mass spectrometry (CIMS), Aerodyne Research, Inc.) and a house-built quadrupole CIMS were used for laboratory characterizations of reagent ions for the measurement of HO2
Summary
Hydroperoxy radicals (HO2) play an important role in the photochemistry of the troposphere. Both techniques directly measure HO2 but their applicability to atmospheric observations is limited, due to either instrumentation needs or low time resolution More recent methods such as peroxy radical chemical amplification (PERCA) (Cantrell and Stedman, 1982; Cantrell et al, 1984; Liu et al, 2009; Horstjann et al, 2014), chemical ionization mass spectrometry (CIMS) (Hanke et al, 2002; Edwards et al, 2003; Hornbrook et al, 2011; Kim et al, 2013; Wolfe et al, 2014), and laser induced fluorescence (LIF) (Stevens et al, 1994; Brune et al, 1995; Griffith et al, 2013; Walker et al, 2015) provide lower detection limits at high temporal resolution.
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