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). HO2 is a reservoir species for OH, the primary daytime oxidant, and facilitates the photochemical production of ozone via its reaction with NO. Additionally, the relative abundance of HO2 to NOx plays a critical role in the fate of peroxy radicals (RO2) and the production of low-volatility products in secondary organic aerosol (SOA) formation (Orlando and Tyndall, 2012; Ziemann and Atkinson, 2012). For instance, reactions of HO2 with RO2 serve as the main source of atmospheric organic hydroperoxides, which are important constituents of SOA (Docherty et al., 2005).

HO2 is also critical for the evaluation of model photochemical schemes. Because HO2 is short lived (Heard and Pilling, 2003), HO2 measurements allow for evaluation of model photochemistry with the exclusion of confounding phenomena, such as atmospheric transport. However, accurate measurements have proven difficult due to the naturally low abundance of HO2. HO2 also possess weak spectral lines, which has posed challenges for spectroscopic techniques. Nevertheless, some spectroscopic measurements have been made, primarily in the laboratory. Radford et al. (1974) employed laser magnetic resonance (LMR) spectroscopy in the laboratory to measure HO2 while Mihelcic et al. (1985, 2003) used matrix isolation electron spin resonance (MIESR). The latter requires HO2 to be collected for a period of 30 min on a D2O matrix at 77 K before detection. 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. However, these techniques do not measure HO2 directly. Instead, the aforementioned techniques require that HO2 be titrated with NO, which may introduce additional complexity, primarily from reactions of NO with RO2. The PERCA technique, for example, exploits the radical chain reactions of HO2 with NO and OH with CO to produce multiple NO2 molecules from each HO2 present in the sample. The production of NO2 molecules is proportional to the contact time between the added reagents and the sample gas. Because multiple NO2 molecules are produced from each HO2, the signal is effectively amplified. The NO2 has then traditionally been detected using luminol, though some recent measurements employ cavity ring-down spectroscopy (Liu et al., 2009; Horstjann et al., 2014). If organic peroxy radicals are present, the addition of NO to the sample gas results in additional production of HO2; therefore the technique allows for the measurement of HO2 + RO2. As of yet, speciation of HO2 from other peroxy radicals has not been successful, though previous attempts have been made (Miyazaki et al., 2010).

Chemical ionization techniques such as ROx Chemical Conversion/CIMS (ROxMAS) (Hanke et al., 2002) and Peroxy radical CIMS (PerCIMS) (Edwards et al., 2003; Hornbrook et al., 2011; Kim et al., 2013; Wolfe et al., 2014) also rely on addition of NO to the sample gas. The OH produced from HO2 titration subsequently reacts with SO2 which is added to the inlet to produce H2SO4. The H2SO4 is then ionized by nitrate ions (NO3-) to produce a stable HSO4- product ion for detection. Because NO is added, PerCIMS suffers from positive artifacts from the contribution of the RO2 + NO reaction to the measured HO2 concentration. Previous attempts to limit the contribution of the RO2 + NO reaction have been made. For example, Hornbrook et al. (2011) successfully speciated HO2 and HO2 + RO2 measurements by modulating the relative NO and O2 concentrations in the reaction region, suppressing the conversion of some peroxy radicals to ∼ 15 %. However, the effectiveness of the oxygen dilution modulation scheme depends largely on the chemical structure of the hydrocarbons in the sample. Unsaturated hydrocarbons, such as isoprene, have additional pathways to the formation of HO2 which are not suppressed by the oxygen dilution modulation scheme.

Unlike CIMS techniques and chemical amplification, the laser induced fluorescence technique does not require multiple conversion steps. It only requires the titration of HO2 to OH, which is subsequently detected using laser excitation at 308 nm. The relative simplicity of the technique allows for more effective control of reaction time, which can be used to minimize RO2 + NO reactions. Nevertheless, LIF instruments have previously been shown to be susceptible to similar artifacts, with magnitudes dependent on the effective reaction time allowed after NO addition before detection and on peroxy radical precursor composition of the sample gas (Fuchs et al., 2011; Whalley et al., 2013).

Given the uncertainties associated with indirect methods of HO2 measurement and the time resolution required for atmospheric measurements, a direct fast time resolution measurement of HO2 would benefit efforts aiming to measure and model HO2 in order to understand atmospheric photochemistry. The exclusion of measurement artifacts would aid in evaluating the gap between measured and modeled HO2 concentrations in forested regions, where measured to modeled HO2 ratios are highly variable (Stone et al., 2012). In this work, 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. This technique provides selective, fast time resolution measurements of HO2. Ambient measurements were conducted in Atlanta in June 2015 to demonstrate instrument performance. Important measurement considerations and future improvements are also discussed.

A high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS, Aerodyne Research, Inc.) and a house-built quadrupole CIMS were used for laboratory characterizations of reagent ions for the measurement of HO2. Ambient data were collected using the HR-ToF-CIMS exclusively. The HR-ToF-CIMS consists of an atmospheric pressure interface with five differentially pumped stages, utilizing two scroll pumps and a multi-stage turbomolecular pump. The instrument design has been described in detail by Bertram et al. (2011). Figure 1 shows a schematic diagram of the HR-ToF-CIMS. HO2 is measured by introducing 2 standard L min-1 of sample into the ion–molecule reaction region through a 0.5 mm orifice and mixed with Br- reagent produced by passing 10 sccm of a 0.2 % CF3Br / N2 mixture carried by ∼ 2 standard L min-1 N2 gas (99.999 %, Airgas) through a cylindrical 10 mCi 210Po alpha radiation source. The ion–molecule reaction region (IMR) has a residence time of 0.07 s and is operated at a pressure of 100 mbar. The contents of the IMR were sub-sampled through a 0.3 mm critical orifice leading to the instrument's ion optics. HO2 present in the sample cluster with Br- to form Br-(HO2) adducts which are transmitted by a series of ion optics and speciated by the time-of-flight spectrometer. The isotopic abundance of bromine is such that the adduct is detected at two nominal mass-to-charge ratios, m/z 112 and m/z 114 corresponding to the 79Br and 81Br isotopes, respectively. The adducts have fractional mass-to-charge ratios of 111.9165 and 113.9144 Th. In order to demonstrate the generalizability of the technique to instruments with unit mass resolution, only the unit mass resolution data are used for ambient measurements, though the high-resolution capabilities were exploited to diagnose and address possible artifacts during the method development. The high-resolution mass spectra are also used to unambiguously identify the BrHO2- adducts. In contrast to the HR-ToF-CIMS, the quadrupole CIMS was operated at an ion–molecule reaction region pressure of 15 torr. The sample and N2 flow rates were both 2 standard L min-1.

HO2 were generated for the evaluation of chemical ionization schemes and instrument calibrations by water photolysis at 184.9 nm using a mercury UV lamp as previously described by a number of studies e.g., (Tanner et al., 1997; Holland et al., 2003; Smith et al., 2006; Dusanter et al. 2008). Air was humidified by passing the gas stream through glass bubblers and diluted in dry air from a pure air generator (AADCO 747-14) to vary the relative humidity of the gas. The gas stream was then introduced into a black anodized aluminum square flow tube (15.6 mm × 15.6 mm × 520 mm) and exposed to UV radiation through a small slit. HO2 concentrations were calculated using Eq. (), Δ[HO2]=ϕσI[H2O]Δt, where ϕ, σ, I, and Δt represent the quantum yield, water absorption cross-section, UV lamp photon flux at 184.9 nm, and irradiation time, respectively. The quantum yield was assumed to be unity and a value of 7.22 × 10-20 cm2 was used for the water absorption cross section (Creasey et al., 2000). The lamp photon flux was measured using a Hamamatsu Phototube (Hamamatsu Photonics), and found to be 2.6 × 1013 photons cm-2 s-1. A bandpass filter (HORIBA Scientific) was used to selectively transmit at 185 nm. The dew point of the gas was measured using a LICOR LI-840A CO2 / H2O gas analyzer. The irradiation time was calculated based on the flow velocity which was measured using a Dwyer pitot tube and a magnehelic pressure sensor. Flow velocities varied between 400 and 800 cm s-1 (Re > 4000) to promote plug flow conditions. The slit allowing light into the tube was located such that the distance before irradiation after entry into the tube was 10 times the hydraulic diameter, allowing the flow profile to fully develop before exposure to UV light. Plug flow conditions were confirmed by measuring the flow velocity both at the center line and near the wall of the tube, showing no measurable differences. The water vapor mixing ratios varied between 0.66 and 8.20 ppt. The time after irradiation before introduction into the instrument was minimized (τ ∼ 60 ms) to avoid additional HO2 generation from OH oxidation of trace CO present in the N2 gas. It was calculated that less than 8 % of the OH formed would react with CO to produce additional HO2 assuming a CO concentration of 500 ppb. Addition of 40 ppm of C3F6 as an OH scavenger had no effect on the HO2 signal intensity, confirming the lack of contribution from OH oxidation to HO2 production. The HO2 concentration was kept low (2–45 ppt) to calibrate for the HO2 levels observed during ambient sampling and to avoid non-linearity in the calibration curve due to depletion of HO2 through HO2 radical–radical recombination. At the HO2 mixing ratios employed, less than 1 % of HO2 are estimated to be lost to recombination. The overall calibration uncertainty is 18 % (1σ). The contribution of the different parameters to the overall uncertainty is given in Table S1 in the Supplement. A conservative estimate of 20 % is used in this work.

Prior to the selection of Br- for our ionization scheme, a number of negative reagent ions were evaluated for their ability to detect HO2. HO2 were generated using the procedure in Sect. 3. HO2 mixing ratios were typically in excess of 300 ppt for initial reagent ion evaluation, and varied by varying the gas humidity and velocity. NO was added in excess (2–4 ppm) to obtain the instrument background. NO was also added in small concentrations and in increasing increments to roughly test the kinetics suggested by the HO2 signal response to additions of varying concentrations of NO, as additional confirmation that the analyte being observed corresponded to HO2. Tests were conducted primarily at room temperature (293 K). The humidity of the gas stream was determined by the amount of water vapor added to produce HO2. No additional sources of water vapor were present, nor was water directly added to the IMR. The reagent ions evaluated included O2-, SF6-, Cl-, and I-. Because of their low electron affinities, O2- and SF6- were utilized in an attempt to produce the HO2- ion directly via charge exchange. However, HO2- was not observed in laboratory characterization experiments. It is likely that the HO2- ion formed but was not detected due to its low electron affinity (Ramond et al., 2002), which results in high reactivity. The SF6- ionization did yield a cluster at mass-to-charge 52, which was assigned as HO2F- in the high-resolution mass spectrum. However, the signal was not quantitatively reproducible and did not remain constant for a given HO2 concentration. Additionally, the form of the cluster is more likely to be O2-(HF) than F-(HO2) (Seeley et al., 1996), which may compromise the selectivity of the measurement, as O2- ions are not uniquely formed from the ionization of HO2.

Chloride and iodide reagent ions were generated from HCl and CH3I mixtures, respectively. Full mass spectra obtained using the quadrupole CIMS identified Cl-(HCl), Cl-(HNO3), and Cl-(CF3COOH) as prominent ions. However, the Cl-(HO2) cluster was not observed. The characterizations involving chloride reagent ions in the laboratory were conducted using the quadrupole CIMS. Unlike the other ions, which were evaluated with the HR-ToF-CIMS as well as the quadrupole CIMS, Cl- was not revisited with the HR-ToF-CIMS instrument.

The I- reagent ion, which has been used extensively to measure both organic and inorganic species (Huey et al., 1995; Slusher et al., 2004; Lee et al., 2014; Woodward-Massey et al., 2014; Brophy and Farmer, 2015; Faxon et al., 2015; Nah et al., 2016; Lee et al., 2016), was found to cluster with HO2, appearing at mass-to-charge 160, consistent with observations by Veres et al. (2015). However, we observed that addition of NO2 (Scott-Marrin, 100 ppm v/v N2) to a clean N2 gas matrix resulted in an increase of 1 cps per ppb NO2 per 106 cps I- in the m/z 160 signal. A 20 ppb addition results in a 20 cps increase in the m/z 160 signal, equivalent to ∼ 4 ppt of HO2. The addition of NO2 showed an increase in a peak (m/z 159.9896) not associated with HO2 that we could not identify. The addition of NO2 did not affect the high-resolution I-(HO2) signal but is expected to be a significant artifact for instruments of low resolving power.

Unlike the other reagent ions, the Br- ionization scheme was found to be sensitive to HO2, the measurements were reproducible, and there were no observed positive artifacts from NO2 or O3, making this an ideal scheme for measurements of HO2. To evaluate potential positive artifacts as observed using iodide, NO2 was added to the N2 gas sample and measured using the bromide reagent, but no increase in the Br-(HO2) (nominal m/z 112 and m/z 114) signals was observed. Parts per million mixing ratios of ozone were also introduced into the inlet in a clean N2 matrix but did not cause any changes in the Br- (HO2) cluster signal. While Br- has the disadvantage of having a ∼ 50 % natural isotopic abundance with nominal m/z 79 and 81, HO2 calibrations performed as described in Sect. 4 showed similar absolute sensitivities for the I-(HO2) and 79Br-(HO2) clusters using I- and Br- reagents, respectively. A synthesized mixture containing primarily 79Br could nearly double the sensitivity at the m/z 112 cluster if necessary, giving Br- a potential advantage over I- with respect to sensitivity. The 79Br-(HO2) cluster was used preferentially over the 81Br-(HO2) cluster for ambient data because m/z 114 has a contribution from the isotope of a large m/z 113 CF3COO- signal which arises from impurities in PFA Teflon^™. Iodide ionization was attempted once more during ambient sampling, which will be discussed in Sect. 5.3.

To demonstrate the applicability of the Br- ionization scheme to ambient HO2 measurements, a field study was conducted in June 2015 (9–25 June 2015) in Atlanta at an urban site located on the roof (30–40 m above ground) of the Ford Environmental Science & Technology building on the Georgia Tech campus, which has been used for previous ambient studies (Hennigan et al., 2008; Xu et al., 2015a, b). The site is about 840 m west of Interstate 75/85 and can therefore be affected by traffic emissions. The instrument was located outside in an enclosure, allowing for a short 1 cm inner diameter Teflon^™ inlet of approximately 13 cm in length. The residence time of ambient sample in the tube was short (0.3 s) which helps to minimize HO2 surface losses on the sample tubing. Data was collected at a 1 Hz frequency and averaged to 1 min. A solenoid valve was used to perform periodic additions of 10 sccm of an NO / N2 mixture (Scott-Marrin, 810 ppm) into the sample stream every 10 min on a 10 % duty cycle to obtain the measurement background. The m/z 112 signal was normalized to a 79Br- count of 106 cps to account for temporal changes in reagent ion abundance.

Various co-located instruments were deployed for simultaneous measurements of O3, NO, NO2, and HNO4. NO concentrations were measured using a Teledyne 200EU chemiluminescence monitor while NO2 was measured by a Cavity Attenuated Phase Shift NO2 monitor (Aerodyne Research, Inc.). Ozone was measured using a Teledyne Model T400 UV absorption analyzer. Pernitric acid (HNO4), formed from the reaction of HO2 with NO2 was also monitored using a house-built quadruple CIMS with an iodide-adduct ionization scheme and observed at m/z 206. A similar configuration of the instrument has been described previously by Slusher et al. (2004). Previous measurements of HNO4 using I- have been conducted by Veres et al. (2015). Meteorological data, including temperature and humidity, were recorded using a Vantage Pro2 weather station. An additional UV sensor was employed with the Vantage Pro2 weather station to obtain an UV index measurement between 280 and 360 nm.

HO2 is an important contributor to photochemistry in the atmosphere. In this work, we investigated the feasibility of a direct chemical ionization measurement of HO2. We evaluated a number of negative reagent ions (O2-, SF6-, Cl-, I-, and Br-) using a HR-ToF-CIMS and found that detection of HO2 using charge exchange ionization is not feasible in the real atmosphere. However, ionization of HO2 via clustering was found to be a promising mechanism for the direct measurement of atmospheric HO2. Among the reagent ions evaluated, Br- was found to be the best candidate for the measurement of HO2, providing improved selectivity over I-. The HO2 sensitivities as measured at m/z 160 and m/z 112 using iodide and bromide, respectively, were found to be similar, giving Br- a potential advantage in sensitivity, as an isotopically pure CF379Br mixture should nearly double the sensitivity at m/z 112. Using Br- also allows for the measurement of HO2 at a lower m/z which may decrease the likelihood of measurement interferences and reduce ambiguity in peak identification, as a smaller number of possible chemical formulas for ion products are possible. Furthermore, Br has a high electron affinity, which makes the production of small charged ions from ionization and collisional dissociation unlikely.

Ambient measurements were conducted in Atlanta in June 2015 to demonstrate the performance and capability of the instrument. The sensitivity using Br- (5.1 ± 1.00 cps ppt-1 per 10679Br ion counts) was sufficient for ground-based measurements as the observed baselines were relatively low. Furthermore, the absolute sensitivity for HO2 may also be significantly improved by using a radioactive source with higher activity, provided that measures are taken to suppress the increased background due to internal HO2 generation. The measured HO2 diurnal profile behaves in a manner consistent with the NOx and HNO4 abundance, though there exist no previous measurements for HO2 in Atlanta available for a more quantitative comparison. Future work will focus on optimizing the instrument sensitivity to HO2, conducting instrument intercomparisons, and further exploring Br- ionization for the measurement of other atmospherically important species.

The data presented in this paper are available upon request from the corresponding author.