Measurements of hydroperoxy radicals (HO2) at atmospheric concentrations using bromide chemical ionization mass spectrometry

Abstract. Hydroxyl and hydroperoxy radicals are key species for the understanding of atmospheric oxidation processes. Their measurement is challenging due to their high reactivity, therefore very sensitive detection methods are needed. Within this study, the measurement of hydroperoxy radicals (HO2) using chemical ionization combined with an high resolution time of flight mass spectrometer (Aerodyne Research Inc.) employing bromide as primary ion is presented. The 1σ limit of detection of 4.5 × 107 molecules cm−3 for a 60 s measurement is below typical HO2 concentrations found in the atmosphere. The detection sensitivity of the instrument is affected by the presence of water vapor. Therefore, a water vapor dependent calibration factor that decreases approximately by a factor of 2 if the water vapor mixing ratio increases from 0.1 to 1.0 % needs to be applied. An instrumental background most likely generated by the ion source that is equivalent to a HO2 concentration of 1.5 ± 0.2 × 108 molecules cm−3 is subtracted to derive atmospheric HO2 concentrations. This background can be determined by overflowing the inlet with zero air. Several experiments were performed in the atmospheric simulation chamber SAPHIR at the Forschungszentrum Jülich to test the instrument performance by comparison to the well-established laser-induced fluorescence (LIF) technique for measurements of HO2. A high linear correlation coefficient of R2 = 0.87 is achieved. The slope of the linear regression of 1.07 demonstrates the good absolute agreement of both measurements. Chemical conditions during 15 experiments allowed testing the instrument’s behavior in the presence of atmospheric concentrations of H2O, NOx and O3. No significant interferences from these species were observed. All these facts are demonstrating a reliable measurement of HO2 by the chemical ionization mass spectrometer presented.


pollutants in the atmosphere. Primary sources of OH radicals are mainly ozone photolysis and in polluted environments also nitrous acid (HONO) photolysis can be of importance. Organic pollutants are oxidized by OH to produce organic peroxy radical species (RO 2 ) and also hydroperoxy radicals (HO 2 ). OH and HO 2 radicals are closely inter-connected by a radical chain reaction, in which OH is reformed by the reaction of HO 2 with nitric oxide (NO): As the atmospheric lifetime of HO 2 radicals is typically up to a factor 10 longer than that of OH radicals, HO 2 can be regarded as an important chemical reservoir for hydroxyl radical (OH). Atmospheric NO concentrations are often sufficiently high to maintain an efficient OH production by the reaction of HO 2 with NO, so that R1 provides a large portion of the total OH production. Measurements of both species are needed to analyze the OH radicals budget.
The majority of the techniques currently applied to measure atmospheric concentrations of HO 2 radicals use chemical 10 conversion, which is an indirect measurement. In chemical amplifying systems, a radical reaction cycle between OH and HO 2 is established by adding two reactants. The concentration of the product species is therefore amplified compared to the small, initial HO 2 concentration in the sampled air.
PEroxy RadiCal Amplification (PERCA) instruments make use of NO and CO for the conversion of HO 2 to OH and OH to HO 2 , respectively. One NO 2 molecule is produced in each reaction cycle so that the initially small HO 2 concentration is 15 amplified as NO 2 , which is then detected by a luminol detector, fluorescence or absorption methods. Because RO 2 is also converted to HO 2 in the reaction with NO, these instruments measure the sum of RO 2 and HO 2 . Typically an amplification of roughly a factor of 100 is achieved to produce a measurable amount of NO 2 (Cantrell et al., 1984;Hastie et al., 1991;Clemitshaw et al., 1997;Burkert et al., 2001;Sadanaga et al., 2004;Mihele and Hastie, 2000;Green et al., 2006;Andrés-Hernández et al., 2010). 20 Alternatively to CO, SO 2 can be used in the chemical amplifier system (Reiner et al., 1997;Hanke et al., 2002;Edwards et al., 2003;Hornbrook et al., 2011). The high sensitivity of CIMS measurement using NO − 3 as primary ion allows to detect H 2 SO 4 produced in the reaction of SO 2 with OH. Amplification factors of approximately 10 are sufficient is this case. Like in the PERCA instrument, RO 2 is also converted to HO 2 in the reaction with NO in these instruments. However, Hornbrook et al. (2011) developed a method to distinguish between HO 2 and RO 2 by operating the instrument at different chemical conditions 25 (varying NO, SO 2 and O 2 concentrations), thereby changing the relative sensitivities for HO 2 and RO 2 .
Laser-induced fluorescence (LIF) is a sensitive technique for OH radical measurements and it is used for the indirect detection of HO 2 by its conversion into OH after reaction with NO. The concurrent conversion of some specific RO 2 radicals can contribute to the HO 2 signal (Fuchs et al., 2011;Whalley et al., 2013;Lew et al., 2018). This can be minimized by reducing the NO concentration added to the sampled air for the conversion of HO 2 to OH, but on the cost of a reduced sensitivity. A 30 comparison of three LIF instruments in 2010 before the RO 2 interference was discovered showed significant differences in measured HO 2 concentration in experiments in the SAPHIR chamber (Fuchs et al., 2010). This could have been partly due to interferences from RO 2 , but measurements also differed depending on the water vapor concentration.
Several drawbacks are connected with existing HO 2 detection methods. The PERCA systems exhibit a strong water vapor dependence of the amplification factor. In addition, chemical conversion of HO 2 by the reaction with NO used in all instruments can lead to the concurrent conversion of RO 2 .
Previous work by Veres et al. (2015) showed that HO 2 radicals can be detected with a CIMS instrument using iodide as primary ion. Sanchez et al. (2016) demonstrated for the first time that this approach can also be used with Br − . The HO 2 5 radicals are directly measured by a mass spectrometer as an ion cluster formed with bromide ions. In this study, the direct measurement of atmospheric concentrations of HO 2 radicals using Br-CIMS is presented. A detailed characterization of the instrument has been performed. Further, the inter-comparison with an LIF based HO 2 measurement is used to identify potential interferences.

Chemical ionization mass spectrometry technique
The instrument used for the detection of the Br − · HO 2 cluster consists of a custom-built ion flow tube ( Fig. 1) that is mounted upstream of a commercial, high resolution time-of-flight mass spectrometer (TOF-MS, Aerodyne Res.). For the detection of reactive HO 2 radicals, losses in inlets can play a significant role. As radical species are easily lost by contact on walls, the inlet of the instrument is designed to sample air directly into the ion flow tube without additional inlet lines. The TOF-MS is 15 equipped with an atmospheric pressure ionization (APi) transfer stage providing the ion transfer from the ion flow tube to the detector. The TOF mass analyzer (Tofwerk Ag, Switzerland) has a mass resolution better than 2000.
Ambient air containing HO 2 (flow rate 3.4 slm) is sampled through a 0.7 mm skimmer nozzle and is mixed with the bromide ions in the ion flow tube shown in Fig. 1. The ion flow tube has an inner diameter of 22 mm and a length of 130 mm. The distance between the ion source and the nozzle downstream is 100 mm. The ion flow tube is kept at a constant pressure of 20 120 hPa using a butterfly control valve upstream of a scroll pump. Assuming that 5.4 slm of gas are passing through the ion flow tube without considering the complex fluid dynamics in the ion flow tube, the mean residence time is 4 ms. Longer versions of the ion flow tube of up to twice its size were tested, but a reduced sensitivity for HO 2 was found. Downstream of the ion flow tube, the sampled air enters a commercially available transfer stage (CI-API transfer stage, Aerodyne Research Inc.) through a nozzle with 0.5 mm diameter. The transfer stage consists of two quadrupoles and direct current transfer optics 25 that guide the ions to the TOF analyzer.
Bromide ions easily clusters with polar species e.g. acids (Caldwell et al., 1989). This enables their detection in the gas phase including HO 2 , which is a relative strong acid (the binding energy is 353 kcal mol −1 Harrison (1992)). In order to produce Br − ions, a gas flow of 2 slm nitrogen is mixed with 10 sccm of a 0.4 % mixture of CF 3 Br in nitrogen (Air Liquide Deutschland GmbH, N 2 99.9999 % purity). The resulting gas mixture of approximately 20 ppmv CF 3 Br in nitrogen is supplied to the 30 370 MBq 210 Po ion source to generate bromide ions.
The isotopic pattern of bromide (approx. 1 79 Br : 1 81 Br) provides additional information if a signal detected at a certain mass contains a cluster with bromide, because similar signals need to be contained at two masses (m/z and m/z+2). Therefore, HO 2 · Br − is detected on masses 112 and 114 with similar intensities. Both signals can be used for the data evaluation in order to improve the signal-to-noise ratio. The data are analyzed using the following procedure. 30 mass spectra measured with a time resolution of 2 s are summed up to improve the signal-to-noise ratio (cf. Sect. 3.2). The HO 2 · Br − ion cluster ion count rate (m/z 112) is normalized to the count rate of the primary ion (m/z 79). The isotopic signal at a mass-to-charge ratio of 114 and 81 are treated in the same 5 way. The signal at both isotopic masses of the HO 2 · Br − ion cluster are compared to check for possible interference from ions not containing a bromide molecule. In the following step, a water vapor dependent sensitivity is applied to convert the signal to a HO 2 concentration. Details about the water vapor dependent sensitivity are presented in Sect (not containing bromide) is interfering. In this study, only data from one of the two isotopes (m/z 112 and 79) are discussed for simplicity.

Calibration source
For calibrating the HO 2 -CIMS instrument's sensitivity the same radical source is used as for calibration of the LIF instrument that is in operation at Forschungszentrum Jülich (Fuchs et al., 2011). This is possible because the designs of the inlet nozzle 5 and flow rates of both instruments are similar. The LIF is sampling 1.0 slm and the CIMS instrument is sampling 3.4 slm.
Both flows are much smaller than the total flow through the calibration source. The calibration source provides a laminar gas stream of humidified synthetic air at a flow rate of 20 slm. The gas supply device for the calibration source allows for systematic variation of the water vapor concentration. During calibrations, the water vapor concentration is altered from 0.1 to 1.6 %, in order to determine the humidity dependence of the instrument's sensitivity. Water vapor is photolysed at 185 nm at 10 atmospheric pressure using a penray lamp leading to the production of equal concentrations of OH and HO 2 radicals (Fuchs et al., 2011). The radical concentration that is provided by the calibration source is calculated from the UV intensity that is monitored by a photo-tube detector, the flow rate and water vapor concentration. The photo-tube signal is calibrated against ozone that is concurrently produced from oxygen photolysis by the 185 nm radiation. An absorption cell in-between the UV lamp and the photolysis region can be filled with a N 2 O / N 2 mixture to vary the UV intensity, as N 2 O is a strong absorber 15 at this wavelength. If excess CO is added to the synthetic air provided to the calibration source, OH is converted to HO 2 , so that the HO 2 concentration is doubled compared to the operation without CO. Typically, the calibration is performed at HO 2 concentrations between 5 × 10 8 and 1 × 10 10 molecules cm −3 .

HO 2 detection by laser-induced fluorescence
The LIF instrument uses two detection channels to detect OH and HO 2 simultaneously. The LIF instrument has been described 20 in detail by Holland et al. (2003), Fuchs et al. (2011), andTan et al. (2017).
For the HO 2 measurement, a gas stream of ambient air is expanded in to the sample cell at 4 hPa. NO is added to the sampled air for the conversion of HO 2 to OH (Reaction R1). The NO concentration is adjusted to provide a HO 2 conversion efficiency of approximately 10 % in order to minimized concurrent RO 2 conversion (Fuchs et al., 2011). The OH radicals are excited by a laser pulse at 308 nm, provided by a dye laser system. Ozone can be photolysed at 308 nm, which can lead 25 to a small interference from ozone that is subtracted from the measured signal. For the experiments discussed here, 50 ppbv O 3 gave a signal that is equivalent to a HO 2 concentration of 3 × 10 6 cm −3 . The sensitivity of the HO 2 LIF detection is water vapor dependent due to the quenching of the OH fluorescence by water. The change in the sensitivity is calculated from quenching constants. Both corrections are taken into account. The accuracy of the LIF HO 2 measurement is ±10 % from the uncertainty of the calibration. The typical precision of measurements gives an limit of detection of 1 × 10 7 mol cm −3 (2σ) for 30 a 80 s measurement (Tan et al., 2017).

SAPHIR
SAPHIR is an atmospheric simulation chamber at the Forschungszentrum Jülich. The chamber has been described in detail by Rohrer et al. (2005). It consists of a double-wall FEP film of cylindrical shape (length 18 m, diameter 5 m, volume 270 m 3 ).
It is equipped with a a shutter system that can be opened to expose the chamber air to natural sunlight. Synthetic air used in the experiments is produced from liquid nitrogen and oxygen of highest purity (Linde, purity <99.9999 %). A combination of 5 sensitive measurement instruments allows for studying chemical systems under well-defined, atmospheric conditions and trace gas concentrations. SAPHIR has proven to be a valuable tool for inter-comparison of different measurement techniques (Fuchs et al., 2012;Dorn et al., 2013;Fuchs et al., 2010;Apel et al., 2008), as it is ensured that all instruments can sample the same air composition.
For this study, measurements were performed during a series of experiments in the SAPHIR chamber in May and June 2017.

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The focus of the experiments was to study the chemistry of two classes of oxidation products of isoprene: the isoprene hydroxyhydroperoxides (ISOPOOH) and the isoprene epoxydiols (IEPOX). In addition, reference experiments without addition of VOCs, as well as experiments with isoprene were performed. These experiments were used to compare the performance of the CIMS and the LIF instrument at atmospheric HO 2 concentrations, testing various conditions, e.g. presence of ozone, NO x species and different water concentrations. 15 The CIMS was mounted at the bottom of the chamber, 4 m away from the LIF instrument. The ion flow tube setup shown in Fig. 1 was directly connected to the chamber, so that the sampling nozzle was sticking into the chamber.
Data from the following instruments are used for the data evaluation and interpretation: The humidity was measured using a Picarro cavity ring-down instrument (G2401 Analyzer). NO and NO 2 were monitored by a Eco Physics chemiluminescence instrument (TR780) and ozone was detected by an UV photometer (41M, Ansyco). 20 3 Characterization of the HO 2 -CIMS

Calibration procedure
In general, the conversion of ion count rates measured by a CIMS instrument to concentrations of the detected molecule requires regular calibrations of the sensitivity. For calibrating the HO 2 sensitivity, we utilized a radical source as described in Sect. 2.2. Figure 2 shows the measured, normalized ion count rates measured by the CIMS, when the calibration source was 25 operated at a constant water vapor mixing ratio of 1.0 %. The HO 2 concentration was varied by changing the UV radiation intensity, which was achieved by varying the N 2 O concentration in the absorption cell. A linear behavior for the normalized count rate is observed in a range of 3.0 × 10 8 to 1.3 × 10 9 HO 2 molecules cm −3 . The slope of the linear regression gives the calibration factor of 6.8 × 10 −12 cm −3 ncps −1 . The intercept of 5.1 × 10 −4 ncps of the linear fit indicates a HO 2 background signal. No background correction of the CIMS signal (see below) is applied here.

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Alternatively, the HO 2 provided by the calibration source can be varied by changing the water mixing ratio at constant UV intensity. The HO 2 concentration provided by the calibration source is well characterized for different water mixing ratios. This  allows the determination of the water dependency of the CIMS. The water dependent sensitivity is defined by Eq. 1, where c is the sensitivity that depends term on the water concentration.
[HO 2 ] = c(H 2 O) m/z(112) m/z (79) (1) Figure 3 shows the sensitivity determined for each water vapor mixing ratio showing a decreasing sensitivity with increasing water vapor mixing ratio. The water dependent decrease in sensitivity is nearly linear for atmospheric relevant water mixing 5 ratios higher than 0.1 %. Two effects contribute to the water dependence: The HO 2 ion cluster is stabilized by water during the attachment process, as water takes the access energy of the cluster rearrangement during substitution by the analyte molecule.
On the other hand, the HO 2 bromide ion cluster is in a fast equilibrium with polar molecules in the gas phase. If atmospheric water vapor concentrations are present in the ion flow tube, water may substitute HO 2 in the ion cluster. The ion cluster typically has a shell of water molecules at atmospheric conditions, caused by the ions polarity Derpmann 10 et al., 2012;Albrecht et al., 2014).
As indicated in R2, an excess of water can push the reaction equilibrium in the reverse direction. Thereby, the cluster switching from HO 2 · Br − to H 2 O · Br − causes a decrease in sensitivity.  Further, the HO 2 radical itself can form a water cluster (Aloisio and Francisco, 1998;Kanno et al., 2005;Stone and Rowley, 2005). This, for example, leads to an enhancement of the HO 2 self reaction of up to a factor of two for atmospheric conditions (Stone and Rowley, 2005). However, only a fraction of the HO 2 (20 % at 297 K and 50 % humidity) is attached to a water molecule at atmospheric conditions (Kanno et al., 2005). The concentration of HO 2 water radical clusters is further reduced because of the lower pressure in the ion flow tube along with a lower partial water pressure. Therefore, compared to dry 5 conditions an roughly 10x increased sensitivity at humid conditions is likely mainly caused by the ion water cluster.
A direct calibration for dry conditions was not possible with the radical source, because the calibration source needs water to generate HO 2 . The sensitivity of the instrument was also characterized by the production of HO 2 from the ozonolysis of 2,3 dimethyl-2-butene, that was added in a concentration of 30 ppbv to a mix of synthetic air and 200 ppbv ozone. The radical source was used as a flow-tube to overflow the inlet of the instrument with this gas mixture. 0.2 % CO was added to scavenge 10 OH radicals produced from the ozonolysis reaction by a fast conversion of OH to HO 2 . The water mixing ratio was altered during the ozonolysis experiment from 0.0 to 0.6 %. Assuming that the HO 2 concentration from the ozonlysis is constant, the relative change in the signal gives the relative change of the instrument sensitivity. In addition, calibration measurements using the water photolysis were performed for water vapor mixing rations higher than 0.1 %, so that the water dependence of the sensitivity determined by the two methods can be compared. As shown in Fig. 3 drops by nearly an order of magnitude in the absence of water vapor. It is therefore beneficial to add water to the ion flow tube to maintain an high instrument sensitivity at very dry conditions of the sampled air.
The water vapor dependence of the sensitivity can be parameterized by a third order polynomial (Eq. 2) for water vapor mixing ratios higher than 0.15 %. This is typically sufficient for atmospheric conditions. At lower water vapor mixing ratios as experienced in the chamber experiments the parameterization in Eq. 3 provides a good fit. S is the signal normalized by the 5 primary ion, a, b, c, d are the fit parameters and H 2 O is the absolute water vapor mixing ratio.
For the chamber experiments, the chamber air was humidified at the beginning of each experiments. At that time, no HO 2 is expected to be present in the chamber. Therefore, the increase in the background signal that has the same water vapor 10 dependence as the sensitivity (see next section) can be used to determine the relative change of the sensitivity on water vapor on a daily basis. All HO 2 data from the chamber experiments shown in Sect. 3.4 were evaluated by applying this procedure.
During the series of chamber experiments presented in Sect. 3.4, calibrations were done in-between the experiments. In the middle of the series of experiments (6 June), settings of the instrument were tuned changing the sensitivity of the instrument.
In total 6 calibrations were performed. 15

Precision of the HO 2 measurement
The precision of the instrument can be demonstrated by the Allan deviation plot shown in Fig. 4. 10 hours of measurement were used for this analysis while the instrument sampled from the calibration source that was operated at constant conditions.
As mentioned above only the signal at mass-to-charge ratio 112 is used for simplicity. The calibration source constantly produced 2.5 × 10 9 HO 2 molecules cm −3 . A minimum integration time of 4 s was used for the evaluation, resulting in an 20 Allan deviation of 1.7 × 10 8 HO 2 molecules cm −3 . With increasing integration time, the Allan deviation follows Gaussian noise demonstrating the statistically nature of the instrument's noise. An Allan deviation of 4.5 × 10 7 HO 2 molecules cm −3 is achieved, if the measurement is averaged over 60 s. This is a sufficient detection limit for atmospheric measurements. Lower detection limits can be achieved, if, for example, an integration time of 10 min is acceptable. In addition, the use of both isotopic signals at mass-to-charge ratio 112 and 114 would lower the detection limit by a factor of √ 2.

Instrumental background
The instrumental background was characterized in experiments where the inlet was overflown with humidified synthetic air.
This was done either using the radical source as a flow tube when the UV lamp was off or during experiments in SAPHIR, when only humidified synthetic air was present in the chamber. As shown in Fig. 5, the background signal changes similarly with water vapor for both experimental conditions. The shape of the water vapor dependence is consistent with the assumption that a constant HO 2 concentration (1.5±0.2×10 8 molecules cm −3 ) is internally produced in the instrument, which is detected according to the water vapor dependence of the instrument sensitivity discussed above. Therefore, the background can be be subtracted from the measured HO 2 concentration after applying the water vapor dependent calibration factor. The value of the background needs to be regularly determined. For chamber experiments reported here, the background signal was measured in 5 the clean dark chamber at the start of each experiment.
In turn, the change in the background signal with changing water vapor reflects the relative change in the instrument sensitivity. This is especially relevant for the experiments in the SAPHIR chamber, because the chamber air was humidified starting from dry synthetic air at the start of the experiments. Once the water addition was started the signal was rising steep and decreased slightly at higher water concentration, as shown in Fig. 5. No trend of the background signal over a period of 2 month 10 was observed. The day-to-day variability of the background (in total 16 experiments) was within a range of ±12 % during 2 months of measurements at the chamber.

Potential interference from ozone
Ozone is known to be an interference in some HO 2 LIF instruments due to the photolysis of O 3 by the 308 nm excitation laser (Holland et al., 2003). The potential ozone interference in the CIMS HO 2 detection was investigated in laboratory experiments. 15 Ozone was added to humidified synthetic air (water vapor mixing ratios 0.2 and 2.6 %). For both conditions no increase of the background signal could be observed for ozone mixing ratios of up to 400 ppbv.  During experiments in the SAPHIR chamber, instrument background effects can only be determined for periods of the experiments without the presence of reactants, when no HO 2 was present. Typically, ozone was added in a concentration of 100 to 200 ppbv. Although no artefacts were found in the laboratory characterization, an increase in the background upon ozone addition was observed in two of 12 experiments in SAPHIR. For both experiments, the chamber was first humidified and ozone was added afterwards. This appears as an increased intercept of 2.3×10 8 and 1.0×10 8 HO 2 molecules cm −3 in the 5 linear regression between LIF and CIMS HO 2 data for the experiments of 21 June and 26 June (Fig. 7), respectively. The data of the LIF instrument were corrected for a maximum ozone interference of 0.05 × 10 8 and 0.15 × 10 8 HO 2 molecules cm −3 on these days, respectively. This correction is much smaller than the HO 2 concentration observed by the CIMS instrument, so that it can be excluded that differences are due to systematic errors in the data of the LIF instrument.
In the correlation plot (Fig. 8), including all experiments, this additional background was subtracted. The increased back-

Comparison of CIMS and LIF HO 2 measurements
A time series for a typical experiment is shown in Fig. 6. The HO 2 production was initiated with the injection of ozone and the opening of the chamber roof providing UV light to the chamber. An addition of CO further boosted the HO 2 production, which dropped upon closing of the roof. However, HO 2 was still produced via radical chemistry in the dark. After the injection of water the CIMS shows a stable signal with a small offset. During the experiment the LIF and CIMS data reveal a good 5 correlation having similar errors. This experiment was performed without the addition of a volatile organic compound (VOC), as well as, two other experiments marked with "None" in Fig. 7. Figure 7 displays the correlation between HO 2 measurements by the CIMS and the LIF instrument for all day-long photooxidation experiment in the SAPHIR chamber performed in this study. The chemical composition was varied between experiments by changing for example the NO mixing ratio.  injection of IEPOX can be attributed to the interference from IEPOX, because IEPOX was injected in the dark chamber so that no HO 2 is expected to be present. This gives the relationship between the signal observed at the IEPOX mass (m/z 197) to the interference signal from IEPOX at the HO 2 mass (m/z 112). During the photo-oxidation of IEPOX, when also HO 2 is present, the interference signal can be subtracted from the signal at the HO 2 mass by scaling the initial interference signal by the relative change on m/z 197. The correction improves the correlation of the CIMS and the LIF but the absolute agreement is 5 still not as good (slope of the regression 0.86; coefficient of determination 0.79) compared to the other experiments. However, a correction was performed for all experiments with IEPOX injection. The corrections are in the order of or smaller than the HO 2 measurements, and works best for the experiment with the lowest IEPOX concentration. It is worth noting that IEPOX concentrations were at least 10 times higher than typically found in the atmosphere. Kaiser et al. (2016) found IEPOX concentrations of 1 ppbv during a campaign in a forest in the South-East US where isoprene, the precursor of IEPOX, was the 10 13 Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2018-195 Manuscript under review for journal Atmos. Meas. Tech. concentrations were much higher (several ppbv) than typically found in the atmosphere (less than 1 ppbv Kaiser et al. (2016)), so that no significant impact for atmospheric conditions is expected.
All concurrent measurements of the two instruments for HO 2 by CIMS and LIF, in the photo-oxidation experiments are summarized in the correlation plot shown in Fig. 8. In general, the correlation fit shows that there is an excellent agreement of both instruments giving a slope of linear regression of 1.07 and the linear correlation coefficient R 2 is 0.87. Experiments 10 investigating the photo-oxidation of IEPOX and ISOPOOH are color-coded and are excluded from the correlation fit. However, using all data for the correlation fit leads to similar result (slope of linear regression of 1.05; coefficient of correlation R 2 = 0.89).
Correlation of individual experiments (Fig. 7, e.g. 21 June and 26 June) give partly significant offsets in the regression analysis of up to 2.3 × 10 8 cm −3 HO 2 . One possible reason could be the procedure, how the water vapor dependence of the 15 instrument sensitivity was derived. This was done by using the relative change of the a presumably constant instrumental HO 2 background during the humidification of the clean chamber air. The water vapor concentration was measured at a different location in the chamber. Therefore, there is the potential that the water vapor concentration measurement in the chamber was not representative for the water concentration in the ion flow tube of the instrument. In this case, the determination of the relative change of the instrument's sensitivity would fail and could results in an offset in the evaluation of data during the 20 experiments. As seen in the experiment, shown in Fig. 6, there is a small offset that starts with the humidification. To avoid this effect in the future, a humidity sensor will be implemented at the ion flow tube.

Conclusion and Outlook
Chemical ionization was applied to measure atmospheric HO 2 concentrations using bromide ions as reagent. Laboratory characterization experiments and measurements in the atmospheric simulation chamber SAPHIR in Jülich were used to check 25 the instruments applicability for atmospheric measurements. The performance of the CIMS instrument is comparable with measurements by a laser-induced fluorescence instrument. A water vapor dependence of the instrument sensitivity needs to be taken into account in the evaluation of data because the sensitivity of the instrument changes by roughly a factor of 2 for atmospheric water vapor concentrations between 0.2 and 1.4 %. Also a water vapor dependent background signal is observed.
The change of the background signal with increasing water vapor, however, is explained by the water vapor dependence of 30 the sensitivity. Therefore, the assumption is that the background consists of constant HO 2 production in the instrument. This background was stable within ±12 % during two months of measurements and no further trend was identified. The background signal and the instrument sensitivity needs to be quantified on a daily basis. No significant interference from trace gases NO, NO 2 , O 3 , CO, isoprene and ISOPOOH were found for atmospheric conditions. Only for non-atmospheric high IEPOX concentrations of several ppbv artificial signals were found that scaled with the IEPOX concentration. The HO 2 measurements correlate well with the LIF measurements. A slope of the linear regression of 1.07 was determined and a linear correlation coefficient (R 2 ) of 0.87 was found. With a detection limit of 4.5 × 10 7 molecules cm −3 for a 60 s measurement the instrument is suitable to measure typical HO 2 concentrations in the atmosphere.

5
Further improvements of the instrument sensitivity might be expected, if wall contact of the sampled air including HO 2 is further minimized. This could be achieved by applying a sheath flow of pure nitrogen along the surface of the ion flow-tube.