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

Abstract

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 ionisation combined with a high-resolution time-of-flight mass spectrometer (Aerodyne Research Inc.) employing bromide as the primary ion is presented. The sensitivity reached is equal to 0.005×108 HO2 cm−3 for 106 cps of bromide and 60 s of integration time, which is below typical HO2 concentrations found in the atmosphere. The detection sensitivity of the instrument is affected by the presence of water vapour. Therefore, a water-vapour-dependent calibration factor that decreases approximately by a factor of 2 if the water vapour 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 in comparison to the well-established laser-induced fluorescence (LIF) technique for measurements of HO2. A highly 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 experiments allowed for testing the instrument's behaviour in the presence of atmospheric concentrations of H2O, NOx, and O3. No significant interferences from these species were observed. All of these facts demonstrate a reliable measurement of HO2 by the chemical ionisation mass spectrometer presented.

Molecular composition and volatility of multi-generation products formed from isoprene oxidation by nitrate radical

“…The ToF-MS was operated in 'V' mode with the mass resolution power between 3000-4000 Th/Th. In order to reduce the losses of HO2 radicals and HOM on the tubing, a customized inlet (Albrecht et al, 2019) was directly connected to the chamber. The CIMS was operated in negative ion mode using Bras the reagent ion, which is selective to polar species such as acids, hydroxy or nitrooxy carbonyls, as well as HO2 radicals (Albrecht et al, 2019;Ng et al, 2008;Rissanen et al, 2019;Riva et al, 2019).…”

“…Bromide ions were generated by passing a mixture of 10 standard cubic centimeters per minute of 0.4% CF3Br in nitrogen and 2 standard liter per minute nitrogen through a 370 MBq 210 Po source (Type P-2021-5000, NDR Static Control LLC, USA), resulting in ~10 5 ion counts per second (Albrecht et al, 2019). In our system, most compounds were detected as adducts with Br -, but some strong acidic compounds like nitric acid were also detected as deprotonated ions.…”

Multi-scheme chemical ionization inlet (MION) for fast switching of reagent ion chemistry in atmospheric pressure chemical ionization mass spectrometry (CIMS) applications

“…In principle, a mass spectrometer is a universal detector with applicability mainly controlled by two factors: (i) volatility, and (ii) ionizability of the analyte (Baeza-Romero et al, 2011;McLafferty, 2011). In practice, the sampled material must be volatilized into the gas phase and then ionized by a suitable method.…”

Calibration of an airborne HO<sub><i>x</i></sub> instrument using the All Pressure Altitude-based Calibrator for HO<sub><i>x</i></sub> Experimentation (APACHE)

“…Other methods have been successfully implemented to measure HO x . Chemical ionization mass spectrometry (CIMS) (Cantrell et al, 2003;Mauldin et al, 2004;Sjostedt et al, 2007;Dusanter et al, 2008;Kukui et al, 2008;Albrecht et al, 2019) and differential optical absorption spectroscopy (DOAS) (Brauers et al, 1996(Brauers et al, , 2001Schlosser et al, 2007) have also been used in the measurement of HO x in the field and in intercomparison projects with LIF instrumentation. However, low atmospheric concentrations of HO x (Schlosser et al, 2009) and potential interferences (Faloona et al, 2004;Fuchs et al, 2011Fuchs et al, , 2016Mao et al, 2012;Novelli et al, 2014) can make HO x measurements especially challenging.…”