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.
Highlights
Understanding of the oxidation processes in the atmosphere requires sensitive measurements of the radical species involved
The primary sources of OH radicals are mainly due to ozone photolysis and, in polluted environments, nitrous acid (HONO) photolysis can be of importance
OH and HO2 radicals are closely interconnected by a radical chain reaction, in which OH is reformed by the reaction of HO2 with nitric oxide (NO): HO2 + NO → OH + NO2
Summary
Understanding of the oxidation processes in the atmosphere requires sensitive measurements of the radical species involved. The high sensitivity of chemical ionisation mass spectrometer (CIMS) measurements using nitrate (NO−3 ) as the primary ion allows for the detection sulfuric acid (H2SO4) produced in the reaction of SO2 with OH. Laser-induced fluorescence (LIF) is a sensitive technique for OH radical measurements and it is used for the indirect detection of HO2 by its conversion into OH after reaction with NO. Sanchez et al (2016) demonstrated that the most promising ionisation technique is the detection of the bromide cluster with HO2 In their work they showed that a sufficient sensitivity for atmospheric measurements can be achieved and no significant interference from NOx, HCHO, SO2, or O3 is present. Experiments in the atmospheric simulation chamber SAPHIR were performed at atmospheric gas mixtures and radical concentrations
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