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

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 x 10(8) HO2 cm(-3) for 10(6) 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) x 10(8) 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 Julich 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 R-2 = 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 O-3. 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.