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. Hydroxyl and hydroperoxy radicals are key species for the understanding ofatmospheric oxidation processes. Their measurement is challenging due totheir 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-flightmass spectrometer (Aerodyne Research Inc.) employing bromide as the primary ionis presented. The sensitivity reached is equal to 0.005×108 HO2  cm −3 for 10 6  cps of bromide and 60 s of integration time, which isbelow typical HO2 concentrations found in the atmosphere. Thedetection sensitivity of the instrument is affected by the presence of watervapour. Therefore, a water-vapour-dependent calibration factor that decreasesapproximately by a factor of 2 if the water vapour mixing ratio increases from0.1  % to 1.0  % needs to be applied. An instrumental background, mostlikely generated by the ion source that is equivalent to a HO2 concentration of ( 1.5 ± 0.2 ) × 10 8 molecules cm−3 , issubtracted to derive atmospheric HO2 concentrations. This backgroundcan be determined by overflowing the inlet with zero air. Several experimentswere performed in the atmospheric simulation chamber SAPHIR at theForschungszentrum Julich to test the instrument performance in comparisonto the well-established laser-induced fluorescence (LIF) technique formeasurements of HO2 . A highly linear correlation coefficient of R2=0.87 is achieved. The slope of the linear regression of 1.07demonstrates the good absolute agreement of both measurements. Chemicalconditions during experiments allowed for testing the instrument's behaviour inthe 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 bythe chemical ionisation mass spectrometer presented.

atmospheric measurement techniques

Measurements of hydroperoxy radicals (HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;) at atmospheric concentrations using bromide chemical ionisation mass spectrometry