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<p>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 (<span class="inline-formula">HO₂</span>) 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 <span class="inline-formula">0.005×10⁸</span> <span class="inline-formula">HO₂</span>&thinsp;cm<span class="inline-formula">⁻³</span> for 10<span class="inline-formula">⁶</span>&thinsp;cps of bromide and 60&thinsp;s of integration time, which is below typical <span class="inline-formula">HO₂</span> 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&thinsp;<span class="inline-formula">%</span> to 1.0&thinsp;<span class="inline-formula">%</span> needs to be applied. An instrumental background, most likely generated by the ion source that is equivalent to a <span class="inline-formula">HO₂</span> concentration of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>(</mo><mn mathvariant="normal">1.5</mn><mo>±</mo><mn mathvariant="normal">0.2</mn><mo>)</mo><mo>×</mo><msup><mn mathvariant="normal">10</mn><mn mathvariant="normal">8</mn></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="79pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="d17ed23b129252ba5aebbaaaa3c31c27"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-891-2019-ie00001.svg" width="79pt" height="15pt" src="amt-12-891-2019-ie00001.png"/></svg:svg></span></span>&thinsp;<span class="inline-formula">molecules cm⁻³</span>, is subtracted to derive atmospheric <span class="inline-formula">HO₂</span> 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 <span class="inline-formula">HO₂</span>. A highly linear correlation coefficient of <span class="inline-formula"><i>R</i>²=0.87</span> 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 <span class="inline-formula">H₂O</span>, <span class="inline-formula">NO_<i>x</i></span>, and <span class="inline-formula">O₃</span>. No significant interferences from these species were observed. All of these facts demonstrate a reliable measurement of <span class="inline-formula">HO₂</span> by the chemical ionisation mass spectrometer presented.</p>