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<p>Chemical ionisation mass spectrometry (CIMS) using <span class="inline-formula">I⁻</span> (the iodide anion), hereafter I-CIMS, as a primary reactant ion has previously been used to measure <span class="inline-formula">NO₃</span> and <span class="inline-formula">N₂O₅</span> both in laboratory and field experiments. We show that reports of large daytime mixing ratios of <span class="inline-formula">NO₃</span> and <span class="inline-formula">N₂O₅</span> (both usually present in detectable amounts only at night) are likely to be heavily biased by the ubiquitous presence of <span class="inline-formula">HNO₃</span> in the troposphere and lower stratosphere. We demonstrate in a series of laboratory experiments that the CIMS detection of <span class="inline-formula">HNO₃</span> at <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>m</mi><mo>/</mo><mi>z</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="279239c08ce5ed5dd0fbeb16cbabd960"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-5319-2021-ie00006.svg" width="23pt" height="14pt" src="amt-14-5319-2021-ie00006.png"/></svg:svg></span></span> 62 using <span class="inline-formula">I⁻</span> ions is efficient in the presence of peroxy acetyl nitric anhydride (PAN) or peroxyacetic acid (PAA) and especially <span class="inline-formula">O₃</span>. We have characterised the dependence of the sensitivity to <span class="inline-formula">HNO₃</span> detection on the presence of acetate anions (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">3</mn></msub><msup><msub><mi mathvariant="normal">CO</mi><mn mathvariant="normal">2</mn></msub><mo>-</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="51pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="12116abfd19a39506a0d5a640e4eaefd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-5319-2021-ie00007.svg" width="51pt" height="15pt" src="amt-14-5319-2021-ie00007.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M18" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>m</mi><mo>/</mo><mi>z</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="7dc7cb769bb742b7e5caf4e3949b8538"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-5319-2021-ie00008.svg" width="23pt" height="14pt" src="amt-14-5319-2021-ie00008.png"/></svg:svg></span></span> 59, from either PAN or PAA). The loss of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">3</mn></msub><msup><msub><mi mathvariant="normal">CO</mi><mn mathvariant="normal">2</mn></msub><mo>-</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="51pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="fb6d8d7a7419664dbb3edfa51feb3cdd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-5319-2021-ie00009.svg" width="51pt" height="15pt" src="amt-14-5319-2021-ie00009.png"/></svg:svg></span></span> via conversion to <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M20" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn></msub><mo>-</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="c3a4747f2c8783874abb0846591684f5"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-5319-2021-ie00010.svg" width="30pt" height="15pt" src="amt-14-5319-2021-ie00010.png"/></svg:svg></span></span> in the presence of <span class="inline-formula">HNO₃</span> may represent a significant bias in I-CIMS measurements of PAN and PAA in which continuous calibration (e.g. via addition of isotopically labelled PAN) is not carried out. The greatest sensitivity to <span class="inline-formula">HNO₃</span> at <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>m</mi><mo>/</mo><mi>z</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="0d76c8bd8de2f967436a9f4b1d52f77e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-5319-2021-ie00011.svg" width="23pt" height="14pt" src="amt-14-5319-2021-ie00011.png"/></svg:svg></span></span> 62 is achieved in the presence of ambient levels of <span class="inline-formula">O₃</span> whereby the thermodynamically disfavoured, direct reaction of <span class="inline-formula">I⁻</span> with <span class="inline-formula">HNO₃</span> to form <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M27" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn></msub><mo>-</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="91231030ab72dfd8d13e6de2d14574f5"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-5319-2021-ie00012.svg" width="30pt" height="15pt" src="amt-14-5319-2021-ie00012.png"/></svg:svg></span></span> is bypassed by the formation of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M28" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><msub><mi mathvariant="normal">IO</mi><mi>x</mi></msub><mo>-</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="26pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="70b59f3c967a9fdfb72e0945b1f97c3d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-5319-2021-ie00013.svg" width="26pt" height="14pt" src="amt-14-5319-2021-ie00013.png"/></svg:svg></span></span>, which reacts with <span class="inline-formula">HNO₃</span> to form, for example, iodic acid and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M30" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn></msub><mo>-</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="4b63ac2ccaf7dd01277385a46736260c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-5319-2021-ie00014.svg" width="30pt" height="15pt" src="amt-14-5319-2021-ie00014.png"/></svg:svg></span></span>. The ozone and humidity dependence of the detection of <span class="inline-formula">HNO₃</span> at <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M32" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>m</mi><mo>/</mo><mi>z</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="335a219683ee8afed0badec468c9a644"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-5319-2021-ie00015.svg" width="23pt" height="14pt" src="amt-14-5319-2021-ie00015.png"/></svg:svg></span></span> 62 was characterised in laboratory experiments and applied to daytime, airborne measurements in which good agreement with measurements of the <span class="inline-formula">I⁻</span>(<span class="inline-formula">HNO₃</span>) cluster ion (specific for <span class="inline-formula">HNO₃</span> detection) was obtained. At high ozone mixing ratios, we show that the concentration of <span class="inline-formula">I⁻</span> ions in our ion–molecule reactor (IMR) is significantly depleted. This is not reflected by changes in the measured <span class="inline-formula">I⁻</span> signal at <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M38" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>m</mi><mo>/</mo><mi>z</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="eca55b4362726df35abcff0f4a24ff8d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-5319-2021-ie00016.svg" width="23pt" height="14pt" src="amt-14-5319-2021-ie00016.png"/></svg:svg></span></span> 127 as the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M39" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><msub><mi mathvariant="normal">IO</mi><mi>x</mi></msub><mo>-</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="26pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="f65904c10f6924e431010e2b92b94ed4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-5319-2021-ie00017.svg" width="26pt" height="14pt" src="amt-14-5319-2021-ie00017.png"/></svg:svg></span></span> formed does not survive passage through the instrument but is likely detected after fragmentation to <span class="inline-formula">I⁻</span>. This may result in a bias in measurements of trace gases using I-CIMS in stratospheric air masses unless a calibration gas is continuously added or the impact of <span class="inline-formula">O₃</span> on sensitivity is characterised.</p>