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<p>The impact of heterogeneous uptake of HO<span class="inline-formula">₂</span> on aerosol surfaces on radical concentrations and the O<span class="inline-formula">₃</span> production regime in Beijing in summertime was investigated. The uptake coefficient of HO<span class="inline-formula">₂</span> onto aerosol surfaces, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">HO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="7e7365fbdd9769573ed8089ca146a8a7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-5679-2023-ie00001.svg" width="24pt" height="12pt" src="acp-23-5679-2023-ie00001.png"/></svg:svg></span></span>, was calculated for the AIRPRO campaign in Beijing, in summer 2017, as a function of measured aerosol soluble copper concentration, [Cu<span class="inline-formula">²⁺</span>]<span class="inline-formula">_eff</span>, aerosol liquid water content, [ALWC], and particulate matter concentration, [PM]. An average <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">HO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="54f14db4c4cb485fb9e6aaee5d984ebc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-5679-2023-ie00002.svg" width="24pt" height="12pt" src="acp-23-5679-2023-ie00002.png"/></svg:svg></span></span> across the entire campaign of <span class="inline-formula">0.070±0.035</span> was calculated, with values ranging from 0.002 to 0.15, and found to be significantly lower than the value of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">HO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub><mo>=</mo><mn mathvariant="normal">0.2</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c95de63b04c22a916fc2883ceb57eb7b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-5679-2023-ie00003.svg" width="52pt" height="14pt" src="acp-23-5679-2023-ie00003.png"/></svg:svg></span></span>, commonly used in modelling studies. Using the calculated <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">HO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="ed5c3b2d830439cea88ec3156f37ea15"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-5679-2023-ie00004.svg" width="24pt" height="12pt" src="acp-23-5679-2023-ie00004.png"/></svg:svg></span></span> values for the summer AIRPRO campaign, OH, HO<span class="inline-formula">₂</span> and RO<span class="inline-formula">₂</span> radical concentrations were modelled using a box model incorporating the Master Chemical Mechanism (v3.3.1), with and without the addition of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">HO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="7aeed63908ac7cafa6c7b34a499b0555"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-5679-2023-ie00005.svg" width="24pt" height="12pt" src="acp-23-5679-2023-ie00005.png"/></svg:svg></span></span>, and compared to the measured radical concentrations. The rate of destruction analysis showed the dominant HO<span class="inline-formula">₂</span> loss pathway to be HO<span class="inline-formula">₂</span> <span class="inline-formula">+</span> NO for all NO concentrations across the summer Beijing campaign, with HO<span class="inline-formula">₂</span> uptake contributing <span class="inline-formula">&lt;0.3</span> % to the total loss of HO<span class="inline-formula">₂</span> on average. This result for Beijing summertime would suggest that under most conditions encountered, HO<span class="inline-formula">₂</span> uptake onto aerosol surfaces is not important to consider when investigating increasing O<span class="inline-formula">₃</span> production with decreasing [PM] across the North China Plain. At low [NO], however, i.e. <span class="inline-formula">&lt;0.1</span> ppb, which was often encountered in the afternoons, up to 29 % of modelled HO<span class="inline-formula">₂</span> loss was due to HO<span class="inline-formula">₂</span> uptake on aerosols when calculated <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M29" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">HO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="3bf20b14fe58c5015340719fb217f73a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-5679-2023-ie00006.svg" width="24pt" height="12pt" src="acp-23-5679-2023-ie00006.png"/></svg:svg></span></span> was included, even with the much lower <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M30" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">HO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="8b136d55403521e6f9ce747703af498c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-5679-2023-ie00007.svg" width="24pt" height="12pt" src="acp-23-5679-2023-ie00007.png"/></svg:svg></span></span> values compared to <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M31" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">HO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub><mo>=</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="35pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="7ee6064c3b7c7e953fa9b01be76c8e92"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-5679-2023-ie00008.svg" width="35pt" height="12pt" src="acp-23-5679-2023-ie00008.png"/></svg:svg></span></span> 0.2, a result which agrees with the aerosol-inhibited O<span class="inline-formula">₃</span> regime recently proposed by Ivatt et al. (2022). As such it can be concluded that in cleaner environments, away from polluted urban centres where HO<span class="inline-formula">₂</span> loss chemistry is not dominated by NO but where aerosol surface area is high still, changes in PM concentration and hence aerosol surface area could still have a significant effect on both overall HO<span class="inline-formula">₂</span> concentration and the O<span class="inline-formula">₃</span> production regime.</p> <p>Using modelled radical concentrations, the absolute O<span class="inline-formula">₃</span> sensitivity to NO<span class="inline-formula">_<i>x</i></span> and volatile organic compounds (VOCs) showed that, on average across the summer AIRPRO campaign, the O<span class="inline-formula">₃</span> production regime remained VOC-limited, with the exception of a few days in the afternoon when the NO mixing ratio dropped low enough for the O<span class="inline-formula">₃</span> regime to shift towards being NO<span class="inline-formula">_<i>x</i></span>-limited. The O<span class="inline-formula">₃</span> sensitivity to VOCs, the dominant regime during the summer AIRPRO campaign, was observed to decrease and shift towards a NO<span class="inline-formula">_<i>x</i></span>-sensitive regime both when NO mixing ratio decreased and with the addition of aerosol uptake. This suggests that if [NO<span class="inline-formula">_<i>x</i></span>] continues to decrease in the future, ozone reduction policies focussing solely on NO<span class="inline-formula">_<i>x</i></span> reductions may not be as efficient as expected if [PM] and, hence, HO<span class="inline-formula">₂</span> uptake to aerosol surfaces continue to decrease. The addition of aerosol uptake into the model, for both the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M46" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">HO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="a5d13e77e48374dda8eff88be7206bc4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-5679-2023-ie00009.svg" width="24pt" height="12pt" src="acp-23-5679-2023-ie00009.png"/></svg:svg></span></span> calculated from measured data and when using a fixed value of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M47" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">HO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub><mo>=</mo><mn mathvariant="normal">0.2</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="26c91797cfc5db9d663ac88236a18e05"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-5679-2023-ie00010.svg" width="52pt" height="14pt" src="acp-23-5679-2023-ie00010.png"/></svg:svg></span></span>, did not have a significant effect on the overall O<span class="inline-formula">₃</span> production regime across the campaign. While not important for this campaign, aerosol uptake could be important for areas of lower NO concentration that are already in a NO<span class="inline-formula">_<i>x</i></span>-sensitive regime.</p>