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<p>Atmospheric models often underestimate particulate sulfate, a major component in ambient aerosol, suggesting missing sulfate formation mechanisms in the models. Heterogeneous reactions between SO<span class="inline-formula">₂</span> and aerosol play an important role in particulate sulfate formation and its physicochemical evolution. Here we study the reactive uptake kinetics of SO<span class="inline-formula">₂</span> onto aerosol containing organic peroxides. We present chamber studies of SO<span class="inline-formula">₂</span> reactive uptake performed under different relative humidity (RH), particulate peroxide contents, peroxide types, and aerosol acidities. Using different model organic peroxides mixed with ammonium sulfate particles, the SO<span class="inline-formula">₂</span> uptake coefficient (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">SO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="29ace9adccb0489b55da43a8b0d8b25a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-6647-2021-ie00001.svg" width="23pt" height="12pt" src="acp-21-6647-2021-ie00001.png"/></svg:svg></span></span>) was found to be exponentially dependent on RH. <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">SO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="ddc34999a27253cd37cc52fbe68de2e8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-6647-2021-ie00002.svg" width="23pt" height="12pt" src="acp-21-6647-2021-ie00002.png"/></svg:svg></span></span> increases from 10<span class="inline-formula">⁻³</span> at RH 25 % to 10<span class="inline-formula">⁻²</span> at RH 71 % as measured for an organic peroxide with multiple O–O groups. Under similar conditions, the kinetics in this study were found to be structurally dependent: organic peroxides with multiple peroxide groups have a higher <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">SO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="6a669af0bc9106415cb1891de177087e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-6647-2021-ie00003.svg" width="23pt" height="12pt" src="acp-21-6647-2021-ie00003.png"/></svg:svg></span></span> than those with only one peroxide group, consistent with the reactivity trend previously observed in the aqueous phase. In addition, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">SO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="5c11bac1e5dfc130eedfa594dc1f9017"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-6647-2021-ie00004.svg" width="23pt" height="12pt" src="acp-21-6647-2021-ie00004.png"/></svg:svg></span></span> is linearly related to particle-phase peroxide content, which in turn depends on gas–particle partitioning of organic peroxides. Aerosol acidity plays a complex role in determining SO<span class="inline-formula">₂</span> uptake rate, influenced by the effective Henry's Law constant of SO<span class="inline-formula">₂</span> and the condensed-phase kinetics of the peroxide–SO<span class="inline-formula">₂</span> reaction in the highly concentrated aerosol phase. These uptake coefficients are consistently higher than those calculated from the reaction kinetics in the bulk aqueous phase, and we show experimental evidence suggesting that other factors, such as particle-phase ionic strength, can play an essential role in determining the uptake kinetics. <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">SO</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="3618f60c7f8bb3066f3927a6e59d86d2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-6647-2021-ie00005.svg" width="23pt" height="12pt" src="acp-21-6647-2021-ie00005.png"/></svg:svg></span></span> values for different types of secondary organic aerosol (SOA) were measured to be on the order of 10<span class="inline-formula">⁻⁴</span>. Overall, this study provides quantitative evidence of the multiphase reactions between SO<span class="inline-formula">₂</span> and organic peroxides, highlighting the important factors that govern the uptake kinetics.</p>