Comment on acp-2021-672

<strong class="journal-contentHeaderColor">Abstract.</strong> Glyoxal (CHOCHO), the simplest dicarbonyl in the troposphere, is a potential precursor for secondary organic aerosol (SOA) and brown carbon (BrC) affecting air quality and climate. The airborne measurement of CHOCHO concentrations during the KORUS-AQ (KORea–US Air Quality study) campaign in 2016 enables detailed quantification of loss mechanisms pertaining to SOA formation in the real atmosphere. The production of this molecule was mainly from oxidation of aromatics (59 %) initiated by hydroxyl radical (OH). CHOCHO loss to aerosol was found to be the most important removal path (69 %) and contributed to roughly <span class="inline-formula">∼</span> 20 % (3.7 <span class="inline-formula">µ</span>g sm<span class="inline-formula">^−3</span> ppmv<span class="inline-formula">^−1</span> h<span class="inline-formula">^−1</span>, normalized with excess CO) of SOA growth in the first 6 h in Seoul Metropolitan Area. A reactive uptake coefficient (<span class="inline-formula"><i>γ</i></span>) of <span class="inline-formula">∼</span> 0.008 best represents the loss of CHOCHO by surface uptake during the campaign. To our knowledge, we show the first field observation of aerosol surface-area-dependent (<span class="inline-formula"><i>A</i>_surf</span>) CHOCHO uptake, which diverges from the simple surface uptake assumption as <span class="inline-formula"><i>A</i>_surf</span> increases in<span id="page806"/> ambient condition. Specifically, under the low (high) aerosol loading, the CHOCHO effective uptake rate coefficient, <span class="inline-formula"><i>k</i>_eff,uptake</span>, linearly increases (levels off) with <span class="inline-formula"><i>A</i>_surf</span>; thus, the irreversible surface uptake is a reasonable (unreasonable) approximation for simulating CHOCHO loss to aerosol. Dependence on photochemical impact and changes in the chemical and physical aerosol properties “free water”, as well as aerosol viscosity, are discussed as other possible factors influencing CHOCHO uptake rate. Our inferred Henry's law coefficient of CHOCHO, <span class="inline-formula">7.0×10⁸</span> M atm<span class="inline-formula">^−1</span>, is <span class="inline-formula">∼</span> 2 orders of magnitude higher than those estimated from salting-in effects constrained by inorganic salts only consistent with laboratory findings that show similar high partitioning into water-soluble organics, which urges more understanding on CHOCHO solubility under real atmospheric conditions.