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<p>Continuous aerosol and cloud condensation nuclei (CCN) measurements carried out at the ground observational facility situated in the rain-shadow region of the Indian subcontinent are illustrated. These observations were part of the Cloud Aerosol Interaction Precipitation Enhancement Experiment (CAIPEEX) during the Indian summer monsoon season (June to September) of 2018. Observations are classified as dry–continental (monsoon break) and wet–marine (monsoon active) according to the air mass history. CCN concentrations measured for a range of supersaturations (0.2&thinsp;%–1.2&thinsp;%) are parameterized using Twomey's empirical relationship. CCN concentrations at low (0.2&thinsp;%) supersaturation (SS) were high (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>&gt;</mo><mn mathvariant="normal">1000</mn><mspace width="0.125em" linebreak="nobreak"/><mrow class="unit"><msup><mi mathvariant="normal">cm</mi><mrow><mo>-</mo><mn mathvariant="normal">3</mn></mrow></msup></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="63pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="5d36040742b9727a30e068b43ef0c62f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-7307-2020-ie00001.svg" width="63pt" height="13pt" src="acp-20-7307-2020-ie00001.png"/></svg:svg></span></span>) during continental conditions and observed together with high black carbon (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow><mtext>BC</mtext><mo>∼</mo><mn mathvariant="normal">2000</mn><mspace linebreak="nobreak" width="0.125em"/><mrow class="unit"><mi mathvariant="normal">ng</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">3</mn></mrow></msup></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="88pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="9921e953a2116123777481c43a0ff83c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-7307-2020-ie00002.svg" width="88pt" height="15pt" src="acp-20-7307-2020-ie00002.png"/></svg:svg></span></span>) and columnar aerosol loading. During the marine air mass conditions, CCN concentrations diminished to <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>∼</mo><mn mathvariant="normal">350</mn><mspace width="0.125em" linebreak="nobreak"/><mrow class="unit"><msup><mi mathvariant="normal">cm</mi><mrow><mo>-</mo><mn mathvariant="normal">3</mn></mrow></msup></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="57pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="454ba4058746fc6d67d4fb9423fdb00f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-7307-2020-ie00003.svg" width="57pt" height="13pt" src="acp-20-7307-2020-ie00003.png"/></svg:svg></span></span> at 0.3&thinsp;% SS and low aerosol loading persisted (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mtext>BC</mtext><mo>∼</mo><mn mathvariant="normal">800</mn><mspace width="0.125em" linebreak="nobreak"/><mrow class="unit"><mi mathvariant="normal">ng</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">3</mn></mrow></msup></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="82pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="984130d47df8cab3be934d5375caf774"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-7307-2020-ie00004.svg" width="82pt" height="15pt" src="acp-20-7307-2020-ie00004.png"/></svg:svg></span></span>). High CCN activation fraction (AF) of <span class="inline-formula">≅0.55</span> (at 0.3&thinsp;% SS) was observed before the monsoon rainfall, which reduced to <span class="inline-formula">≅0.15</span> during the marine air mass and enhanced to <span class="inline-formula">≅0.32</span> after that. There was mostly monomodal aerosol number size distribution (NSD) with a mean geometric mean diameter (GMD) of <span class="inline-formula">≅85</span>&thinsp;<span class="inline-formula">nm</span>, with least (<span class="inline-formula">≅9</span>&thinsp;%) contribution from nucleation mode (<span class="inline-formula">&lt;30</span>&thinsp;<span class="inline-formula">nm</span>) particles persisted before the monsoon, while multimode NSD with <span class="inline-formula">≅19</span>&thinsp;% of nucleation mode particles was found during the marine air mass. Critical activation diameters (<span class="inline-formula"><i>d</i>_cri</span>) for 0.3&thinsp;% SS were found to be about 72, 169, and 121&thinsp;<span class="inline-formula">nm</span> prior to, during, and after the marine conditions, respectively. The better association of CCN with aerosol absorption, and the concurrent accumulation mode particles during continental conditions, points to the possibility of aged (oxygenated) carbonaceous aerosols enhancing the CCN activity prior to the marine conditions. An enhancement in CCN concentrations and <span class="inline-formula"><i>k</i></span> values during the daytime along with absorption Ångström exponent was observed during the marine conditions. Best closure obtained using measured critical diameter and ammonium sulfate composition during continental conditions emphasizes the role of aged aerosols contributing to the accumulation mode, enhancing the CCN efficiency. The overestimation of CCN and less hygroscopicity of accumulation mode aerosols during the marine air mass indicate the role of size-dependent aerosol composition in CCN activity during the period.</p>