1 CCN characteristics during the Indian Summer Monsoon over a rain- shadow region Venugopalan Nair Jayachandran 1 , Mercy Varghese 1 , Palani Murugavel 1 , Kiran S. Todekar 1 , Shivdas P. Bankar 1 , Neelam Malap 1 , Gurunule Dinesh 1 , Pramod D. Safai 1 , Jaya Rao 1 , Mahen Konwar 1 , Shivsai Dixit 1 , Thara V. Prabha 1 , 5 1 Indian Institute of Tropical Meteorology, Pune, India. Correspondence to: V. Jayachandran (jayanspl@gmail.com) Abstract. Continuous aerosol and Cloud Condensation Nuclei (CCN) measurements carried out at the ground observational facility situated in the rain-shadow region of the Indian sub-continent are illustrated. These observations were part of the Cloud-Aerosol Interaction Precipitation Enhancement EXperiment (CAIPEEX) during the Indian Summer Monsoon season 10 (June to September) of 2018. Observations are classified as dry-continental (monsoon break) and wet-marine (monsoon active) according to air mass history. CCN concentrations measured for a range of supersaturations (0.2-1.2 %) are parameterized using Twomey’s empirical relationship. CCN concentrations even at low (0.2 %) supersaturation (SS) were high (>1,000 cm - 3 ) during continental conditions associated with high black carbon (BC~2,000 ng m -3 ) and columnar aerosol loading. During the marine air mass conditions, CCN concentrations diminished to ~350 cm -3 at 0.3 % SS and low aerosol loading persisted 15 (BC~900 ng m -3 ). High CCN activation fraction (AF) of ~0.55 (at 0.3 % SS) were observed before the monsoon rainfall, which reduced to ~0.15 during the monsoon and enhanced to ~0.32 after that. Mostly mono-modal aerosol number-size distribution (NSD) with a mean geometric mean diameter (GMD) of ~85 nm, with least (~9 %) contribution from nucleation mode (<30 nm) particles persisted before monsoon, while multi-mode NSD with ~19 % of nucleation mode particles were found during the monsoon. Critical activation diameters (d cri ) for 0.3 % SS were found to be about 72, 169, and 121 nm prior, during and, 20 after the monsoon, respectively. The estimated d cri were inversely correlated to the AF, linearly up to 100 nm and non-linearly beyond that. The better association of CCN with aerosol absorption, and the concurrent accumulation mode particles during continental conditions, point to the possibility of aged (oxygenated) carbonaceous aerosols enhancing the CCN activity. The enhancement in CCN concentration and k-values during daytime along with the increase in absorption Angstrom exponent, indicate the freshly emitted local anthropogenic aerosols dominated by organics reducing the CCN AF during the monsoon. 25 Best closure obtained using measured critical diameter, and ammonium sulfate composition during continental conditions emphasize the role of aged aerosols contributing to the accumulation mode, enhancing the CCN efficiency. But the over- estimation of CCN and less hygroscopicity of accumulation mode aerosols during the monsoon point to the significant role of size-dependent aerosol composition in CCN activity during the period. 30 https://doi.org/10.5194/acp-2020-45 Preprint. Discussion started: 3 February 2020 c  Author(s) 2020. CC BY 4.0 License.