Cloud condensation nuclei characteristics during the Indian summer monsoon over a rain-shadow region

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 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 %–1.2 %) are parameterized using Twomey's empirical relationship. CCN concentrations at low (0.2 %) supersaturation (SS) were high ( > 1000 cm - 3 ) during continental conditions and observed together with high black carbon ( BC ∼ 2000 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 ( BC ∼ 800 ng m - 3 ). High CCN activation fraction (AF) of ≅0.55 (at 0.3 % SS) was observed before the monsoon rainfall, which reduced to ≅0.15 during the marine air mass and enhanced to ≅0.32 after that. There was mostly monomodal aerosol number size distribution (NSD) with a mean geometric mean diameter (GMD) of ≅85 nm , with least ( ≅9  %) contribution from nucleation mode ( nm ) particles persisted before the monsoon, while multimode NSD with ≅19  % of nucleation mode particles was found during the marine air mass. Critical activation diameters ( dcri ) for 0.3 % SS were found to be about 72, 169, and 121  nm 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 k  values during the daytime along with absorption Angstrom 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.