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 (>1000cm-3) during
continental conditions and observed together with high black carbon
(BC∼2000ngm-3) and columnar aerosol loading. During the marine air
mass conditions, CCN concentrations diminished to ∼350cm-3 at 0.3 % SS and
low aerosol loading persisted (BC∼800ngm-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
(<30 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 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.
Cloud condensation nuclei characteristics during the Indian summer monsoon over a rain-shadow region
