CCN characteristics during the Indian Summer Monsoon over a rain-shadow region

Abstract

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 (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 (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 (