Decoding Aerosol-to-CCN activation: Insights into size and composition effects using a long-term dataset from a high-altitude site in the Western Ghats, India

The study presents a long-term evaluation of physical and chemical properties of ambient aerosols and their effect on the Cloud Condensation Nuclei (CCN) activity observed over a high-altitude site in the Western Ghats of India. High time-resolution data from co-located instruments, including High Resolution Time of Flight Aerosol Mass Spectrometer (HR-TOF AMS), CCN counter, Scanning Mobility Particle Sizer (SMPS) etc., were collected from January 2020 to October 2020 at High Altitude Cloud Physics Laboratory (HACPL), Mahabaleshwar. The mean ambient aerosol concentration was 5625/cm 3 , 5016/cm 3 1495/cm 3 and 3016/cm 3 with the mean geometric mean diameter (GMD) as 88.9 nm, 86 nm, 66.4 nm and 93.8 nm during the winter, pre-monsoon, monsoon and post-monsoon seasons, respectively. The high aerosol concentration with low GMD events was observed more during winter and pre-monsoon, suggesting influence from anthropogenic emissions as well as new particle formation. The activation fraction (AF), defined as the ratio of CCN to total aerosol, exhibited a significant positive correlation ( r = 0.701, p < 0.05, z-score = 0.84) with GMD. Higher AF values within a given GMD range were associated with increased inorganic aerosol content (mainly sulfate) and lower aerosol number concentrations. Conversely, lower AF values corresponded with higher aerosol number concentrations and a dominance of organic aerosols. Notably, cases with elevated oxygenated organic content (higher f 44 ) exhibited enhanced CCN activation, even when inorganic fractions were low. This further illustrated that the role of highly oxygenated organics in CCN activities cannot be ignored. These results highlight the critical role of both particle size distribution and chemical composition—particularly the presence of highly oxygenated organics—in determining CCN activation potential.