Physical characterization and hygroscopicity of ambient aerosols and their role in CCN closure over Western Ghats, India

Three years (2021 – 2023) of in-situ ground-based observations from a high-altitude site in Western Ghats, India, have been utilized to study the cloud condensation nuclei (CCN) activation and closure during the monsoon (JJAS, June to September), including its transition periods (May and October). Aerosol-CCN characteristics showed an apparent temporal variation between monsoon and its transition periods. The estimated aerosol bulk hygroscopicity (κ) showed a narrow (broad) distribution during monsoon (transition periods). Aitken mode was dominant during JJAS, while Aitken-accumulation mode was dominant during the transition periods, suggesting that these modes primarily control the bulk hygroscopicity of the total aerosol population. These further indicate the increased wet scavenging of larger aerosol particles during JJAS compared to transition periods. The CCN closure study was performed using different κ values. The analysis showed a good closure in JJAS when κ = 0.15 was considered, while the transition periods showed a good closure at various κ and supersaturation (SS) values. This suggests that the CCN closure is less sensitive to the change in SS or κ during JJAS than the transition periods. Hence, the assumption of a single κ (e.g., κ = 0.15) or a narrow range of κ (= 0.15 – 0.20) can be considered during monsoon, while the same assumption cannot be applied during the transition periods for the study region. To the best of our knowledge, this is the first such study using an extensive set of ground-based observations from the Indian region.