Cloud and Aerosol Characteristics During Dry and Wet Days of Southwest Monsoon over the Rain Shadow Region of Western Ghats, India

In situ measurements of aerosol particle chemistry and cloud microphysics made during the Cloud–Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) in 2015 over the Western Ghats and its rain shadow region are presented in this study. The high ratio of cloud condensation nuclei (CCN) to large sized aerosol (above 0.1 µm) concentrations indicates Aitken mode aerosols as the major contributor towards the observed CCN. Morphology and chemical composition of airborne aerosol samples collected from different altitudes and the warm cloud layers indicated distinct particle chemistry on the dry and wet days. The majority of the particles sampled were heterogeneous and internally mixed with two or more aerosol species indicating multiple sources and atmospheric aging, even during the wet days. Si-rich particles (up to 73%) were the dominant species in samples collected during dry days when the shallow clouds with narrow drop size distribution were observed. A higher concentration of Na-rich particles (up to 50%) was observed on wet days when the clouds were majorly multiple layers of stratus, which had broader cloud droplet spectra. Internally mixed carbonaceous and iron/cobalt-rich aerosols from local pollution sources were found on both wet and dry days suggesting boundary layer venting of aerosols. Large droplets were observed near the cloud base over the Western Ghats compared to those formed over the rain shadow region.