Hygroscopic growth and CCN activation of aerosols during Indian Summer Monsoon over a rain-shadow region

Abstract

Hygroscopic growth factor and cloud condensation nuclei (CCN) activation properties of aerosols along with their physical and absorption properties were measured during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over a rain-shadow region, Solapur, India. The hygroscopicity of aerosols at both sub- and supersaturated regimes and the responsible factors are investigated during the Indian Summer Monsoon (ISM). During the intermittent rainy, wet, and cloudy days the total aerosol (≈ 966 cm−3), CCN (≈ 587 cm−3 at 0.4% supersaturation, S) number concentrations, and black carbon (≈ 482 ng m−3) mass concentrations were less, which increased to the mean values of ≈ 3754 cm−3, ≈ 974 cm−3, and ≈ 892 ng m−3, respectively during the dry and clear-sky period. Multiple mode aerosol number size distribution prevailed irrespective of the meteorological conditions, while a prominent nucleation mode was observed during the dry conditions. The nucleation mode was consistent from 09:00 hours local time (IST) during the dry conditions, apart from those associated with the local emissions during the sunset. The nucleation mode associated with the sharp enhancement in absorption Angstrom exponent after the BC peak during the early morning hours of dry days indicated the secondary organic aerosol formation. The hygroscopic growth factor measurements at 50 and 150 nm and the hygroscopicity parameter-ΚHTDMA values were spread over a wide range. But the mean and the median values were similar for both wet and dry periods. However, the hygroscopicity derived from CCN measurements-ΚCCN at 0.2 and 1.0% Ss were higher than the ΚHTDMA values. The high ΚCCN (≈ 0.44) during wet conditions reduced to ≈ 0.28 during the dry conditions for 1.0% S (corresponding to ~40 nm). The hygroscopicity and the CCN efficiency of aerosols were high during daytime and from midnight to sunrise for the dry and the wet periods, respectively.