Closure and parameterization of sub-micron aerosols’ hygroscopicity and it’s seasonal variability over the Western Ghats, India

Hygroscopicity of atmospheric aerosol primarily depends on the size and chemical composition of the particle and is important for estimating anthropogenic aerosol radiative forcing. Hygroscopicity is highly related to the activation of aerosols to Cloud Condensation Nuclei (CCN), and hence plays a crucial role in cloud formation and modulating its properties. However, due to limitations of measurement techniques, seasonal variation in size segregated aerosol hygroscopicity (k) is not available over the Indian region.  This study presents ‘k’ as derived from a Humidified Tandem Differential Mobility Analyzer (HTDMA) over the High Altitude Cloud Physics Laboratory (HACPL) in the Western Ghats, India for more than a year (from May 2019 to May 2020).  The average hygroscopicity values of aerosol particles of diameters 32, 50, 75, 110, 150, 210, and 260 nm at 90% RH conditions are 0.189, 0.177, 0.163, 0.170, 0.183, 0.199, 0.207 respectively during the entire observation period.  k was observed to decrease with an increase in size in the Aitken mode regime (32-75 nm) and an increase in the accumulation mode (110-260 nm).  Seasonal variation of hygroscopicity for a wide range of particle diameters is reported which is highly demanding as there is a change in the air mass flow pattern in each of the seasons.  The diurnal cycle of hygroscopicity showed a prominent peak during the midnight to early morning hours followed by a decrease in the forenoon hours and a secondary peak in the afternoon hours.  k is found to be higher in pre-monsoon as compared to the winter season as Chl is approximately 3% higher in pre-monsoon and NH₄Cl is highly hygroscopic among the assumed chemical composition.  Assuming the internal and external mixing of aerosols, the closure study yields chemically derived hygroscopicity (k_chem) overestimates as compared to k_HTDMA though the assumption of external mixing of aerosols improved the values of predicted k. CCN derived hygroscopicity (k_ccn) underestimates as compared to hygroscopicity derived from HTDMA measurements. Both k_chem and k_ccn are found to follow the similar diurnal variation of k_HTDMA. Thus, the k_chem and   can be used as a proxy of  in the absence of direct HTDMA measurements. Using k_chem and  in numerical models will propagate systematic bias in aerosol to CCN activation processes so a parameterization of hygroscopicity with dry diameter of sub-micron particles is developed and that could be used for a closer real-atmospheric value of hygroscopicity.

Keywords: Hygroscopicity, Parameterization, sub-micron aerosols