CCN estimates from bulk hygroscopic growth factors of ambient aerosols during the pre-monsoon season over Central Nepal

Ambient and dry aerosol size distribution were measured simultaneously using two SMPS with sizes in the range 14 nm – 340 nm during the pre-monsoon season over Central Nepal (Dhulikhel and Besisahar). Chemical composition of the aerosols was determined from the filter samples taken during the sampling period. The diameter growth factors (DGFs) were estimated using 1) inverse cumulative distribution of the wet (ambient) and dry distribution, and 2) ratio between the modes of the fitted lognormal distribution for the wet and dry distribution. A simple model based on Köhler theory for NH4SO4 aqueous solution with an insoluble core was used to explain the observed growth factors of ambient particles. Preliminary results show the existence of the aerosols in metastable state owing to the strong diurnal cycle of relative-humidity, which also partly explains the consistent haze build up in this region. The critical supersaturation for droplet activation is estimated based on the k-Kohler theory (Petters and Kreidenweis 2008 and Petters and Kreidenweis 2007, using the bulk hygroscopicity parameter ( ) obtained from the DGF. The CCN spectra at each critical supersaturation were obtained by integrating the fitted aerosol size distribution from the smallest dry diameters that activate at the given supersaturation. The CCN spectra were then fitted with the modified power law parameterization of Cohard et al. (1998), and the estimated parameter during the sampling period are presented.