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

Aerosol size distribution (ambient and dried) and chemical composition were measured simultaneously using two Scanning Mobility Particle Sizers (SMPSs) and filters during the pre-monsoon season (April–June 2009) at two locations in Central Nepal: Dhulikhel, an urban site in the Kathmandu valley, and Besisahar, a rural village in the Marsyangdi valley. Diameter growth factors (DGF) were estimated using the larger mode (around 100 nm) of the aerosol size distributions. The measured DGF suggest that the aerosols were in metastable state below 80% RH owing to the strong diurnal cycle of relative humidity (RH) at Dhulikhel, while no discernible growth was observed for Besisahar. Assuming ideal behavior of water-soluble organic compounds and using their fraction as determined by water extraction of filters produced the best agreement for a two-component hygroscopic growth model with the observed DGFs. This finding together with the DGF and chemical composition data were used to determine the aerosol bulk hygroscopicity parameter, κ . The κ parameter suggests that aerosols in this region are less hygroscopic than aerosols reported in previous studies from other regions. The estimated κ and the vertically scaled aerosol size distribution were then used to estimate the CCN spectra. These are the first estimates of CCN spectra from ground-based observations for the Central Himalaya region.