CCN estimates from bulk hygroscopic growth factors of ambient aerosols during the pre-monsoon season over Central Nepal Prabhakar Shrestha a, * , Ana P. Barros a , Andrei Khlystov b a Pratt School of Engineering, Duke University, Durham, NC 27708, USA b Environmental and Industrial Sciences Division, Research Triangle Institute, 3040 Cornwallis Rd., PO Box 12194, Research Triangle Park, NC 27709, USA highlights < Aerosol Hygroscopicity and CCN spectra estimated for Central Nepal. < DGF and Chemical Composition data used for hygroscopicity estimates. < Aerosols found to be less hygroscopic and in metastable state. < Two-component hygroscopic growth model used to explain growth factor. < Effect of WSOC solubility on CCN estimates also addressed. article info Article history: Received 19 August 2011 Received in revised form 24 October 2012 Accepted 27 October 2012 Keywords: Hygroscopicity Aerosol size distribution CCN Nepal abstract 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 (AprileJune 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, k. The k parameter suggests that aerosols in this region are less hygroscopic than aerosols reported in previous studies from other regions. The estimated k 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. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The aerosol size distribution and chemical composition were measured concurrently during the 2009 pre-monsoon season at two relatively low altitude sites in Central Nepal during the JAMEX09 (Joint Aerosol Monsoon Experiment 2009) campaign [Fig. 1 , Shrestha et al. (2010)]. This region experiences a strong aerosol seasonal cycle, with aerosol concentration peaking during the pre-monsoon and winter season (Shrestha and Barros, 2010). At the sampling sites, aerosol number concentrations exhibited a strong diurnal cycle, and the day to day variability in the time series of observed aerosol composition consisted of concentration peaks associated with synoptic scale pollution events and the subsequent scavenging by rainfall (Shrestha et al. (2010); Brun et al., 2011). The variability in the number concentration and chemical properties of aerosol influence the CCN spectra, and therefore can potentially affect cloud microphysical processes. Previously, numerous modeling studies have been conducted to investigate the sensitivity of cloud microphysical processes to CCN (e.g. Pinty et al. (2001), Thompson et al. (2004), Muhlbauer and Lohmann (2008), Lynn et al. (2007), Cheng et al. (2006), Fan et al. (2007), Tao et al. (2007), Iguchi et al. (2008), Saleeby et al. (2009), among many others). In order to perform such studies, either the aerosol size distribution and its hygroscopicity, or the CCN spectra are required. * Corresponding author. Current address: University of Bonn, Meteorological Institute, Bonn, Germany. E-mail addresses: pshrestha@uni-bonn.de, prabshr@gmail.com (P. Shrestha). Contents lists available at SciVerse ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2012.10.042 Atmospheric Environment 67 (2013) 120e129