# Investigation of Aerosol Climatology and Long-Range Transport of Aerosols over Pokhara, Nepal
## 1 Introduction  
Atmospheric aerosols have a significant impact on the Earth's atmospheric radiation budget, due to their direct scattering and absorption characteristics, as well as an indirect impact on microphysics and clouds' formation [1; 2; 3]. In recent decades, there has been increasing concern about aerosols' impact on melting snow and ice in the high Himalaya and over the Tibetan Plateau [4; 5; 6], with indications that a significant portion of the aerosols arrived from the Indo-Gangetic Plains [7; 8]. To date, though, therehave been only limited studies of aerosols over the Himalayan foothills immediately south of high Himalaya, on their route from the IGP towards the high mountains [5; 9]. The measurement of atmospheric aerosol physical and optical properties can provide knowledge of understanding the role of aerosols in the climate system. In the last few decades, various techniques have been used to measure and characterize the atmospheric aerosols from ground, aircraft, and satellite measurements [10; 11; 12; 13; 14]. Aerosol Robotic Network (AERONET) is one of the global ground-based networks used to monitor atmospheric aerosol optical properties and is maintained by the National Aeronautics and Space Administration (NASA). It has adopted the robotic Cimel Sun photometer throughout its network, to measure direct and diffuse solar radiation, which is then used to retrieve the aerosol products on the AERONET website [12; 15]. The network provides a readily accessible public domain database of atmospheric column integrated aerosol optical properties, such as aerosol optical depth (\(\tau_{\text{AOD}}\)) measured in the ultraviolet to infrared wavelengths. Moreover, \(\tau_{\text{AOD}}\) represents the columnar aerosol content resulting from the different aerosol populations in the atmospheric column above the measurement site [12; 14; 16]. The spectral variation of \(\tau_{\text{AOD}}\), which can be characterized by the Angstrom parameters (\(\alpha\) and \(\beta\)), can give a picture of aerosol size distribution and aerosol loading in the atmospheric column [14; 16; 17]. These are important parameters in the determination of aerosol size distribution. The variability of aerosol size distribution is a good indicator for the sources of aerosols, such as those generated from urban and industrial, biomass burning, combustion of fossil fuels, and airborne soil particles [17]. Many investigations have previously used the information on particle size and spectral aerosol optical characteristics to validate the dominant aerosol type [17; 18; 19]. Remote sensing techniques are important tools to monitor aerosol optical properties globally, by validating the accuracy of measurements [12; 20; 21]. The establishment of AERONET sites has provided an opportunity to study the aerosol loading due to local and long-range transportation over the region [13; 18]. A combined analysis of ground-based column integrated aerosol optical data, atmospheric simulations such as the back trajectory analysis from Hybrid Single-Particle Lagrangian Integrated Trajectory Model (HYSPLIT) and the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observation data of atmospheric pollutants can be used to study the spatial and temporal variation of aerosol optical properties, aerosol types, and source [13; 22; 23]  
Previous studies in Nepal have presented aerosol characteristics, but did not investigate long-term aerosol optical properties and aerosol sizes based on the spectral aerosol optical depth [24; 25; 26; 27]. A number of studies have shown that Nepal's atmosphere is affected by a variety of emissions, such as those produced by biomass burning, agricultural burning, and domestic uses of biofuels, which are taking place in its large neighboring countries [28; 29]. Particularly the Terai--the flat part of southern Nepal that lines the northern edge of the Indo-Gangetic Plains (IGP)--is heavily affected by trans-boundary pollution from the rest of the IGP.