# Investigation of Aerosol Climatology and Long-Range Transport of Aerosols over Pokhara, Nepal
## 4 Conclusions  
We have been able to study a cluster analysis of the aerosol climatology over Pokhara, an AERONET site located on a Himalayan foothill, based on long-term aerosol optical properties, size spectra and regional aerosol sources, and the long-range transport of aerosols over the observation site. The variation of AOD spectra and the magnitude of AOD are strongly associated with the change in seasons that brings different air masses over the observation site. The maximum \(\tau_{0.50}\) in the pre-monsoon season (\(0.75\pm 0.15\)), followed by winter (\(0.43\pm 0.12\)), post-monsoon (\(0.39\pm 0.08\)), and monsoon (\(0.27\pm 0.13\)) seasons, show different aerosol loading, by a varying amount of wind-driven long-range transport of aerosols (mainly dust and aerosols from biomass burning, urban-industrial activities). The strong correlation between \(\tau_{0.50}\) and \(\beta\) also shows the association of seasonal aerosol loadings to influence the \(\tau_{0.50}\). The air masses back trajectory and local and regional fire spots were supported, to investigate the effect of transboundary air pollution, from different sectors over Pokhara. We find that Pokhara receives dominating westerly air masses during the post-monsoon season, indicating a significant effect of biomass burning, such as crop residue burning over the northeast part of Pakistan. Both westerly and southwesterly air masses crossing over the Thar desert of India affect the area during the pre-monsoon and winter seasons. The pre-monsoon season (April and May) is mainly crop harvesting time, such as wheat, in the northwest area of Pakistan, and the smoke of burning crop residue easily diffuses to the atmosphere to be transported to Pokhara. The monsoon season is significantly associated with the transport of aerosols from the side of the Bay of Bengal. The trajectory analysis provides a unique background knowledge of aerosol components, which arrives over Pokhara by mixing with anthropogenic pollution background and natural aerosols, while passing over the heavily polluted IGP or coming from the IGP. The study of spectral variation of AOD by using \(\alpha^{\prime}\),the first derivative of the angstrom exponent (\(\alpha\)) with the wavelength in log scale distinguishes the aerosol size distribution, even with a similar value of \(\alpha\). Furthermore, \(\alpha^{\prime}\) is found to be more sensitive for months when there is a transition from one season to another. This investigation suggests that the analysis of \(\alpha^{\prime}\) gives a more sensible complement of \(\alpha\) to characterize more fully wavelength dependence of AOD and the comparative influence of aerosol size spectra of two modes in aerosol loading. Intermediate AAE values range from 1.38 to 1.78, and the increasing and decreasing nature of SSA with wavelengths (analyzed only for winter and pre-monsoon seasons due to limitations of availability of data) proved beneficial for identifying the aerosols over Pokhara as a mixed type of aerosols, such as dust, aerosols from biomass burning and urban-industrial activities. Figure 9: **(a)** The corrected reflectance true-color MODIS satellite image of the day 27 October 2017, shows intensive air pollution plumes over the IGP. **(b)** NOAA HYSPLIT MODEL Back Trajectories ending at 0600 UTC on 27 October 2017. Horizontal and vertical scales of 400 miles on the left figure are shown. We consider the presented methodology as a useful tool for the calibration of aerosol optical properties, and advancing knowledge on aerosol climatology over the Himalaya region of Nepal. [PERSON] is the prime author of this paper and was involved in preparing the manuscript, data and graphical analysis. [PERSON] was responsible for analyzing aerosol data and supporting the preparation of the manuscript. [PERSON]. ([PERSON]), the Pokhara AERONET site's Principal Investigator, was responsible for the site's establishment of and also critically reviewed the manuscript. [PERSON] supported the cluster Analysis and fire spots.