# Investigation of Aerosol Climatology and Long-Range Transport of Aerosols over Pokhara, Nepal
## 3 Results and Discussion
### Angstrom Exponents, Curvature of AOD Spectra, SSA, AAOD and AAE
The significantly low \(\tau_{0.50}\) in monsoon compared to other seasons is associated with significant rainfall in this season [25; 44]. We also analyzed \(\alpha\) computed at different spectral bands, as shown in Figure 5b to identify the aerosol types. The difference in \(\alpha\) at spectral bands \((\alpha_{(0.675-0.87)}-\alpha_{(0.34-0.38)})\) was significantly higher in post-monsoon and winter seasons, indicating a size distribution dominated by fine modes (Figure 5b). The difference lowers in months of pre-monsoon seasons, supporting the company of coarse mode as well. This difference is negative in the monsoon season, which indicated domination by coarse mode aerosols [17; 19]. The aspect of change in aerosol size spectrum, based on Angstrom parameters, is also examined by using aerosol volume size distribution data obtained from AERONET over Pokhara site. July to September data are not analyzed due to the limitation of availability of data in AERONET. Figure 6 shows the monthly mean of the aerosol volume size distribution for different seasons, along with \(\alpha\), and \(\alpha\), and reveals a bimodal structure of aerosol sizes for each month. It is observed from Figure 6 that the volume size distribution for the months are significantly dominated by accumulation mode of aerosols (with higher \(\alpha\) ), both modes of aerosol sizes (with the medium \(\alpha\) ), and coarse modes (with smaller \(\alpha\) ). For this study, July to September volume size distribution data are not available for comparison. Figure 5: Time series of (**a**) Aerosol Optical Depth at 0.50 \(\upmu\)m and Ångström Turbidity Parameter \(\beta\) and curvature of the curve. (**b**) Ångström Exponent in different wavelength pairs. Figure 7 shows mean values of SSA and AAOD for five different months of winter and pre-monsoon season to study absorbing aerosols. These months are chosen because of the data availability in the AERONET site. SSA's spectral variation, the top two figures, shows that SSA increases from 0.44 to 0.675 \(\upmu\)m, and then decreases at higher wavelengths. SSA values are found as 0.89 (January), 0.88 (February), 0.87 (March), 0.87 (April), and 0.87 (May) at wavelength 0.44 \(\upmu\)m, and indicate the presence of reasonably strong absorbing aerosol components. Figure 7, bottom two figures, shows AAOD spectra. Previous studies reported that SSA spectra of different AERONET locations with dust containing aerosols have increased SSA with increasing wavelength, while locations dominated by urban industrial or biomass burning decrease with increasing wavelength [2; 4; 45; 46]. This trend was also used in the past studies to differentiate between carbonaceous aerosols and dust in different locations [2; 10; 47; 48]. The behaviors of increasing of SSA spectra up to 0.675 \(\upmu\)m resemble aerosol components from dust aerosols, and decreasing from 0.675 \(\upmu\)m to 1.02 \(\upmu\)m shows similar to that of aerosol products of biomass burning, urban and industrial activities. Similar results can also be found in previous studies to define a mixed type of aerosols [45; 46]. Figure 6: The monthly averaged columnar volume particle size distribution in the range of sizes 0.05 \(\upmu\)m \(\leq\) r \(\leq\) 15 \(\upmu\)m, for different seasons, as indicated. The spectral dependence of AAOD was also used to compute the absorption Angstrom exponent (AAE), using a linear regression fit on the logarithmic scale plot [2,45,46]. AAE data were found for January (\(1.71\pm 0.04\)), February (\(1.50\pm 0.03\)), March (\(1.42\pm 0.03\)), April (\(1.38\pm 0.03\)) and May (\(1.47\pm 0.03\)), with ranges \(1.38\) to \(1.71\). Various studies have been done in the IGP region and different locations of AERONET to investigate the AAE based on types of aerosols [2,49,50,51]. These previous investigations have shown that AAE values vary from \(1.2\) to \(3\) for dust, \(0.75\) to \(1.3\) for urban and industrial aerosols, and \(1.2\) to \(2\) for biomass burning.