# Investigation of Aerosol Climatology and Long-Range Transport of Aerosols over Pokhara, Nepal
## 3 Results and Discussion  
### Variability of Spectral Column AOD and Precipitable Water  
In this section, we present the monthly and seasonal variability of spectral AOD and the temporal variability of AOD, by comparing with columnar precipitable water vapor (PW). Figure 2 shows the monthly mean spectral AOD values, \(\tau_{\text{AOD}}\), over the 9 years from 2010 to 2018, at seven different wavelengths--0.34 \(\upmu\)m, 0.38 \(\upmu\)m, 0.44 \(\upmu\)m, 0.50 \(\upmu\)m, 0.675 \(\upmu\)m, 0.87 \(\upmu\)m, and 1.02 \(\upmu\)m. Our statistical analysis excluded monthly averaged aerosol data from any months that had \(\tau_{\text{AOD}}\) data for less than ten days. Due to the above constraint on data availability, the mean values for each calendar month across the nine years were computed from a varying number of months, as given in the parenthesis: January (6), February (8), March (8), April (7), May (8), June (5), July (4), August (3), September (4), October (6), November (6), and December (7). Level 2 aerosol data were very scarce on the AERONET website for the rainy season months of June, July, and August, due to cloud screening of the data and the requirement for the sun photometer to remain parked in a protected position whenever the rain sensor was wet. The general trend of spectral variations shows that the \(\tau_{\text{AOD}}\) is higher at a shorter wavelength and decreases at longer wavelengths. We found the spectral \(\tau_{\text{AOD}}\) highest in April, followed by March, May, February, June (January, November), (December, October) September and (August and July). Months placed in parentheses have almost identical spectral \(\tau_{\text{AOD}}\) at all wavelengths. In the monsoon season, the gradient of spectral \(\tau_{\text{AOD}}\) in the longer wavelength decreases. Distinct features of seasonal \(\tau_{\text{AOD}}\) can be observed in Figure 2, with the highest aerosol loading in the months of pre-monsoon  
Figure 2: Spectral variation of monthly mean aerosol optical depth (AOD), over nine years between 2010–2018, in different seasons. season followed by winter, post-monsoon, and monsoon season. In the months of two seasons, winter and post-monsoon, the aerosol loadings are not significantly deviated from each other. The temporal variation of light attenuation is also observed using a monthly mean AOD at 0.50 \(\upmu\)m(\(\tau_{0.50}\)), along with a column-averaged precipitable water level (PW) in centimeters for different months and seasons (Figure 3). The \(\tau_{0.50}\), ranges from 0.31 to 0.56 in winter season with average value of 0.43 \(\pm\) 0.12, 0.63 to 0.92 in pre-monsoon season with average value of 0.75 \(\pm\) 0.15, 0.18 to 0.47 in monsoon season with average value of 0.27 \(\pm\) 0.13, and 0.33 to 0.45, in post-monsoon season with average value of 0.39 \(\pm\) 0.08. Variation of PW shows a similar trend of rainfall in Pokhara with an increase from months of winter to monsoon seasons, and then decreasing in post-monsoon season [40]. In monsoon season, the variation on \(\tau_{0.50}\) is significantly affected by the rainfall. The variations of \(\tau_{0.50}\) and PW show two different characters. Between December and April, these two parameters correlate significantly, with high R\({}^{2}\) (0.91) and low \(p\)-values (0.01), and while including data of May R\({}^{2}\) drops to 0.35 and \(p\)-value increases to 0.21. However, in monsoon and the post-monsoon months, they vary inversely with R\({}^{2}\) equal to 0.43 and \(p\)-value 0.15. Previous studies have shown that atmospheric water vapor can serve as a medium for igniting multiphase reaction to the formation of gas to particle transformation, and play a key role for hygroscopic growth of aerosols, which ultimately affects the aerosol optical properties [41; 42]. In this study, we argue that \(\tau_{0.50}\), which has a significant correlation with PW, from December to April, is mainly associated with the actual aerosol loading, and might not be linked with aerosol particles' hygroscopic effect growth.