Characterization of Aerosol Physical Properties and its Impact on Solar Radiation over Nepal

Abstract

This thesis focuses on characterizing the physical and optical properties of aerosols over Nepal by including the Himalayan region (Jomsom, Kyanjin Gompa, and EVK2_ CNR), the Hilly region (Pokhara, and KTM-Bode), and the Terai region (Lumbini). NASA's (National Aeronautics and Space Administration) AERONET (Aerosol Robotic Network) provides a significant amount of aerosol data for analysis for these locations, covering the country's largest and most strategic areas. PurpleAir, a low-cost instrument installed in Pokhara and Kathmandu, provides continuous one-minute average data. These data were analyzed and compared with satellite data from MODIS (Moderate Resolution Imaging Spectroradiometer). An analysis of HYSPLIT back trajectory, such as cluster analysis, was used to identify aerosol particle sources. Pyranometer data were used to observe global solar radiation (GSR). Different statistical tools, such as Root Mean Square Error (RMSE), standard deviation, and correlation coefficient, were used to analyze aerosol optical properties over multiple years. Based on AERONET data, the four metrics aerosol optical depth (AOD), precipitable water (PW), angstrom exponent (AE), and single scattering albedo (SSA) are presented in this study. There was a maximum PW of 5.87 ±0.12 cm in Lumbini in July and an average of 3.25±0.21 cm, whereas the minimum was observed in EVK2_CNR with an average of 0.23 ±0.02 cm. An analysis of the AE can provide crucial information on the size distribution of aerosols based on their dependence on a wavelength. Except for EVK2_CNR, all observed stations have an average AE value greater than one. Jomsom has the highest average (1.52 ±0.18), and EVK2_CNR has the lowest (0.77± 0.34). A clear domination of fine-mode aerosols can be seen between 440 and 870 nm in the wavelength range. There is a tendency for AOD to decrease with the altitude of the geographical locations. Monthly average values of AOD 500 nm decrease with altitude from Lumbini (0.71 ± 0.19) to EVK2_CNR (0.04 ± 0.01). Accordingly, Lumbini has the highest pollution level, and EVK2_CNR has the lowest pollution level. SSA varied between 0.79 and 0.96 for all observation periods except for EVK2_CNR, with an average of 0.68± 0.17 and varies between 0.27 to 0.98. Monsoon and post-monsoon seasons have higher values, while winter and pre-monsoon seasons have lower values. SSA shows different monthly variations showing the variability of aerosol optical characteristics. The maps plotted based on MODIS data for AOD from 2015 to 2020 show that AOD is highest in the southern part of the country, relatively low in the middle hills, and lowest in the high mountains. Additionally, AOD is relatively lower in 2020, indicating a reduction in anthropogenic aerosols due to the COVID-19 lockdown. Pokhara's aerosol product time-series analysis from 2010-2018 shows lower aerosol loading during the morning and evening compared to mid-day. As demonstrated by the correlation between AOD at 500 nm and Precipitable Water level (PW) from December to April (0.96), aerosol and water vapor accumulation in the atmosphere significantly increases during the winter and pre-monsoon seasons and varies inversely from May to November, with a correlation coefficient of -0.61. A five-day back trajectory cluster analysis reveals that air masses reaching Pokhara valley comes from the Indo Gangetic Plains (IGP) region during winter and West Indian plains (West India, Pakistan) during the pre-monsoon when strong western disturbances prevail. During monsoon, most air mass arrives east of the IGP region and the Bay of Bengal. Analysis of daily average data for Global Solar Radiation (GSR) in Pokhara shows that maximum GSR values gradually increased from January to March, then decreased in April, then increased again in June. It gradually decreases from July to December. Almost all months had mean values that were lower than their median values. March has the highest average [19.19±4.61 MJ/m2], and January has the lowest average [9.31±4.39 MJ/m2] with yearly average [14.48±4.31 MJ/m2 ]. Comparison of GSR data with AOD data shows, the amount of solar radiation reaching the ground decreased with an increase in aerosol loading in the vertical column. While investigating the reliability of PurpleAir measurements, this study compares column-integrated aerosol data from MODIS with surface-level aerosol concentration. PurpleAir Hourly averaged PM2.5 levels fluctuate bimodally in all seasons, with the peak values in the rush hours of morning and evening. In Pokhara, PM2.5 values range from a minimum of 22.78 ±3.23 µg m–3 in the summer to a maximum of 101 ±26.31 µg m–3 in the winter. During pre-monsoon and post-monsoon, it was observed to be 55.58 ±11.42 µg m–3 and 45.46 ±12.16 µg m–3 respectively. During the summer, PM2.5 levels are lowest due to rain and diffusion in the vertical atmosphere.