Comparing Surface and Column-integrated Aerosol Loadings with Relative Humidity as a Correction Factor

The comparison of concentration of surface PM 2.5 with Aerosol Optical Depth (AOD) retrieved from MODIS was made in Kathmandu in the year 2020. The hourly and seasonal averaged PM 2.5 was correlated with temporal AOD and corrected using relative humidity, which improved their correlation significantly. It was found that the correlation was comparatively low for the months with higher average temperature.

The bimodal variation is seen in the hourly average PM2.5, with greatest levels in the morning and evening. A minimal concentration is seen in the afternoon and early morning, reflecting the impact of morning and evening cooking and transportation. As a result, it is possible to link the anthropogenic effect to the overall air pollution at the surface level. During the monsoon season, there was a significant drop in the surface level particle content, demonstrating the impact of rainfall and the atmospheric spread of air pollution. After March, there was a noticeable decrease in PM2.5 variability, which can be related to the COVID-19 lockdown's impact on traffic. The contribution of trans-boundary aerosols over the observation site is shown using trajectory cluster analysis.