Variability of Surface Level Particulate Matter in Kathmandu and Pokhara and an Overview of Origin of Pollutants

Abstract

The concentration of fine and coarse particulate matter at the surface level in Pokhara and Pulchowk (Kathmandu Valley) of Nepal was monitored from January to September 2020 using particulate matter sensors. Size-segregated particulate matter with diameters less than or equal to 1 micrometer (PM 1.0), PM 2.5, and PM 10 are analyzed hourly, daily, and seasonally. Pokhara is observed to be less polluted than Pulchowk, with an average of 58.96 μg /m3 during winter and 38.50 μg /m3 pre-monsoon. Pulchowk PM 2.5 averaged 99.64 μg /m3 during the winter season, December to February, and 55.94 μg /m3 during the pre-monsoon season, March to May. PM concentrations vary bimodally and decrease significantly in the daytime until 5 pm local time. There was a significant decrease in PM 2.5 after the last week of March in both cities, resulting from the COVID-19 lockdown, which indicates that most surface-level aerosol particles are anthropogenic such as traffic and cooking activities in the evenings and mornings might have significantly affected particulate matter concentrations. Based on seasonal variation, Pulchowk PM 2.5 leads to Pokhara by 1.63 times in winter, 1.46 times in pre-monsoon, 1.25 times in summer, and 1.32 times in post-monsoon, indicating Pokhara is less polluted than Pulchowk. In both cities, PM 2.5 contributes equally to PM 10, with fraction of 0.89 (0.86), 0.87 (0.85), 0.94 (0.92), and 0.76 (0.83), with the numbers in parenthesis representing Pulchowk and Pokhara, respectively.