Seasonal and diurnal variations in methane and carbon dioxide in the Kathmandu Valley in the foothills of the central Himalayas

Kathmandu Valley exhibits significantly higher levels of methane (CH4) and carbon dioxide (CO2) compared to global background sites. Annual averages measured at Bode, a semi-urban location in the valley, were 419.3 ppm for CO2 and 2.192 ppm for CH4—substantially above levels recorded in Mauna Loa (USA) and Waliguan (China). Seasonal variations showed that CO2 peaked during the pre-monsoon season due to forest fires and agro-residue burning, while CH4 exhibited minimal seasonal fluctuation. Diurnal patterns revealed pronounced morning peaks for all three gases (CO2, CH4, and CO), with a notable dip in the afternoon, attributed to local emissions such as residential cooking, vehicle exhaust, and brick kiln activities during winter. Simultaneous measurements at a rural site (Chanban) confirmed that urban areas like Bode experience 3.8–64% higher gas concentrations, underscoring the valley's unique geography and pollution dynamics.

Mahata, K. S., Panday, A. K., Rupakheti, M., Singh, A., Naja, M., & Lawrence, M. G. (2017). Seasonal and diurnal variations in methane and carbon dioxide in the Kathmandu Valley in the foothills of the central Himalayas. Atmospheric Chemistry and Physics, 17(20), 12573–12596. https://doi.org/10.5194/acp-17-12573-2017