The role of wildfires in surrounding regions in exacerbating air pollution in the central Himalayas of Nepal

Wildfires, primarily over the Indo-Gangetic Plain, southern hilly region of Nepal, cause exceptionally high levels of pollution in the Central Himalayan region during the pre-monsoon season. Although wildfire smoke is one of the main sources of pollution, little research has been done on it. During the pre-monsoon season of 2021, we studied a hazardous level of air pollution in Kathmandu Valley, Nepal, using multiple datasets, including in-situ measurements, reanalysis data, and backward trajectory analysis. A total of 13 days exceeded the extreme pollution level of 134.29 μg/m³ (Mean + 2 * S.D.), with the highest daily concentration reaching 305 μg/m³. We found that smoke transported from nearby and transboundary wildfires was the main cause of the hazardous pollution in the valley. Furthermore, we discovered that the number of wildfires in the source region during that year was the highest on record. A strong correlation existed between daily active fire counts and valley pollution levels. The larger correlation among nearby locations indicates a greater responsibility for the increasing pollution, and it also reflects the rapid movement of pollutants into the valley. Due to the bowl-shaped structure of the valley, pollution accumulated and showed a considerable influence from nearby wildfires when it lagged by two days, reaching a maximum of 0.89 (p<0.05). Because of the calm wind conditions in the valley, a diurnal pollution pattern from the previous days persisted, although it was insufficient to entirely flush pollutants from the valley. Additionally, we noticed that synoptic and mesoscale dynamics in the area regulate the transport of pollutants to the valley. Since wildfire pollution affects people and economic activity in this region, conclusions drawn from research like ours may serve as a starting point for the implementation of legislation aimed at reducing the effects of wildfires and the air pollution they cause.