An episode of transboundary air pollution in the central Himalayas during agricultural residue burning season in North India

The Indo-Gangetic Plain experiences extremely hazardous air pollution levels during the months of October and November due to post-harvest agricultural residue burning (ARB), mainly in the northwest Indian states of Punjab and Haryana. The impact of the resultant pollutant transport on the air quality of the central Nepali Himalayas has received little study thus far. We used multiple datasets including in-situ measurements, satellite observations, reanalysis data and back-trajectory analysis to investigate a recent episode of elevated air pollution in Kathmandu, Nepal, during November 4-7, 2020, in which the level of particulate matter (PM2.5) doubled in a few days. We found the transboundary transport of aerosols from ARB in northwest India was responsible for the spike in PM2.5. We also found that although ARB fires occur throughout the post-monsoon season, the transport of pollutants beyond the Mahabharat range in Nepal is episodic and is controlled by synoptic and mesoscale dynamics in the region. Under specific wind conditions, pollutants concentrated in the southern Mahabharat range can be transported to Kathmandu and higher elevations, mainly through the river valleys. When combined with remote sensing data and back-trajectory models, even coarse resolution reanalysis data can provide valuable insights to understand the distribution and transport mechanism of air pollution in the Nepali Himalayas and the Tibetan Plateau, where direct measurements are sparse. Since pollution from ARB impacts people in several countries in this region, insights provided by analyses such as ours may provide a starting point for multilateral dialogue and cooperation to devise and implement policies to mitigate the impact of transboundary air pollution on human health.