High resolution Air Quality simulation in the Himalayan valleys, a case study in Bhutan

Abstract. Our study focuses on Bhutan, a highly mountainous country where governmental authorities are increasingly monitoring air pollution. To support further analysis and the monitoring strategy, we present the first high-resolution air quality simulations with the chemistry transport model WRF-CHIMERE over the western region of Bhutan at a spatial resolution of roughly 1 km. Increasing the horizontal resolution of the model improve the performances, decreases potential errors due to too important spatial average of meteorological and emissions data having an high spatial variability. However, the air pollutant emissions must be improved at fine scale with better proxy, particularly for industries where improvement are still required. For the first time, we propose high resolution maps of air pollution (concentrations and deposition fields). Our simulations confirm that Bhutan valleys also suffer from air pollution mainly due to PM_2.5 (concentrations exceeding 20 µg m⁻³) dominated by carbonaceous species, largely above the World Health Organization guidelines. Wildfires and anthropogenic activities release large amount of carbonaceous species and can also impact the glaciers by atmospheric fallout. Wildfires can locally contribute to 20 % of the total PM_2.5 concentrations over a 15 days period, and theoretically, black carbon can be transported up to the highest peaks. Ecosystems are at risks with deposition fluxes of sulfur and nitrogen species comparable with other locations at risk in the world.