Radiative impacts of particulate matter in a Himalayan valley: A modelling case study of the Khumbu Valley, Nepal.

Air pollution in mountain ecosystems has recently received particular attention. The peculiar and complex topography of such regions, combined with region-specific heating practices, has been shown to significantly reduce air quality levels, particularly in locations and communities situated on mountain valley floors.

The Khumbu Valley, located in the Himalayan ridge, connects the Indo-Gangetic Plain to the Nepal Climate Observatory – Pyramid (NCO-P) observation site at the foothills of Mount Everest (5079 m a.s.l). It often experiences high levels of particulate matter, including carbonaceous aerosols species (e.g. black carbon), which are largely modulated by the typical mountain valley circulation. These aerosols can be transported into the Khumbu valley from the Indo-Gangetic plain through thermally driven up-valley flows. However, the extent to which such circulation is directly impacted by absorbing and scattering aerosol compounds is currently unknown.

In this study, we conducted a one-month regional chemical transport model (CTM) simulation using the WRF-CHIMERE model at 1 km horizontal grid spacing, centered over the Khumbu Valley. The resolution was chosen to best account for the valley wind circulation typical of the region, while maintaining a trade-off with computational demands. We evaluated the impact of aerosols on meteorology due to aerosol-radiation interactions (ARI) over the Khumbu Valley and quantified its overall absolute magnitude. The pre-monsoon month of April was chosen as the period when transport of particulate matter from the Indo-Gangetic Plain is at its peak. Our results indicated that the model was able to reproduce the influx of particulate matter from the Indo-Gangetic Plain, with the modelled midday average peak in line with measurements at the NCO-P site. Accounting for ARI in the meteorological host model indicated a statistically significant cooling of the valley induced by aerosols, with potential implications for valley wind circulation. Given the extent of the Himalayan range, the results presented here may have implications for future climate scenarios, as aerosol-radiation interactions are often not resolved in coarse Earth system model applications.