Simulations of cumulative and relative impact of aerosol species over the Hindu Kush Himalayan region: validation, implications on glacier runoff, and source control

The Hindu Kush Himalayan (HKH) region holds substantial strategic significance owing to its extensive reserves of pristine water in the form of glaciers. In recent years, significant levels of particulate pollution have been documented within the high-altitude regions of the Himalayas. The influence of the albedo changes due to aerosol pollutants deposition on the glacial mass balance due to an excess and earlier snow melting, and thereby an earlier glacier runoff, is expected to impact the downstream hydrology. This is specifically of concern for the HKH region as the Himalayan glaciers are the source of major rivers in South Asia. The remote topography and severe weather conditions prevalent in the Himalayan region, however pose challenges to obtaining consistent spatial-temporal measurements of atmospheric aerosol concentration and their presence in snow. The simulated aerosol species concentration, using atmospheric chemical transport models (CTMs), which is validated by measurements, can be utilized to predict the spatial mapping of aerosol species distribution over the HKH region. In order to spatially map the estimates of atmospheric aerosol species concentration and their concentration in snow as adequately as possible, including the corresponding snow-albedo reduction over the HKH region, an integrated approach merging the relevant information from observations with a relatively consistent atmospheric chemical transport model estimates are applied in the present study.

We examine the cumulative and relative impact of aerosol species over the HKH region, including aerosol concentration in the snow, impacts on snow albedo re duction (SAR) and enhanced annual glacier snowmelt runoff identifying the hotspot locations. This is done evaluating aerosols transport simulations corresponding to dust, sulfate, and organic carbon (OC) aerosols relative to black carbon (BC) in free-running (freesimu) atmospheric general circulation model (GCM) and application of constrained (constrsimu) aerosol simulations, aerosol-snow radiative interaction model, and a novel hypsometric glacier energy mass balance model. Estimates for aerosol species concentrations from freesimu demonstrated increased accuracy at high-altitude (HA) stations compared to their performance at low-altitude (LA) stations. Conversely, estimates from constrsimu exhibited notably better performance at LA stations. The pre-monsoon aerosol species deposited in snow (> 850 µg kg⁻¹ over Gangotri and Chorabari) were the highest among glaciers, being about 40% greater than winter with OC including BC over selected glaciers and dust across all glaciers as compared to sulphate being twice larger than winter. The annual runoff increase (ARI) from the cumulative impact due to all aerosols showed the most significant ARI for Pindari glacier (about 500 mm w.e. y⁻¹), with five out of the nine glaciers, including Sankalpa, Milam, Gangotri, and Chorabari, had an ARI exceeding 300 mm w.e. y−1. Analysis from source- and region-tagged simulations indicates about 50 to 60% of aerosols-induced ARI can be mitigated by controlling BC aerosols over the region originating from open-biomass burning emissions mainly in the Indo Gangetic plain (IGP) and far-off region.