Simulations of aerosol species over Hindu Kush Himalayan sites: validation, sources, and implications on glacier runoff

Abstract:

We present an analysis of aerosol transport simulations carried out in Laboratoire de Me ́te ́orologie Dynamique (LMD-ZT) General Circulation Model (GCM) over the Hindu Kush Himalayan (HKH) region. Spatial distribution of the winter season mean aerosol optical depth (AOD) from the model displayed spatial variation with the highest value being near the foothills adjoining the Indo-Gangetic plain (IGP) and the lowest being over east to 85°E and north to 22°N. The seasonal mean of GCM estimated AOD mirrored well the measured AOD. The pre-monsoon mean AOD was estimated to be 20% higher than that of the winter mean. About 75% of the simulated AOD was found to be from anthropogenic emissions and was mostly constituted of sulfate (45-65%), followed by organic carbon (OC) (40-50%), and black carbon (BC) (4-8%). One of the prominent features was the discrete spatial gradient in the distribution of sulfate and BC aerosols thereby indicating their origin from different source regions, possibly remote regions for sulfate and that from neighboring IGP region for BC. A substantial variation was also seen in the spatial distribution of aerosol single scattering albedo (SSA) between the HKH region and the neighboring regions of IGP. GCM estimates of seasonal mean SSA agreed relatively well with that obtained from observations. The lowest value of SSA over HKH region was inferred due to emissions from biomass combustion; it was also found to be relatively lower due to aerosol emissions originating in Africa and West Asia, and India than that due to rest of the world. Furthermore, the relative percentage of snow albedo reduction (SAR) due to the deposition of different aerosol species were the highest over the region confined between 27°N-29°N and 78°E-82°E with their values being respectively 1-2.5%. SAR over different glaciers were further used as input to a glacial mass balance model, and sensitivity studies of surface runoff to aerosol deposition is carried out and validated with available observational data revealing annual increase in surface runoff from glaciers being as high as 5-105 mm of water equivalent. The analysis of source and region specific contribution to atmospheric aerosols led to identifying the potential emission sources being from IGP for OC and sulfate, whereas transported aerosols from far off regions for Dust aerosols.