Sources, aerosols chemical composition, and their radiative effects over the Hindu Kush Himalayan (HKH) region

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. Aerosol radiative forcing was found to be positive at the top-of-atmosphere (TOA) indicating a net warming effect. Notably, the value of aerosol radiative forcing at TOA during pre-monsoon was about 4-6 times the value during the winter season. A strong influence of transport from far-off regions, which contributed as high as 60% of the total positive radiative effects was inferred. Furthermore, the relative percentage of snow albedo reduction (SAR) and the annual increase in surface runoff due to BC deposition were the highest over the region confined between 27°N-29°N and 78°E-82°E with their values being respectively 13-15% and 5-7 inches of water equivalent. The radiative transfer calculations are further used as input to a hydrological model, and sensitivity studies of surface runoff to aerosol deposition is carried out and validated with available observational data.