The Himalayan region is sensitive to a variety of aerosols emitted in India, Pakistan and parts of China. The central Himalayan region, which includes Nepal and the northern part of India, merges into the densely populated and highly polluted Indo-Gangetic Basin (IGB).

Because of the combined effects of IGB topography and the Himalayan mountains, these aerosols can be lifted up to high altitudes. The back-air trajectory analysis outlined in this study suggests that north-west India contributes significantly to atmospheric aerosols (particularly black carbon emitted by abundant agricultural fires), which are frequently transported to the Himalayan region (see figure). These emissions may be key to understanding the impact of black carbon aerosols on the Indian summer monsoon in the region, the precipitation pattern over the Himalayas and frozen water storage.

The study was undertaken by Atul Kumar Srivastava at the Indian Institute of Tropical Meteorology and colleagues from the University of Tokyo, the Space Physics Laboratory, Trivandrum, India, and the Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital, India. The study region, Manora Peak, is a high-altitude, sparsely inhabited region in the Indian Himalayan foothills in the central Himalayas.

The researchers made a first attempt to assess black-carbon aerosols and their radiative impact by using simultaneously measured aerosol chemical composition and black-carbon mass concentrations from July 2006 to May 2007. The team also quantified how black-carbon aerosols contribute to radiative forcing using model studies.

The results show that, compared with urban regions, the mean black-carbon mass concentration over the region is much lower (about 1.0 ±0.68 μgm–3), with a relatively small contribution (<~3%) to the total aerosol mass. Modelling indicated that the small amount of black carbon observed over the Himalayan region contributed as much as 17% to the total aerosol loading (in terms of aerosol optical thickness). This exerted a relatively large surface heating (~45%) compared to the total aerosols, and contributed as much as 70% to the total atmospheric forcing at Manora Peak. These results may have implications for the strength of Himalayan glaciers, monsoon circulation and precipitation over India.

"Our knowledge of several climate issues, mainly on inter-annual and intra-seasonal scales, is rather poor and needs to be addressed on the basis of long-period investigations to improve the understanding of the regional climate over the climatically sensitive Himalayas," said Srivastava.