Persistent, Widespread, and Strongly Absorbing Haze Over the Himalayan Foothills and the Indo-Gangetic Plains

Abstract

We examine the impact of the Atmospheric Brown Clouds on the direct radiative forcing of the Himalayan foothills and the Indo-Gangetic Plains (IGP) regions, home for over 500 million S. Asians. The NASA-Terra MODIS satellite data reveal an extensive layer of aerosols covering the entire IGP and Himalayan foothills region with seasonal mean AODs of about 0.4 to 0.5 in the visible wavelengths (0.55 micron), which fall among the largest seasonal mean dry season AODs for the tropics. We show new surface data which reveal the presence of strongly absorbing aerosols that lead to a large reduction in solar radiation fluxes at the surface during the October to May period. The three-year mean (2001 to 2003) October to May seasonal and diurnal average reduction in surface solar radiation for the IGP region is about 32 (±5) W m )2 (about 10% of TOA insolation or 20% of surface insolation). The forcing efficiency (forcing per unit optical depth) is as large as )27% (note that the forcing is negative) of top-of-atmosphere (TOA) solar insolation, and exceeds the forcing efficiency that has been observed for other polluted regions in America, Africa, East Asia, and Europe. General circulation model sensitivity studies suggest that both the local and remote influence of the aerosol induced radiative forcing is to strengthen the lower atmosphere inversion, stabilize the boundary layer, amplify the climatological tendency for a drier troposphere, and decrease evaporation. These aerosol-induced changes could potentially increase the life times of aerosols, make them more persistent, and decrease their single scattering albedos, thus potentially leading to a detrimental positive feedback between aerosol concentrations, aerosol forcing, and aerosol persistence. In addition, both the model studies and observations of pan evaporation suggest that the reduction in surface solar radiation may have led to a reduction in surface evaporation of moisture. These results suggest the vulnerability of this vital region to air pollution related direct and indirect (through climate changes) impacts on agricultural productivity of the region.

Similarities and differences of aerosol optical properties between southern and northern sides of the Himalayas

“…3) is quite low at QOMS_CAS and EVK2-CNR, but much higher at Pokhara. There may be a local contribution from Pokhara City (with population of 200 000), but Pokhara is also in a region of Nepal with significant rural biomass burning, and downwind of the IndoGangetic Plains, which are often covered in haze during winter and spring (Ramanathan and Ramana, 2005). At QOMS_CAS, the maximum monthly mean of 0.08 occurs in July, and the maximum monthly median of 0.08 also occurs in July.…”

Absorbing aerosols facilitate transition of Indian monsoon breaks to active spells

“…The greater magnitude of positive AI over the north and north-western part of India and northern-most Arabian Sea clearly shows that these regions are dominated by the presence of mostly absorbing type of aerosols. Observations show that the IGP experiences a very persistent and heavy aerosol burden (e.g., Ramanathan and Ramana 2005) and the composition in large part made up of absorbing aerosols (single-scattering albedo as low as 0.85, Ramanathan et al 2001). The remaining parts of the oceanic and continental regions are covered by lessabsorbing or scattering types of aerosols.…”

Estimation of NO<sub>x</sub> emissions from Delhi using Car MAX-DOAS observations and comparison with OMI satellite data

“…The growing population and human activities increase the atmospheric pollution which is e.g. the cause of the dense fog, smog and haze during winter season (December and January) every year (Singh et al, 2004(Singh et al, , 2005Ramanathan and Ramana, 2005;Gautam et al, 2007). In the years 2005 and 2006, about 4.8 million vehicles were in operation in Delhi.…”