Aerosols, the important constituents of our atmosphere, indicate a colloidal system of particulate, gaseous and volatile organic compounds. Aerosols play a significant role in affecting adversely the radiative balance of the Earth as well as the air temperature. Moreover, these not only influence the visibility and overall air quality, but also adversely affect health of living organisms in an ecosystem. In this context, the present attempt at Mohal (1,154 m, 77.12 degrees E, 31.91 degrees N) in the Kullu valley of Himachal Pradesh in the northwestern Himalayan region explains the ever increasing columnar aerosols, their relationship with black carbon (BC) aerosols, impact of local meteorological conditions, long range transport sources and their collective impact on radiative forcing and resultant temperature rise. The aerosol optical depth (AOD) having been under observation for the last half a decade (2006-2010) shows higher values at shorter wavelengths and lower at longer wavelengths. At a representative wavelength of 500 nm, AOD is found to be increasing at the rate of 1.9 % per annum from 2006 to 2010. Overall, AOD values in all the wavelengths (380-1,025 nm) were found between 0.238-0.242, reflecting an increasing trend at the rate of 0.84 % per annum. The monthly mean concentration of BC aerosols is noticed maximum with 6,617 ng m(-3) in January, 2010. The pollution loads in terms of AOD values translate into a temperature rise by similar to 0.54 K day(-1). The local as well as transported aerosols together contribute to the existing aerosols in the present study region. The local sources possibly belong to anthropogenic aerosols including vehicular emissions, biomass burning (like fuel wood for cooking), forest fires, open waste burning, etc. While the transported aerosols most probably include fine mineral dust from the desert regions and the sulphate aerosol from the oceanic regions with the movement of air masses prior to the western disturbances and monsoonal winds in the region.

We use GEOS-Chem chemical transport model simulations of sulfate-ammonium aerosol data from the NASA ARCTAS and NOAA ARCPAC aircraft campaigns in the North American Arctic in April 2008, together with longer-term data from surface sites, to better understand aerosol sources in the Arctic in winter-spring and the implications for aerosol acidity. Arctic pollution is dominated by transport from mid-latitudes, and we test the relevant ammonia and sulfur dioxide emission inventories in the model by comparison with wet deposition flux data over the source continents. We find that a complicated mix of natural and anthropogenic sources with different vertical signatures is responsible for sulfate concentrations in the Arctic. East Asian pollution influence is weak in winter but becomes important in spring through transport in the free troposphere. European influence is important at all altitudes but never dominant. West Asia (non-Arctic Russia and Kazakhstan) is the largest contributor to Arctic sulfate in surface air in winter, reflecting a southward extension of the Arctic front over that region. Ammonium in Arctic spring mostly originates from anthropogenic sources in East Asia and Europe, with added contribution from boreal fires, resulting in a more neutralized aerosol in the free troposphere than at the surface. The ARCMS and ARCPAC data indicate a median aerosol neutralization fraction [NH4+]/(2[SO42-] + [NO3-]) of 0.5 mol mol(-1) below 2 km and 0.7 mol mol(-1) above. We find that East Asian and European aerosol transported to the Arctic is mostly neutralized, whereas West Asian and North American aerosol is highly acidic. Growth of sulfur emissions in West Asia may be responsible for the observed increase in aerosol acidity at Barrow over the past decade. As global sulfur emissions decline over the next decades, increasing aerosol neutralization in the Arctic is expected, potentially accelerating Arctic warming through indirect radiative forcing and feedbacks. (C) 2011 Elsevier Ltd. All rights reserved.