Long-term record of aerosol optical properties and chemical composition from a high-altitude site (Manora Peak) in Central Himalaya

Abstract

Abstract. This MS reports on a long-term study of aerosol optical properties and chemical composition, conducted during February 2005–July 2008, from a high-altitude site (Manora Peak, ~2000 m a.s.l.) in the central Himalaya. The chemical analyses suggest that, on average, total carbonaceous aerosols (TCA) and water-soluble inorganic species (WSIS) contribute nearly 25% and 10% of the total suspended particulate (TSP) mass, respectively. Both, TSP and aerosol optical depth (AOD) exhibit significant increase during summer months, with simultaneous increase in the abundance of mineral dust under the prevailing south-westerly winds and long-range transport from desert regions (from middle-East and Thar Desert in western India). The temporal variability in the abundance pattern of carbonaceous species (EC, OC) is also significantly pronounced, with lower concentrations occurring during summertime (April–June) and monsoon (July–August) and relatively high during post-monsoon (September–November) and wintertime (December–March). The WSOC/OC ratios (range: 0.32 to 0.83) during summer and post-monsoon suggest significant contribution from secondary organic aerosols. The mass fraction of absorbing EC (elemental carbon) ranges from less than a percent (during summer and monsoon) to as high as 7.6% (during winter) and absorption coefficient (babs, at 678 nm) varied as 0.9–33.9 Mm−1 (1 Mm−1=10−6 m−1). The linear regression analysis between (babs and EC concentration (μgC m−3) yields a slope of 12.2(±2.3) m2 g−1, referred as mass absorption efficiency (σabs) of EC. However, temporal data suggests lower σabs values during winter and higher in summer and post-monsoon. The change in the mixing state of aerosols and/or variability in the emission sources could be a plausible reason for the variability in σabs at this high-altitude site (Manora Peak).