Abstract
Abstract. A long-term study, conducted from February 2005 to July 2008, involving chemical composition and optical properties of ambient aerosols from a high-altitude site (Manora Peak: 29.4° N, 79.5° E, ~1950 m a.s.l.) in the central Himalaya is reported here. The total suspended particulate (TSP) mass concentration varied from 13 to 272 μg m−3 over a span of 42 months. Aerosol optical depth (AOD) and TSP increase significantly during the summer (April–June) due to increase in the concentration of mineral dust associated with the long-range transport from desert regions (from the middle-East and Thar Desert in western India). The seasonal variability in the carbonaceous species (EC, OC) is also significantly pronounced, with lower concentrations during the summer and monsoon (July–August) and relatively high during the post-monsoon (September–November) and winter (December–March). On average, total carbonaceous aerosols (TCA) and water-soluble inorganic species (WSIS) contribute nearly 25 and 10% of the TSP mass, respectively. The WSOC/OC ratios range from 0.36 to 0.83 (average: 0.55 ± 0.15), compared to lower ratios in the Indo-Gangetic Plain (range: 0.35–0.40), and provide evidence for the enhanced contribution from secondary organic aerosols. The mass fraction of absorbing EC ranged from less than a percent (during the summer) to as high as 7.6% (during the winter) and absorption coefficient (babs, at 678 nm) varied between 0.9 to 33.9 Mm−1 (1 Mm−1=10−6 m−1). A significant linear relationship between babs and EC (μgC m−3) yields a slope of 12.2 (± 2.3) m2 g−1, which is used as a measure of the mass absorption efficiency (σabs) of EC.
Highlights
The rapid urbanization and industrial growth in the south and south-east Asia have lead to a substantial increase in the atmospheric abundance of aerosols on a regional scale (Adhikary et al, 2007; Jethva et al, 2005; Rengarajan et al, 2007)
The formation of Atmospheric Brown Cloud (ABC), a thick and grey-brownish haze over the Indo-Gangetic Plain (IGP) and the Indian Ocean during the late NE-monsoon (January–March) has been a subject of significant debate in recent years (Bonasoni et al, 2010; Ramanathan and Carmichael, 2008; Ramanathan et al, 2005; Ramanathan and Ramana, 2005)
Based on the real-time measurements and retrieval of the data from satellites, recent studies have inferred that Black carbon (BC) mass concentration contributes ∼5–10% of the total aerosol optical depth (AOD) over the central Himalaya (Pant et al, 2006; Ramana et al, 2004) and relatively low single scattering albedo (SSA) suggests the absorbing nature of aerosols (Gautam et al, 2009; Hyvarinen et al, 2009; Ramana et al, 2004; Sagar et al, 2004)
Summary
The rapid urbanization and industrial growth in the south and south-east Asia have lead to a substantial increase in the atmospheric abundance of aerosols on a regional scale (Adhikary et al, 2007; Jethva et al, 2005; Rengarajan et al, 2007). The high atmospheric aerosol loading over south-east Asia, persistent throughout the year, can further induce dimming of the solar radiation at the surface and counter the influence of warming caused by greenhouse gases (Carmichael et al, 2009). Black carbon (BC), a major absorbing species in the atmosphere with a warming potential of ∼55% in comparison to that of CO2 (Carmichael et al, 2009), plays a significant role in Earth’s radiation budget, regional climate and hydrological cycle (Jacobson, 2001; Lelieveld et al, 2001; Menon et al, 2002; Ramanathan and Carmichael, 2008). It has been argued that an increase in the atmospheric concentration of BC and its deposition to snow surfaces can decrease the snow-albedo leading
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