Characteristics of aerosol black carbon mass concentration over a high altitude location in the central Himalayas from multi-year measurements

Multi-year measurements of near surface aerosol black carbon (BC) mass concentration, made from a high altitude station at Manora Peak (29.4° N, 79.5° E, 1958 m msl) in the Central Himalayas, using a 7-channel Aethalometer for 38 months from November 2004 to December 2007, are examined. Temporally, BC exhibited well-defined diurnal variations, comprising of a single prominent peak occurring in the late afternoon (before sunset) hours during the months from October to March, while these variations were insignificant during April to September. These were found to be closely associated with the dynamics of atmospheric boundary layer (ABL), and pose a form that is distinctly different from those reported for the plains. BC mass concentrations were always higher by a factor of about 2 during daytime than the concentrations during nighttime. Seasonally, BC mass concentrations were higher during spring (with a mean value of 1.34 ± 0.05 μg m-3), which are attributed to lifting up of pollutants from the valley (below the mountain peak) by the convective boundary layer and increased local emissions. The concentration decreased by a factor of 2 in summer (0.53 ± 0.02) and recovered during autumn (1.03 ± 0.04). The long-term average value was 0.99 ± 0.02 μg m-3. Examination of the wavelength dependence revealed that BC observed at this location is generally dominated by fossil fuel combustion