Mass absorption efficiency of elemental carbon (EC) in ambient aerosols: Characterization from high-altitude sites in India

The mass absorption efficiency (MAE, σabs) is an important parameter for the determination of atmospheric black carbon (BC) mass concentration by optical instruments. Recent studies suggest that σabs is highly variable in space and time depending on the type of emission sources of carbonaceous aerosols and analytical methods employed. We have studied changes in σabs of elemental carbon (EC) from two high-altitude sites in India; Mt Abu (coordinates; 1700 m asl): located in the Thar desert of western India (dominated by mineral dust) and Manora Peak (coordinates; 2200 m asl): located in the southern slopes of Himalaya (influenced by anthropogenic sources in the Indo-Gangetic Plain. Based on the data set collected during 2005-2006, we document large temporal variability in σabs of EC (range: 3.6-13.7 m2g-1) at Mt Abu. The average σabs (9.8±2.1 m2g-1) is similar to the most accepted value of 10 m2g-1 for ambient aerosols. Analytical data suggest that mineral dust contributes ~70% of the total suspended particulate (TSP) mass. Assuming it to be externally mixed with carbonaceous species and its absorption efficiency of 0.009 m2g-1, absorption due to mineral dust is no more than ~10% at Mt Abu. In contrast, long-term (2005-2008) average of 12.8±2.9 m2g-1 for σabs (range: 6.1-19.1 m2g-1) is documented at Manora Peak. We suggest that relatively high σabs of EC at Manora Peak is attributed to the internal mixing of carbonaceous aerosols and anthropogenic species (NO3- and SO4--) during transport from the Indo-Gangetic Plain. These results have implications to large seasonal variability in the aerosol absorption properties (due to change in the morphological feature of EC) and atmospheric radiative forcing on a regional scale.