Atmospheric abundances of primary and secondary carbonaceous species at two high-altitude sites in India: sources and temporal variability

Abstract

Based on the time-series analyses of bulk-aerosol samples, we report on the large-scale temporal variability in the atmospheric abundances of elemental carbon (EC) and organic carbon (OC) at two high-altitude sites, Manora Peak (1950 m asl in north India) and Mt. Abu (1680 m asl in western India). The total suspended particulate (TSP) mass concentration in the ambient atmosphere also exhibits large seasonal variability at both the sites; varying from 13.4 to 432.3 µg m^-3 at Mt. Abu and 12.7 to 271.7 µg m^-3 at Manora Peak. The relatively high abundance of TSP, occurring during Apr-Jun, is associated with enhanced contribution from mineral dust. Both, OC and EC abundances at Manora Peak are nearly 2-3 times higher than those at Mt. Abu; the minimum concentrations occurring during the high-dust season (Apr-Jun) and monsoon season (Jul-Aug) and maximum in winter months (Dec-Mar). At Mt. Abu, annual-average abundances of OC (range: 0.9-12.3 µg m^-3; Av = 3.7 µg m^-3) and EC (range: 0.06-2.3 µg m^-3; Av = 0.5 µg m^-3) account for about 10 and 2% of the TSP, respectively. In contrast, annual-average concentrations of OC and EC at Manora Peak are 8.7 µg m-3 (range: 2.0-22.3 µg m^-3) and 1.1 µg m^-3 (range: 0.14-2.7 µg m^-3), respectively; and account for about 14 and 2% of the TSP. The OC/EC ratios at the two sites (Manora Peak, range: 4.8-14.9 and Mt. Abu, range: 3.0-11.5) are significantly higher compared to those reported in the literature (2.0-3.0) for the urban regions. The high OC/EC ratios and low EC concentrations are attributed to relative dominance of organic carbon derived from biomass burning (crop waste). The average contribution of total carbonaceous aerosols (TCA; TCA = 1.6 × OC + EC) to TSP is ~24% at Manora Peak and ~16% at Mt. Abu is only 15%. The relatively high contribution of TCA, at Manora Peak, is influenced by the regional emission sources in north India. The contribution of secondary organic carbon (SOC) to OC, calculated based on minimum OC/EC ratio method, averages around 27% at Manora Peak and 16% at Mt. Abu; and brings to focus its significant role on a regional scale. The low EC concentration together with significant contribution of OC and SOC to TCA and their temporal variability suggests reassessment of relative amounts of absorbing (BC) and scattering (OC) species used in the radiative forcing models on a regional scale.