Absorption coefficient and site-specific mass absorption efficiency of elemental carbon in aerosols over urban, rural, and high-altitude sites in India

Abstract

Temporal and spatial variability in the absorption coefficient (b_abs, Mm^-1) and mass absorption efficiency (MAE, σ_abs, m²g^-1) of elemental carbon (EC) in atmospheric aerosols studied from urban, rural, and high-altitude sites is reported here. Ambient aerosols, collected on tissuquartz filters, are analyzed for EC mass concentration using thermo-optical EC-OC analyzer, wherein simultaneously measured optical-attenuation (ATN, equivalent to initial transmittance) of 678 nm laser source has been used for the determination of MAE and absorption coefficient. At high-altitude sites, measured ATN and surface EC loading (EC_s, μg cm^-2) on the filters exhibit linear positive relationship (R² = 0.86-0.96), suggesting EC as a principal absorbing component. However, relatively large scatter in regression analyses for the data from urban sites suggests contribution from other species. The representative MAE of EC, during wintertime (Dec 2004), at a rural site (Jaduguda) is 6.1 ± 2.0 m²g^-1. In contrast, MAE at the two high-altitude sites is 14.5 ± 1.1 (Manora Peak) and 10.4 ± 1.4 (Mt. Abu); and that at urban sites is 11.1 ± 2.6 (Allahabad) and 11.3 ± 2.2 m²g^-1 (Hisar). The long-term average MAE at Manora Peak (February 2005 to June 2007) is 12.8 ± 2.9 m²g^-1 (range: 6.1-19.1 m²g^-1). These results are unlike the constant conversion factor used for MAE in optical instruments for the determination of BC mass concentration. The absorption coefficient also shows large spatiotemporal variability; the lower values are typical of the high-altitude sites and higher values for the urban and rural atmosphere. Such large variability documented for the absorption parameters suggests the need for their suitable parametrization in the assessment of direct aerosol radiative forcing on a regional scale.