Absorbing and scattering properties of boundary layer aerosols over Dibrugarh, Northeast India
The absorbing aerosols, primarily black carbon (BC), play a unique and important role in the Earth’s climate system primarily by warming the atmosphere. This warming effect contrasts with the cooling effect of aerosols such as sulphates that are mostly of the scattering type. With a view to studying the characteristics of both absorbing and scattering aerosols within the boundary layer, collocated measurements using an aethalometer and a nephelometer were carried out over Dibrugarh (27.3° N, 94.6° E, 111 m amsl), Northeast India. The diurnal variation of BC mass concentration (M BC) shows a primary peak during late evening (2000–2200 local time (LT)) while a weak secondary peak is observed in the morning (0600–0800 LT). A seasonal shift in diurnal peak M BC was also observed. Both diurnal and seasonal variations in the scattering coefficient (β sca) resemble that of M BC. It may, therefore, be inferred that the majority of both absorbing and scattering types of aerosol prevalent over the study location have common production sources. The seasonal spectral variation in absorption coefficient (β abs) shows monotonic decrease from shorter to longer wavelength in all seasons. The wavelength dependence of absorption by aerosols, as obtained from the absorption Ångström exponent (α abs), indicates a stronger presence of absorbing aerosols originating from biomass burning than those originating from fossil fuel burning over Dibrugarh. The high values of single-scattering albedo (SSA) obtained over Dibrugarh reveal that the scattering type of aerosol is predominant in the ambient air. SSA, together with M BCm, is a useful parameter for estimation of radiative forcing and hence the climatic impact of aerosols.
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Document Type: Research Article
Affiliations: Centre for Atmospheric Studies, Dibrugarh University, Dibrugarh, 786 004, India
Publication date: July 18, 2014