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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 22 | Copyright
Atmos. Chem. Phys., 15, 12731-12740, 2015
https://doi.org/10.5194/acp-15-12731-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 17 Nov 2015

Research article | 17 Nov 2015

Direct radiative effect by brown carbon over the Indo-Gangetic Plain

A. Arola1, G. L. Schuster2, M. R. A. Pitkänen1,3, O. Dubovik4, H. Kokkola1, A. V. Lindfors1, T. Mielonen1, T. Raatikainen5, S. Romakkaniemi1, S. N. Tripathi6,7, and H. Lihavainen5 A. Arola et al.
  • 1Finnish Meteorological Institute, Kuopio, Finland
  • 2NASA Langley Research Center, Hampton, VA, USA
  • 3Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
  • 4Laboratoire d'Optique Atmosphérique, Université de Lille1/CNRS, Villeneuve d'Ascq, France
  • 5Finnish Meteorological Institute, Helsinki, Finland
  • 6Department of Civil Engineering, Indian Institute of Technology, Kanpur, India
  • 7Centre for Environmental Science and Engineering, Indian Institute of Technology, Kanpur, India

Abstract. The importance of light-absorbing organic aerosols, often called brown carbon (BrC), has become evident in recent years. However, there have been relatively few measurement-based estimates for the direct radiative effect of BrC so far. In earlier studies, the AErosol RObotic NETwork (AERONET)-measured aerosol absorption optical depth (AAOD) and absorption Angstrom exponent (AAE) were exploited. However, these two pieces of information are clearly not sufficient to separate properly carbonaceous aerosols from dust, while imaginary indices of refraction would contain more and better justified information for this purpose. This is first time that the direct radiative effect (DRE) of BrC is estimated by exploiting the AERONET-retrieved imaginary indices. We estimated it for four sites in the Indo-Gangetic Plain (IGP), Karachi, Lahore, Kanpur and Gandhi College. We found a distinct seasonality, which was generally similar among all the sites, but with slightly different strengths. The monthly warming effect up to 0.5 W m−2 takes place during the spring season. On the other hand, BrC results in an overall cooling effect in the winter season, which can reach levels close to −1 W m−2. We then estimated similarly also the DRE of black carbon and total aerosol, in order to assess the relative significance of the BrC radiative effect in the radiative effects of other components. Even though BrC impact seems minor in this context, we demonstrated that it is not insignificant. Moreover, we demonstrated that it is crucial to perform spectrally resolved radiative transfer calculations to obtain good estimates for the DRE of BrC.

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There have been relatively few measurement-based estimates for the direct radiative effect of brown carbon so far. This is first time that the direct radiative effect of brown carbon is estimated by exploiting the AERONET-retrieved imaginary indices. We estimated it for four sites in the Indo-Gangetic Plain: Karachi, Lahore, Kanpur and Gandhi College.
There have been relatively few measurement-based estimates for the direct radiative effect of...
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