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

Atmos. Chem. Phys., 15, 12731–12740, 2015 www.atmos-chem-phys.net/15/12731/2015/ doi:10.5194/acp-15-12731-2015 © Author(s) 2015. CC Attribution 3.0 License. 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 1 Finnish Meteorological Institute, Kuopio, Finland 2 NASA Langley Research Center, Hampton, VA, USA 3 Department of Applied Physics, University of Eastern Finland, Kuopio, Finland 4 Laboratoire d’Optique Atmosphérique, Université de Lille1/CNRS, Villeneuve d’Ascq, France 5 Finnish Meteorological Institute, Helsinki, Finland 6 Department of Civil Engineering, Indian Institute of Technology, Kanpur, India 7 Centre for Environmental Science and Engineering, Indian Institute of Technology, Kanpur, India Correspondence to: A. Arola () Received: 1 June 2015 – Published in Atmos. Chem. Phys. Discuss.: 10 August 2015 Revised: 22 October 2015 – Accepted: 26 October 2015 – Published: 17 November 2015 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 AERONETretrieved 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. 1 Introduction Aerosols affect the Earth’s climate both directly (by scattering and absorbing radiation) and indirectly (by serving as nuclei for cloud droplets). Currently, aerosol forcing is the largest uncertainty in assessing the anthropogenic climate change (Myhre, 2013). Specifically, the role of carbonaceous aerosols is poorly understood. These particles can be divided into two categories: (1) black carbon (BC) is the main absorbing component present in atmospheric aerosols; and (2) organic carbon (OC) represents a significant and sometimes major (20–90 %) mass fraction of the sub-micron aerosol (Kanakidou et al., 2005; Zhang et al., 2007). Organic carbon has been most often assumed, in global models for instance, to be a non-absorbing or only slightly absorbing component. However, there is growing evidence that a substantial amount of organic aerosols absorb at UV and visible wavelengths, particularly strongly at shorter wavelengths (e.g., Kirchstetter et al., 2004; Martins et al., 2009). Nevertheless, so far there have been only relatively few measurementbased estimates for the direct radiative effect (DRE) of absorbing organic carbon, often called brown carbon, BrC. Published by Copernicus Publications on behalf of the European Geosciences Union. 12732 Both Chung et al. (2012) and Feng et al. (2013) exploited AErosol RObotic NETwork (AERONET) measurements to derive the radiative effect by BrC; the former used an approach to separate dust and carbonaceous aerosols based on the AERONET-measured absorption Angstrom exponent (AAE), while the latter accounted for shortwave enhanced absorption by BrC in their global model and demonstrated an improved correspondence of modeled aerosol absorption optical depth (AAOD) and AERONET measurements, when BrC absorption was included in the model. The approach of Chung et al. (2012) has evident difficulties in separating dust and carbonaceous aerosols by using an AAE, and arguably an approach using AERONET-retrieved imaginary indices of refraction would be more justified, as discussed also in Schuster et al. (2015a, b). We estimated the BrC fractions by using the method of Schuster et al. (2015a) for four AERONET sites in the Indo-Gangetic Plain (IGP), Karachi, Lahore, Kanpur and Gandhi College, and then calculated the corresponding radiative effect by BrC. We moreover calculated similarly the DRE of BC and total aerosol, in order to assess the relative significance of the BrC radiative effect in carbonaceous or total aerosol radiative effects. 2 Data and methods 2.1 AERONET data AERONET (AErosol RObotic NETwork) is a globally distributed network of automatic Sun and sky scanning radiometers that measure at several wavelengths, typically centered at 0.34, 0.38, 0.44, 0.50, 0.67, 0.87, 0.94, and 1.02 µm. The AERONET UV filters (340 and 380 nm) have a full width at half maximum (FWHM) of 2 nm as compared to 10 nm for all other channels. All of these spectral bands are utilized in the direct Sun measurements, while four of them are also used for the sky radiance measurements, 0.44, 0.67, 0.87 and 1.02 µm. Spectral aerosol optical depth (AOD) is obtained from direct Sun measurements, and inversion products of other aerosol optical properties, such as single scattering albedo (SSA), refractive indices and the columnintegrated aerosol size distributions above the measurement site, are provided at the sky radiance wavelengths (Holben et al., 1998). The estimated uncertainty in AOD (Level 2) is 0.01–0.02 and is primarily due to the calibration uncertainty (Eck et al., 1999). The uncertainty in the complex index of refraction depends on AOD; Dubovik et al. (2000) estimated errors on the order of 30–50 % for the imaginary part and 0.04 for the real part of the refractive index for the cases of high aerosol loading (AOD at 440 nm larger than 0.5). Aerosol loading is very high in the IGP region; therefore, these uncertainty estimates are likely representative for our AERONET sites as well. Atmos. Chem. Phys., 15, 12731–12740, 2015 A. Arola et al.: BrC radiative effect over the IGP Since the shortest sky radiance wavelength is 440 nm, AERONET wavelengths are not ideal for detecting BrC absorption, which is (...truncated)