Limitation of the Use of the Absorption Angstrom Exponent for Source Apportionment of Equivalent Black Carbon: a Case Study from the North West Indo-Gangetic Plain Saryu Garg, † Boggarapu Praphulla Chandra, † Vinayak Sinha, † Roland Sarda-Esteve, ‡ Valerie Gros, ‡ and Baerbel Sinha* ,† † Department of Earth and Environmental Sciences, Indian Institute of Science Education and Research Mohali, Sector 81, S.A.S Nagar, Punjab 140306, India ‡ LSCE, Laboratoire des Sciences du Climat et de l’Environnement, CNRS-CEA-UVSQ, Orme des Merisiers, F91191 Gif-sur-Yvette, France * S Supporting Information ABSTRACT: Angstrom exponent measurements of equivalent black carbon (BC eq ) have recently been introduced as a novel tool to apportion the contribution of biomass burning sources to the BC eq mass. The BC eq is the mass of ideal BC with defined optical properties that, upon deposition on the aethalometer filter tape, would cause equal optical attenuation of light to the actual PM 2.5 aerosol deposited. The BC eq mass hence is identical to the mass of the total light-absorbing carbon deposited on the filter tape. Here, we use simultaneously collected data from a seven-wavelength aethalometer and a high-sensitivity proton-transfer reaction mass spectrometer installed at a suburban site in Mohali (Punjab), India, to identify a number of biomass combustion plumes. The identified types of biomass combustion include paddy- and wheat- residue burning, leaf litter, and garbage burning. Traffic plumes were selected for comparison. We find that the combustion efficiency, rather than the fuel used, determines α abs, and consequently, the α abs can be ∼1 for flaming biomass combustion and >1 for older vehicles that operate with poorly optimized engines. Thus, the absorption angstrom exponent is not representative of the fuel used and, therefore, cannot be used as a generic tracer to constrain source contributions. ■ INTRODUCTION Black carbon (BC) consists of optically absorbing particles produced from incomplete combustion. 1,2 The presence of fine particulate matter, such as black carbon, in the atmosphere impairs visibility 1,2 and human health. 3,4 BC also alters the radiative budget of Earth through direct, semidirect, and indirect effects. 2,5,6 The direct radiative forcing term of BC includes the effects of atmospheric warming, 2,5−7 decrease in the albedo of ice cover due to black carbon deposited on ice, 8 and surface dimming due to scattering and absorption of incoming solar radiation. 2 Semidirect forcing effects include the impact of cloud burning and BC-induced perturbations of the atmospheric temperature structures on cloud cover. 9 Indirect effects comprise the modification of cloud properties and cloud cover due to perturbation of the number density, size distribution, and chemical properties of the cloud condensation nuclei population. 2 Recently, black carbon has attracted a lot of attention because it combines two interesting properties. First, it is the second most important climate warming agent after carbon dioxide, with a total climate forcing of +1.1 Wm 2− (a range of 0.17−2.1 W m −2 ) 2,6 and second, it has a short atmospheric lifetime of days to weeks. 1,2,10 Identifying black-carbon sources for targeted mitigation has the potential to offset the CO 2 - induced warming in the near and intermediate future because it is a short-lived climate pollutant (SLCP). The current black-carbon emission inventories are highly uncertain, with a range of 2000−29 000 Gg BC emissions per year. 2 The largest uncertainties pertain to domestic biofuel use, open burning (which includes both wild vegetation fires and anthropogenic emissions such as crop residue burning), and industrial coal use (in particular, consumption by small-scale cottage industries in developing economies). 2 The contribution of open waste burning of domestic and industrial waste has been largely neglected in the black-carbon emission inventories due to lack of activity data. 2 Received: August 10, 2015 Revised: December 6, 2015 Accepted: December 11, 2015 Published: December 11, 2015 Article pubs.acs.org/est © 2015 American Chemical Society 814 DOI: 10.1021/acs.est.5b03868 Environ. Sci. Technol. 2016, 50, 814−824