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

Angstrom exponent measurements of equivalent black carbon (BC<sub>eq</sub>) have recently been introduced as a novel tool to apportion the contribution of biomass burning sources to the BC<sub>eq</sub> mass. The BC<sub>eq</sub> 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<sub>2.5</sub> aerosol deposited. The BC<sub>eq</sub> 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<sub>abs,</sub> and consequently, the α<sub>abs</sub> 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.