Angstrom exponent measurements of equivalent black carbon (BCeq) have recently been introduced as a novel tool to apportion the contribution of biomass burning sources to the BCeq mass. The BCeq 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, as the actual PM2.5 aerosol deposited. The BCeq 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 7-wavelength aethalometer and a high sensitivity Proton Transfer Reaction Mass Spectrometer installed at a sub-urban 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 αabs and consequently the αabs 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.