A wide range of globally significant biomass fuels wereburned during the fourth Fire Lab at Missoula Experiment (FLAME-4). Amulti-channel photoacoustic absorption spectrometer (PAS) measured dryabsorption at 405, 532, and 660 nm and thermally denuded (250 °C)absorption at 405 and 660 nm. Absorption coefficients were broken intocontributions from black carbon (BC), brown carbon (BrC), and lensingfollowing three different methodologies, with one extreme being a methodthat assumes the thermal denuder effectively removes organics and the otherextreme being a method based on the assumption that black carbon (BC) has anÅngström exponent of unity. The methodologies employed provide ranges ofpotential importance of BrC to absorption but, on average, there was a difference of a factor of 2 in the ratio of the fraction of absorptionattributable to BrC estimated by the two methods. BrC absorption atshorter visible wavelengths is of equal or greater importance to that of BC,with maximum contributions of up to 92 % of total aerosol absorption at405 nm and up to 58 % of total absorption at 532 nm. Lensing is estimatedto contribute a maximum of 30 % of total absorption, but typicallycontributes much less than this. Absorption enhancements and the estimatedfraction of absorption from BrC show good correlation with the elemental-carbon-to-organic-carbon ratio (EC ∕ OC) of emitted aerosols and weaker correlation withthe modified combustion efficiency (MCE). Previous studies have shown thatBrC grows darker (larger imaginary refractive index) as the ratioof black to organic aerosol (OA) mass increases. This study is consistentwith those findings but also demonstrates that the fraction of totalabsorption attributable to BrC shows the opposite trend: increasing as theorganic fraction of aerosol emissions increases and the EC ∕ OC ratio decreases.
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