1Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming, USA
2NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA
3Observation Based Research, Met Office, Fitzroy Road, Exeter, EX1 3PB, UK
4Transport Emissions, Air Quality and Climate Consulting, Brisbane, Australia
5Department of Chemistry, University of Iowa, Iowa City, Iowa, USA
6Department of Chemistry, University of Montana, Missoula, Montana, USA
anow at: School of Geography and Earth Science, McMaster University, Hamilton, Ontario, Canada
Received: 12 Nov 2016 – Discussion started: 18 Nov 2016
Abstract. A wide range of globally significant biomass fuels were burned during the fourth Fire Lab at Missoula Experiment (FLAME-4). A multi-channel photoacoustic absorption spectrometer (PAS) measured dry absorption at 405, 532, and 660 nm and thermally denuded (250 °C) absorption at 405 and 660 nm. Absorption coefficients were broken into contributions from black carbon (BC), brown carbon (BrC), and lensing following three different methodologies, with one extreme being a method that assumes the thermal denuder effectively removes organics and the other extreme being a method based on the assumption that black carbon (BC) has an Ångström exponent of unity. The methodologies employed provide ranges of potential importance of BrC to absorption but, on average, there was a difference of a factor of 2 in the ratio of the fraction of absorption attributable to BrC estimated by the two methods. BrC absorption at shorter visible wavelengths is of equal or greater importance to that of BC, with maximum contributions of up to 92 % of total aerosol absorption at 405 nm and up to 58 % of total absorption at 532 nm. Lensing is estimated to contribute a maximum of 30 % of total absorption, but typically contributes much less than this. Absorption enhancements and the estimated fraction of absorption from BrC show good correlation with the elemental-carbon-to-organic-carbon ratio (EC ∕ OC) of emitted aerosols and weaker correlation with the modified combustion efficiency (MCE). Previous studies have shown that BrC grows darker (larger imaginary refractive index) as the ratio of black to organic aerosol (OA) mass increases. This study is consistent with those findings but also demonstrates that the fraction of total absorption attributable to BrC shows the opposite trend: increasing as the organic fraction of aerosol emissions increases and the EC ∕ OC ratio decreases.
Revised: 25 Feb 2017 – Accepted: 22 Mar 2017 – Published: 19 Apr 2017
Pokhrel, R. P., Beamesderfer, E. R., Wagner, N. L., Langridge, J. M., Lack, D. A., Jayarathne, T., Stone, E. A., Stockwell, C. E., Yokelson, R. J., and Murphy, S. M.: Relative importance of black carbon, brown carbon, and absorption enhancement from clear coatings in biomass burning emissions, Atmos. Chem. Phys., 17, 5063-5078, doi:10.5194/acp-17-5063-2017, 2017.