Relative importance of black carbon, brown carbon, and absorption enhancement from clear coatings in biomass burning emissions

Rudra P. Pokhrel, Eric R. Beamesderfer, Nick L. Wagner, Justin M. Langridge, Daniel A. Lack, Thilina Jayarathne, Elizabeth A. Stone, Chelsea E. Stockwell, Robert J. Yokelson, Shane M. Murphy
2017 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> 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<span class="thinspace"></span>nm and thermally denuded (250<span class="thinspace"></span>°C) absorption at 405 and 660<span class="thinspace"></span>nm. Absorption coefficients were broken into contributions from black carbon (BC), brown
more » ... arbon (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<span class="thinspace"></span>% of total aerosol absorption at 405<span class="thinspace"></span>nm and up to 58<span class="thinspace"></span>% of total absorption at 532<span class="thinspace"></span>nm. Lensing is estimated to contribute a maximum of 30<span class="thinspace"></span>% 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<span class="thinspace"></span>∕<span class="thinspace"></span>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<span class="thinspace"></span>∕<span class="thinspace"></span>OC ratio decreases.</p>
doi:10.5194/acp-17-5063-2017 fatcat:xk7s6aplancatnwrwv5fzrdioa