10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Pinning Down the Highly Variable Light-absorption Properties of Brown Carbon
RAWAD SALEH, Zezhen Cheng, Khairallah Atwi, University of Georgia
Abstract Number: 367 Working Group: Carbonaceous Aerosol
Abstract Combustion of biomass fuels contributes a significant portion of atmospheric brown carbon (BrC), the light-absorbing fraction of organic aerosols. BrC exhibits highly variable light-absorption properties, with imaginary part of the refractive indices (k) reported in the literature varying over two orders of magnitude. There is a major gap in the understanding of this variability, posing a challenge to accurately representing brown carbon in radiative-transfer calculations.
Here, we present a framework that allows us to pin down the variability in BrC light-absorption properties. We hypothesize that BrC is comprised of black carbon (BC) precursors whose transformation to BC has not seen fruition during combustion. Depending on the combustion conditions, these BC precursors exhibit different maturity levels which dictate their light-absorption properties (k). The more mature are the precursors, the more absorptive (or BC-like) they are. This explains the aforementioned variability in k values reported in the literature: due to the chaotic nature of BrC-producing combustion, different measurements reported in the literature feature widely varying combustion conditions leading to different retrieved k values. More importantly, our hypothesis entails that BrC and BC lie on the same “optical continuum,” and carbonaceous combustion products can be represented as distributions along this continuum.
To validate this hypothesis, we performed controlled combustion experiments in which the combustion conditions (temperature and air/fuel ratio) were varied and the wavelength-dependent k was retrieved from real-time multi-wavelength light-absorption measurements at each condition. We used benzene and toluene, the inception of which during combustion marks the initial critical steps leading to BC formation, as model fuels. By varying the combustion conditions of these two simple molecules, we isolated BrC components with k values spanning the range of values reported in the literature for biomass-burning BrC, a clear evidence for the importance of combustion conditions in dictating BrC light-absorption properties. Specifically, the values we obtained for k at 550 nm (k550) and its wavelength-dependence (w) ranged from k550 = 0.005 and w = 8 to k550 = 0.3 and w = 0.4. For reference, k550 and w of BC reported in the literature are approximately 0.7 and 0, respectively.
Our results unveil a continuum of optical bins defined by k550 and w pairs that progress from lighter BrC (small k550 and large w) to darker BrC (large k550 and small w) to BC. We stress that the darkest BrC we could isolate is optically more similar to BC than the lighter BrC components. This blurs the line between BrC and BC and challenges the wide-spread perception that BrC is negligibly absorbing in the mid- and long-visible wavelengths compared to BC and that its contribution to atmospheric radiative transfer is only important in the short visible and near-UV wavelengths.