Evidence in biomass burning smoke for a light-absorbing aerosol with properties intermediate between brown and black carbon

Abstract

Biomass combustion produces black carbon (BC) and brown carbon (BrC) aerosols that contribute substantially to warming the Earth's atmosphere. Accurate knowledge of their emissions and absorption per unit mass (mass absorption cross-section; MAC) can be used to quantify the radiative impact of these combustion products. We isolated particles generated from laboratory biomass burning fires by morphology and found that some particles from biomass burning do not correspond to either BC or BrC according to common operational definitions. Unlike BrC, these particles strongly absorb red light, with a MAC and spectral dependence of absorption between that of BrC and BC. They also have intermediate volatility: they survive thermodenuding at 250 C but do not heat to incandescence in a single particle soot photometer (SP2) instrument. We also found evidence for intermediate properties in ambient wildfire smoke from the 2013 Rim Fire in California. More work is needed to understand how much this intermediate material contributes to atmospheric light absorption from typical combustion, whether or not it corresponds to "tar balls," and how it may affect previous MAC measurements that were attributed to enhanced absorption by transparent coatings.

Optical and Chemical Analysis of Absorption Enhancement by Mixed Carbonaceous Aerosols in the 2019 Woodbury, AZ, Fire Plume

“…In addition, our MAC (870 nm) in the fire plume was more than an order of magnitude lower than the MAC Org (661 nm) of 0.9–2.0 m 2 /g observed by Adler et al (2019) for laboratory burns. This underscores the need for more field observations to quantify absorption efficiency of OA at red wavelengths to resolve the issue raised by Adler et al (2019).…”

Detection of tar brown carbon with a single particle soot photometer (SP2)

“…The lower values in this range are similar to those of the soluble brC emitted by biomass combustion (Kumar et al, 2018;Li et al, 2019). The higher values in this range correspond to absorption efficiencies comparable to those of soot BC (Alexander et al, 2008;. The broad range reflects a broad range of molecular structures, where larger, more conjugated structures would correspond to more efficient absorption.…”

“…Tar brC emitted from biomass burning may contain impurities of K, Cl, Si, and S (Pósfai et al, 2004;Adachi et al, 2019). Atmospheric tar brC spheres have also been reported which contained impurities of S and Si but not K (Alexander et al, 2008;Zhu et al, 2013), which has been proposed as indicating an origin of residual-fuel (heavy fuel oil) combustion .…”

“…In terms of their optical properties, tarballs may be considered a subset of brC, as their imaginary refractive index decreases with increasing wavelength (Alexander et al, 2008;, which results in a brown appearance at appropriate concentrations (Liu et al, 2016). However, light absorption by tar remains significant even in the near-infrared (about 900 nm wavelength) (Alexander et al, 2008;Hoffer et al, 2016a;. This light absorption has been described by the Tauc band-gap model , which is also applicable to soluble brC (Sun et al, 2007) and which predicts a slow tailing off of absorption with increasing wavelength.…”

“…Atmospheric light-absorbing carbon (LAC) in particulate matter (PM) plays a substantial role in the radiative balance of the earth both directly and by influencing cloud properties (Boucher et al, 2013). While soot black carbon (soot BC) is the best-recognized form of LAC (Bond et al, 2013), increasing attention has recently been paid to the so-called "brown carbon" (Kirchstetter et al, 2004;Laskin et al, 2015) and "tarballs" (Pósfai et al, 2004;Hand et al, 2005;Niemi et al, 2006;Semeniuk et al, 2006;Tivanski et al, 2007;Alexander et al, 2008;Vernooij et al, 2009;Chakrabarty et al, 2010;Adachi and Buseck, 2011;China et al, 2013;Zhu et al, 2013;Tóth et al, 2014;Hoffer et al, 2016a, b;Sedlacek III et al, 2018; which possess substantially different physical properties than BC. The term brown carbon is canonically used to refer to the collection of substantially light-absorbing organic molecules found in PM, while the term tarballs refers to the insoluble amorphouscarbon spheres which may be produced by the pyrolysis of high-molecular-weight fuels such as biomass (Tóth et al, 2014) or heavy fuel oil .…”