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

Abstract

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.

Long-term measurements (2010–2014) of carbonaceous aerosol and carbon monoxide at the Zotino Tall Tower Observatory (ZOTTO) in central Siberia

“…From prescribed burns conducted in boreal forests (northern Ontario, Canada), Mazurek et al (1991) reported a range of 10-18 and 21-95 for full-flaming and smoldering fire conditions, respectively. A chamber experiment conducted with typical Siberian biomass (pine) showed that the OC / EC ratios for PM 10 in fresh (aged) smoke for flaming and smoldering are 0.3 (2.3) and 181 (126), respectively (Popovicheva et al, 2015). In addition, the OC / EC ratios for fresh forest debris smoke were found to be 0.6 and 35 in flaming and smoldering fires, respectively.…”

“…In addition, due to the lensing effect, organic coatings can substantially modify the optical properties of EC in mixed particles. Thus, laboratory experiments have shown that the light absorption enhancement factor for internal mixtures of EC with organic and inorganic coatings can range from 1.3 to 3.5 (Schnaiter et al, 2005;Mikhailov et al, 2006;Zhang et al, 2008;Shiraiwa et al, 2010;Pokhrel et al, 2017).…”

Exploring the observational constraints on the simulation of brown carbon

“…Previous modeling studies have typically used either the lower or higher bound from laboratory studies to estimate the minimum or maximum absorption properties of BrC (Feng et al, 2013;Lin et al, 2014). In addition, it is unclear what fraction of the OA is BrC and how this differs with source and ambient combustion conditions (Pokhrel et al, 2017). In laboratory studies organics are not always fully soluble; typically, 40-90 % of the total material can be extracted, depending on the solvent (e.g., ∼ 40 % can be extracted in water and more than 90 % can be extracted in methanol; Chen and Bond, 2010).…”

Aerosol optical properties and trace gas emissions by PAX and OP-FTIR for laboratory-simulated western US wildfires during FIREX

“…Thus, the B abs contribution from BC at 401 nm can be calculated from 2.17 times B abs at 870, and any additional B abs at 401 can be assigned to BrC subject to limitations due to "lensing" by coatings discussed elsewhere (Pokhrel et al, 2016;Lack and Langridge (2013)). Pokhrel et al (2017) found that coatings typically accounted for much less than 30% of absorption in room burn smoke 1-2 hours old and coatings are likely much less important in 5 s old stack burn smoke (Akagi et al, 2012).…”

“…Coating effects are very difficult to deconvolve from BrC effects even with additional instruments that were not available during 20 the stack burns (Pokhrel et al, 2017). This adds some uncertainty to the BrC attribution, but not to the absorption measurements themselves.…”