Infrared-absorbing carbonaceous tar can dominate light absorption by marine-engine exhaust

Abstract

Ship engines in the open ocean and Arctic typically combust heavy fuel oil (HFO), resulting in light-absorbing particulate matter (PM) emissions that have been attributed to black carbon (BC) and conventional, soluble brown carbon (brC). We show here that neither BC nor soluble brC is the major light-absorbing carbon (LAC) species in HFO-combustion PM. Instead, "tar brC" dominates. This tar brC, previously identified only in open-biomass-burning emissions, shares key defining properties with BC: it is insoluble, refractory, and substantially absorbs visible and near-infrared light. Relative to BC, tar brC has a higher Angstrom absorption exponent (AAE) (2.5-6, depending on the considered wavelengths), a moderately-high mass absorption efficiency (up to 50% of that of BC), and a lower ratio of sp 2-to sp 3-bonded carbon. Based on our results, we present a refined classification of atmospheric LAC into two sub-types of BC and two sub-types of brC. We apply this refined classification to demonstrate that common analytical techniques for BC must be interpreted with care when applied to tar-containing aerosols. The global significance of our results is indicated by field observations which suggest that tar brC already contributes to Arctic snow darkening, an effect which may be magnified over upcoming decades as Arctic shipping continues to intensify.

A Decadal Climatology of Chemical, Physical, and Optical Properties of Ambient Smoke in the Western and Southeastern United States

“…We also calculate AAE smoke for 440/675 nm, 675/870 nm, and 870/1,020 nm for the three regions in Table S1. AAE smoke increases from shorter wavelength pairs to longer wavelength pairs in each region, showing a trend in AAE similar to the trend reported for tar brown carbon in Corbin et al (2019). Figure 5 shows our estimates for the normalized smoke size distribution for each region using the differences between the mean volume size distributions for smoky and nonsmoky days, normalizing each size bin by the total volume concentration ( Figure S9 shows the average volume size distributions on the smoky and nonsmoky days in the three regions that are used to create Figure 5).…”

“…Our AAE smoke is 1.74, 1.55, and 1.89 in the PW, SW, and SE, respectively. AAE smoke increases from shorter wavelength pairs to longer wavelength pairs in each region, showing a trend in AAE similar to the trend reported for tar brown carbon in Corbin et al (2019). 74-6.98 (McMeeking et al, 2014), while measurements from FLAME-4 had a wider range of values across burns (0.85-4 for most fuels, with peat and chamise burning ranging from 4-10; Pokhrel et al, 2016).…”

Variability in the mass absorption cross-section of black carbon (BC) aerosols is driven by BC internal mixing state at a central European background site (Melpitz, Germany) in winter

“…BC absorbs light broadly from near-UV to near-infrared wavelengths and thus has a weak spectral dependence (AAE ≈ 1; van de Hulst, 1957). By contrast, light absorbing organics (Corbin et al, 2019), can exhibit substantial light absorption at near-UV and blue wavelengths while being negligible at red to near-UV wavelengths, which normally results in AAE larger than 1.…”

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

“…3). It may be speculated that these plateaus reflect the breakup of tar particles into more and less absorbing parts, with the more absorbing part being completely evaporated and the less absorbing part passing through the laser unchanged, in analogy to the particle breakup that is observed for heavily coated rBC (Sedlacek et al, 2012;Dahlkötter et al, 2014;Moteki et al, 2014).…”

“…4b rules out the hypothesis that this region reflects extremely thickly coated soot. While soot coatings ideally undergo complete evaporation in the SP2 prior to incandescence, extremely thick coatings may result in particle breakup (Moteki and Kondo, 2007;Sedlacek et al, 2012;Dahlkötter et al, 2014). That is, the coating may fragment and generate a secondary particle large enough to generate a scattering signal in the SP2.…”