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

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 sp2- to sp3-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. Ship emissions are dominated by a different type of particle than once thought. Ships typically use heavy fuel oil for combustion, releasing small light-absorbing particles such as black and brown carbon (brC). However, the properties and composition of such ship-related carbonaceous emissions is not well-known, complicating the quantification of their climatic impacts. Using various analytical techniques, Joel Corbin from the Paul Scherrer Institute, Switzerland and National Research Council Canada, and colleagues, demonstrate that light-absorbing particulates from heavy fuel oil are dominated by tar brC, an insoluble particulate with strong absorbance in the near infrared, previously associated only with biomass burning. This newly recognized source of tar brC motivates the identification and quantification of tar particulates in other environments affected by ship emissions, and necessitates a refined classification of light-absorbing particulates to avoid misinterpreting their source.