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 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.
Highlights
Heavy fuel oil (HFO), a blend of the residual and distillate fractions of crude oil, is the most widely-used marine-engine fuel globally.[1]Unburnt HFO is toxic; past HFO spills have caused substantial environmental damage that persisted for decades.[1]
The combination of properties we identify for HFO-particulate matter (PM) tar brown carbon (brC) means that the majority of common analytical techniques for quantifying airborne or in-snow light-absorbing carbon (LAC) are substantially biased in its presence
We have demonstrated that tar brC may be produced by biomass burning, and by internal combustion engines operated on HFO
Summary
Heavy fuel oil (HFO), a blend of the residual and distillate fractions of crude oil, is the most widely-used marine-engine fuel globally.[1]Unburnt HFO is toxic; past HFO spills have caused substantial environmental damage that persisted for decades.[1]. Micron-sized char BC has an AAE close of a PM sample after rapid heating to ~4000 K, and laserto 0 (Fig. 1; see SI for details), substantially different from that of soot BC, such that any light-absorption-based measurement of BC
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