Extremely low-volatility organic coating leads to underestimation of black carbon climate impact

Black carbon (BC) aerosols play an important role in climate systems. Estimating BC’s radiative effect requires knowledge of its mixing state and light absorption enhancement resulting from coatings by other materials. Observational studies have reported much lower light absorption enhancement factors (Eabs) than expected from laboratory studies and model estimates. This has led to an intensive debate on the true magnitude of BC’s climate impact. We find that the observed, apparently small Eabs cannot reflect the overall effect of BC coatings, as it does not include the persisting absorption enhancement by extremely low-volatility (eLV) organics that remain on the BC particles after passing through thermodenuders. Our observations show that eLV organics are extensively present in BC coatings, with a mass fraction of ∼5%–100%. Accounting for these eLV coatings, the observation estimates of Eabs increase substantially to up to 1.8–2.0. Our results highlight a strong radiative warming effect from atmospheric BC.