A Climate-Forcing Perspective on Black Carbon Aging (Invited)

Black carbon is a strong climate warmer, and this fact has generated interest in understanding its chemistry and physics. The set of processes commonly known as "aging" describes the transformation of these particles from the time they are emitted to the time they are removed from the atmosphere. This conversion is a profound one. The particles begin as fluffy aggregates with low oxygen content that refuse to interact with water and have very low single-scattering albedo (about 0.25). By the end of their lifetimes, the particles are hydrophilic enough for rainout. Their absorption, scattering, and single-scattering albedo increase. Removal and optical properties are major determinants of climate forcing. Vapor deposition, coagulation, and oxidation contribute to black carbon aging. Laboratory and field studies are assessing these important microphysical processes. We address the questions: Can we ever have enough knowledge to limit uncertainties in modeled climate forcing? If so, do we already have it? The transitions from hydrophobic to hydrophilic and from less-absorbing to more-absorbing appear to be threshold changes, rather than infinite continua. This parallels the parameterization of even simple global models, and can be adequately represented by detailed microphysical schemes, so that there is some hope for modeled forcing. The "specific forcing contribution," in GJ/g emitted, is the energy added to the Earth system by a packet of emissions. We briefly demonstrate the sensitivity of this measure in different world regions to these critical inputs. Then, we discuss how well these models can be constrained based on what's known from laboratory and field measurements.