Grantee Research Project Results

Black carbon (BC) particles play a significant role in climate forcing, yet the effects of aging – atmospheric processes that affect the mass, size, shape, and chemical composition of aerosol particles – on their radiative properties are poorly constrained. This combined laboratory and modeling study will provide new insights into the detailed effects of atmospheric aging on the climate impacts of black carbon particles.

In Year 2 of this project we continued our work on BC heterogeneous aging, examining in detail the kinetics of such processes, plus the changes to chemical composition and water-uptake properties of the particles upon aging. Results indicate that heterogeneous oxidation is a rapid enough process to compete with other aging mechanisms (condensation, coagulation), but also that changes to water-uptake properties are likely dominated by condensation processes. We have also carried out the first study of the aging of BrC particles, finding that their chemical composition changes dramatically after the equivalent of a few days of oxidation, and more importantly that oxidation appears to degrade their light-absorbing properties. Additional experimental work has focused on measurements of the mass spectra and morphologies of BCcontaining particles from various sources. Modeling work has focused on improving the simulation of BC in a global model (GEOS-Chem) and developing an accompanying simulation of BrC. We updated the properties and aging mechanism for BC particles in the model and found that the short BC lifetime is critical to capturing more remote measurements. Our estimate of the direct radiative forcing of BC is considerably lower that the most recent estimate from the IPCC. We attribute this difference to previous overestimates of the BC lifetime as well as a misattribution of BrC absorption to BC.

In Year 3 of this project, laboratory experiments will focus on measurements of changes to optical properties of BC (and BrC) upon oxidative and condensational aging, and parameterization of these changes for use in global models. Future modeling work will include efforts to further develop our simulation of BrC based on observational constraints, as well as further investigation of the impacts of aging, as represented by the laboratory experiments, on the global burden and forcing from BC.