Soot particle variations affect climate modeling

Science

Black carbon, or soot, enters the air mainly as a byproduct of fuel combustion and absorbs sunlight, leading to a strong warming effect on the Earth’s atmosphere. The exact atmospheric impact, however, remains uncertain and not well understood. This analysis shows that a particle’s shape and chemical composition can vary significantly, creating discrepancies between standard model predictions and ambient observations.

Impact

Modeling and laboratory studies suggest that black carbon absorbs more strongly when mixed with other aerosol components, but some ambient observations show more variable and weaker absorption enhancement. Through a combination of measurement and modeling, this research provides a framework that explains globally disparate observations and can be used to improve estimates of black carbon’s global radiative effect.

Summary

The researchers used laboratory data from the Fourth Boston College-Aerodyne Black Carbon Experiment to strengthen estimates from the particle-resolved Particle Monte Carlo Model for Simulating Aerosol Interactions and Chemistry (PartMC-MOSAIC). They found that lower-than-expected enhancements in ambient measurements result from a combination of two factors. First, models often assume a spherical particle coated by other organic materials in the air, an approximation that generally overestimates light absorption. Second, and more importantly, models do not adequately consider heterogeneity in composition from particle to particle. This second factor leads to substantial overestimation of absorption by the total particle population, with greater heterogeneity associated with larger model-measurement differences. Accounting for these two effects—deviations from the core-shell approximation and variability in per-particle composition—reconciles absorption enhancement predictions with laboratory and field observations and resolves the apparent discrepancy.

The microscopy analysis was partially conducted at EMSL, the Environmental Molecular Sciences Laboratory, as part of its Biogeochemical Transformations and Isotope & Chemical Analysis Integrated Research Platforms. The team included scientists from Brookhaven National Laboratory; Aerodyne Research; Boston College; University of California, Davis; Michigan Technological University; EMSL; and University of Georgia.