Radiative absorption enhancements by black carbon controlled by compositional heterogeneity
Fierce, Laura — Brookhaven National Laboratory
Area of research
This paper addresses a critical gap in understanding the radiative impacts of black carbon (BC), an aerosol species estimated as the second most important climate warming agent after CO2. Modeling and laboratory studies suggest that black carbon absorbs more strongly when mixed with other aerosol components, but some ambient observations suggest more variable and weaker absorption enhancement. We show that the lower-than-expected enhancements in ambient measurements result from particle-to-particle compositional heterogeneity and, to a lesser extent, deviation from the core-shell approximation.
Large discrepancies between standard model predictions and regionally specific observations—often with observed absorption lower than expected—raise questions about current understanding of black carbon absorption and its atmospheric impacts. Through a combination of measurement and modeling, this work provides a framework that explains globally disparate observations and that can be used to improve estimates of black carbon’s global radiative effect.
A critical gap in quantifying black carbon’s radiative effect on climate is predicting enhancements in light absorption that result from internal mixing between BC and other aerosol components. Modeling and laboratory studies show that BC, when mixed with other aerosol components, absorbs more strongly than pure, uncoated BC; however, some ambient observations suggest more variable and weaker absorption enhancement. We show that the lower-than-expected enhancements in ambient measurements result from a combination of two factors. First, the often used spherical, concentric core-shell approximation generally over-estimates the absorption by BC. Second, and more importantly, inadequate consideration of heterogeneity in particle-to-particle composition engenders substantial over-estimation in absorption by the total particle population, with greater heterogeneity associated with larger model-measurement differences. We show that accounting for these two effects--variability in per-particle composition and deviations from the core-shell approximation--reconciles absorption enhancement predictions with laboratory and field observations and resolves the apparent discrepancy. Further, our consistent model framework provides a path forward for improving predictions of BC's radiative effect on climate.