Constraining Carbonaceous Aerosol Climate Forcing by Bridging Laboratory, Field and Modeling Studies

Friday, 19 December 2014
Manvendra Krishna Dubey1, Allison C Aiken1, Shang Liu1, Rawad Saleh2, Christopher D Cappa3, Leah R Williams4, Neil McPherson Donahue5, Kyle Gorkowski6, Nga Lee Ng7, Claudio Mazzoleni8, Swarup China8, Noopur Sharma9, Robert J Yokelson10, James D Allan11 and Dantong Liu12, (1)Los Alamos National Laboratory, Los Alamos, NM, United States, (2)Carnegie Mellon University, Center for Atmospheric Particle Studies, Pittsburgh, PA, United States, (3)University of California Davis, Civil and Environmental Engineering, Davis, CA, United States, (4)Aerodyne Research Inc., Billerica, MA, United States, (5)Carnegie Mellon Univ, Pittsburgh, PA, United States, (6)Carnegie Mellon University, Pittsburgh, PA, United States, (7)Georgia Institute of Technology, Atlanta, GA, United States, (8)Michigan Technological University, Houghton, MI, United States, (9)Michigan Technological Univ, Houghton, MI, United States, (10)University of Montana, Department of Chemistry, Missoula, MT, United States, (11)University of Manchester, Manchester, M13, United Kingdom, (12)University of Manchester, School of Earth, Atmospheric and Environmental Sciences, Manchester, United Kingdom
Biomass and fossil fuel combustion emits black (BC) and brown carbon (BrC) aerosols that absorb sunlight to warm climate and organic carbon (OC) aerosols that scatter sunlight to cool climate. The net forcing depends strongly on the composition, mixing state and transformations of these carbonaceous aerosols. Complexities from large variability of fuel types, combustion conditions and aging processes have confounded their treatment in models. We analyse recent laboratory and field measurements to uncover fundamental mechanism that control the chemical, optical and microphysical properties of carbonaceous aerosols that are elaborated below:
  1. Wavelength dependence of absorption and the single scattering albedo (ω) of fresh biomass burning aerosols produced from many fuels during FLAME-4 was analysed to determine the factors that control the variability in ω. Results show that ω varies strongly with fire-integrated modified combustion efficiency (MCEFI)—higher MCEFI results in lower ω values and greater spectral dependence of ω (Liu et al GRL 2014). A parameterization of ω as a function of MCEFI for fresh BB aerosols is derived from the laboratory data and is evaluated by field data, including BBOP. Our laboratory studies also demonstrate that BrC production correlates with BC indicating that that they are produced by a common mechanism that is driven by MCEFI (Saleh et al NGeo 2014).  We show that BrC absorption is concentrated in the extremely low volatility component that favours long-range transport. 
  2. We observe substantial absorption enhancement for internally mixed BC from diesel and wood combustion near London during ClearFlo. While the absorption enhancement is due to BC particles coated by co-emitted OC in urban regions, it increases with photochemical age in rural areas and is simulated by core-shell models. We measure BrC absorption that is concentrated in the extremely low volatility components and attribute it to wood burning. Our results support enhanced light absorption by internally mixed BC parameterizations in models and identify mixed biomass and fossil combustion regions where this effect is large.

We unify the treatment of carbonaceous aerosol components and their interactions to simplify and verify their representation in climate models, and re-evaluate their direct radiative forcing.