New Observational Parameterizations to Constrain Climate Forcing by Black, Brown and Organic Carbon

 

Authors

Manvendra K. Dubey — Los Alamos National Laboratory
Shang Liu — Los Alamos National Laboratory
Allison C Aiken — Los Alamos National Laboratory
Chris Cappa — University of California, Davis
Robert James Yokelson — University of Montana
Claudio Mazzoleni — Michigan Technological University
Neil Donahue — Carnegie Mellon University
Leah R Williams — Aerodyne Research Inc
Timothy B Onasch — Aerodyne Research
Sonia Kreidenweis — Colorado State University

Category

Absorbing Aerosol

Description

(Left) Aerosol Single Scatter Albedo (780nm) as a function of Modified Combustion Efficiency for 20 fuels over a range of conditions and a fit to the data (Liu et al GRL 2014) (right) Observed absorption enhancement at 405 nm by independent approaches (direct thermal denuded studies in boxes and MAC (PASS3abs/SP2mass) derived in blue dots) at Detling as a function of coating thickness (Rbc) with colors denoting organic content (yellows have more OC). Evidence of coating enhancement (boxes) and brown carbon absorption (blue dots) are evident and analyzed by models (Liu et al in prep Nature 2014)
Accurate assessment of the radiative forcing carbonaceous absorbing aerosols emitted by fossil energy and fires that could be the 2nd largest warming agent and are very uncertain is a focus of ASR. Elucidating the effects of both the combustion mechanisms that regulate the primary emissions in the near field and atmospheric microphysical and photochemical aging on the optical properties are critical to quantify their climate impacts. We report the following validated parameterizations by integrating laboratory & field observations: (1) SSA dependence on biomass combustion efficiency: Single scattering albedo (ω) of fresh biomass burning (BB) aerosols produced from 92 controlled laboratory combustion experiments of 20 different woods and grasses was analyzed 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. A parameterization of ω as a function of MCEFI for fresh BB aerosols is derived from the laboratory data and is evaluated by field observations from two wildfires. The parameterization suggests that MCEFI explains 60% of the variability in ω, while the 40% unexplained variability could be accounted for by other parameters such as fuel type. Our parameterization provides a promising framework that requires further validation and is amenable for refinements to predict ω with greater confidence, which is critical for estimating the radiative forcing of BB aerosols. We plan to extend our evaluations to BBOP. (2) Enhanced light absorption by internally mixed atmospheric black carbon: We report the first direct in vivo evidence of substantial absorption enhancement of up to 50% for internally mixed black carbon (BC) at and around London during Clerflo. While the absorption enhancement is due to coated BC particles at emission in urban regions, the absorption enhancement increases with photochemical aging in rural areas, consistent with theoretical predictions and laboratory experiments. Our field results support parameterizations of enhanced light absorption by internally mixed BC in climate models and identifies mixed biomass and fossil combustion regions where this effect is large. (3) Quantifying the role of brown carbon: Indirect spectral approaches in the shortwave in conjunction with constrained Mie modeling will be used to infer brown carbon forcing in fires and urban regions.

Lead PI

Manvendra K. Dubey — Los Alamos National Laboratory