xuan wang and colette l. heald 7th international geos-chem user’s meeting, may 5, 2015 this work...
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Xuan Wang and Colette L. Heald
7th International GEOS-Chem User’s Meeting, May 5, 2015
This work is funded by U.S. EPA
Simulating Brown Carbon and its Direct Radiative Forcing:
From “Bottom-up” to “Top-down”
Brown Carbon: The Absorbing Organics
• Recent studies show some organic aerosols can absorb light (so-called ”brown
carbon”: BrC) (Arola et al., 2011; Hecobian et al., 2010; Chakrabarty et al., 2010 etc.)
• Usually found in biomass burning and biofuel emissions.
• Most absorbing at UV wavelengths, leading to a high absorption angstrom exponent
(AAE).
• BrC contributes to global warming but its effect has not been well estimated.
Visible SpectrumUV IR
BC absorption
BrC absorption
Estimating BrC: Building a Bottom-up Simulation
Optical Properties
Aerosol Mass
Estimating BrC: Building a Bottom-up Simulation
Optical Properties
Aerosol Mass
Measured Imaginary part of refractive Index in biomass burning/biofuel sources
Estimating BrC: Building a Bottom-up Simulation
Assume 50% of biofuel/biomass burning POA to be primary BrC
Assume aromatic SOA to be secondary BrC
(Wang et al., 2014)
Assumption for GEOS-Chem
Optical Properties
Aerosol Mass
Mass Absorption Coefficient = 1 m2/g at 440nm for primary BrC
0.3 m2/g at 440nm for secondary BrC
Assumption for GEOS-Chem
Building a Bottom-up Simulation
• The global mean absorbing DRF of BrC is estimated to be +0.07 Wm-2
• Other model studies of BrC are also built on simple assumptions or single
experiments (Feng et al., 2013; Lin et al., 2014; Lu et al., 2015.)
0.001 0.01 0.1 1
Simulated Annual Average Direct Radiative Effect (DRE) of BrC
(Wang et al., 2014)
Deriving BrC Absorption from Measurements
• Previous work assumes AAE=1 for BC to separate BrC (Chung et al., 2012; Yang et al., 2009; Liu et al., 2015 etc)
Wavelength
Is the AAE = 1 Assumption Reasonable?
• Previous work assumes AAE=1 for BC to separate BrC (Chung et al., 2012; Yang et al., 2009; Liu et al., 2015 etc)
• Theoretically, the AAE is sensitive to size, mixing state and the chosen wavelength
• In field observations, AAE usually ranges from 0.55 to 1.7 (Bahadur et al., 2012; Gyawali et al., 2009; Lack and Langridge, 2013)
Mie Calculation
BC (core) diameter, nm
AAE
Refractive Index (BC) = 1.95-0.79i (Bond and Bergstrom, 2006; Bond et al., 2014)
2014 AERONET AAODBC + BrC
BC only (range defined
by Mie calculations)
Developing a New Method to Derive BrC
Define: Wavelength Dependence of AAE (WDA) WDA
2014 AERONET AAODBC + BrC
BC only (range defined
by Mie calculations)
Developing a New Method to Derive BrC
Derived BrC: A Top-down Constraint on Regional Sources
Relationship between derived BrC AAOD at 440nm and observed AAOD at 675nmfrom AERONET 2 years monthly data (2011-2012)
R2 = 0.75Slope = 0.50
Select data in North America only, and dominated by biofuel emissions
R2 = 0.91Slope = 0.30
R2 = 0.75Slope = 0.50
Derived BrC: A Top-down Constraint on Regional Sources
R2 = 0.86Slope = 0.54
R2 = 0.75Slope = 0.50
Select data in East Asia only, and dominated by biofuel emissions
Derived BrC: A Top-down Constraint on Regional Sources
R2 = 0.84Slope = 0.55
R2 = 0.75Slope = 0.50
Select data in Europe only, and dominated by biofuel emissions
Derived BrC: A Top-down Constraint on Regional Sources
R2 = 0.91Slope = 0.42
R2 = 0.75Slope = 0.50
Select data in dominated by biomass burning emissions globally
Derived BrC: A Top-down Constraint on Regional Sources
Summary and Future Work
• The global mean BrC absorption that we derive from AERONET is ~30% lower than
the bottom-up simulation.
• We could use the regional source relationships shown above to constrain model
simulations of BrC.
Summary and Future Work
• The global mean BrC absorption that we derive from AERONET is ~30% lower than
the bottom-up simulation.
• We could use the regional source relationships shown above to constrain model
simulations of BrC.
• Our AERONET estimate for BrC absorption is only for one wavelength (440 nm),
we need additional information on the wavelength dependence of BrC absorption
(the AAE) to estimate the global radiative impacts of BrC.
Summary and Future Work
Surface observation in GOAMAZON campaign (http://www.arm.gov/sites/amf/mao/)
• The global mean BrC absorption that we derive from AERONET is ~30% lower than
the bottom-up simulation.
• We could use the regional source relationships shown above to constrain model
simulations of BrC.
• Our AERONET estimate for BrC absorption is only for one wavelength (440 nm),
we need additional information on the wavelength dependence of BrC absorption
(the AAE) to estimate the global radiative impacts of BrC.
BrC AAE for 370/430nm
Days in 2014
BC AAE for 430/880nm
Derived BrC-AAOD at 440nm in 2012
MAMDJF
SONJJA
Selected AERONET sites with significant BrC influenceSites affected largely by dust have been removed