1. 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”

2. 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 Spectrum UVIR BC absorption BrC absorption

3. Estimating BrC: Building a Bottom-up Simulation Optical Properties Aerosol Mass

4. Estimating BrC: Building a Bottom-up Simulation Optical Properties Aerosol Mass Measured Imaginary part of refractive Index in biomass burning/biofuel sources

5. 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 m 2 /g at 440nm for primary BrC 0.3 m 2 /g at 440nm for secondary BrC Assumption for GEOS-Chem

6. 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 0.11 Simulated Annual Average Direct Radiative Effect (DRE) of BrC (Wang et al., 2014)

7. Deriving BrC Absorption from Measurements Wavelength

8. Is the AAE = 1 Assumption Reasonable? Mie Calculation BC (core) diameter, nm AAE Refractive Index (BC) = 1.95-0.79i (Bond and Bergstrom, 2006; Bond et al., 2014)

9. 2014 AERONET AAOD BC + BrC BC only (range defined by Mie calculations) Developing a New Method to Derive BrC

10. 2014 AERONET AAOD BC + BrC BC only (range defined by Mie calculations) Developing a New Method to Derive BrC

11. Derived BrC: A Top-down Constraint on Regional Sources Relationship between derived BrC AAOD at 440nm and observed AAOD at 675nm from AERONET 2 years monthly data (2011-2012) R 2 = 0.75 Slope = 0.50

12. Select data in North America only, and dominated by biofuel emissions R 2 = 0.91 Slope = 0.30 R 2 = 0.75 Slope = 0.50 Derived BrC: A Top-down Constraint on Regional Sources

13. R 2 = 0.86 Slope = 0.54 R 2 = 0.75 Slope = 0.50 Select data in East Asia only, and dominated by biofuel emissions Derived BrC: A Top-down Constraint on Regional Sources

14. R 2 = 0.84 Slope = 0.55 R 2 = 0.75 Slope = 0.50 Select data in Europe only, and dominated by biofuel emissions Derived BrC: A Top-down Constraint on Regional Sources

15. R 2 = 0.91 Slope = 0.42 R 2 = 0.75 Slope = 0.50 Select data in dominated by biomass burning emissions globally Derived BrC: A Top-down Constraint on Regional Sources

16. 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.

17. 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.

18. 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

19. Derived BrC-AAOD at 440nm in 2012 MAMDJF SONJJA Selected AERONET sites with significant BrC influence Sites affected largely by dust have been removed