1. BC: Composition, Structure, and Light Absorption
On Black Carbon Absorption: Atmospheric Heating and Snow-Albedo Feedback *Kuo-Nan Liou Joint Institute for Regional Earth System Science and Engineering (JIFRESSE) and Atmospheric and Oceanic Sciences Department University of California, Los Angeles, CA, USA _________________ *Research work supported in part by NSF and DOE
BC: Composition, Structure, and Light Absorption
BC: Atmospheric Heating and Implications
BC and Mountain-Snow-Albedo Feedback
Summary
**Aerosol Workshop, Academia Sinica, Taipei, Nov 5-6, 2012

2. Black Carbon (BC): Definition, Shape, Composition, and Size Distribution
BC: A solid form of mostly pure carbon that absorbs solar radiation at all wavelengths. Is produced by incomplete combustion
Soot: A complex light – absorbing mixture of mostly BC and organic carbon (OC), and usually includes other inorganic materials, such as metal and sulfate
(Shiraiwa et al., 2008）

3. Left panel: Real and imaginary refractive indices for BC taken from Krekov (1993), d’Almeida et al. (1991), and Bond and Bergstrom (2006, visible). Right panel: Real and imaginary refractive indices for ice; original (Warren 1984) and updated (Warren and Brandt 2008) values.

4. Construction of aggregates based on stochastic processes using homogeneous and shell spheres (smooth and irregular) as building blocks (Liou et al. 2010, 2011): closed and open cells, and observed soot.
Light absorption and scattering by small irregular particles based on the geometric-optics and surface-wave approach verified by comparison with existing results for columns and plates (Liou, Takano and Yang 2011).

5. BC shape and mixing state effects on its single-scattering properties
Spectral extinction coefficient, asymmetry factor, and single-scattering albedo (ssa) for polydisperse spheres with an effective radius of mm taken from tabulated values given in d’Almeida et al. (1991) (in the current UCLA GCM). Results for aggregates, having an equivalent spherical radius and the same mass and volume, generated from the stochastic procedures, and followed by light absorption and scattering calculations based on the geometric-optics/surface-wave approach developed by Liou et al. (2011). At visible wavelengths, ssa differences between the two shapes are ~ 0.1. Also, internal mixing state absorbs more radiation, as shown in the bottom panel.

6. Reflection (Albedo), absorption, and transmission for a soot layer as a function of aerosol mass path (AMP) on a black surface using a solar zenith angle of 600. The 0.03 mm radius is the mean observed equivalent radius for BC aerosols. Note substantial differences between the two BC shapes using diffusion limited aggregate and equal-mass (and equal-volume) spheres. Optical depth t can be obtained by t = ae AMP, where ae is the specific extinction coefficient (m2/g). The adding-doubling method was used for radiative transfer calculations.

7. Light Absorption & Scattering by BC/Dust
Direct Radiative Forcing & Regional Climate
BC: Highly Absorbing
Dust: Absorbing & Scattering
0.5 μm
Sunlight
Forward Scattering
Absorption
Absorption of Sunlight by BC/Dust
0.5 μm
Atmospheric Heating
Vertical Temperature Profile
Scattering
Absorption: Transform to Heat
Regional Circulation
Scattering: Redirect the energy in different directions
Solar Dimming at the Surface
Regional Surface Temperature & Precipitation

8. Precipitation
Surface Air Temperature
simulated (w =0.88)
simulated (w =0.88)
Simulated annual mean differences in (a) precipitation (%) and (c) surface air temperature (K) between Experiments B and A, along with the observed (b) precipitation (%) and (d) surface air temperature anomalies (K) over China in the 1990s. Exp A consists of 10% BC and 90% non-absorbing aerosols (w = 0.92). Exp B consists of 15% BC and 85% of non-absorbing aerosols (w =0.88). All based on the data listed in d’Almeida et al. (1991) for spherical particles. The sea surface temperature, greenhouse gases, and other forcings are fixed in these two experiments so that aerosols are the only forcings in 5-year simulations (after Gu, Liou et al. 2010). The bottom panels are simulation results using aggregates (w =0.87).
observed
observed
simulated (w =0.87)
simulated (w =0.87)

9. (a) Averaged (with standard deviations noted) vertical profiles of the BC mass mixing ratio (ng/kg) over land based on the HIPPO-1 data from ~ 0.3 to 14 km altitude measured from a Single-Particle Photometer (SP2) in the HIAPER aircraft (Schwarz et al. 2010; Ginoux et al. 2006). (b) Corresponding atmospheric heating rates (K/day) representing three latitudinal belts for clear conditions, including only BC aerosols in the calculations (m0 = 0.5).
(a)
(b)

10. Human and Natural Drivers of Climate Change

11. Left panel: Observed and modeled snow grains for fresh, aging, and old snow conditions with the inclusion of BC/dust in a model based on stochastic processes. Below are Spectral snow albedo (m0 = 0.5; semi-infinite optical depth) values calculated from the adding-doubling radiative transfer method for pure snow and three BC depositions.
rsnow = 100 mm

12. Visible single-scattering co-albedo (the ratio of absorption and extinction coefficients) and snow albedo as a function of soot and dust equivalent radii for a snow grain of 50 mm in equivalent radius for pure and contaminated conditions (m0 = 0.5 and optically semi-infinite snow layer). Large differences in snow albedo are shown with external and internal mixing cases. A 1 mm soot particle internally mixed with snow grains could effectively reduce snow albedo as much as 5-10% (Liou et al. 2011).

13. The effect of internal (left panel) and external (right panel) mixings in snow grains on the spectral asymmetry factor (upper panel), single-scattering co-albedo (middle panel) and snow albedo (m0 = 0.5, optically semi-infinite layer; lower panel) covering 0.2 to 5 mm solar spectrum. The snow grain size is 100 mm with 3 BC sizes of 0.1, 1, and 10 mm. Internal mixing cases absorb more radiation than their external counterparts. For application to CLM-WRF, total BC deposition can be converted to a mean BC size.

14. Connection to Surface Energy Balance Equation
(Community Land Model, CLM <-> WRF)
Basic Equation
3D Mountain Effects
External & Internal Mixing of BC in Snow Grains (work in progress)

15. An Illustration of Mountains/Snow-Albedo Feedback due to Absorbing Aerosols
Anthropogenic
(BC/Dust)
Decrease in Snow Grain Purity (External/ Internal Mixing)
Decrease in Snow Albedo/Cover (Snow is less Bright)
Absorbs more Incoming Sunlight
Surface Warming
Mountain Effect
Wet/Dry Deposition
3D Radiative Transfer
Positive Feedback ?
Global Warming
(CO2)
Known

16. On Black Carbon Absorption: Some Remarks
Aggregates (BC) absorb more radiation than the equivalent spherical particles (same volume and mass) commonly used in climate models. Internal mixing (BC core and non-absorbing shell) absorbs more radiation than pure BC.
An example of aggregates vs. spheres for BC in GCM simulations illustrates the significance of single-scattering albedo in climate study.
BC vertical profiles affect the vertical heating rates and the consequence of temperature fields and circulation patterns.
Mountain snow albedo reduction appears to be linked to absorbing aerosols, principally BC, based on satellite data analysis over the Tibetan Plateau and the Sierras.
Introduction of the concept of aerosols/mountain-snow/albedo feedback as a regional Earth system.