Atmospheric aerosols are particles in the air that can affect climate in various ways. They can cool the climate by reflecting sunlight, but also impact clouds and precipitation. Aerosols have likely offset some warming from greenhouse gases in the past, but exactly how much is unclear. The presenter studies aerosols using climate models to better understand their effects on climate and how their future reduction may influence additional warming from rising carbon dioxide levels.

1. Atmospheric Aerosols:
What they are and why/how I study them
Geeta Persad
PECS Student Presentation

2. What is an aerosol?
• Particles in the atmosphere, both natural and anthropogenic
Organic
Matter
Sea Salt
Black Carbon
Sulfate
DUST
http://en.wikipedia.org http://www.andaman.org/BOOK/originals/Weber-Toba/pinatubo.jpg

3. Why might we care about aerosols?
Aerosols reduce the amount of solar radiation at the
surface by scattering and absorbing sunlight.
• Impact on energy sources: solar power yield,
agriculture
• Air quality impacts on health and quality of life
• Product of energy sources: coal, oil and gas,
biomass burning
• Climate

4. How can aerosols affect the climate?
• Direct Effects: Scattering/reflection of solar radiation
by particles cools the planet.

5. How can aerosols affect the climate?
• Indirect Effects: Aerosols that can condense water
form the nucleus for cloud droplets.
http://earthobservatory.nasa.gov/Library/Aerosols/ http://earthobservatory.nasa.gov/Library/Aerosols/
Large Droplets Lower Surface Area Small Droplets Higher Surface Area
Lower Reflectivity Higher Reflectivity
Shorter Lifetime Longer Lifetime
More Precipitation Less Precipitation

6. How can aerosols affect the climate?
• Semi-Direct Effects: Atmospheric heating from
absorbing aerosols can change cloud field.
Incoming Solar
Saturated
Vapor
Pressure
Re-emitted
Infrared

7. How can aerosols affect the climate?
• Clouds are a major regulator of Earth’s climate.

8. How can aerosols affect the climate?
• Aerosols have
counteracted CO2
warming in the past,
but by exactly how
much is unclear.
• Aerosols are
predicted to
decrease in the
future.
• How much additional
CO2 warming will this
decrease “reveal”?

9. General Circulation Models
Components:
-Ocean
-Land
-Sea Ice
-Atmospheres
Aerosols
Clouds

10. Radiative Forcing of Black Carbon
Characteristics of an optimal forcing calculation:
– (1) proxy for the change in global surface temperature
– (2) means for comparing and aggregating results across models
or Fi, Fa or Fs
(IPCC, 2007: Chapter 2; Hansen et al., 2005)

11. Radiative Forcing of Black Carbon
Characteristics of an optimal forcing calculation:
– (1) proxy for the change in global surface temperature
– (2) means for comparing and aggregating results across models
or Fi, Fa or Fs
Good at (2), Bad at (1) Good at (1), Bad at (2)?

12. Black Carbon/Climate Interactions
Incoming Solar
Semi-direct Effect
Re-emitted
Infrared
• Black carbon heating can affect clouds dynamically and
thermodynamically
•Clouds are a major source of uncertainty in GCMs

13. Radiative Flux Perturbation
• Subsiding regions primarily respond via semi-direct effect,
• Convective regions respond primarily via dynamical impacts.
• Robust semi-direct effect dominates the RFP signature.
(Persad et. al; Journal of Climate, 2012)

14. Radiative Flux Perturbation
Deep Convective Regions
Radiative Flux Perturbation (W/m2) Change in Mid Cloud Amount (%)
• For black carbon above the cloud layer, there is an increase in
outgoing shortwave radiation.
• Increase in radiation is concurrent with an increase in middle cloud
amount.

15. Radiative Flux Perturbation
Deep Convective Regions
Change in Temperature (K) in convective mass flux* in Cloud Amount (%)
• Dominated by dynamical effects of black carbon heating.
• Decrease in cloud amount at level of black carbon heating due to inversion and
detrainment into large-scale environment.
• Above detrainment, latent heat release balances decreased convective heating,
maintaining high cloud level.
• Mixture of first order physics and parameterization.
* Units of (10 3 kg m 2 s 1)

16. Solar Dimming and Brightening
(Wild, M.; BAMS, 2012 = Great review article)

17. Solar Dimming and Brightening
(Wild, M.; BAMS, 2012 = Great review article)

18. Solar Dimming and Brightening

19. Solar Dimming and Brightening
NH SW Surface Rad. - Clear NH SW Surface Rad. - Cloudy
•Understanding why the model does or does not recreate the solar
dimming and brightening trend may be useful for analyzing the
performance of different mechanisms in the model.

20. Solar Dimming and Brightening
•NH Precipitation Anomalies: Driven by aerosols? How and why?

21. Summary
• Aerosols = major issue for climate and the environment
• Aerosols counteract CO2 warming
• ?? How much ??
• ?? Why exactly ??
• In order to predict future climate change due to CO2, we need to
understand how past climate has changed due to CO2, which requires
separating out the effect of aerosols.
• My work:
• Focused on addressing this issue using general circulation models.
• Using observed trends to validate model processes to get a better
understanding of how the real world responds to aerosols.