Atmospheric Aerosols:What they are and why/how I study them

Geeta PersadPECS Student Presentation

What is an aerosol?

http://en.wikipedia.org

DUST

Sea Salt

• Particles in the atmosphere, both natural and anthropogenic

http://www.andaman.org/BOOK/originals/Weber-Toba/pinatubo.jpg

Sulfate

OrganicMatter

Black Carbon

Why might we care about aerosols?

Aerosols reduce the amount of solar radiation at thesurface 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

How can aerosols affect the climate?• Direct Effects: Scattering/reflection of solar radiation

by particles cools the planet.

How can aerosols affect the climate?

Large Droplets Lower Surface AreaLower ReflectivityShorter Lifetime

More Precipitation

http://earthobservatory.nasa.gov/Library/Aerosols/

Small Droplets Higher Surface Area

Higher ReflectivityLonger Lifetime

Less Precipitation

http://earthobservatory.nasa.gov/Library/Aerosols/

• Indirect Effects: Aerosols that can condense waterform the nucleus for cloud droplets.

How can aerosols affect the climate?• Semi-Direct Effects: Atmospheric heating from

absorbing aerosols can change cloud field.

Re-emittedInfrared

Incoming Solar

SaturatedVaporPressure

How can aerosols affect the climate?

• Clouds are a major regulator of Earth’s climate.

How can aerosols affect the climate?

• Aerosols havecounteracted CO2warming in the past,but by exactly howmuch is unclear.

• Aerosols arepredicted todecrease in thefuture.

• How much additionalCO2 warming will thisdecrease “reveal”?

General Circulation ModelsComponents:

-Ocean

-Land

-Sea Ice

-Atmospheres

Aerosols

Clouds

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)

Radiative Forcing of Black Carbon

or Fi, Fa or Fs

Good at (2), Bad at (1) Good at (1), Bad at (2)?

Characteristics of an optimal forcing calculation:– (1) proxy for the change in global surface temperature– (2) means for comparing and aggregating results across models

Black Carbon/Climate Interactions

Re-emittedInfrared

Incoming Solar

Semi-direct Effect

• Black carbon heating can affect clouds dynamically andthermodynamically•Clouds are a major source of uncertainty in GCMs

Radiative Flux Perturbation

(Persad et. al; Journal of Climate, 2012)

• Subsiding regions primarily respond via semi-direct effect,• Convective regions respond primarily via dynamical impacts.• Robust semi-direct effect dominates the RFP signature.

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 inoutgoing shortwave radiation.

• Increase in radiation is concurrent with an increase in middle cloudamount.

Radiative Flux Perturbation

Deep Convective RegionsDeep 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)

Solar Dimming and Brightening

(Wild, M.; BAMS, 2012 = Great review article)

Solar Dimming and Brightening

(Wild, M.; BAMS, 2012 = Great review article)

Solar Dimming and Brightening

Solar Dimming and BrighteningNH SW Surface Rad. - Clear NH SW Surface Rad. - Cloudy

•Understanding why the model does or does not recreate the solardimming and brightening trend may be useful for analyzing theperformance of different mechanisms in the model.

Solar Dimming and Brightening

•NH Precipitation Anomalies: Driven by aerosols? How and why?

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 tounderstand how past climate has changed due to CO2, which requiresseparating 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 betterunderstanding of how the real world responds to aerosols.