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

Geeta PersadPrinceton University

Program in Atmospheric and Oceanic Sciences

• What are aerosols? What are their sources? Whatare the different types?

• How do aerosols impact the climate?• Why is understanding them important for climate

change prediction?• How do we learn about them?• How do we address them?

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

What is an aerosol?

Black CarbonDust

Sea SaltSulfate

Why might we care about aerosols?

Aerosols change the amount of solar radiation thatreaches the surface and that leaves the atmosphereby scattering and absorbing sunlight.

• Product of energy sources: coal, oil and gas,biomass burning

• Impact on energy sources: solar power yield,agriculture

• Air quality impacts on health and quality of life• Climate

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

by particles cools the planet, absorption warms.

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”?

(IPCC, 2007)

Motivation

!T = " ! F

Global mean temperaturechange due to forcing

Climate sensitivity

Radiative Forcing

A Global Aerosol Picture

Observations

General Circulation ModelsComponents:

-Ocean

-Land

-Sea Ice

-Atmospheres

Aerosols

Clouds

Solar Dimming

(Wild, 2012 BAMS)

Solar DimmingSurface Solar Radiation Anomaly (W/m2)

Ano

mal

y (W

/m2 )

Obs. AM3 AM2.1

Impact on Precipitation

(Bollasina et al, 2011)

Solutions

Solutions

Co-benefit or not?

(Mickley et al, 2012, Atmos. Env.)

Summary

• Aerosols have both natural and human sources.• Aerosols can impact climate by directly interacting

with sunlight or by interacting with clouds.• Aerosols generally cool the planet, but certain types

can also warm the planet.• Major uncertainty exists about the cancellation of

the warming and cooling aspects of aerosols.• Aerosols can reduce precipitation and impact

human health.• Solutions exist for removing certain aerosols that

have both health and climate benefits.• Aerosol removal may also reveal more warming.