1. Aerosols Kenneth Hunu and Partner
Dynamics of Climate Variability and Climate Change
5 December 2006

2. What are aerosols?
Aerosols: liquid or solid particles suspended in air
Natural aerosols:
sources: volcanoes, sea spray from ocean, dust
coarse in size (~2 microns)
Anthropogenic aerosols:
sources: combustion of fossil fuels and biomass
fine in size (~ 0.2microns)
Aerosols have a short lifespan in the atmosphere

3. Main Point 1: Aerosols influence the Earth’s radiation budget
Direct effect: Increased albedo- reflecting incoming sunlight
Semi-direct effect: absorption of sunlight by black carbon and soot warms the atmosphere thereby reducing convection.
First indirect effect: increase number of cloud droplets thereby increasing cloud’s reflectivity and cooling the earth.
Second indirect effect: smaller size of anthropogenic aerosols makes it difficult for cloud droplets to gather enough mass to fall as rain - cools the earth and decreases precipitation efficiency of cloud.
Aerosols have a net cooling effect
Aerosol cooling expected to decrease in the future

4. Main Point 2: Aerosols affect the hydrologic cycle
Smaller cloud condensation nuclei from man-made sources
Results in smaller cloud droplets (indirect effect)
Harder to form raindrops (lowered precipitation efficiency)
Potential to rain less over polluted regions
Potential to have polluted rain over clean areas
This results in a net cooling effect and reduced precipitation

5. Main Point 3: The magnitude of aerosol forcing on climate is uncertain
Uncertainties make it hard to calculate exact aerosol forcing
Difficult to determine the second indirect effect
Incomplete understanding about cloud dynamics
Resolution of models too coarse
Net cooling effect of aerosols results in a ‘radiative’ forcing of around 0 to –4.4 W/m2

6. References
Andreae, M.O., Jones, C.D., Cox, P.M. (2005). “Strong present-day aerosol cooling implies a hot future.” Nature, 435,
Ramanathan, V., Crutzen, P.J., Kiehl, J.T., Rosenfeld, D. (2001). “Aerosols, Climate, and the Hydrological Cycle.” Science, 294(7):
Anderson, T.L., Charlson, R>J., Schwarzt, S.E., Knutti, R., Boucher, O., Rodhe, H., Heintzenberg, J. (2003). “Climate Forcing by Aerosols—a Hazy Picture.” Science, 300,
Dufresne, J.L., Quaas, J., Boucher, O., Denvil, S., Fairhead, L. (2005). “Contrasts in the effects on climate on anthropogenic sulfate aerosols between the 20th and 21st century.” American Geophysical Union, 32, L21703.
Ackerman, A.S., Toon, O.B., Stevens, D.E., Heymsfield, A.J., Ramamathan, V., Welton, E.J. (2000). “Reduction of Tropical Cloudiness by Soot.” Science, 288,
Penner, J.E., Quaas, J., Storelvmo, T., Takemura, T., Boucher, O., Guo, H., Kirkevag, A., Kristjansson, J.E., Seland, O. (2006). “Model intercomparison of indirect aerosol effects.” Atmospheric Chemistry and Physics, 6,
Kaufman, Y.J., Tanre, D., Boucher, O. (2002). “A satellite view of aerosols in the climate system.” Nature, 419,
Penner J.E, Chuang C.C, Grant K.(1998). “Climate Dynamics” Springer

7. Questions?