AEROSOL & CLIMATE(IN THE ARCTIC)

Pamela Lehr

METEO 6030

Spring 2006

http://www.arctic.noaa.gov/images/polarbear

TALK OUTLINE

1. Aerosol Basics

2. The Arctic

3. Aerosol in the Arctic (Recent Studies)

4. Summary

http://www.photolib.noaa.gov/corps/

AEROSOL REFRESHER COURSE

• Definition: liquid or solid particles suspended in the air

• Scatter and absorb radiation

• Aerosol distribution largely determines cloud droplet distribution

• Chemical composition is important: solubility, radiative properties

Things to keep in mind about aerosol:

• Spatial and temporal variability– Temporal: relatively short lifetime (hours to weeks)– Spatial: depends on emission location, transport, lifetime

• Sources– Emitted directly – dust, sea salt, organics, soot– New particle formation (such as gas-to-particle conversion) –

sulfates, organics

CLIMATE IMPACTS:cooling? warming? NEXT: 5 EFFECTS TO REMEMBER

Aerosol “____________” Effect• Direct: clear sky scattering by aerosol →

cooling

• 1st Indirect: – Constant LWC– Increased CCN → increased cloud droplet

concentration → increased cloud albedo → cooling

• 2nd Indirect (Cloud Lifetime): – Increased CCN → increased cloud droplet

concentration → decreased precipitation efficiency → cloud lasts longer → cooling

• Semi-Direct: – Absorbing aerosol (BC) → warm the cloud

→ decrease in life of cloud → warming

• LW Indirect: – Thin clouds, low emissivity– Increased CCN → increased emissivity →

possible warmingwww.iac.ethz.ch/groups/lohmann

What’s going on in the Arctic?

• First we need to think about conditions and climate in the Arctic

• Climate change, aerosol effects, etc., are often talked about in global averages

• But conditions can vary greatly by region– Pollution sources (anthropogenic aerosol and

aerosol precursors) – Regional Climate

Winter

• Polluted Dec – April/May• Pollution mostly confined

to lowest 5 km• Low cloud frequency

~35%• Clear skies 30-40%• Pollution sources: mostly

northern Europe and Asia• SO2 from burning of fossil

fuels & smelting of sulfide ores

• Inversions not uncommon

Summer• Unpolluted • Low cloud frequency ~75%• Clear skies 5-8%

The Arctic

(Barrie, L. 1986)

Cloud Emissivity and Surface LW Heating in the Arctic – Role of Aerosol

The Idea:• Thin clouds tend to have emissivities <

1 (not BB)• Increase in CN (decrease in effective

radius (re)) → increase in emissivity in thin clouds

• If cloud is warmer than surface & emissivity is increased → warmer surface and colder troposphere (and vice versa)

• Most effective during polar night and when there are low sun angles (i.e., common conditions in the Arctic)

(Garrett, et al. 2002)

More from the emissivity study…

• Figure below shows the sensitivity of surface CRFLW due to a decrease in re versus liquid water path (LWP) (x-axis):

– Translation: shift to smaller re in thin clouds leads to greater LW surface flux = surface warming

• Possibility of Arctic warming due to changes in emissivity in addition to GHG warming is noteworthy

(Garrett, et al. 2002)

(Garrett, et al. 2002)

Indirect Effect in the Arctic? A Recent Study

• Low, thin clouds• There is a wavenumber

range sensitive to cloud droplet size

• More negative slope associated with higher CN concentrations (smaller re)

• Check for a significant increase in emissivity at same time

• Is it likely due to a shift in the distribution’s re ?

• Conclusion: Yes!• Increase on the order of 3.4

W m-2

(Lubin & Vogelmann, 2006)

SUMMARY

• 5 aerosol effects:– Direct– 1st Indirect– 2nd Indirect– Semi-direct– LW Indirect

• Aerosol effect in arctic: surface warming (not during summer months)

• Studies indicate warming could be of climatological significance, though more studies need to focus on the winter months when the LW effect dominates

QUESTIONS?

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REFERENCES

Barrie, L. A. Arctic air pollution: an overview of current knowledge. Atmos. Environ. 20, 643-663 (1986).

Garrett, T., Radke, L. F. & Hobbs, P. V. Aerosol effects on cloud emissivity and surface longwave heating in the Arctic. J. Atmos. Sci. 59, 769-778 (2002).

Lubin, D. & Vogelmann, A. M. A climatologically significant aerosol longwave indirect effect in the Arctic. Nature 439, 453-456 (2006).

IPCC Third Assessment 2001,Scientific Basiswww.ipcc.ch