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Aerosols and climate

Aerosols and climate. Rob Wood, Atmospheric Sciences. What are aerosols?. Solid or liquid particles suspended in air Sizes range from a few nm to a few thousand nm  Huge range of masses. Where do aerosols come from?.

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Aerosols and climate

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  1. Aerosols and climate Rob Wood, Atmospheric Sciences

  2. What are aerosols? • Solid or liquid particles suspended in air • Sizes range from a few nm to a few thousand nm  Huge range of masses

  3. Where do aerosols come from?

  4. Estimate of Present-Day Global Emission of Major Aerosol Types (in Tg/year)

  5. 1 10 100 103 104 105 106 [nm]

  6. Scattering efficiency per mass 10 100 1000 10000 Particle diameter [nm] Aerosols and light scattering • Scattering of solar radiation per unit mass is maximum when particle size is close to solar wavelengths ( ~ 400-800 nm) • Particles in the size range 200-2000 nm are most efficient scatterers • Particle surface area more important than mass

  7. Scattering efficiency per mass 10 100 1000 10000 Particle diameter [nm] Surface area and mass • M = (4/3)  r3 • A = 4 r2 • A/M  1/r • when r «  scattering efficiency increases with r4  r<100 nm particles don’t matter

  8. Aerosols and light scattering • If aerosol particle is soluble then size will depend upon water uptake • Water uptake depends upon relative humidity (RH) • For given number of particles, scattering will increase with RH • But with no particles there is no scattering despite what the RH is

  9. Size as a function of RH

  10. Chemical composition affects growth marine airmass 3 2 1 growth factor r/rdry polluted airmass RH

  11. What determines scattering? • For particles of size > 200 nm, surface area determines scattering from one particle • Multiply this by the number concentration of particles to get overall scattering per unit volume of air • Atot  N r2, Mtot  N r3  Atot  N1/3 M2/3 • Need to know both total aerosol mass and the numberconcentration

  12. Importance of sulfate • Sulfate aerosols dominate the scattering over continental regions • Anthropogenic sources comparable to natural sources • Regionally, this scattering can seriously degrade visibility • Globally, this scattering reduces solar radiation entering the climate system by 2 W m-2 • Studies suggest that about 0.3-0.8 W m-2 of this scattering are from anthropogenic aerosols, mostly sulfate (“direct” effect of aerosols)

  13. Beijing after rain Beijing during dry period

  14. Solar radiation reaching the surface

  15. Global Sulfur Budget Rates in Tg S yr-1 SO42- t = 3.9d SO2 t = 1.3d cloud 42 OH H2SO4(g) 18 8 4 NO3 OH (CH3)2S 64 dep 6 dry 44 wet 10 dep 27 dry 20 wet (DMS) t = 1.0d 22 Phytoplankton Volcanoes Combustion/ Smelters

  16. Global Sulfur Emission Patterns Chin et al. [2000]

  17. SO2 sources by type

  18. Direct effect of aerosols upon solar radiation • Aerosols scatter solar radiation • More particles  more scattering • Some scattered radiation returns to space • Reduction in amount entering climate system •  Cooling effect

  19. Aerosol optical depth (measure of scattering)

  20. How well do we understand human influence? • Direct radiative forcing (effect of anthropogenic emissions on aerosol scattering) from different models

  21. Trends in U.S. NOx AND SO2 Emissions

  22. Aerosol-cloud-climate interactions

  23. More aerosol-Climate effects

  24. Cloud droplets form on aerosol particles • For soluble aerosols, particle size increases with RH • When a parcel of air rises, it cools and the RH increases • When RH>1, the soluble aerosols can “activate” (grow unstably into much larger cloud droplets) • The RH at which an aerosol activates is strongly dependent upon its size • Larger aerosols activate first and can consume vapor, preventing the smaller ones from activating • The number of cloud droplets is strongly controlled by the number of soluble aerosols

  25. What determines N in warm clouds? Cloud droplet concentration [cm-3] Aerosol concentration (r>0.1 micron) [cm-3]

  26. Link between number of cloud droplets and cloud albedo • Similar arguments as for light scattering due to aerosols • For given amount of liquid water, increased N gives smaller average size of droplets • Smaller droplets have larger surface area • More reflective cloud, more albedo

  27. Most well-understood aerosol-cloud-climate effects (indirect effects)

  28. Global Sulfur Emission Patterns Chin et al. [2000]

  29. MODIS, data courtesy of NASA

  30. IPCC, 2007

  31. Putting it all together

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