7. Aerosols and Climate

Transcription

1 7. Aerosols and Climate I. Scattering 1. When radiation impinges on a medium of small particles, scattering of some of the radiation occurs in all directions. The portion scattered backward is called the albedo. 2. Single scattering implies only a single interaction between particle and radiation. If the incident radiation is travelling in the direction with angle θ with the vertical, the fraction which reaches the surface without being scattered or absorbed is: I = exp ( -τ ) I 0 0 cos θ θ Where τ 0 is called the (normal) optical thickness of the layer

2 I = exp ( -τ ) I 0 0 cos θ Single scattering is applicable when the layer is optically thin, for many purposes when τ 0 < 0.1. But when θ=> π/2, cos θ => 0 and no layer is optically thin. 3. Multiple scattering occurs when there are more than one interaction between radiation and particles in a layer. 4. Rayleigh scattering: particles (like molecules) that are small compared to the wavelength of the radiation produce Rayleigh scattering. The radiation induces an electric dipole in the particle which then reradiates. With I o the incident radiation, the scattered radiation is proportional to the volume 2, so it increases rapidly with size, and proportional to λ -4 : I s a 6 λ 4 r 2 (1+cos 2 θ)i o a=particle radius λ=wavelength r=scattering distance

3 In the visible, λ=4x10-5 cm (blue) and λ=8x10-5 cm (red). The scattering is therefore 16 times larger for blue than red. The scattering distribution is the same in the forward and backward directions, so all points of the sky are essentially sources of predominantly blue scattered light. At twilight, the longer ray path through the atmosphere (r) causes most of the scattered blue radiation to be dispersed from the beam before it reaches the observer, so light then reflected from clouds or scattered to the observer from the haze layer appears reddish.

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5 5. Mie Scattering: when the particle dimensions are large compared with λ, it is no longer sufficient to consider merely an induced dipole. The effect is to increase the amount of scattering in the forward direction. This is partly due to diffraction of photons around the particles. The maximum efficiency for Mie scattering occurs when a~ λ. For a>> λ, the efficiency goes to 2 one part of photons passing through the particle, while the other for photons diffracted around it. Scattering Efficiency Q e 2 r= λ 6. If the particle absorbs radiation, the forward scattering is not greatly affected as the absorption first becomes > 0 (diffraction part is untouched), while the backward radiation is diminished. However, when absorption becomes very large, the particle will act more like a reflector, and backward scattering will be greatly increased. r

6 7. Define a size parameter X=2πa/ λ. Rayleigh scattering gives reasonable accuracy when X<0.3. In a particle-free atmosphere, single scattering theory with Rayleigh scattering results in a small error in the visible, but appreciable errors in the UV.

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8 II. Atmospheric scattering 1. Atmospheric aerosols: a solid or liquid particle that is small enough to be suspended in a gaseous medium (small settling velocity).

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11 2. Most aerosols lie between µm, varying approximately as r -4, where r is the particle radius. There are about g of aerosols in the atmosphere, with g/yr produced, resulting in a lifetime in the troposphere on the order of hours to days removed by rain, and fallout of large particles due to their faster settling speed. Stoke s Law for terminal velocity equates the buoyant force/unit mass to the viscous force/unit mass: g( ρ-ρ ) = ρ 6πηwr ρ 4/3πr 3 Where η=viscosity coefficient; r=particle radius; w= vertical (fall) velocity. So as r increases, the settling velocity increases. By 10µm, Particles are falling at a speed of 2km/day. w= 2 9 gr2 (ρ- ρ ) η

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13 3. Total extinction due to atmospheric scattering depends on the cross sectional area of the particles, the scattering efficiency, and the number distribution: Total extinction πr 2 Q e ( r ) n(r) λ n(r) is number distribution The result is that the total extinction varies as the scattering efficiency- dominance by size range near that of the wavelength, because the more numerous small particles have much lower efficiency, and smaller cross-sectional areas. 4. As infrared radiation from the earth peaks near 10 µm, it is larger than most of the dry aerosols, so they act like Rayleigh scatterers with small effect. The effect on short wave radiation is somewhat greater, although even here more than 95% is scattered forward. 5. Aerosols are thought to be poor absorbers, a few % in the infrared, even less in the visible, but these values depend on the exact aerosol composition. For example, black soot particles from biomass burning can be strong absorbers in the visible.

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15 Global Aerosol distribution Green - carbonaceous particles (biomass burning) Red desert dust particles Blue sulfate aerosols (industrial air pollution)

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19 Ship Tracks First Indirect Effect

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23 Second Indirect effect Rainfall production

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32 III. Volcanoes and scattering 1. General levels for volcanic aerosol injection: 7-15 km (troposphere, short lifetime because soon washed out); 20-27km (stratosphere, long lasting); 40-50km (only a small amount ever gets this high. El Chichon 1982 Pinatubo 1991

33 2. The residence time in the stratosphere is less than one year for 2-5µm particles, years for particles of µ m.

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36 El Chichon

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38 3. For perhaps 6 months after a volcanic eruption, the stratosphere is dominated by silicate particles, µm basaltic glass, obsidian, Al 2 O 3. These are good scatterers up to 10 µm (and thus 10x better than the normal scattering efficiency in the infrared), with 75% of the scattering being forward. They are also better absorbers in the visible. After large eruptions, the scattering optical thickness can be τ>0.1 (about 20x larger than normal in the visible).

39 4. After 6 months, increased sulfate production, presumably due to SO 2 emissions increases scattering in the visible, and slightly increases absorption in the infrared.

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47 5. To assess the impact of volcanoes on the climate, it is necessary to know the ratio of absorption/backscatter for both short and long wave radiation, relative to the normal situation. 6. It is also necessary to compare these values to the surface albedo. If the surface albedo is very high (snow), warming can occur if absorption approximately equals backscatter, because that which is now being absorbed was previously lost to space. With a low surface albedo (oceans) most of the incident radiation was being absorbed anyway. So in this case the absorption needs to be greater than the backscatter to produce a net heating.

48 Stratospheric Temp change So/4[1-A] = τσt 4

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53 GeoEngineering

54 Homework: Rosenfeld et al., 2008: Flood or Drought: How do aerosols affect precipitation? SCIENCE, 321,