# Aerosol and climate Chul Eddy Chung ( 정 철 ) GIST, Korea.

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Aerosol and climate Chul Eddy Chung ( 정 철 ) GIST, Korea

How does aerosol change climate? 1.Direct effect: Aerosol absorbs or scatters radiation in the atmosphere. Aerosols, except dust, interfere mainly with solar radiation. 2.Indirect effect: Some aerosols act as cloud condensation nuclei (CCN), thus affecting cloud albedo and lifetime (so-called indirect effect). 3.Darkening effect: Dark color aerosols can be deposited on sea ice, snow packs and glaciers, thus darkening the snow and ice surfaces, and enhancing the absorption of sunlight. Dark aerosols can go inside of a raindrop and enhance solar radiation absorption. 4.Semi-direct effect: Some of the aerosols can absorb sunlight efficiently and heat the atmosphere. This can burn cloud.

External change: Increased CO2, increased aerosol, etc. Radiative forcingGlobal Ts change Radiative forcing, Climate forcing

AOD (Aerosol Optical Depth) When a beam of light is attenuated, we call this attenuation extinction. Extinction is a result of scattering plus absorption. Aerosols can scatter and absorb light, and the attenuation due to aerosol is called aerosol extinction. Aerosol extinction will weaken the light intensity from I λ to I λ + dI λ after traversing a thickness ds in the direction of its propagation. λ represents wavelength. Then, the following equation holds: d I λ = −k λ ρI λ ds (1) where ρ is the density of the material, and k λ denotes the mass extinction cross section (in units of area per mass). k λ ρ is referred to as the aerosol extinction coefficient, whose units are given in terms of length (typically, cm −1 ). The aerosol extinction coefficient is the sum of the aerosol scattering coefficient and the aerosol absorption coefficient.

AOD vs. aerosol mass Aerosol speciesMEC (m 2 /g) SO 4 (sulfate)6.1 NO 3 (nitrate)5.5 OM (Organic Matter)4.5 BC (Black Carbon)15.0 2 : Estimated by Magi (2009, 2011)

Annual-mean clear-sky aerosol forcing as a function of AOD at 550 nm. The simulation is made with a Monte-Carlo radiation model as in Chung et al. (2005), which only considered solar radiation. Specified parameters are SSA=0.19 at all the wavelengths, ASY=0.7 at 550 nm, α=1.4, and land surface albedo of 0.15 at a latitude of 21˚N.

AOD (Aerosol Optical Depth) = AOT

SSA (Single Scattering Albedo) The SSA is defined as the ratio of the scattering to the extinction. Extinction is the sum of scattering and absorption. When photons are scattered, the wavelength remains unchanged. SSA is a function of wavelength. Aerosol speciesSSA at 550nm BC0.16~0.20 OM0.80~0.98 dust0.90~0.97

10  What is BC?  So-called soot emitted during incomplete combustion of hydrocarbon is blackish or brownish in color. Soot quickly evolves into aggregates of fine particles [Katrinak et al. 1993]. These fine particles are called spherules or monomers. Aggregate soot particle consists of pure carbon and other minor components, and was referred to as soot carbon by Andreae and Gelencsér [2006]. We refer to this soot carbon as black carbon (BC) here. A BC particleMonomer or spherule

11  One of the issues in understanding the wide range of the model based estimates is whether BC is internally mixed or externally mixed with other aerosols in the calculation.  Studies (e.g., Moffet and Prather, 2009) have shown that BC particles commonly exist in the ambient atmosphere as coated by scattering material such as sulfate, organics, nitrate.  Many of the model based estimates treat BC as 100% externally mixed with other aerosols [Textor et al. 2006].  When BC is coated by scattering material, BC forcing is estimated to be larger [Jacobson 2001]. BC Internally mixed BC

SSA and aerosol forcing

SSA (Single Scattering Albedo)

ASY (Asymmetry Parameter) When aerosols scatter light, the phase function describes the angular distribution of scattered energy. The phase function P(cosΘ) is a normalized function, such that where Θ refers to the angle between the direction of incoming light and that of the scattered light. When Θ π/2. The asymmetry parameter, or asymmetry factor, g is defined as follows:

ASY and aerosol forcing

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