A Quote from A satellite view of aerosols in the climate system by Yoram J' Kaufman et al'

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• A Quote from A satellite view of aerosols in the
  climate system by Yoram J. Kaufman et al.
• During the last century, the Earths surface
  temperature increased by 0.6 C, reaching the
  highest levels in the last millennium.
• This rapid temperature change is attributed to a
  shift of less than 1 in the energy balance
  between absorption of incoming solar radiation
  and emission of thermal radiation from the Earth
  system.

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• Current Radiation Balance
• So 1368 W m-2
• Planetary Albedo a 0.7
• Solar Heating of the Earth System
• So/4 x a 240 W m-2
• OLR Solar heating 240 W m-2
• ? Brightness Temperature Tb 255 K
• Global warming in the last century 0.6 K
• ? Tb 255.6 K ? OLR 242 W/m2 ? DOLR 2 W m-2
  
• DOLR / OLR 1

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Estimated radiative forcing between 1850 and the
present due to human activites
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• Greenhouse gases, CO2, CH4, N2O
• Life-time 100 years
• ? homogenous spatial distribution
• Optical properties are well understood
• Aerosols
• Life-time 7 days (similar to water vapor)
• ? large spatial and temporal distributions
• Chemical, physical, and optical properties
• are not well understood

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• Aerosol Direct Radiative Effects
• Cooling effect
• reflection of solar radiation by aerosols
• cooling the surface
• Warming effect
• absorption of solar radiation by aerosols
• warming of the atmosphere
• cooling of the surface
• net warming effect
• Impact on
• radiation budgets of the earth-atmosphere
  system
• atmospheric stability and surface heat budgets

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• Aerosol Indirect Radiative Effects
• Aerosols serve as cloud condensation nuclei
• ? more cloud drops ?smaller drop size
• ? suppressed precipitation
• ? high cloud albedo
• ? cooling of the climate
• Compared to the aerosol direct radiative
  forcing,
• these indirect radiative effects are poorly
  understood.

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• Natural vs. Anthropogenic Aerosols
• The particle size of natural aerosols are
  generally larger than anthropogenic aerosols.
• If aerosol particle size can be inferred from
  satellite measurements, it is possible to
  separately retrieve the natural and anthropogenic
  aerosols.

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• Summary
• Aerosols have the effects of
• cooling the earth-atmosphere system
• stabilizing the atmosphere
• reducing precipitation
• Note The aerosol IR warming effect is very
  difficult to assess and, accordingly, has been
  ignored in nearly all studies.

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Sources and Types of Aerosols

• Fossil fuel combustion
• sulphate, carbonaceous material, black carbon
• Biomass burning
• smoke containing light-absorbing black carbon
• Wind blown dust
• Large particles, containing light-absorbing iron
  oxide
• Oceanic aerosols
• Coarse salt particles, sulfates (DMS), mostly
  light-scattering with little absorption
• Volcanic eruptions
• Dust, sulphate, nitrate,
• (Please read the article I distributed to you in
  class.)

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Gas-to-Particle Conversion (GPC)

• Heterogeneous nucleation
• growth of existing nuclei
• large particles 0.1-1.0 mm.
• Homogeneous nucleation
• formation of new particles
• small particles lt 0.1 mm.
• Aerosols involved in GPC
• sulphate, nitrate,
• carbonaceous and black carbon

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• Factors Affecting Aerosol Optical Thickness,
  Single Scattering Albedo, and Asymmetry Factor
• Amount (total column, vertical distribution)
• Particle size distribution (single mode, duel
  modes)
• Particle shape (spherical, irregular)
• Chemical composition (index of refraction)

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From Kaufman et al., A satellite view of aerosols
in the climate system. Nature, 2002.
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Aerosol Size Distribution

• Why is aerosol size important?
• It affects the absorption and
• scattering of light.
• For small aerosols,
• ? more reflection (larger ta)
• less absorption (smaller wa),
• less forward scattering (larger ga)

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• Observations of Aerosols
• Short Life-time of aerosols
• ? Large spatial and temporal variations
• ? Difficult to measure aerosols
• Derivations of Aerosol Properties
• ? Retrieval from ground measurements
• ? Retrieval from satellite observations
• ? Simulations using transport-chemical models

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• Ground Measurements
• Sunphotometers
• Seven spectral channels in UV,
• visible, and near IR solar spectral regions
• UV 0.34, 0.38, 0.44 mm
• Visible 0.50, 0.67 mm
• Near IR 0.87, 1.02 mm
• (We have sunphotometer measurements at Taipei
  and Tainan)

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• Direct sun radiance measurement, R(qo)
• Estimate aerosol optical thickness, ta, from
• R(qo) exp-(th2oto3ta)/cos(qo)
• qo is the solar zenith angle.
• th2o, to3, ta are optical thickness of water
• vapor, ozone, and aerosol.
• Diffuse sky radiance measurement, R(q, l)
• q is the zenith angle, l the azimuth angle
• Estimate aerosol single-scattering albedo and
• asymmetry factor (or particle size) from R(q, l)

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Satellite Retrieval

• Aerosols are retrieved from reflected solar
  radiation measured by satellite.
• Reflected solar radiation depends on aerosol
  optical properties and the surface albedo.
• Aerosols cannot be retrieved from satellite
  measurements of the emitted radiation in IR and
  microwave spectral channels.
• Emitted radiation in the IR and microwave
  channels depends not only on aerosol optical
  properties and the surface albedo but also on
  atmospheric temperature.

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Satellite Retrieval (Continued)

• Aerosol optical properties vary with wavelength
• optical depth
• single-scattering albedo
• asymmetry factor
• To retrieve these parameters, it requires
  measurements at different wavelength and angles
• Spectral observations
• Angular observations

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• Multiple channels
• Reflection of solar radiation by aerosols
  depends on particle size and wavelength.
• Over oceans, this property is used to
  distinguish small pollution particles from coarse
  sea-salt and dust particles.
• Retrieved optical thickness over high reflective
  land surface. A channel in the near infrared is
  only weakly affected by aerosols, which is used
  for estimating surface reflectivity.
• Multiple angles
• Satellite measured radiation is a function of
  aerosols and surface reflectivity.
• The contribution of surface reflectivity
  (aerosol) to the satellite-measured radiation
  depends weakly (strongly) on the variation of
  angles.
• This property is used to separate the surface
  effect from the aerosol effect and to estimate
  the surface reflectivity and aerosol optical
  thickness.
• Polarization
• High degree of polarization for large particles.
  Light reflected from the land surface is
  non-polarized.
• This property is used to retrieve large dust
  particles.

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Channel selection

• The satellite measured solar radiation depends
  on
• Aerosol particle size,
• Aerosol absorption,
• Aerosol total extinction,
• Surface reflectivity.
• These property can be estimated by properly
  choosing satellite channels.

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• Upper panels of the previous picture
• Small smoke particles
• Reflection by aerosols is high in the visible
  but is low
• in the near IR.
• Middle panels
• Large dust particles
• Reflection by aerosols is high in both visible
  and near IR.
• Lower panels
• Small smoke particles
• Reflection by aerosols is high in the visible
  but is low
• in the near IR.
• High surface albedo in both visible and near IR.

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Model Simulations of Aerosols(chemical-transport
model)

• Aerosol processes in the chemical-transport
  model
• Sources (types and amount)
• Gas-to-particle conversion (chemical
  transformation)
• Transport (use wind field from an atmospheric
  model)
• Deposition (due to gravity and precipitation)
• Model outputs
• Spatial and spectral distributions of aerosol
  optical thickness, single-scattering albedo, and
  asymmetry.

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Calculations of Aerosol Radiative Forcing

• Use aerosol optical properties either retrieved
  from satellite observations or simulated using a
  chemical-transport model
• Compute radiative fluxes using a radiation model
  with temperature, humidity, and cloud fields
  simulated by an atmospheric model.
• Two sets of radiation model calculations. One
  with aerosol included, and the other without
  aerosols included.
• Radiative flux difference between these two sets
  of calculations is the aerosol radiative forcing.

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Aerosols (Summary)

• Various sources/types of aerosols
• Fossil fuel combustions, dust, smoke, sea salt
• Large temporal and regional variations
• Short life time, 10 days
• Difficult to differentiate between aerosols and
  thin cirrus
• Difficult to retrieve aerosol properties over
  land
• High surface albedo
• Differences between various data sets of
  satellite-retrieved, as well as model-calculated
  aerosol optical thickness are large.
• Impact of aerosols on thermal IR is neglected.
• Potentially, aerosols could have a large impact
  on regional and global climate.