Optical properties

1

• Optical properties
• of aerosol constituents
• Cathrine Lund Myhre and
• Claus J. Nielsen
• Department of Chemistry
• University of Oslo

2

• Aerosols in the atmosphere
• effects
• types
• uncertainties
• Experimental method
• Reflection spectroscopy
• Kramers Kronig transformations
• Optical properties of
• H2SO4/H2O
• HNO3/H2O
• (NH4)HSO4
• MSA (CH3)HSO3
• Oxalic acid (COOH)2
• Conclusions

3
Effects of aerosols in the atmosphere

• Climate
• Direct effect Absorbing and scattering radiation
  
• Indirect effect Change in cloud microphysics
• Ozone
• Heterogeneous reactions
• Uptake and removal of gasses

4

• Stratospheric aerosols
• Assumed to be of only natural sources
• H2O
• H2SO4/H2O
• HNO3/H2O
• H2SO4/HNO3/H2O

• Tropospheric aerosols
• Anthropogenic and natural sources
• Inorganic H2SO4, (NH4)HSO4, CH3SO3H, HNO3,
  (NH4)NO3, sea-salt, mineral dust, and mixtures
• Carbonaceous Soot, oxidation products as
  phthalic acid, pinonic acid and dicarboxylic
  acids (succinic acid, oxalic acid and more)
• Mixtures

5
Why study optical properties?

• IPCC (2001)
• There are large uncertainties connected to the
  impact of aerosols on climate
• the optical properties extinction coefficient,
  single scattering albedo, and asymmetry factor
  are of major importance in the quantification of
  the climate impact of aerosol
• Interpretation of remote sensing observations of
  aerosols, both in the stratosphere and in the
  troposphere
• Satellite and airborne observations by optical
  scattering techniques have commonly been used in
  measurements of stratospheric and tropospheric
  aerosols

6

• The complex index of refraction together with the
  particle size distribution determines all the
  aerosol optical properties.
• Redemann, et al., J. of Geophys. Res., 2000
• The modelling of the optical properties of
  aerosols relies on the knowledge of the complex
  index of refraction
• The interpretation of the remote sensing studies
  is dependant of the aerosols complex index of
  refraction
• The complex index of refraction
• Ñ(n) n(n) ik(n)

7
Cumulus clouds embedded in an absorbing aerosol
layer

• Evaporation of clouds
• Prevent cloud formation
• Affect local vertical circulations
• The complex index of refraction determines the
  absorption and scattering properties of the
  aerosol
• N(n)n(n)ik(n),
• S.E. Schwartz and P. Buseck,
• Science, 12. May, 2000, vol. 288

8
Reduction of tropical cloudness by soot
A. S. Ackerman et al., Science 288, 1042, (2000).
9

• Laboratory
• specular reflection spectroscopy in the infrared
  and UV/VIS region of pure and mixed aerosol
  components, a bulk study
• Modelling
• Mie scattering, extinction coefficient, single
  scattering albedo, and asymmetry factor with new
  data
• Test widely used assumptions in climate modelling
• Database

10
Method
11
From reflectance measurements to absorption and
refractive index
58.5 wt H2SO4/H2O, T293 K
Kramers-Kronig transformation
12
HNO3/H2OT293 K
13
H2SO4/H2OT293 K
14
H2SO4/H2O 48.6 wt
15
(NH4)HSO4/H2OT293 K
16
MSA (CH3)HSO3/H2OT293 K
17
Oxalic acid (COOH)2/H2OT293 K
18
Conclusions Optical properties in the
infrared region

• Inorganic constituents
• H2SO4/H2O
• The the absorption and refractive index, varies
  significantly with concentration and temperature,
  particularly in the concentration region 40-65
  wt.
• HNO3/H2O
• The optical properties are mainly determined by
  the concentration. To a good approximation n(v)
  and k(n) is independent of temperature.
• (NH4)HSO4/H2O
• The change in optical properties compared to pure
  water may have significant influence on the
  radiative properties of the aerosols. Optical
  properties of (NH4)2SO4 and mixtures are required.

19
Conclusions Optical properties in the
infrared region

• Organic constituents
• MSA
• The results indicate a significant change in the
  optical properties compared to water and aqueous
  sulfuric acid.
• Oxalic acid
• The first results of the refractive and
  absorption index in the infrared region of this
  component indicate that the difference between
  the optical constants of oxalic acid and water is
  small