Spectroscopy of Trace Elements in the Atmosphere

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Spectroscopy of Trace Elements in the Atmosphere

• Presentation by Nicola Lumley

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What are Trace elements in the atmosphere?

• Nitrogen, Oxygen, Water Vapour and Argon make up
  99.6 of the atmosphere
• Less than 0.5 is composed of several hundred
  trace elements
• CO2 is most abundant at 360ppm in lower
  atmosphere
• Includes green house gases such as methane 1.7ppm
  and CO 0.1ppm
• NO and NO2 are present in the stratosphere
• Tropospheric Ozone 0.02ppm
• Pollutants such as CFCs, benzene, mercury, lead

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Why do we need to monitor them?

• Maintain energetic balance of the earth
• Surface temp is 33K higher than without some of
  these gases (greenhouse effect)
• However the increase of many trace elements leads
  to global warming
• Many other trace elements are harmful Mercury,
  lead, benzene
• Tropospheric ozone causes smog and is potentially
  harmful
• Stratospheric ozone blocks harmful radiation but
  decreased by the increase of chlorine free
  radicals from CFCs
• O3 Cl
  ----gt ClO O2
  ClO O ---gt Cl O2

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Types of spectroscopy used

• FTIR (Fourier Transform Infrared) spectroscopy
• Space born methods ATMOS (Atmospheric Trace
  MOlecule Spectroscopy Experiments)
• LIDAR method (LIght Detection And Ranging)

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FTIR Spectroscopy
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FTIR Spectroscopy

• Converts interference patterns into a spectrum
  using algorithms based on Fourier Transforms
• Sun (12100/cm) or moon (770/cm) as light source
• Intensity of the absorption is proportional to
  the concentration
• Collects data in IR (700/cm)to UV (33000/cm) most
  trace elements absorb in IR range
• Vertical concentration can be found by looking at
  the shape of the spectral lines Doppler
  broadening (above 40km) and Pressure broadening
  (below 10km)

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Resolution

• Calibrated using the spectrum from samples of
  known concentration at two extreme temperatures
• Resolution depends on signal to noise ratio
• SNR ? (t/tA)1/2
• t measurement time, tA time to
  measure one channel
• Quality of beam splitter and mirrors
• x is measured using a laser
• Position of reading high altitude
• Solar resolution 0.0035/cm, lunar resolution
  0.02/cm

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Ozone results
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ATMOS (Atmospheric Trace MOlecule Spectroscopy
Experiments)

• Uses Michelson Interferometer (600 to 5000/cm)
  resolution 0.015/cm
• Also provides information about temperature
• Only sunset and sunrise
• Better resolution above Tropopause due to limb
  path
• First flight in 1985
• 1193 atmospheric spectra recorded of
  unprecedented quality (plus 1474 solar)

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LIDAR (LIght Detection And Ranging)

• Several different types Plain, Ramon, Resonance
• Use ruby or neodymium lasers
• LIDAR Equation
• Different types of scattering Mie, Raleigh,
  Ramon, Florescence

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Plain LIDAR

• Elastic scattering with aerosols Raleigh and Mie
• vr v0
• Signal is recorded as a function of time
• Maps the distribution in the troposphere and
  stratosphere

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Ramon LIDAR

• Uses spectrometer to record the frequencies
  reflected and shifted due to Ramon scattering
• Concentration ? intensity
• Selection rules
• ?v 0, 1 ?J 0, 2
• Detects large concentrations CO2 SO2 H2O
• Requires high powered lasers and large telescopes

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Resonance LIDAR

• Laser and detector frequencies match the
  absorption
• Stimulates resonance scattering which increases
  power
• More accurate in lower atmosphere, however in
  upper atmosphere quenching occurs

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References

• Springer- Verlag Topics in Applied Astrophysics
  Vol 14 laser monitoring or the atmosphere
• Clark Hester- Spectroscopy in Environmental
  Science
• Arndt Meier- Reports on Polar Research