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Spectroscopy of Trace Elements in the Atmosphere. Presentation by Nicola Lumley. 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

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Spectroscopy of Trace Elements in the Atmosphere


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what are trace elements in the atmosphere
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 CFC’s, benzene, mercury, lead
why do we need to monitor them
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 CFC’s

O3 + Cl ----> ClO + O2 ClO + O ---> Cl + O2

types of spectroscopy used
Types of spectroscopy used
  • FTIR (Fourier Transform Infrared) spectroscopy
  • Space born methods: ATMOS (Atmospheric Trace MOlecule Spectroscopy Experiments)
  • LIDAR method (LIght Detection And Ranging)
ftir spectroscopy1
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)
resolution
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
atmos atmospheric trace molecule spectroscopy experiments
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)
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

LIDAR (LIght Detection And Ranging)
plain lidar
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
ramon lidar
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
resonance lidar
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
references
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