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Spectroscopy of Trace Elements in the Atmosphere

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

  2. 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

  3. 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

  4. Types of spectroscopy used • FTIR (Fourier Transform Infrared) spectroscopy • Space born methods: ATMOS (Atmospheric Trace MOlecule Spectroscopy Experiments) • LIDAR method (LIght Detection And Ranging)

  5. FTIR Spectroscopy

  6. 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)

  7. 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

  8. Ozone results

  9. 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)

  10. 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)

  11. 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

  12. 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

  13. 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

  14. 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

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