Ground and Satellite Observations of Atmospheric Trace Gases

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Ground and Satellite Observations of Atmospheric
Trace Gases

• George H. Mount
• Laboratory for Atmospheric Research
• WSU
• 13 April 2007

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Outline

• Aura/OMI - Ozone Monitoring Instrument on the
  Aura satellite
• Aura ground truth/validation of data
• MFDOAS instrument for urban airshed trace gas
  measurements and satellite validation
• NASA INTEX B results, PNNL, spring 2006

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Aura/OMI

• WSU involvement began in 1997 at the inception of
  the OMI project - Dutch instrument on NASA bird,
  small US team
• double channel spectrograph covering 270 - 510 nm
  at 0.7nm resolution
• launched in July 2004
• measures column O3, NO2, BrO, OClO, CH2O, SO2,
  aerosol indices
• data products
• atmospheric column of above gases
• trop NO2
• trop O3 - not yet routinely available
• aerosol data

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trace gas measurements observing the Earths
backscattered uv/visible radiation
Sun
Sunlight passes through the atmosphere,
reflects off clouds and the surface, and is
scattered back into the instrument field of view.
Molecular spectral absorption is proportional to
the concentration of the gas doing the absorbing
along the path.
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Observing Principle for OMI
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GOME



Sciamachy

OMI


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TOMS
single pixel size for four satellite instruments
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OMI single pixel
OMI pixel 12 km x 13 km superposed onto the
Seattle airshed (zoom mode)
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• New WSU MFDOAS Instrument
• WSU was funded 3 years ago to develop a new
  ground based instrument that would mimic the
  satellite measurements from the ground support
  validation of the Aura satellite data from the
  ground
• new instrument uses the molecular spectrum of the
  sky and direct sun as light sources for
  measurement of urban air pollution
• scans the sky at low elevation angles where the
  tropospheric air mass is significantly enhanced
• completing development at WSU as we speak
• ground based campaign at NASA Goddard Space
  Flight Center 7-21 May
• ground based campaign at NASA Jet Propulsion
  Laboratory 1-15 July
• was fielded in prototype form during the NASA
  INTEX at Pacific Northwest National Laboratory in
  central Washington spring 2006

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MF-DOAS geometryfor sky-viewing mode
stolen from Platt group
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WSU MFDOAS instrument
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WSU MFDOAS data during INTEX B at PNNL,
Richland, WA
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OMI data during INTEX over PNNL, WA spring
2006 comparison with MFDOAS shows a 20 bias
between OMI and MFDOAS with OMI underestimating
NO2 column. Other validations show a similar
underestimate.
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• Things to consider in using satellite data
• basic limitations
• can only observe when clear - problem in
  Pacific NW in winter
• tied to equator crossing time (1345h for Aura)
  for a polar orbiter
• cannot get more than a couple of orbits of data
  before the urban area rotates
• out of the FOV - orbital period 90 minutes
• cannot observe at night for most instruments
• accuracy of the trop result depends on removing
  the strat overburden (if exists)
• trace gases
• ozone and NO2 total columns are done well from
  space
• problem is the stratospheric overburden --gt
  must separate strat/trop
• this is not an easy problem - cloud slicing,
  use of other instruments onboard
• RT codes have advanced this a lot in the last 5
  years (spherical codes)
• CO can be done fairly easily in the IR part of
  the spectrum
• SO2 and HCHO are very hard due to low levels in
  the trop - few ppbv sensitivity
• aerosols are measured by using uv radiances and
  work well

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• temporal resolution
• orbital period of 90 min --gt get a picture
  each period until airshed
• rotates out of the swath - larger swath angles
  are better --gt information
• on multiple orbits
• may take several days to build up an image due
  to spatial scanning swath
• satellite moves at 7 km/sec --gt ground track
  always moves at that speed
• integration time to get good s/n - sets
  footprint in velocity vector
• direction (NS)
• short int. times produce low s/n, especially in
  the Huggins O3 bands
• time to get complete global coverage - e,g, GOME
  3d

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to Farrens talk
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• Spectroscopic Technique for Air Pollution
  Measurements
• measure the absorbed spectrum of sunlight
  reflected from the Earths surface
• many tropospheric trace gases have complicated
  molecular spectra allowing
• identification and quantification of
  concentration, e.g.
• need a telescope to look down on the Earth
• to collect light
• to image the Earth surface onto the
  spectrograph over
• a wide field of view (e.g. for OMI 114
  2600 km) at high
• spatial resolution (e.g. for OMI 12 km x 24
  km pixel size in global mode)

NO2 photoabsorption cross section
Harder, Brault, Johnston, and Mount, 1997
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• basic physics is very simple
• solar light traverses an atmospheric path
  Sun --gt Earth surface, then
• reflecting Earth surface/clouds --gt satellite
  sensor
• measurement of spectrum of incoming light -
  solar irradiance spectrum
• measurement of spectrum of light into sensor
  (reflected solar
• spectrum from Earth Earth radiance spectrum)
• ratio of Earth radiance spectrum to solar
  spectrum
• elimination of spectrum of solar spectrum (to
  first order)
• reveals absorption spectrum of atmospheric
  molecules of interest
• absorbance depth is proportional to the
  abundance of that molecule
• along the absorption path
• note this is NOT a tropospheric concentration
• technique only useful if there is
  differential absorption from
• the molecule doing the absorbing - a smooth
  continuum spectrum with no
• spectral structure only depresses the entire
  intensity of the spectrum

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• Current Satellite Data - TOMS, GOME, TES, MOPITT,
  Sciamachy
• spatial footprint
• currently operational satellite doing air
  pollution work
• Sciamachy footprint 30 km x 60 km -
  scanning mirror for swath
• GOME 80 km x 340 km - scanning mirror for
  swath
• TOMS 50 km x 200 km - scanning mirror for
  swath
• MOPITT 22 km x 22 km - scanning mirror for
  swath - CO
• TES 5 km x 9 km ozone
• temporal resolution at time of overpass
  (typically about 130PM)
• get a picture each orbit of 90 min duration
  when airshed underneath
• with a large swath, get several orbits of data
  sequentially over airshed
• with small swath, may take several days to
  build a picture (GOME -3d)
• time of day is restricted depends on
  equatorial time transit
• e.g. for Aura, it is 1345 h --gt obsv at same
  time of day each day

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WSU MFDOAS data from PNNL during INTEX
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DOAS Theory

• Measure wavelength dependent light intensity
  (I?) as light passes through the air mass
• Initial intensity (Io ?) decreases in the
  airmass due to
• absorption by the trace gases,
• scattering by molecules and aerosol particles
• trace gases can be detected in the ratio of I ?
  to Io? as a function of wavelength due to their
  unique absorption features

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DOAS Theory Beer-Lambert Law

• Theory
• Reality

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Air Mass Factors Geometrical Approach
Total Slant Column
Recent Developments in DOAS an Overview. Ulrich
Platt. Institut für Umweltphysik, Universität
Heidelberg