Multi-axis differential absorption spectroscopy (MAX-DOAS) at the And

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Multi-axis differential absorption spectroscopy
(MAX-DOAS) at the Andøya Rocket Range in February
and March 2003

• Folkard Wittrock, Hilke Oetjen, Andreas Richter,
  and John P. Burrows

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Structure

• Focus of DOAS observations
• DOAS analysis
• Introduction to MAX-DOAS
• Selected results from Andøya campaign
• BREDOM
• Summary and outlook

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DOAS observations Focus

• Determination of atmospheric trace gases
• Ozone, NO2, NO3, OClO, BrO, IO, HCHO
• Validation of satellites (GOME, SAGE, SCIAMACHY)
  and of model calculations
• standard data products
• vertical columns of ozone and NO2
• slant columns of OClO, BrO, HCHO and IO
• further data products
• vertical columns of BrO, OClO, HCHO
• tropospheric amounts e.g. for NO2, BrO

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Differential Optical Absorption Spectroscopy
(DOAS)

• Only narrowband, i. e. differential structures
  of the spectra are used to detect the absorbers.
• Broadband absorption and other broadband
  structures like instrumental features and
  features caused by Mie and Rayleigh scattering
  are removed by a polynomial.
• Basically, all kind of light sources can be
  used.
• Here Light scattered in the zenith or at
  the horizon

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DOAS Equation

• Radiation in matter is attenuated according to
  the Lambert-Beer-Law
• The current intensity I(l) is compared to a
  reference I0(l)
• A polynomial is added to compensate for
  scattering and other broadband structures
• Slant columns of several absorbers (with known
  absorption cross sections si) can be retrieved
  simultaneously
• The slant column is the density of the absorber
  along the photon path and depends on the SZA

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Multi Axis (MAX)-DOAS

• Extension of the light path in the troposphere
• High sensitivity for tropospheric absorbers,
  similar path through the stratosphere
• 3 angles to describe the geometry
• 1. elevation angle 2. SZA 3.
  relative azimuth

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Radiative Transfer and Vertical Columns

• Light path is simulated by a radiative transfer
  model
• The air mass Factor (AMF) weights the absorption
  due to the changing SZA, viewing angle, azimuth
• Vertical column (V) is the vertical density of
  the absorber
• Here SCIATRAN (CDIPI-Version) by A. Rozanov
• Input

• viewing geometry
• profile of absorbers
• T and p profiles

• wavelength region
• albedo
• aerosols

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2 Instruments Asymmetric Czerny-Turner
Spectrometer CCD

• CCDs
• 1320x400 pixel
• 1024x256 pixel
• Wavelength
• region
• 325 413 nm
• 330 490 nm
• FWHM
• 0.45 nm
• 0.60 nm
• Integration
• time
• 1 min

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Multi-Axis Telescopeon the roof of the old
Lidar-Building

• Commercial stainless steel box
• Shadings to avoid direct sun
• Pointing of telescope towards NNW

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MAX-DOAS telescope
Automated measurements in 4 directions 3,
7.5, 12.5 and zenith

• Different viewing directions given by moving
  mirror
• Box is heated to ensure operation of the motor
  and ice free windows
• Calibration unit with lamps Tungsten and HgCd
• Quartz fiber bundle adapter

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MAX-DOAS measurements data analysis

• Basic idea
• Use O4 to find correct model settings
• derive slant columns of O4 for given directions
• simulate O4 slant columns (vertical column is
  known) and vary aerosol scenario and surface
  albedo until closure for all lines of sight and
  also solar zenith angles is obtained
• Use information from different lines of sight to
  derive profile information for the absorber of
  interest
• simulate absorbers slant column using aerosol
  and albedo settings from O4 retrieval and vary
  profile until good agreement with retrieved slant
  columns is reached for all directions
• RTM SCIATRAN full spherical, multiple
  scattering, refraction

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MAX-DOAS measurements O4
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MAX-DOAS measurements NO2
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MAX-DOAS measurements BrO
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Andøya MAX-DOAS Summary and Outlook

• Results
• Multi Axis DOAS measurements from 4 different
  groups are being performed during the campaign
• O3, NO2, BrO columns can be retrieved
• tropospheric amounts of BrO and NO2 have been
  estimated
• Andoya measurements are very valuable to check
  the consistency of different MAX-DOAS setups
  Analysis still ongoing
• Problems
• clouds make the analysis very complex
• automated profile retrieval still under
  development

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Acknowledgments

• We like to thank the whole ARI staff for there
  great support and especially
• Reidar, Michael, June, Petter
• Tusen takk!

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Bremian DOAS network for atmospheric measurements
(BREDOM)

• Three tropical stations
• Similar setup for all measurement sites
• High-sensitivity DOAS-instruments for stand-alone
  operation
• Multiple viewing directions (MAX-DOAS)
• Retrieval of ozone and NO2 as well as minor
  absorbers (e.g. BrO, OClO, SO2, HCHO)

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What is the rationale behind BREDOM?

• For the validation of satellites (e.g. SCIAMACHY
  on ENVISAT), we need
• long term validation
• of many trace species
• on a global scale
• From recent validations (e.g. GOME) we learned,
  that
• DOAS UV/visible instruments can provide such
  validation
• that tropical stations are mandatory
• that tropospheric products need extra validation
• Build on the existing DOAS network, extend the
  range of species, add stations in the tropics,
  add capability to monitor troposphere

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Bremian DOAS network for atmospheric measurements
(BREDOM)

• Three tropical stations
• Similar setup for all measurement sites
• High-sensitivity DOAS-instruments for stand-alone
  operation
• Multiple viewing directions (MAX-DOAS)
• Retrieval of ozone and NO2 as well as minor
  absorbers (e.g. BrO, OClO, SO2, HCHO)

• Campaign instrument
• Kaashidhoo (5 N, 73 W, 5m asl) February
  March 1999
• Po area (46 N, 9 E, 400m asl) JulyAugust
  2002,
• SeptemberOctober 2003
• Andoya (69 N, 16 E, 20m asl) FebruaryMarch
  2003