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Introduction to Measurement Techniques in Environmental Physics Summer term 2006 Postgraduate Programme in Environmental Physics University of Bremen Atmospheric Remote Sensing II Christian von Savigny. Overview – Lecture 2.

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Introduction to Measurement Techniques in

Environmental Physics

Summer term 2006

Postgraduate Programme in Environmental Physics

University of Bremen

Atmospheric Remote Sensing II

Christian von Savigny


Overview – Lecture 2

  • Principle of wavelength pairing to remotely sense atmospheric O3 and also other constituents
  • Overview of retrieval Techniques for atmospheric remote sensing of atmospheric constituents
      • Emission measurements
      • Nadir-Backscatter
      • Occultation
      • Limb-scattering
  • Retrieval example: onion-peel inversion of solar occultation measurements

Cross secton 



Note: Absorption cross section typically also dependent on x

Complication: Light path dependent on 

Known from measurements of solar spectrum

Absorber column amount along effective light path

Principle of wavelength pairs (online - off-line)


Dobson’s spectrophotometer I

Quartz plates

Ajustable wedge

Fixed slits


Detector: photomultiplier

Org. photographic plate

Detailed description also to be found in script


Dobson’s spectrophotometer II

Used wavelength pairs

Longest existing ozone time series


Overview of satellite observations geometries

Measured signal:

Directly transmitted solar radiation

Measured signal:

Reflected and scattered sunlight

Measured signal:

Scattered solar radiation


Ozone measurements using emissions

  • Usually measure longwave radiation thermally emitted by the atmosphere along theline of sight of the instrument
  • Infrared (9.6 μm ozone band), microwave, and NIR wavelengths (760nm, 1270 nm)
  • Limb sounding or nadir sounding viewing geometries
  • Used to retrieve ozone profiles and total columns

Example:Mesospheric O3 profiles retrieved from O2(1) measurements

Main source of O2(1) in dayglow: photolysis of ozone

O3 + h O2(1)JO3

O2(1) O2 + h AO2


chemistry !!

Change in O2(1):

AO2[O2(1)]: volume emission rate, measured quantity

Steady state:[O3] = AO2[O2(1)] / JO3  retrieve [O3]


Backscatter UV Ozone measurements I

  • Solar UV radiation reflected from the surface and backscattered by atmosphere or clouds is absorbed by ozone in the Hartley-Huggins bands (< 350 nm)
  • Note:
    • most ozone lies in stratosphere
    • most of the backscattered UV radiation comes from the troposphere
    • little absorption by ozone occurs in the troposphere
    • little scattering occurs in the stratosphere
    • radiation reaching the satellite passes through the ozone layer twice
  • Backscatter UV measurements allow retrieval of total O3 columns and also vertical profiles, but with poor vertical resolution (7 – 10 km)
  • Measure O3 slant column and use Radiative transfer model to convert to vertical column

Backscatter UV Ozone measurements II


BUV (Backscatter Ultraviolet) instrument on Nimbus 4, 1970-1977

SBUV (Solar Backscatter Ultraviolet) instrument on Nimbus 7, operated from 1978 to 1990

SBUV/2 (Solar Backscatter Ultraviolet 2) instrument on the NOAA polar orbiter satellites: NOAA-11 (1989 -1994), NOAA-14 (inorbit)can measure ozone profiles as well as columns

TOMS (Total Ozone Mapping Spectrometer)first on Nimbus 7, operated from 1978 to 1993. Then three subsequent versions: Meteor 3 (1991-1994), ADEOS (1997), Earth Probe (1996-). Measures total ozone columns.

GOME (Global Ozone Monitoring Experiment)launched on ESA's ERS-2 satellite in 1995employs a nadir-viewing BUV technique that measures radiances from 240 to 793 nm. Measures O3 columns and profiles, as well as columns of NO2, H2O, SO2, BrO,OClO.


Solar occultation measurements I

I0() spectrum at the highest tangent altitude with negligible atmospheric extinction

I(,zi) spectrum at tangent altitude zi within the atmosphere

LoS: line of sight

With kext, being the total extinction coefficient at position s along the line of sight LoS.

Extinction is usually due to Rayleigh-scattering, aerosol scattering and absorption by minor constituents:


Absorption cross-section

O3 Number density

Solar occultation measurements II

Due to the different spectral characteristics of the different absorbers and scatterers the optical depth due to, e.g., O3 can be extracted.

If we assume that the cross-section does not depend on x, i.e., not on temperature and/or pressure, then

With the column density c(zi)

The measurement provides a set of column densities integrated along the line of sight for different tangent altitudes zi.

 Inversion to get vertical O3 profile

Disadvantage of solar occultation measurements:

The occultation condition has to be met: Measurements only possible during orbital sunrises/sunsets

For typical Low Earth Orbits there are 14–15 orbital sunrises and sunsets per day

 poor geographical coverage


Solar occultation measurements III


SAGE (Stratospheric Aerosol and Gas Experiment) Series provided constinuous observations since 1984 to date

Latest instrument is SAGE III on a Russian Meteor-3M spacecraft

HALOE (Halogen Occultation Experiment) on UARS (Upper Atmosphere Research Satellite) operated from 1991 until end of 2005, employing IR wavelengths

POAM (Polar Ozone and Aerosol Measurement) series use UV-visible solar occultation to measure profiles of ozone, H2O, NO2, aerosols

GOMOS (Global Ozone Monitoring by Occultation of Stars) on Envisat will performs UV-visible occultation using stars

SCIAMACHY (Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY) on Envisat performs solar and lunar occultation measurements providing e.g., O3, NO2, and (nighttime) NO3 profiles.


Modelled limb-radiance profiles

At 280 nm

Optically thin regime


Optically thick regime

Limb scatter measurements I

    • Radiation is detected which is scattered into the field of view of the instrument along the line of sight and also transmitted from the scattering point to the instrument
    • Solar radiation pickts up absorption signatures along the way
    • Also: Light path can be modified by atmospheric absorption
    • Vertical profiles of several trace constituents can be retrieved fom limb- scattered spectra emplying a radiative transfer model (RTM) to simulate the measurements
  •  Retrieval without forward model not possible

Limb scatter measurements II


SME (Solar Mesosphere Explorer)launched in 1981, carried the first ever limb scatter satellite instruments. Mesospheric O3 profiles were retrieved using the Ultraviolet Spectrometer and stratospheric NO2 profiles were retrieved using the Visible Spectrometer

MSX satellite – launched in 1996 , carried a suite of UV/visiblesensors (UVISI)

SOLSE(Shuttle Ozone Limb Sounding Experiment) flown on the Space Shuttle flight in 1997. Provided good ozone profiles with high vertical resolution down to the tropopause

OSIRIS (Optical Spectrograph and Infrared Imager System) launched in 2001 on Odin satellite. Retrieval of vertical profiles of O3, NO2, OClO, BrO

SCIAMACHY (Scanning Imaging Absorption SpectroMeter for Atmospheric CHartographY), launched on Envisat in 2002. Retrieval of vertical profiles of O3, NO2, OClO, BrO and aerosols


Advantages and disadvantages of measurement techniques:


Emission•doesn’t require sunlight •slightly less accurate than

•long time series backscatter UV

•simple retrieval technique •long horizontal path for limb obs.

•provides global maps twice

a day (good spatial coverage)

Backscatter UV•accurate • requires sunlight, so can’t be

• long time series used at night or over winter poles

• good horizontal resolution • poor vertical resolution below the

due to nadir viewing ozone peak (~30 km) due to the

effects of multiple scattering and

reduced sensitivity to the profile shape

Occultation• simple equipment • can only be made at satellite

• simple retrieval technique sunrise and sunset, which limits

• good vertical resolution number and location of meas.

• self-calibrating • long horizontal path

Limb Scattering• excellent spatial coverage • complex viewing geometry

• good vertical resolution

• data can be taken nearly • poor horizontal resolution









Example: Onion peel inversion of occultation observations





xi : O3 concentration at altitude zi

yj : O3 column density at tangent height THj







The matrix elements aij correspond to geometrical path lengths through the layers


Unconstrained least squares solution

Inverse of A exists if the determinant of A is not zero

Standard approach: least squares solution (assume N=M)



This leads to:


Constrained least squares solution

Add additional condition to constrain the solution, e.g.:

 is a smoothing or constraint coefficient coefficient