1. 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
Date
9 – 11
11 – 13
14 – 16
April 19
Atmospheric Remote Sensing I (Savigny)
Oceanography (Mertens)
Atmospheric Remote Sensing II (Savigny)
April 26
DOAS (Richter)
Radioactivity
(Fischer)
Measurement techniques in Meteorology (Richter)
May 3
Chemical measurement techniques (Richter)
Soil gas ex- change (Savigny)
Measurement Techniques in Soil physics (Fischer)

2. 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

3. Principle of wavelength pairs (online - off-line)
Cross secton  1 2
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

4. Dobson’s spectrophotometer I
Quartz plates
Ajustable wedge
Fixed slits
Prisms
Detector: photomultiplier
Org. photographic plate
Detailed description also to be found in script

5. Dobson’s spectrophotometer II
Used wavelength pairs
Name
WL 1 [nm]
WL 2 [nm] A
305.5
325.4 B
308.8
329.1 C
311.45
332.4 D
317.6
339.8 C’
453.6
Longest existing ozone time series

6. Overview of satellite observations geometries
Measured signal:
Reflected and scattered sunlight
Measured signal:
Directly transmitted solar radiation
Measured signal:
Scattered solar radiation

7. Ozone measurements using emissions
Usually measure longwave radiation thermally emitted by the atmosphere along the line 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
Simplified
chemistry !!
Change in O2(1):
AO2[O2(1)]: volume emission rate, measured quantity
Steady state: [O3] = AO2[O2(1)] / JO3  retrieve [O3]

8. 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

9. Backscatter UV Ozone measurements II
Examples:
BUV (Backscatter Ultraviolet) instrument on Nimbus 4,
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 ( ), NOAA-14 (in orbit) can measure ozone profiles as well as columns
TOMS (Total Ozone Mapping Spectrometer) first on Nimbus 7, operated from 1978 to Then three subsequent versions: Meteor 3 ( ), ADEOS (1997), Earth Probe (1996-). Measures total ozone columns.
GOME (Global Ozone Monitoring Experiment) launched on ESA's ERS-2 satellite in 1995 employs 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.

10. 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:

11. 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.
Absorption cross-section
O3 Number density
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

12. Solar occultation measurements III
Examples:
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.

13. Limb scatter measurements I
“Knee”
Optically thin regime
Optically thick regime
At 280 nm
Modelled limb-radiance profiles
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

14. Limb scatter measurements II
Examples:
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/visible sensors (UVISI)
SOLSE (Shuttle Ozone Limb Sounding Experiment) flown on the Space Shuttle flight in 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 Retrieval of vertical profiles of O3, NO2, OClO, BrO and aerosols

15. Advantages and disadvantages of measurement techniques:
TECHNIQUE ADVANTAGES DISADVANTAGES
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
continuously

16. Example: Onion peel inversion of occultation observations
(TH1)
(TH2)
(TH3)
(TH4)
(TH5) •
Sun
xi : O3 concentration at altitude zi
yj : O3 column density at tangent height THj
Earth x5 x4 x3 x2 x1
The matrix elements aij correspond to geometrical path lengths through the layers

17. Unconstrained least squares solution
Inverse of A exists if the determinant of A is not zero
Standard approach: least squares solution (assume N=M)
Minimize:
by:
This leads to:

18. Constrained least squares solution
Add additional condition to constrain the solution, e.g.:
 is a smoothing or constraint coefficient coefficient

19. End of lecture