introduction to measurement techniques in environmental physics summer term 2006
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Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
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)
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
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
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Principle of wavelength pairs (online - off-line)
C
ross
sec
ton
1 2
dxxndxxn
eI
I
I
I 21
20
10
2
1
dxxn
eII2
202
dxxn
eII1
101
dxxn
eI
I
I
I 21
20
10
2
1
dxxn
I
I
I
I21
20
10
2
1 lnln
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
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Dobson’s spectrophotometer I
Quartz platesAjustable wedgeFixed slitsPrismsDetector: photomultiplier
Org. photographic plate
Detailed description also to be found in script
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Dobson’s spectrophotometer II
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’ 332.4 453.6
Longest existing ozone time series
Used wavelength pairs
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Overview of satellite observations geometries
Measured signal:Reflected and scattered sunlight
Measured signal:Directly transmitted solar radiation
Measured signal:Scattered solar radiation
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Change in O2(1):
AO2[O2(1)]: volume emission rate, measured quantity
Steady state: [O3] = AO2[O2(1)] / JO3
retrieve [O3]
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
ΔOAOJdt
ΔOd 12O3O
12
23
Simplified chemistry !!
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
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 stratospheremost of the backscattered UV radiation comes from the tropospherelittle absorption by ozone occurs in the tropospherelittle scattering occurs in the stratosphereradiation 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
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Examples:
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 (in orbit) 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 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.
Backscatter UV Ozone measurements II
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
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
λI
THλ,Iexp
0
i)THτ(λ,- i
)LoS(TH
λext,
i
dxxkexp 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:
xkxkxkxk gasesλ
aerosolλ
Rayleighλλext,
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
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 II
If we assume that the cross-section does not depend on x, i.e., not on temperature and/or pressure, then
iO
THLoS
Oiozone THcσdxxnσTHλ,τ
3
i
3 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
Due to the different spectral characteristics of the different absorbers and scatterers the optical depth due to, e.g., O3 can be extracted.
LoS
Oiozone dxxnxσTHλ,τ
3
Absorption cross-section
O3 Number density
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
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.
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
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
0
10
20
30
40
50
60
70
80
90
10-6
10-5
10-4
10-3
10-2
10-1
305 nm
280 nm
Limb radiance [a.u.]
Tang
ent h
eigh
t [km
] “Knee”
Optically thin regime
Optically thick regime
At 280 nmModelled limb-radiance profiles
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
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 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
Limb scatter measurements II
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
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 twicea 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 thedue to nadir viewing ozone peak (~30 km) due to the
effects of multiple scattering andreduced 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 resolutioncontinuously
Advantages and disadvantages of measurement techniques:
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Example: Onion peel inversion of occultation observations
Earth x1x2x3x4x5
(TH1)(TH2)(TH3)(TH4)(TH5)
•
•
•
My
y
y
y
y
3
1
1
Nx
x
x
x
x
3
1
1
xy
A
xi : O3 concentration at altitude zi
yj : O3 column density at tangent height THj
MNMM
N
N
aaa
aaa
aaa
21
22221
11211
A
a11 a21/2a22
a32/2
The matrix elements aij correspond to geometrical path lengths through the layers
N
jjiji xay
1
Sun
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
yx -1A
Inverse of A exists if the determinant of A is not zero
Standard approach: least squares solution (assume N=M)
Mixay j
N
jijii ,,1,
1
Minimize:
by:
M
i
M
i
N
jijiji yxa
1 1
2
1
22
01
2
1
M
i
N
jijij
k
yxax
This leads to:
01 1
M
iik
N
jijij ayxa
yx TT AAA
yx TT AAA
-1
Unconstrained least squares solution
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Constrained least squares solution
M
i
N
jjj
M
i
N
jijij
N
jji xxyxaxx
1
2
11
1
2
1
2
1
2
Add additional condition to constrain the solution, e.g.:
is a smoothing or constraint coefficient coefficient
02
11
1
2
1
N
jjj
M
i
N
jijij
k
xxyxax
02 111 1
kkk
M
iik
N
jijij xxxayxa
0 xyx
HAAA TT yx TT AHAA
-1
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