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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 I
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
General principles of Remote Sensing
Radiation Source
Interaction with atmospheric constituents
(may also be radiation source)
Dispersive element
Radiation detector
Instrument
Interaction of radiation with the
atmosphere
Uncalibrated raw data
Calibration procedure
Calibrated spectra / radiances
A priori information
Retrieval procedure
Inversion from radiation spectra to species of interest
Forward model
Interaction of radiation with the
atmosphere
AD converter
Data product of interest
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Overview – Lecture 1
• Introduction
• Brief summary of relevant aspects of radiative transfer
• Radiation-dispersing devices
• Radiation detectors
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Distinction of In-situ and Remote Sensing Techniques
In situ Remote sensing (using EM radiation)
Target directly accessible
Target NOT directly accessible
Active Passive
- taking samples: e.g., air to determine O3, CO2 concen-
trations etc.
- using thermometers,barometers, hygrometers etc.
- using electromagnetic
radiation: e.g., • Rocket-borne Lyman-
hygrometer• Balloon-borne DOAS
with white cell
- RADAR (Radiation Detection and Ranging)
- LIDAR (Light Detection and Ranging)
Lidars are used to measure profiles of temperatures, O3, stratospheric aerosols, to detect polar stratospheric clouds, polar mesospheric clouds and tropospheric cloud top heights (ceilometers)
RADARs are used to measure cloud structure, cloud top - bottom. Doppler RADARs for wind-speed measurements
Measurement of radiation originating in the atmosphere / the surface / the sun and interacting with the target (atmosphere, ocean, surface).
Used is: Microwave, sub-mm, thermal, IR, UV/Vis radiation
Platforms:Ground-based, aircraft, balloon, rocket, satellite
Platforms:Ground-based, aircraft, balloon, rocket, satellite
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Examples of remotely sensed atmospheric fields I
RADAR measurements of cloud structure
Measurement type: Ground-based active remote sensing
Instrument: GKSS Radar
Measured quantity: Cloud structure, cloud top/bottom height(backscattered RADAR radiation)
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Examples of remotely sensed atmospheric fields II
http://ww
w.iu
p.physik.uni-b
reme
n.de/scia-a
rc/
Global measurements of stratospheric ozone profiles
Measurement type: Satellite-based passive remote sensing
Instrument: SCIAMACHY/Envisat
Measured quantity: Stratospheric ozone profiles(from backscattered solar radiation)
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Examples of remotely sensed atmospheric fields III
http://ww
w.iu
p.physik.uni-b
reme
n.de/g
ome
nrt/
Global measurements of total ozone columns
Measurement type: Satellite-based passive remote sensing
Instrument: GOME/ERS-2
Measured quantity: Total ozone columns(from backscattered solar radiation)
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Examples of remotely sensed atmospheric fields IV
Measurement type: Satellite-based passive remote sensing
Instrument: SCIAMACHY/Envisat
Measured quantity: Mesopause (about 87 km) temperature(from atmospheric airglow emissions)
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
The electromagnetic spectrum
100 m 10-4 cm-1
10 MHz
10 m 10-3 cm-1 Radio
100 MHz
1 m 10-2 cm-1
1 GHz
10 cm 0.1 cm-1
10 GHz Microwave 1 cm 1 cm-1
100 GHz
1 mm 10 cm-1
1 THz sub-mm – Far IR 0.1 mm 100 cm-1
10 THz
10 μm 1000 cm-1 Thermal IR
al IR 100 THz
Near IR 1 μm 104 cm-1
1000 THz Ultraviolet
100 nm 105 cm-1
Wavelength Frequency Wave number
Visible 400-700 nm
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
The optical (UV-visible-NIR) spectral range
1 nm 10 nm 100 nm 200 nm 400 nm 700 nm
5 m
VisibleVacuum UV Near UV NIR IREUVX-rays
100 nm 400 nm320 nm280 nm
UV AUV C UV B
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Advantages of Remote Sensing ?
• Measurements in inaccessible areas possible
• No perturbation of the observed air volume
• Remote sensing facilitates creation of long time series and extended measurement areas
• Satellite-based remote sensing measurements allow global observations
• Measurements can usually be automated
• In many applications several parameters can be measured at the same time
• On a per measurement basis, remote sensing measurements usually are less expensive than in-situ measurements
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Disadvantages of Remote Sensing ?
• Remote sensing measurements are always indirect measurements
• The electromagnetic signal is often affected by several factors/processes, and not only by the object of interest
• Satellite-borne instruments cannot be calibrated any more on-ground
Instrument degradation leads to retrieval errors
• Usually, additional assumptions and models are needed for the interpretation of the measurements
• Often relatively large measurement areas / volumes
• Validation of remote sensing measurements is a major task and often not possible in a strict sense
• Estimation of the remote sensing retrieval errors is difficult
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Summary of relevant radiative processes
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Basic Processes of Radiative Transfer
• Absorption by molecular species and particulates (aerosols)
1) Ionization - dissociation
2) Electronic transitions
3) Vibrational transitions
4) Rotational transitions
• Scattering by molecular species and aerosols (elastic/inelastic)
1) Rayleigh scattering (elastic)
2) Mie scattering (elastic)
3) Raman scattering (inelastic)
• Emission of radiation
• Reflection of radiation
i
i
s
e
outini r
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Absorption of radiation
Absorption of EM radiation travelling through a medium is mathematically described by Lambert-Beer’s Law:
I0 Initial intensityI(x) Intensity at x(,x) Absorption cross section at
wavelength and xn(x) Absorber number density at x
x
I(x)
I0
x1
I(x1)
n constant along light path
The exponent = n x is dimensionless and is called optical depth (optical density)
nxσλ,0λ
λeIxI
If << 1, then the medium is optically thin
If >> 1, then the medium is optically thick or opaque
Also used: absorption coefficient = n Unit: [] = m-1
Then: = x
If n and constant along the light path:
x
xdxnx
eIxI 00,
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Summary: Main features of Rayleigh and Mie Scattering
Rayleigh Mie
Radius / Wavelength
r << r >>
Phase function P11() (1 + cos2 ) Highly variable, depending on = 2r / Strong forward peak
Asymmetry parameter
g = 0 g > 0
Polarization = 0, : LP = 0 = ± /2 : LP 1
Generally depolarizing,
but variable
Spectral depedence
R -4 M -m
m : Ångstrom exponent
(-1 < m < 4)
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Polarization of Rayleigh-scattered radiation
211 cos1
4
3P
Const.
2cos
Polarized perpendicular to scattering plane
Polarized parallel to scattering plane
Unpolarized radiation
Fig. from Liu, An introduction to atmospheric radiation
Due to the symmetry of Rayleigh phase function the asymmetry parameter g is:
0cos4
0
11
dPgRayleigh
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Rotational Raman Scattering
• In addition to elastic Rayleigh and Mie scattering, inelastic rotational Raman scattering on air molecules is also important in the atmosphere.
• Raman scattering moves energy from the incoming wavelength to neighbouring wavelengths and thus changes the spectral distribution in the scattered light.
• Raman scattering is:
- non polarizing
- isotropic
- proportional to -4
- responsible for about 4% of all Rayleigh scattered light
Slide courtesy of A. Richter
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Instrumentation for remote sensing measurements
Atmospheric remote sensing methods usually require spectrally resolved radiation measurements
spectrally dispersing elements required
The standard radiation-dispersing devices are:
•Prisms•Gratings•Michelson Interferometers•Fabry-Perot Interferometers
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Prism spectrometer
The prisms exloits refraction in media with different refractive indices n for spectral dispersion:
’
nprism > nmedium
Refraction is described by Snell’s law:
n
n
sin
sin
n = c0 / c is the refractive indexc0 is the speed of light in vaccum
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
For constructive interference, the path difference between two neighboring grating rules has to be a multiple of the wavelength:
Diffraction by a grating
Gratings are the most common dispersing elements used in remote sensing instruments:
g
m
g
g distance between grating grooves
m diffraction order
wavelength
g
mλsinαm
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Resolving power of a grating (/)
Consider a grating with n rules and rule distance g :
1
n
g
Maximum 1st order: n
Maximum mth order: mn
Maximum condition:g
mλ
ng
mnλsinα
Minimum condition is: mn + = mn‘ with ’ = +
Then: mn + = mn + mn
= mn or / = mn
Resolving power depends on the number of rules and the order, but NOT on the distance between the rules
Rayleigh criterion:
Interference maximum of 1 must fall onto 1st minimum of 2
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Fourier Transform Spectrometers (FTS of FTIR)
Michelson interferometer
Measured is the intensity of the two interfering light beams as a function of the position x of the movable mirror: I(x) is called interferogram
The spectrum S() is the Fourier-transform of I(x)
x
Movable mirror
Fixed mirror
Source
Beam splitter
Detector
L1/2
L2/2
I(x)
FTS = Fourier Transform Spectrometer / FTIR = Fourier Transform InfraRed Spectrometer
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Fabry-Perot Interferometers and etalons
tn
n’A
B
D
EC
Optical path difference: d = n (BD + DE) – n’ BC
Now: BD = DE
and BC = BE sin’ BE = 2 BD sin
’
d = 2 n BD – 2 n’ BD sin’ sin
With: BD = t / cos and n sin = n’ sin’
cosθ
θsin12nt
cosθ
θ2ntsin
cosθ
2ntd
22 cosθtn2
Fabry-Perot-Etalon: t = const.
Fabry-Perot-Interferometer: t variable
n: refractive index of material
If d = m (with integer m), then constructive interference and radiance maximum
λm
’
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Monochromators and spectrometers (I)
Monochromators are single color, tunable optical band pass filters
Spectrometers measure a continuous spectral range simultaneously
Note: Depending on the type of detector, a prism or grating instrument can be a monochromator or a spectrometer
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Radiation detectors
A radiation detector should fulfill the following requirements:
• Linearity: (output signal intensity)
• Fast response
• Large dynamic range
• Low noise level
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Radiation detectors I: Photomultiplier tubes (PMTs)
Advantages:
• High sensitivity
• Fast response
Disadvantages:
• High voltages required
• Only single wavelength measured
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Radiation detectors II: Photodiodes (PDs)
• Noise by thermal e- crossing between valence and conduction band
• Cooling detector by 7 K reduces thermal noise by a factor of 2
• Largest wavelength detectable determined by width of band gap
Advantages:
• Cheap
Disadvantages:
• Only single wavelength measured
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Radiation detectors III: Photodiode Arrays (PDAs)
• Sizes: 256 - 2048 pixels
• Integration of signal over time
• Photons create e--hole pairs that diffuse to next p-n junction & decharge it
During readout the capacitors are sequentially charged
Advantages:
• Measure many wavelength simultaneously
Disadvantages:
• Lower sensitivity than Photomultipliers
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
Radiation detectors IV: Charge Coupled Devices (CCDs)
• Sizes: 256 256 to 4096 4096 pixels
• e- are collected in uncharged depletion zones
• Read out: charges are shifted sequantially from row to row.
Lowest row is readout and digitized.
Advantages:
• High sensitivity
• 2D imaging spectrometers
Disadvantages:
• Low capacity, i.e. frequent readout necessary
• Long readout time (up to several seconds)
Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006
End of lecture