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 Soil gas ex- change (Savigny) Measurement Techniques in Soil physics (Fischer)
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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
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