Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006 Introduction to Measurement Techniques in Environmental

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)


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


Overview – Lecture 1

• Introduction

• Brief summary of relevant aspects of radiative transfer

• Radiation-dispersing devices

• Radiation detectors


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


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)


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)


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


Instrument: GOME/ERS-2

Measured quantity: Total ozone columns(from backscattered solar radiation)


Examples of remotely sensed atmospheric fields IV


Instrument: SCIAMACHY/Envisat

Measured quantity: Mesopause (about 87 km) temperature(from atmospheric airglow emissions)


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


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


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


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


Summary of relevant radiative processes


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


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,


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)


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


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


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


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


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


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


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


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

’


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


Radiation detectors

A radiation detector should fulfill the following requirements:

• Linearity: (output signal intensity)

• Fast response

• Large dynamic range

• Low noise level


Radiation detectors I: Photomultiplier tubes (PMTs)

Advantages:

• High sensitivity

• Fast response

Disadvantages:

• High voltages required

• Only single wavelength measured


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


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


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)


End of lecture

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Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006 Introduction to Measurement Techniques in Environmental