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 Date9 – 1111 – 1314 – 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 O 3, CO 2 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, O 3, 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 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 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 cm MHz 10 m cm -1 Radio 100 MHz 1 m cm -1 1 GHz 10 cm 0.1 cm GHz Microwave 1 cm 1 cm GHz 1 mm 10 cm -1 1 THz sub-mm – Far IR 0.1 mm 100 cm THz 10 μm 1000 cm -1 Thermal IR al IR 100 THz Near IR 1 μm 10 4 cm THz Ultraviolet 100 nm 10 5 cm -1 Wavelength Frequency Wave number Visible 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 nm200 nm 400 nm 700 nm 5  m VisibleVacuum UV Near UV NIRIR EUVX-rays  100 nm400 nm320 nm280 nm UV AUV CUV 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 ii ss  ee  out  in ii r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: I 0 Initial intensity I(x)Intensity at x  (,x)Absorption cross section at wavelength and x n(x)Absorber number density at x x I(x) I0I0 x1x1 I(x 1 ) n constant along light path The exponent  =  n x is dimensionless and is called optical depth (optical density) 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:

Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006 Summary: Main features of Rayleigh and Mie Scattering RayleighMie Radius / Wavelength r << r >> Phase function P 11 (  )  (1 + cos 2  ) Highly variable, depending on  = 2  r / Strong forward peak Asymmetry parameter g = 0g > 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 Const. 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:

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:  ’’ n prism > n medium Refraction is described by Snell’s law: n = c 0 / c is the refractive index c 0 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

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 1 st order:   n Maximum m th order:   mn Maximum condition: 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 1 st 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 L 1 /2 L 2 /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 t n n’ A B D E C Optical path difference: d = n (BD + DE) – n’ BC Now: BD = DE andBC = BE sin  ’ BE = 2 BD sin  ’’ d = 2 n BD – 2 n’ BD sin  ’ sin  With: BD = t / cos  and n sin  = n’ sin  ’ 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 ’’

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: 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