DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Remote sensing in the UV-vis • Remote sensing by satellites • The inversion problem • The forward model • DOAS technique
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Passive remote sensing Sun
> Earth
?? ??
Lamp
>
> Satellite > Scientist
?? ??
Object
> Detector > Analysis
> measure radiation > infer information on quantities that affect the radiation Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Ultraviolet / visual / near-infrared Reflected sunlight Absorption from atmospheric entry to exit trace gases (O3, NO2, SO2, H2O, CH4, CO, CO2, N2, …) SCIAMACHY/ENVISAT
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
The inversion problem in the retrieval
Inversion
Forward Model Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Retrieval y = F(x) y: vector, measured, x: vector, to be derived F: forward model Auxiliary information: • Measurement error: Sy • Best guess for x: x0 Default method Non-linear least squares - iteratively find minimum of cost function: CF = (y – F(x))T Sy–1 (y – F(x)) (Levenberg-Marquardt) Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Well-posed problems
total column retrieval
Differential Optical Absorption Spectroscopy: fitting absorption structures
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Ill-posed problems Profile retrieval > more information requested as available Least squares gives problems > noise amplification
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Example Nadir ozone profile retrieval
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Ozone profile from nadir 270
280
290
300
310 nm
e n o oz
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Noise amplification Simple two layer model: λ1
: 1.00 x1 + 1.00 x2 = I1 ± ∆I
λ2
: 0.99 x1 + 1.01 x2 = I2 ± ∆I
Pick numbers: x1,2 = 10;
I1,2 = 20;
E(∆I) = 1
Solution: x1 + x2 = 20 ± 1 Day 3
x1 - x2 = 0 ± 141
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Solution: regularisation Extra term in cost function Optimal Estimation (y – F(x))T Sy–1 (y – F(x)) + (x – xa) T Sa–1 (x – xa) xa : a-priori, Sa : a-priori error covariance ¾ Damps unrealistic solutions ¾ Based on Bayes theorem: P(x|y) = P(x)P(y|x)/P(y) P probability density function See e.g. Rodgers Inverse Methods for atmospheric sounding Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Optimal Estimation Linear forward model ( linearize y = F(x) ) y = Kx Analytic solution for CF minimum: xˆ = xa + S a K T ( KS a K T + S y ) −1 ( y − Kxa )
Moderately non-linear case: apply iteratively
Information from a-priori Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
Information from measurement 12
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
GOME
Balloon
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Forward Model Atmospheric Radiation Transfer (UV-VIS nadir)
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
The stage…
θ I(z,θ,φ) Radiance Plane parallel atmosphere
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Radiation transfer: processes
Absorption O3
Scattering
N2
(or O2, or cloud, or aerosol)
O3
N2
Extinction = Absorption + Scattering
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Radiation Transfer Equation Optical depth: dτ = -ext dz = -(abs + scat) dz TOA: τ = 0, Surface:
τ = τ*
dI = − eI + sJ , dz dI µ = I − ωJ, ω = s e dτ
µ
=
P = (scatterin
g function)
θ s = scattering µ = cos θ Day 3
∫ d Ω ' P (Ω ' , Ω ) I (Ω ),
J = (Source)
= P(cos θ s ),
angle,
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Passive remote sensing in the solar spectral range
The source of light is the sun: • Solar spectrum: 0.2 – 3.0 µm, consisting of the: • Ultraviolet: UV < 400 nm • Visible: 400 nm < VIS < 700 nm • Near-Infrared: 700 nm < NIR < 3 µm. Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Earth reflectance spectrum (cloudfree Sahara scene measured by SCIAMACHY)
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Approach for remote sensing of atmospheric composition Choose a quantitative “signature” = unique identification of the quantity of interest: To detect absorbers: use spectral features • Trace gases have spectral absorption lines To detect scattering particles: use brightness + colour + angular features • Clouds: brightness, whiteness, fractal shape, rainbow • Aerosols: colour, polarization Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Detection of trace gases • Trace gases are most easy to detect, because the absorption lines of a molecule are its unique signature. • From the absorption lines the amount of trace gas can be determined. • the deeper an absorption line in the atmospheric spectrum, the more gas there is. • The precise quantitative determination of the total amount of gas depends on: - Vertical distribution of the gas (not known). - Interference with clouds, aerosols. Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Detection of scatterers: clouds and aerosols • Clouds and aerosols give usually a brighter scene, because they scatter more light than the clear atmosphere. • But they are difficult to quantify precisely, because they usually do not have unique scattering features. • Sometimes their angular scattering pattern is unique: - Spherical droplets have rainbows, which are depending on particle size.
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Interaction of solar radiation with the atmosphere sun
satellite atmosphere
O3 clouds NO2 surface
Day 3
aerosol s
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Radiation-matter interaction processes • Rayleigh scattering by air • Absorption by trace gases • Scattering and absorption by aerosol particles • Scattering and absorption by cloud particles • Reflection by the surface. Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Analysis of satellite measurements Requirement: radiative transfer model of the atmosphere = a formula (or a computer code) for describing the transport of sunlight passing through the atmosphere, absorbed by trace gases, scattered by air molecules, clouds and aerosols, reflected by the surface, and finally arriving at the satellite. Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Calculated reflectance spectrum in the UV-VIS
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Ozone
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Ozone absorption spectrum measured in the laboratory
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Reflectance spectrum of the Netherlands (cloudfree) measured by GOME
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Absorption line in spectrum of reflected light Spectrum of atmospheric radiation
λ
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
Spectrum of absorption cross-section per molecule
λ
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Differential Optical Absorption Spectroscopy = DOAS Fit the absorption cross-section spectrum σ(λ) to the logarithm of atmospheric reflectance spectrum R(λ), to find the vertical column density N of the trace gas. Assumption is: R(λ) = R0 (λ) exp (-τs (λ))
where: R (λ) : reflectance with the trace gas R0 (λ) : reflectance without the trace gas τs (λ) : slant optical thickness of trace gas Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
DOAS formula: R(λ) = R0 (λ) exp (-τs (λ)) ⇔ ln R(λ) = ln R0 (λ) –τs(λ) ⇔ - ln R(λ) + ln R0 (λ) = Ns σ(λ) where: ln I0 (λ): low-order polynomial in λ Ns: slant column density of trace gas N = Ns / M: vertical column density of trace gas M = air mass factor Geometric path approximation: M ≅ 1/cos θ + 1/cos θ0 = 1/µ0 + 1/µ Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
DOAS spectral fit of ozone
R(λ) -ln R(λ)+ln R0(λ) Ns σ(λ)
difference (residue)
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Air Mass Factor for ozone Approximation: N = Ns / M = 1/µ0 + 1/µ
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Ozone measurements by SCIAMACHY
20-3-2004 Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
NO2
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
How to measure NO2 from the reflectance spectrum ? GOME, 25 July 1995,The Netherlands
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
DOAS spectral fit of NO2 DOAS FIT
-> Slant column of NO2 Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Retrieval using model informatie and satellite measurements
stratosphere
troposphere
Ntrop vertical=
Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
Ntotal slant – Nstrat slant Mairmass trop 41
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Tropospheric NO2
1. DOAS → slant column (GwinDOAS, developed at BIRA-IASB) 2. Assimilation → strat. slant column (TM4-DAM, developed at KNMI) 3. Modelling
→ tropospheric amf
(DAK, developed at KNMI) Day 3
L4 - Retrieval of UV-Vis - Hennie Kelder
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DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Summary • UV-VIS spectrometry is the preferred method to detect trace gases like ozone and NO2. • A radiative transfer model (including scattering) is needed to interpret these spectra. • There are suitable spectrometers in space: GOME, SCIAMACHY, OMI. • These instruments show important geophysical phenomena: ozone hole, tropospheric pollution.
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L4 - Retrieval of UV-Vis - Hennie Kelder
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