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Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 1
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 2
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Passive remote sensingSun > Earth > Satellite > Scientist
? ? ? ? ? ? ? ?
Lamp > Object > Detector > Analysis
> measure radiation > infer information on quantities that affect the radiation
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 3
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Ultraviolet / visual / near-infraredReflected sunlightAbsorption from atmospheric entry to exittrace gases (O3, NO2, SO2, H2O, CH4, CO, CO2, N2, …)SCIAMACHY/ENVISAT
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 4
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
The inversion problem in the retrieval
Forward Model
Inversion
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 5
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Retrievaly = F(x) y: vector, measured, x: vector, to be derivedF: forward modelAuxiliary information:• Measurement error: Sy
• Best guess for x: x0
Default methodNon-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 6
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 7
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Ill-posed problemsProfile retrieval
> more information requested as available
Least squares gives problems > noise amplification
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 8
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Example
Nadir ozone profile retrieval
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 9
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Ozone profile from nadir
270 280 290 300 310 nm
ozone
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 10
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
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 x1 - x2 = 0 ± 141
Noise amplification
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 11
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Solution: regularisationExtra 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
Damps unrealistic solutionsBased 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 12
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Optimal Estimation
Linear forward model ( linearize y = F(x) )y = Kx
Analytic solution for CF minimum:
Moderately non-linear case: apply iteratively
Information from a-priori Information from measurement
)()(ˆ 1ay
Ta
Taa KxySKKSKSxx −++= −
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 13
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Balloon
GOME
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 14
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 15
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
The stage…
Plane parallel atmosphereRadiance
I(z,θ,φ)
θ
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 16
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Radiation transfer: processes
O3 O3
N2(or O2, or cloud, or aerosol)
N2
Absorption
Scattering
Extinction = Absorption + Scattering
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 17
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Optical depth:
dτ = -ext dz = -(abs + scat) dz
TOA: τ = 0, Surface: τ = τ*
Radiation Transfer Equation
θµθ
θ
ωωτ
µ
µ
cosangle, scattering
),P(cosfunction) g(scatterin
,)(),'('(Source)
,
,
==
==
ΩΩΩΩ==
=−=
+−=
∫
s
sP
IPdJ
esJI
ddI
sJeIdzdI
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 18
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 19
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 20
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 21
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 22
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 23
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 24
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Interaction of solar radiation with the atmosphere
sun satellite
surface
atmosphere
O3
NO2 aerosols
clouds
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 25
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
• Rayleigh scattering by air• Absorption by trace gases• Scattering and absorption by aerosol
particles• Scattering and absorption by cloud
particles• Reflection by the surface.
Radiation-matter interaction processes
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 26
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 27
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Calculated reflectance spectrum in the UV-VIS
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 28
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Ozone
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 29
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Ozone absorption spectrum measured in the laboratory
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 30
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 31
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Absorption line in spectrum of reflected light
λ λ
Spectrum ofatmospheric radiation
Spectrum of absorption cross-section per molecule
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 32
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Differential Optical Absorption Spectroscopy = DOASFit 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 gasR0 (λ) : reflectance without the trace gasτs (λ) : slant optical thickness of trace gas
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 33
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 λNs: slant column density of trace gasN = Ns / M: vertical column density of trace gasM = 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 34
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
DOAS spectral fit of ozone
R(λ)
Ns σ(λ)
-ln R(λ)+ln R0(λ)
difference (residue)
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 35
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 36
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Ozone measurements by SCIAMACHY
20-3-2004
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 37
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
NO2
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 38
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 39
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 40
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 41
DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING
Retrieval using model informatie and satellite measurements
troposphere
stratosphere
Ntrop vertical=Ntotal slant – Nstrat slant
Mairmass trop
Day 3 L4 - Retrieval of UV-Vis - Hennie Kelder 42
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 43
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.