Remote sensing by satellites • The inversion problem • The ... · Ultraviolet / visual /...

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

Passive remote sensingSun > Earth > Satellite > Scientist

? ? ? ? ? ? ? ?

Lamp > Object > Detector > Analysis

> measure radiation > infer information on quantities that affect the radiation

Ultraviolet / visual / near-infraredReflected sunlightAbsorption from atmospheric entry to exittrace gases (O3, NO2, SO2, H2O, CH4, CO, CO2, N2, …)SCIAMACHY/ENVISAT

The inversion problem in the retrieval

Forward Model

Inversion

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)

Well-posed problems total column retrieval

Differential Optical Absorption Spectroscopy: fitting absorption structures

Ill-posed problemsProfile retrieval

> more information requested as available

Least squares gives problems > noise amplification

Example

Nadir ozone profile retrieval

Ozone profile from nadir

270 280 290 300 310 nm

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

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

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

Taa KxySKKSKSxx −++= −

Balloon

Forward Model

Atmospheric Radiation Transfer

(UV-VIS nadir)

The stage…

Plane parallel atmosphereRadiance

I(z,θ,φ)

Radiation transfer: processes

N2(or O2, or cloud, or aerosol)

Absorption

Scattering

Extinction = Absorption + Scattering

Optical depth:

dτ = -ext dz = -(abs + scat) dz

TOA: τ = 0, Surface: τ = τ*

Radiation Transfer Equation

θµθ

ωωτ

cosangle, scattering

),P(cosfunction) g(scatterin

,)(),'('(Source)

ΩΩΩΩ==

sJeIdzdI

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.

Earth reflectance spectrum (cloudfree Sahara scene measured by SCIAMACHY)

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

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.

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.

Interaction of solar radiation with the atmosphere

sun satellite

surface

atmosphere

NO2 aerosols

clouds

• 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

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.

Calculated reflectance spectrum in the UV-VIS

Ozone absorption spectrum measured in the laboratory

Reflectance spectrum of the Netherlands (cloudfree) measured by GOME

Absorption line in spectrum of reflected light

Spectrum ofatmospheric radiation

Spectrum of absorption cross-section per molecule

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

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

DOAS spectral fit of ozone

Ns σ(λ)

-ln R(λ)+ln R0(λ)

difference (residue)

Air Mass Factor for ozone

Approximation: N = Ns / M = 1/µ0 + 1/µ

Ozone measurements by SCIAMACHY

20-3-2004

How to measure NO2 from the reflectance spectrum ?GOME, 25 July 1995,The Netherlands

DOAS spectral fit of NO2

DOAS FIT

-> Slant column of NO2

Retrieval using model informatie and satellite measurements

troposphere

stratosphere

Ntrop vertical=Ntotal slant – Nstrat slant

Mairmass trop

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)

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.

Remote sensing by satellites • The inversion problem • The ... · Ultraviolet / visual /...

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