ESTEC July 2000

Estimation of Aerosol Properties from CHRIS-PROBA Data

Jeff Settle

Environmental Systems Science Centre

University of Reading

ESTEC July 2000

The Importance of Aerosols

Atmospheric Correction of Images

Aerosols and Climate

Aerosols and Air Quality

ESTEC July 2000

The Need for Directional Measurements

Reflection properties of the surface depend on position of the sun, and the geometry of sensing.

Multi-temporal data can be properly evaluated only if they are normalised for these directional effects.

Albedo is determined accurately only by integrating incoming and outgoing flux over all directions.

Information on the structure of vegetation canopies may be retrievable by inversion of directional reflectance

Data driven atmospheric correction is possible

ESTEC July 2000

The Need for Atmospheric Correction

Ground and TOA NDVI valuesGround and TOA Reflectance

Values in Green Light

The main source of error in atmospheric correction is uncertain knowledge of aerosol loading

ESTEC July 2000

Aerosols and Climate

Aerosols have direct and indirect effects on atmospheric radiation

DirectThey scatter and absorb radiation

IndirectThey act as cloud condensation nuclei, and affect the

microphysical structure of the clouds formed

Interaction between aerosols and clouds a major source of uncertainty

ESTEC July 2000

Sul

phat

e A

eros

ols

and

Rad

iati

ve F

orci

ng o

f th

e C

lim

ate

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Global Aerosol Data

Aerosols are highly variable in space and time: concentrations vary by a factor ~1000.

Global climatologies are model based, or extrapolations from a small number of observations.

Aerosol models exist, limited validation.

Observational network (Aeronet) highly skewed

“…tropospeheric aerosol loading is very poorly measured” (NASA 1993, Modeling the Earth System in the Mission to Planet Earth

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Aeronet Network

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Aeronet Sites, Quality Assured

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Correction of ATSR2 Images

ATSR2 characteristics

1 km pixel size

2 view angles (0-20 and 50-55)

4 spectral channels (555, 655, 870, 1600 nm)

Correction approach based on premise that surface reflectance is of the form (shape function) x (spectral function)

ESTEC July 2000

Methodology

The essential method is inversion of a radiative transfer model for the TOA radiance field.

The inversion is constrained by requiring the surface reflectance field to follow a certain generic pattern. A simpler version has been used successfully on ATSR2 data (2 view directions, 4 wavelength channels). It is robust to the aerosol optical depth.

The method is described in North et al (1999) (IEEE Trans. Geoscience and Remote Sensing, 37(1) pp 526-537)

ESTEC July 2000

ATSR-2 Atmospheric Correction(With thanks to Peter North, ITE)

Before Correction After Correction

Green Channel Correction

ESTEC July 2000

ATSR-2 Atmospheric Correction(With thanks to Peter North, ITE)

Before Correction After Correction

NDVI Correction

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ATSR-2 Atmospheric CorrectionBOREAS SSA, 25-9-95 (With thanks to Peter North, ITE)

Top of atmosphere Corrected image

False colour composite:r=1630nm (nadir), g=870nm (nadir), b=555nm (along-track)

ESTEC July 2000

Validation of AATSR atmospheric correction(with thanks to Peter North, ITE)

Aerosol optical thickness Validation against sun photometer data

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Sit

es to

be

Use

d in

th

is S

tudy

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CHRIS has no spectral calibration device on board so we need to find an ‘external’ method of spectral calibration. We aim to determine the spectral displacement of the spectral response curve resulting from launch conditions to within an accuracy of 0.5 nm.

Method: Observe a scene that is spectrally ‘bland’, and preferably dark, through the atmosphere and use observations of a prominent atmospheric absorption feature, matching observed and expected profiles.

The atmospheric absorption feature used is the O2 absorption at 762 nm, The ocean surface is effectively black over the wavelength range 750 - 780 nm.

Wavelength Calibration of CHRIS

ESTEC July 2000

Within a spectral region encompassing just the O2 absorption, locate the detector ‘j’ recording the lowest observed signal and read the signals from adjacent detectors ‘j-2’,’j-1’ and ‘j+1’,’j+2’.

Compare the observed signals with those predicted using Radiative Transfer Theory and the known CHRIS Spectral Response Curves Ri() shifted by a range of possible between ±3.5 nm (See the figure).This is done for a typical range of atmospheric optical depths (i.e. visibilities) - the instrument signal is effectively independent of moisture and ozone content in this spectral range. The predicted signals constitute a Look-Up Table (LUT).

j j+1 j+2j-2 j-1

ObservedDetector

Signal

This dip is due to the O2

absorption

CCD detector cells about the minimumsignal cell ‘j’ - aligned in the spectral direction

Wavelength Calibration of CHRIS Data

ESTEC July 2000

Increasing tonmstep 0.5 nm

nm

Simulated detector signals for an increasing spectral shift at 2 different atmospheric visibilities

Increasing

nm

nm

nm

±NEdL

±NEdL

nm

nm

Visibility 26 km

Visibility 17 km

CCD detector index

Det

ecto

r R

adia

nce W

/cm

2 /sr

/nm

Solar zenith is 40 degreesView zenith is 45 degrees

ESTEC July 2000

The mean rms retrieval accuracy (over all wavelength shifts) of the was found to be better than 0.53 nm in the presence of detector noise. Worst case 1.3 nm (50km visibility).

We found that the method was robust to uncertainties in the (unknown) surface albedo and atmospheric optical depth.

Averaging the darkest pixels in a calibration image will reduce the uncertainty. The method will be extended to include the water vapour absorption profile at 900-1000

Results

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Example Band Set (49 unbinned + 24 binned)

Wavelength (nm)Binned Unbinned

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Wavelength (nm)

Bands at Full Spectral Resolution

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Location of Bands for Over-Land Sites Full Swath, 25m Resolution (Barton)

Wavelength (nm)

The arrows indicate the locations of filters in the Cimel Sun Photometer

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Locations of Bands over Land Aerosol Sites Half Swath, 25m Resolution

Wavelength (nm)

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Band Locations over Aerosol Land Sites Full Swath, 50m Resolution

Wavelength (nm)

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Provisional Location of Bands over Marine Aerosol Sites Full Swath, 25m Resolution

MERIS Bands

Wavelength (nm)

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Candidate Bandset over Water. Half Swath, 25m Resolution

Wavelength (nm)

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Candidate Bandset over Water 50m Resolution, Full swath

Wavelength (nm)