Sustainability of O2 – A band depth with atmospheric changes and its suitability for

aerosol estimation

Barun RayChaudhuriDepartment of PhysicsPresidency University

Kolkata 700 073

Acknowledgement: DST, NRDMS Organizers, Geospatial World Forum 2011

Objective of the work• atmospheric oxygen absorption (O2 – A) at around 760 nm

in the solar radiation spectrum is a good hyperspectral signature for the remote sensing of a lot of atmospheric and surface terrestrial features.– Cloud parameters– Water vapour column– Vegetation fluorescence (Part of the ongoing projectThe present work investigates on • the diurnal, seasonal and atmospheric variations of (O2 – A) band depth and• suitability for aerosol estimation

• What the work does:• Studies the stability and regularity in variation of O2 – A

band depth at 760 nm by measuring its diurnal, seasonal and atmospheric changes

• Justifies the reasonability of the ground-based data with respect to satellite-derived data

• Suggests the O2 – A as a suitable tool for aerosol estimation

• What it does not: Does not report any result on aerosol measurement at some

proper place

Methodology Solar irradiance spectrum at ground surface: at 1 nm resolution throughout (UV-Vis-NIR) range with ASD

FieldSpec spectroradiometer fitted with remote cosine receptor on 25º FOV fibre

Data were collected at different seasons and different atmospheric conditions, generally at solar noon

Measurement during (i) night time and (ii) solar eclipse, [22nd July, 2009 morning, Kolkata (22°39´N, 88°23´E)]

Reflected solar radiance for vegetation: without remote cosine receptor To simulate the effect of uniform

vegetation canopy, fresh large banana leaves were spread over horizontal surface. Calibration with Spectralon white reference panel as usual.

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The sharp absorption band enables a hyperspectral instrument to precisely measure the absorption peak

hence hyperspectral satellite images are likely to yield better information on band depth.

The extraterrestrial irradiance around 760 nm varies steadily with wavelength thereby forming a good baseline for absorption estimation

The oxygen absorption works irrespective of intensity of illumination, solar or any other.

This indicates that the feature of oxygen absorption can be achieved at night time also with artificial radiation source emitting around 760 nm.

Comparison of solar irradiance in dry summer (before rain) with that after continuous rain (after rain)

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Comparison of full-sun and cloud-covered conditions

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Hyperion image: bands 42, 32 and 21 for Kolkata (22°35´ N, 88°24´ E)

(a) rainy season (July 27, 2002) (b) winter season (January 06, 2010)

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d = earth-sun distance in astronomical units

ESUNλ = hyperion mean solar exoatmospheric irradiance (Wm-2µm-1) as function of wavelength and

θ = solar zenith angle  

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Vegetation signatures extracted from Hyperion images

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• Man-made Aerosols: Carbonaceous particles Concrete dust

• Natural Aerosols: Volcanic dust Ocean salt Mineral dust• Importance: Atmospheric radiation balance(i)By reflecting incoming solar radiation (albedo effect)(ii)By arresting the outgoing terrestrial radiation

(greenhouse effect)Marine aerosols in cloud formation

)exp(0 mLL

Aerosol measurements from ground surface

Beer-Bouguer-Lambert law

Total optical depth (τλ) has contributions from:

• Rayleigh scattering • Gaseous absorption• Water vapour absorption• Aerosol scattering

Aerosol measurement from satellite sensor

Radiance detected by the sensor (Lλ)= Radiance leaving object surface + Path radiance (aerosols + Rayleigh)

At NIR water-leaving radiance is negligible

Aerosol optical depth = L λ . Constant

Constant involves ET solar radiance, solar elevation and satellite viewing geometry

Advantages of using Oxygen absorption band:(i)Rayleigh scattering can be neglected at 760 nm w.r.t strong gaseous absorption(ii) Fluctuation due to water vapour absorption need not be considered(iii) Precise, universal location of the absorption band in the spectrum(iv)Even it were some unknown function of optical depth, such as  Lλ = L0λf(τλ) The ratio of the absorption maximum to the baseline at different conditions yields a value proportional to the optical depth.

Optical depths from ground data and Hyperion image

Ground-based measurements:

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Cloudy Sunny Winter Summer 90 min. after sunrise 360 min. after sunrise

Optical depth

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Theoretical variation of airmass (1/cosθ) with solar zenith angle (θ) 5 to 25 degree, equivalent to 1 & ½ hours from mid-sun

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M. R. Pandya, R. P. Singh, K. R. Murali, P. N. Babu, A. S. Kirankumar and V. K. Dadhwal“Bandpass Solar Exoatmospheric Iradiance and Rayleigh Optical thickness of Sensor On Board Indian Remote Sensing Satellites – 1B, -1C, -1D, and P4”; IEEE Tran. Geosci. Reomte Sensing, vol. 40, pp. 714-718, 2002

I. Das, M. Mohan and K. Krishnamoorthy“Detection of marine aerosols with IRS P4-Ocean Colour Monitor”Proc. Indian Acad. Sci. (Earth Planet. Sci.), Vol. 111, No. 4, pp. 425-435, 2002 S. Dey and R. P. Singh“Retrieval of aerosol parameters using IRS P4 OCM data over the Arabian Sea and the Bay of Bengal”Current Sc., vol. 83, pp. 1235-1240, 2002 K. Mishra, V. K. Dadhwal and C. B. S. Dutt“Analysis of marine aerosol optical depth retrieved from IRS-P4 OCM sensor and comparison with the aerosol derived from SeaWiFS and MODIS sensor”J. Earth Syst. Sci., vol. 117, pp. 361–373, 2008. 

The consistency of the present measurements was tested with the following model using IRS P-4 OCM data

Band 7 (748 – 788 nm) includes the O2 – A band.

Spectral bands(nm)

Gain(mWcm-2sr-1μm-1)

Extraterrestrial solar irradiance(mWcm-2μm-1)

Saturation radiance(mWcm-2sr-1μm-1)

Band 1: 404 – 424 49.1 171.38 35.5Band 2: 432 – 452 28.8 184.8 28.5Band 3: 479 – 499 23.54 196.31 22.8Band 4: 502 – 522 22.05 188.39 25.7Band 5: 547 – 567 18.34 185.57 22.4Band 6: 660 – 680 14.1 153.44 18.1Band 7: 748 – 788 6.57 121.67 9.0

Band 8: 847 – 887 10.96 978.9 17.2

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ALGORITHMRadiometrically measured irradiance was averaged over 748 – 788 nm with and without the O2 – A absorption band

In winter, decrease due to oxygen absorption = 9.5% In spring, decrease due to oxygen absorption = 8.7%

In actual satellite, it may be a bit larger. For example,

SeaWiFS band 7 (745 – 785 nm) is almost 13% R. S. Fraser, “The effect of oxygen absorption on band-7 radiance” in SeaWiFS Technical Report Series: Case Studies for SeaWiFS Calibration and Validation, Part 3, NASA Tech. Memo.104566, S. B. Hooker, E. R. Firestone and J. G. Acker, Eds., vol. 27, pp. 16-19, 1995.

 The present model assumes 10% decrease of solar radiance due to oxygen absorption in band 7 (748 – 788 nm) of OCM data, which includes O2 – A absorption,

Similarly, the reflectance of the ‘object’, i.e. vegetation was fixed up

Reflectance from lab measurement: 54% under TH illumination 60% under sunshine for both Band 7 (748 – 788 nm) and Band 8 (847 – 887 nm)

Reflectance from spectral library with ENVI 4.5 (2008) of a number of vegetation species: 50 – 65% for band 7 and 53 – 70% for band 8 Considering these all, the model assumes 60% reflectance for vegetation for both band 7 and band 8.  

Assuming uniform distribution over the hemisphere, radiance entering the atmosphere = irradiance/2π

For band 7: it decreases by 10%, then incidents on vegetation canopy, then 60% of the radiance is reflected back and again decreases by 10% before reaching the satellite For band 8: the 10% decrease is omitted Following the above algorithm:

Calculated Radiance of Band 7 reaching satellite = 9.42 mWcm-2sr-1μm-1, larger than the saturation value. In agreement, saturation is noted in band 7 OCM data for both spring and winter.

Calculated Radiance of band 8 reaching satellite = 9.35 mWcm-2sr-1μm-1. Radiance from satellite image, using DN and gain values, 10.08 mWcm-2sr-1μm-1 in spring and 9.09 mWcm-2sr-1μm-1 in winter, Thus good agreement between the result obtained with the model and that from satellite data

CONCLUSION

The present work investigated on the diurnal, seasonal and atmospheric variationsof the atmospheric oxygen absorption (O2 – A) band depth, a hyperspectral signature at around 760 nm of the solar radiation spectrum The conditions of the band at full-moon night and at solar eclipse were also studied

Active remote sensing may avail of the advantage of O2 – A absorption with any artificial source emitting radiation around 760 nm even in the absence of the sun. The proportional changes in optical depth due to atmospheric variations were studied and compared

It is suggested that the aerosol optical depth can be estimated from this band. This has been justified from both ground based hyperspectral spectroradiometric measurements and Hyperion hyperspectral and OCM multispectral satellite image analyses.

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