Sustainability of O2

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

• Barun RayChaudhuri
• Department of Physics
• Presidency University
• Kolkata 700 073
• Acknowledgement
• DST, NRDMS
• Organizers, Geospatial World Forum 2011

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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
• project
• The present work investigates on
• the diurnal, seasonal and atmospheric variations
  of
• (O2 A) band depth and
• suitability for aerosol estimation

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• 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

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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
  (2239N, 8823E)
• 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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Enlarged view around the oxygen band
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Irradiance measured at night under full moon
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Irradiance measured at solar eclipse
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Enlarged view
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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.
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Comparison of solar irradiance in dry summer
(before rain) with that after continuous rain
(after rain)
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Enlarged view
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Comparison of full-sun and cloud-covered
conditions
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Seasonal variation of solar irradiance
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Diurnal change
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Hyperion image bands 42, 32 and 21 for Kolkata
(2235 N, 8824 E)
(b) winter season (January 06, 2010)
(a) rainy season (July 27, 2002)
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L? DN/40 is the radiance (Wm-2sr-1µm-1)
as function of wavelength d earth-sun
distance in astronomical units ESUN? hyperion
mean solar exoatmospheric
irradiance (Wm-2µm-1) as function of
wavelength and ? 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
• By reflecting incoming solar radiation (albedo
  effect)
• By arresting the outgoing terrestrial radiation
  (greenhouse effect)
• Marine aerosols in cloud formation

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Aerosol measurements from ground surface
Beer-Bouguer-Lambert law

• Total optical depth (t?) has contributions from
• Rayleigh scattering
• Gaseous absorption
• Water vapour absorption
• Aerosol scattering

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Aerosol measurement from satellite
sensor Radiance detected by the sensor (L?)
Radiance leaving object surface Path radiance
(aerosols Rayleigh)
At NIR water-leaving radiance is
negligible Aerosol optical depth L ? .
Constant Constant involves ET solar radiance,
solar elevation and satellite viewing geometry
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• Advantages of using Oxygen absorption band
• 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
• Precise, universal location of the absorption
  band in the
• spectrum
• Even it were some unknown function of optical
  depth,
• such as
•  L? L0?f(t?)
•  
• The ratio of the absorption maximum to the
  baseline at different conditions yields a value
  proportional to the optical depth.

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Optical depths from ground data and Hyperion
image
Ground-based measurements Before rain After rain Cloudy Sunny Winter Summer 90 min. after sunrise 360 min. after sunrise Optical depth 0.435 0.379 0.455 0.381 0.465 0.430 0.496 0.300 change 12.87 16.26 7.53 39.52
Hyperion data Winter Rainy 0.1084 0.1057 2.50
Theoretical variation of airmass (1/cos?) with solar zenith angle (?) 5 to 25 degree, equivalent to 1 ½ hours from mid-sun 9.0
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Theoretical variation of airmass ( 1/cos?)
with solar zenith angle (?)
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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. 361373, 2008.  
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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 35.5
Band 2 432 452 28.8 184.8 28.5
Band 3 479 499 23.54 196.31 22.8
Band 4 502 522 22.05 188.39 25.7
Band 5 547 567 18.34 185.57 22.4
Band 6 660 680 14.1 153.44 18.1
Band 7 748 788 6.57 121.67 9.0
Band 8 847 887 10.96 978.9 17.2
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ALGORITHM Radiometrically 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,
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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.  
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Assuming uniform distribution over the
hemisphere, radiance entering the atmosphere
irradiance/2p 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
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CONCLUSION The present work investigated on the
diurnal, seasonal and atmospheric variations of
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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