Transcript Sustainability of O2 – A band depth with atmospheric

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
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
• 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.
2.0
-2
-1
Radiance (Wm nm )
2.5
1.5
1.0
0.5
0.0
300
400
500
600
700
800
Wavelength (nm)
900
1000
1100
Solar Irradiance (Arb. Unit)
1.6
1.4
1.2
Exoatmospheric
1.0
Ground surface
0.8
0.6
0.4
O2-A
0.2
0.0
300
400
500
600
700
Wavelength (nm)
800
900
1000
Enlarged view around the oxygen band
Solar Irradiance (Arb. Unit)
1.2
Exoatmospheric
1.0
Ground surface
0.8
0.6
0.4
O2-A
0.2
650
700
750
Wavelength (nm)
800
850
Irradiance measured at night under full moon
Lunar irradiance (arb. unit)
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
Wavelength (nm)
800
900
1000
Irradiance measured at solar eclipse
max. eclipse
(i) 30 min. before
(ii) 30 min. after
(iii) solar noon
Normalized Solar Irradiance
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength (nm)
800
900
Enlarged view
Normalized Solar Irradiance
0.8
max. eclipse
(i) 30 min. before
(ii) 30 min. after
(iii) solar noon
0.6
0.4
0.2
0.0
650
700
750
Wavelength (nm)
800
850
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)
Normalized Solar Irradiance
1.0
before rain
0.8
after rain
0.6
0.4
0.2
0.0
300
400
500
600
700
Wavelength (nm)
800
900
1000
Normalized Solar Irradiance
1.0
Enlarged view
before rain
0.8
after rain
0.6
0.4
0.2
650
700
750
Wavelength (nm)
800
850
Comparison of full-sun and cloud-covered conditions
Normalized Solar Irradiance
1.0
sun
cloud
0.8
0.6
0.4
0.2
650
700
750
Wavelength (nm)
800
850
Seasonal variation of solar irradiance
winter
Normalized Solar Irradiance
1.0
0.8
0.6
0.4
summer
autumn
0.2
650
700
750
Wavelength (nm)
800
850
Diurnal change
Normalized Solar Irradiance
0.8
90 min.
210 min.
360 min.
0.7
0.6
0.5
0.4
0.3
0.2
700
720
740
760
Wavelength (nm)
780
800
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)
R
L d
2
ESUN cos
Lλ = DN/40 is the radiance (Wm-2sr-1µm-1)
as function of wavelength
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
Vegetation signatures extracted from Hyperion images
0.40
0.35
Reflectance
0.30
0.25
winter
0.20
rainy
0.15
0.10
0.05
500
550
600
650
700
750
800
Wavelength (nm)
850
900
950
0.45
0.7
0.40
0.6
0.35
0.5
0.30
from spectroradiometry
0.4
0.25
rainy
winter
0.3
0.20
0.2
0.15
0.1
0.10
0.0
400
500
600
700
Wavelength (nm)
800
0.05
900
Reflectance from Hyperion image
Reflectance from spectroradiometry
0.8
• 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
Aerosol measurements from ground surface
Beer-Bouguer-Lambert law
L  L 0 exp(m  )
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:
Optical depth
% change
Before rain
After rain
0.435
0.379
12.87
Cloudy
Sunny
0.455
0.381
16.26
Winter
Summer
0.465
0.430
7.53
0.496
0.300
39.52
90 min. after sunrise
360 min. after sunrise
Hyperion data
Winter
Rainy
Theoretical variation of airmass (1/cosθ)
with solar zenith angle (θ) 5 to 25 degree,
equivalent to 1 & ½ hours from mid-sun
0.1084
0.1057
2.50
9.0
Theoretical variation of airmass ( = 1/cosθ) with solar zenith angle (θ)
4.0
3.5
3.0
1/2 hour from mid-sun
Airmass
2.5
1 & 1/2 hour from mid-sun
2.0
1.5
1.0
0.5
0.0
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75
Solar zenith angle (degree)
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)
Band 1: 404 – 424
Band 2: 432 – 452
Band 3: 479 – 499
Band 4: 502 – 522
Band 5: 547 – 567
Band 6: 660 – 680
Band 7: 748 – 788
Gain
Extraterrestrial
(mWcm-2sr-1μm-1) solar irradiance
(mWcm-2μm-1)
49.1
171.38
28.8
184.8
23.54
196.31
22.05
188.39
18.34
185.57
14.1
153.44
6.57
121.67
Saturation
radiance
(mWcm-2sr-1μm-1)
35.5
28.5
22.8
25.7
22.4
18.1
9.0
Band 8: 847 – 887
10.96
17.2
978.9
Reflectance and normalized DN
1.0
OCM spring
OCM winter
0.8
in sun
0.6
in lab
0.4
0.2
0.0
400
500
600
700
800
Wavelength (nm)
900
1000
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,
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 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.
BEST REGARDS