1. Sustainability of O 2 – 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

2. Objective of the work atmospheric oxygen absorption (O 2 – 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 (O 2 – A) band depth and suitability for aerosol estimation

3. What the work does: Studies the stability and regularity in variation of O 2 – 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 O 2 – A as a suitable tool for aerosol estimation What it does not: Does not report any result on aerosol measurement at some proper place

4. 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, [22 nd 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.

7. Enlarged view around the oxygen band

8. Irradiance measured at night under full moon

9. Irradiance measured at solar eclipse

10. Enlarged view

11. 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.

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

13. Enlarged view

14. Comparison of full-sun and cloud-covered conditions

15. Seasonal variation of solar irradiance

16. Diurnal change

17. 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)

18. L λ = DN/40 is the radiance (Wm -2 sr -1 µm -1 ) as function of wavelength d = earth-sun distance in astronomical units ESUN λ = hyperion mean solar exoatmospheric irradiance (Wm -2 µm -1 ) as function of wavelength and θ = solar zenith angle

19. Vegetation signatures extracted from Hyperion images

21. 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

22. Aerosol measurements from ground surface Beer-Bouguer-Lambert law Total optical depth (τ λ ) has contributions from: Rayleigh scattering Gaseous absorption Water vapour absorption Aerosol scattering

23. Aerosol measurement from satellite sensor Radiance detected by the sensor (L λ ) = 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

24. 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 L λ = L 0λ f(τ λ ) The ratio of the absorption maximum to the baseline at different conditions yields a value proportional to the optical depth.

25. 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/cosθ) with solar zenith angle (θ) 5 to 25 degree, equivalent to 1 & ½ hours from mid-sun 9.0

26. Theoretical variation of airmass ( = 1/cosθ) with solar zenith angle (θ)

27. 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.

28. 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 O 2 – A band. Spectral bands (nm) Gain (mWcm -2 sr -1 μm -1 ) Extraterrestrial solar irradiance (mWcm -2 μm -1 ) Saturation radiance (mWcm -2 sr -1 μm -1 ) Band 1: 404 – 42449.1171.3835.5 Band 2: 432 – 45228.8184.828.5 Band 3: 479 – 49923.54196.3122.8 Band 4: 502 – 52222.05188.3925.7 Band 5: 547 – 56718.34185.5722.4 Band 6: 660 – 68014.1153.4418.1 Band 7: 748 – 7886.57121.679.0 Band 8: 847 – 88710.96978.917.2

30. ALGORITHM Radiometrically measured irradiance was averaged over 748 – 788 nm with and without the O 2 – 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 O 2 – A absorption,

31. 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.

32. 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 -2 sr -1 μm -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 -2 sr -1 μm -1. Radiance from satellite image, using DN and gain values, 10.08 mWcm -2 sr -1 μm -1 in spring and 9.09 mWcm -2 sr -1 μm -1 in winter, Thus good agreement between the result obtained with the model and that from satellite data

33. CONCLUSION The present work investigated on the diurnal, seasonal and atmospheric variations of the atmospheric oxygen absorption (O 2 – 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 O 2 – 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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