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Validation and Refinement of Modis Aerosol Optical Depth Product over Coastal Urban Areas

Barry Gross (CCNY) Brian Cairns (NASA-GISS) Bill Lawrence (Bowie State) Michael Hagigeorgiou (Ugrad CCNY) Min Min Oo (Grad CCNY) Istvan Laszlo (NOAA-NESDIS) Stephan Ungar (NASA-GSFC) Thomas Brakke (NASA-GSFC).

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Validation and Refinement of Modis Aerosol Optical Depth Product over Coastal Urban Areas

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  1. Barry Gross (CCNY)Brian Cairns (NASA-GISS)Bill Lawrence (Bowie State)Michael Hagigeorgiou (Ugrad CCNY)Min Min Oo (Grad CCNY)Istvan Laszlo (NOAA-NESDIS)Stephan Ungar (NASA-GSFC)Thomas Brakke (NASA-GSFC) Validation and Refinement of Modis Aerosol Optical Depth Product over Coastal Urban Areas

  2. Motivation • Air Pollution forcasting (Ozone, Aerosols, etc) has become a major NOAA responsibility in support of EPA • MODIS AOT algorithms are global in nature. • Application to regional areas may be difficult due to local ground albedo anomolies. • Colocated matchups show that MODIS optical depth retrievals often overestimate optical depth measurements on the North East coast in urban areas • Separate aerosol and land contributions using high spatial resolution data to help • Examine urban ground reflectance to tune MODIS • Retrieve aerosols in urban areas

  3. Colocated matchup procedure • Intercompare MODIS Optical Depth with CIMEL optical depth. (Need to use only spatially homogeneous datasets) • CIMEL Optical Depth taken between NYC and Brookhaven (5 hour mean to agree within 10%) • MODIS 10km products. 3 x 3 cells to have std < 20% mean • Few Points satisfy these conditions

  4. CART Site CCNY Site Intercomparision betweenCIMEL Sky radiometer and Satellite

  5. MODIS Aerosol algorithms over land Surface correlations

  6. MODIS-MISR BRDF Productexamining VIS-MIR correlation Red pixels Consistent with MODIS assumptions As we go to Yellow, MODIS Underestimates VIS ground albedo

  7. Path Radiance (Regression) Approach • Many Surfaces have covarying spectral responses between the VIS and MIR • If the spectral responses covary in a similar way (one component model), a strong ‘linear’ correlation between the VIS and MIR bands occurs Single scattering+small Lanbertian surface albedo slope Y intercept

  8. Application and validation of ground reflectance correlations (MODIS over vegetation) 1) Correlation between ground reflection for different channels will result in correlations at the TOA 2) This can be used to separate ground and atmosphere components by plotting the MIR and VIS reflectances and determining their regression coefficients Y intercept gives atmospheric reflection while slope (m) is proportional to the ground correlation Visible / NIR Reflectance MIR Reflectance (2160nm)

  9. Hyperion as model of Special Events Hyperspectral Imager 30 meter pixel resolution S/N between 60 and 150 (oh well) from blue to red.

  10. Probing aerosol retrieval and ground correlations over urban scales New York observed through Hyperion (30 meter resolution) Regions of interest include vegetation (central park – green), Urban areas (red-black) the river (orange), and lower manhattan Scan Line Scan Column

  11. Correlation Coefficient vs wavelength Shadow/Urban effect Urban environments have many wavelength independent reflection (geometric) mechanisms that improve the correlations between the VIS and MIR channels Note a sharp difference for lower Manhattan due to shadowing /urban effect

  12. Total Reflection Single Scattering Hyperion Retrieval TOA Reflectance % Aerosol Reflection based on Aeronet estimates of AOT and Phase Function Rayleigh Scattering Hyperion Retrieval of Aerosol Reflection over heavy urban zone Larger Deviations above 700nm due perhaps to partial Vegetated scenes but Still much smaller than for vegetated scenes themselves

  13. 430nm (Quite noisy) Hyperion Scene Correlation between VIS and MIR Only keep Aerosol Reflection (bad pixels Masked with navy blue) Increased loading to WTC observed WTC

  14. Ground reflection correlation frequency histogramNo angle dependence (Lambertian) assumption Correlation larger than MODIS assumption of 0.5 This has been Observed elsewhere as seen below Light Urban Frequency Central Park Heavy Urban * Source MODIS ATBD Document

  15. Percentage of Optical depthOverestimation Gnd ref 0.02 Gnd ref 0.05 Gnd ref 0.1 Gnd ref 0.2 Percentage Of Delata Tau AOT overestimates seem to be fairly consistent with ground albedo ~0.05 Aerosol optical thickness (Tau) MIR albedo Urban

  16. Conclusions • Spatial Regression over urban areas using high spatial resolution sensors can isolate aerosol Path Radiance directly and help decouple ground albedo from atmosphere. • MODIS correlation coefficients are too low for urban scenes and leads to an overestimate of optical depth from MODIS. • Preliminary Radiative Transfer calculations show that the 30% overestimate of VIS ground reflectance can help explain observed AOD overestimates. • More urban data including high aerosol optical depth events needed.

  17. Additional Slides

  18. Aeronet Optical Depth From GISS 112th St Hyperion Fly By

  19. Water leaving radiance using AVIRIS (subtracting ground decoupled atmosphere signal from total water signal) Obtained using high shadowing Better water leaving retrieval Weak shadowing Poor water leaving retrieval Water leaving Radiance (Reflection Units) shadowing water Little shadowing

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