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Fifth GEMS Science Team Meeting

Learn about aerosols, their properties, and scattering processes in a tutorial by Omar Torres from NASA GSFC and Yonsei University. Understand aerosol microphysical and macro-physical properties, sensing from space, and retrieval methods over land.

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Fifth GEMS Science Team Meeting

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  1. Fifth GEMS Science Team Meeting A tutorial on: UV Aerosol Remote Sensing Omar Torres NASA GSFC Yonsei University Seoul, Korea October 7-10, 2014

  2. What are aerosols? Dust Storm In Irak Aerosol flow from Indian Subcontinent

  3. Fundamental Aerosol Properties Micro-physical properties: -Size: Particle Size distribution, at least two parameters: r0 and σ -Shape: Spherical, non-spherical, aspect ratio,a -Composition: Complex refractive Index m = n + ik n: real component (associated with scattering effects) k: imaginary component (associated with absorption effects) Unlike in trace gas retrieval where the only unknown is the species concentration, the full characterization of aerosol requires information on several particle properties

  4. Atmospheric Scattering Process Single Scattering Multiple Scattering Reasonable approximation at long visible and near IR wavelengths

  5. Types of Atmospheric Scattering Type of scattering is a function of: • the wavelength of the incident radiant energy, and • the molecule size of the gas, dust particle, and/or water vapor droplet encountered.

  6. Angular Dependence of Scattering Backward scattering Θ = 180° Scattering Angle Θ P(Θ) Scattering Phase Function Forward scattering Θ = 0°

  7. Aerosol Scattering Phase Function and Particle Size Backward Scattering Forward Scattering

  8. Scattering Phase Function (P) Inaccessible from ground inaccessible by satellite P depends on: -particle shape -particle size distribution -composition (refractive index) Rayleigh Theory used for molecular scattering Mie Theory used for aerosol spherical particles Non-spherical particles require a different theoretical approaach (T-matrix, others)

  9. Fundamental aerosol microphysical properties Particle size distribution dN/dr Complex refractive Index (wavelength dependent), m = n – ik -Real component, n associated with scattering properties -Imaginary component, k associated with absorption properties Aspect ratio (particle shape parameter), α( 1, for spherical particles) Size parameter, x =2πr/λ, α Adequate Scattering Theory Extinction Efficiency, Qe Extinction coefficient, σe Scattering Efficiency Qs Scattering coefficient, σs Absorption Efficiency, Qa Absorption coefficient, σa Units of σe ,σs and σa are km-1 Aerosol Scattering Phase Function P(Θ) Aerosol optical depth (Vertically Integrated extinction coefficient:τaer = Aerosol Single Scattering Albedo, ω0=(σs/ σe) Aerosol macro-physical properties

  10. Physical Basis of Aerosol Sensing The upwelling UV-VIS reflectance (I) at the top of a cloud-free atmospheric column is given by the summation of three components: I0, Atmospheric component resulting from Rayleigh scattering and gas/particle absorption effects Is, Surface reflection Iaer, Aerosol component resulting from particle scattering and absorption. Iaer IS I0 F0 I = I0 + Is + Iaer Rayleigh scattering attenuation by aerosol absorption

  11. Surface Reflectance 380 nm 440 nm 630 nm

  12. Aerosol Retrieval over Land Spectral dependence of TOA measured radiance In the near UV, ~ 70-80% of the measured signal is Rayleigh scattering, 5% surface reflection At 645 nm, 20% is Rayleigh scattering whereas 50 % is surface reflection. Land surface reflectance effect is minimum in the UV

  13. Uncertainty in retrieved AOD due to uncertainty in prescribed surface albedo Actual Surface Albedo (Rs) : 0.05 ∆Rs = 0.01 Actual AOD: 0.5 388nm 470 nm 645 nm 388 nm 470 nm 645 nm 0.06 0.05 0.04 The uncertainty of retrieved AOD over land is smaller in the near UV because the land reflection component to TOA is significantly smaller than in the visible and near IR.

  14. AOD retrieval over land in the VIS (single view) Aerosol reflectance at 2.1 microns is negligibly small so that TOA measurements are a direct measurement of surface albedo (R2.1) MODIS Algorithm: Relationships between (R2.1) and R0.47 and R0.66 were developed based on observations. R0.47 = 0.25R2.1 R0.66 = 0.5R2.1 (collection 4 model) In collection 5, proportionality constants are a function of geographical location. This parameterization of surface reflectance allows AOT retrievals over most land surfaces. It does not work over deserts. The use of VIS single view observations for aerosol retrieval over land requires the availability of 2.1 μm channel.

  15. Physical Basis of Aerosol Sensing The upwelling UV-VIS reflectance (I) at the top of a cloud-free atmospheric column is given by the summation of three components: I0, Atmospheric component resulting from Rayleigh scattering and gas/particle absorption effects Is, Surface reflection Iaer, Aerosol component resulting from particle scattering and absorption. Iaer IS I0 F0 I = I0 + Is + Iaer Rayleigh scattering attenuation by aerosol absorption

  16. Aerosol Sensing from Space in the near UV Neglecting particle multiple scattering effects, the upwelling reflectance as measured by a satellite, is approximately given by Aerosol single scattering contribution Attenuation of Rayleigh and surface components by aerosol absorption Since I0depends on pressure, then psand paare surface and aerosol layer height pressure levels The net aerosol contribution to the measured reflectance is Aerosol optical depth Solar flux Cosines of satellite and solar zenith angles Single scattering albedo Aerosol scattering phase function

  17. Detecting aerosols using satellite observations (2) Spectral dependence of the aerosol effects The aerosol single scattering term is only weakly wavelength dependent Rayleigh scattering attenuation is a strong function of wavelength. Large sensitivity to aerosol absorption in the UV

  18. Aerosol attenuation term depends on τ, ω0 and aerosol layer height (pa) 0 Percent Attenuation Aerosol height: 3km, τ(380) = 1.0 -15 0 Percent Attenuation ω0(380)= 0.89, τ(380) = 1.0 -15

  19. UV Absorbing Aerosol Index Observation requirements: radiances at two channels in the 340-390 nm range • Qualitative Indicator of presence of absorbing aerosols - Proxy of aerosol absorption at a broader spectral range -Detects absorbing aerosols over oceans and land surfaces -Detects aerosols above highly reflective backgrounds: clouds, snow/ice covered surfaces. -The UVAI magnitude depends mainly on AOD, aerosol layer height and SSA -Quantitative information on aerosol AOD and SSA can be extracted with an inversion procedure CALIOP MODIS RGB OMI UVAI

  20. Retrieving Aerosol Properties from Satellite UV-VIS Observations Retrievable Parameters: Extinction Optical Depth, AOD Single Scattering Albedo, SSA Assumptions are needed on: - Particle shape - Particle size distribution - Composition (i.e., refractive index) - Vertical Distribution Cloud free conditions are required (driven by spatial resolution) Surface effects must be taken into account (choice of wavelength) Land: High surface albedo and BRDF effects Water: Sun’s specular reflection (sunglint), white caps

  21. Choice of wavelengths -Application (Climate, Air Quality Considerations) -Choose spectral regions free of interference by trace gas absorption -Surface Reflective Properties Visible, near IR (0.47, 1 micron) -Climate applications • Molecular scattering low, large sensitivity to aerosol scattering • Multi-wavelength retrievals allows characterization of aerosol size via Extinction Angstrom exponent • Work best over oceans (dark background) Near UV/Deep Blue (340-420 nm) -Land Applications (AQ) -Sensitivity to both absorption and scattering effects -Works over all land surfaces including the normally bright (visible) arid and semi-arid regions. -Absorption sensitivity varies with aerosol layer height.

  22. AOD retrievals over oceans Visible and near-IR: dark surface Sub-kilometer resolution instrument resolution Strong heritage (from early 80’s, AVHRR, MODIS, MiSR) • Robust retrieval algorithms - Multi-wavelength retrievals provide qualitative information on aerosol size (MODIS, MISR) - Multi-wavelength and multiple view angle provide qualitative information on size and particle shape • Accuracy: ~ 0.05

  23. AOD retrievals over land • Surface Reflectance: spectral, spatial and angular variability • Approaches: -Near-UV/DeepBlue: TOMS-OMI (340-390nm), MODIS(412nm) dark surfaces (nUV brightest surface has about 8% reflectivity) -Visible/near IR (MODIS): IR radiance (2.1 μm) is transparent to aerosol effects Uses empirically established vis/IR relationships to account for surface effects - Multiple view angle (MISR): Accurate characterization of surface effects including angular variability

  24. AOD retrieval over land in the near-UV Advantages: - Terrestrial surfaces are dark in the near UV: vegetation1~4%, deserts 6~10% • Sensitivity to Aerosol Single Scattering Albedo • Reduced sensitivity to aerosol scattering phase function effects due to large molecular scattering contribution that smears out particle scattering angular features. The use of near-UV observations for AOD and SSA retrieval requires the availability of information on height of absorbing aerosol types (smoke, dust, volcanic ash)

  25. Aerosol Optical Centroid Height, AOCH (Aerosol Layer Height) Suggested Approaches -O2 A-band (~765 nm) -O2 B-band (~690 nm) -O2-O2 band ~(477 nm) -Raman Scattering (350 nm)

  26. OMAERUV Retrieval Procedure Level2 calibrated radiances at 354 and 388 nm Absorbing Aerosol Index Radiative Transfer Calculations Surface Albedo (TOMS Climatology) Three aerosol types : • Desert Dust • Carbonaceous aerosols • Weakly absorbing Seven aerosol models per type (varying ω0) Cloud Screening -AIRS CO data: -Surface Type Aerosol Type Aerosol Layer Height (CALIOP Climatology) Assumed aerosol parameters: -Particle size distribution -Real comp. refractive index -Relative spectral dep. of imag. refractive index. Inversion Scheme Extinction optical depth Single Scattering Albedo Lookup Tables Retrievals over the ocean only account for the presence of desert dust and carbonaceous aerosols.

  27. OMAERUV-AERONET AOD Comparison at representative sites Desert DustCarbonaceous Aerosols Urban Industrial Aerosols

  28. Validation of OMI Scattering Aerosols (2005 - 2008)

  29. OMAERUV SSA assessment: Comparison at selected AERONET sites 51% (75%) of matched pairs agree within 0.03 (0.05)

  30. Summary Advantages of UV-VIS observations for aerosol retrieval: -UV-VIS information can be used to simultaneously retrieve AOD and SSA. -Low land-surface albedo in the near-UV/DB regions offers a unique advantage for the retrieval of aerosol properties over land. -Reliable retrievals of non-absorbing aerosols are possible provided that sub-pixel cloud contamination can be minimized (fine spatial resolution). Improvements needed on: -Aerosol type selection (in terms of particle size) -Aerosol layer height determination for accurate retrieval of absorbing aerosols -Fine spatial resolution to minimize sub-pixel cloud contamination.

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