1. GE0-CAPE Workshop University of North Carolina-Chapel Hill 18-20 August 2008 Aerosols: What is measurable and by what remote sensing technique? Omar Torres Hampton University (with contributions from Lorraine Remer, Ralph Kahn, Shohba Kondragunta, Sundar Christopher, Ana Prados)

2. Aerosol Types and Origin Aerosol particles larger than about 1  m in size are produced by windblown dust and sea salt from sea spray and bursting bubbles Aerosols smaller than 1 µm are mostly formed by condensation processes such as conversion of sulfur dioxide (SO2) gas to sulfate particles and by formation of soot and smoke during burning processes. After formation, the aerosols are mixed and transported by atmospheric motions and are primarily removed by cloud and precipitation processes.

3. Microphysical Size distribution Refractive index Shape Vertical distribution Macrophysical properties Scattering function (P) Single scattering albedo (SSA) Ext. Optical thickness (AOT) Particle Scattering Theory AOT ext = AOT sca + AOT abs SSA = AOT sca /(AOT sca + AOT abs ) Scattering Phase Function (P) : AERONET Measurements Aerosol Physical Properties

4. Aerosol properties measurable from passive satellite remote sensing -Visible and Near-IR scattering AOT (usually reported as extinction AOT with an assumption on single scattering albedo) by MODIS, MISR. -Observations at 412 nm (Deep Blue) used to derive aerosol AOT and SSA over deserts (MODIS) -Near-UV observations can be used to derive information on aerosol absorption Qualitative (Aerosol Index TOMS / OMI) Quantitative if aerosol vertical distribution is known. Polarization measurements (POLDER, PARASOL, Glory-APS) n/a to GEO-CAPE

5. Retrieval Issues Correction for gas absorption (minor issue) sub-pixel cloud contamination (depends on spatial resolution) (7km resolution better than OMI’s (13X24) but still non-optimum) ‘Surface’ correction: Ocean: -Ocean color effects (chlorophyll conc., dissolved organic matter) - Glint effects (viewing geometry) - White caps (foam reflectance, wind speed) Land : High surface reflectance -Difficulty to separate aerosol signal from bright background in the vis and near IR -Less of a problem in the near UV BRDF effects Angular dependence of surface reflection 380 nm 440 nm 630 nm Surface Albedo from GOME observations

6. Aerosol reflectance at 2.1 microns is negligibly small so that TOA measurements are a direct measurement of surface albedo (R 2.1 ). Relationships between (R 2.1 ) and R 0.47 and R 0.66 were developed based on observations. R 0.47 = 0.25R 2.1 R 0.66 = 0.5R 2.1 Handling of land reflectance issue in MODIS algorithm (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.

7. Current VIS and near-IR satellite aerosol products MODIS aerosol product over oceans: - AOT at 0.47, 0.55, 0.66, 0.87, 1.24, 1.63, 2.13 microns - AOT fine / AOT total - Effective radius (Based on spectral dependence of AOT) MODIS aerosol product over land: - AOT at 0.47 and 0.66 microns: - AOT fine / AOT total -Qualitative Information on particle size distribution via the Angstrom Exponent - AOT- SSA over deserts from Deep Blue Algorithm

8. Ocean product (10kmx10km): –Total Spectral Optical thickness –Effective radius –Optical thickness of small & large modes/ratio between the 2 modes  ~ ±0.03±0.05  (dust excepted) Aerosol Effective Radius 2.00.01.00.51.5 Aerosol Optical Depth 0.80.00.40.20.6 September, 2000 Small Mode Fraction 1.00.00.500.250.75

9. One MODIS Aerosol Type Classification: Low AOT (blue), High AOT+Coarse (green), High AOT+Fine (red) Kaufman et al., JGR, 2005

10. Absorbing aerosols as seen in the near-UV Long-range aerosol transport takes place in the free troposphere, frequently above clouds.

11. The Absorbing Aerosol Index Thus, the AAI definition reduces to: R sfc is a Lambert Equivalent effective surface reflectvity value such that R sfc is assumed wavelength independent

12. Advantages of near-UV aerosol observations Sensitivity to Aerosol Absorption -Aerosol Detection Capability over all surface types: All vegetated surfaces deserts Oceans Ice-snow covered surfaces Above clouds and inter-mingled with clouds -Sensitive to Aerosol Layer Height

13. Quantitative Near UV Retrieval Products (OMI) By means of an inversion algorithm AOD and SSA are derived March 9, 2007 The observed near-UV spectral contrast is conveniently ‘packed’ as the UV Aerosol Index

14. June 27-08 (OMI) July 6-06 (OMI) April 25-98 (TOMS) May 16 1998 (TOMS)Oct 1-07 (OMI) Long range transport of aerosols into GE0-CAPE’s coverage area

15. Aerosol type identification with UV-VIS observations -Near UV-VIS spectral information can be used for aerosol type identification -Requires knowledge of spectral surface albedo

16. Satellite data have gaps due to clouds. Problem is particularly bad for polar-orbiting satellites as they see a particular location on the Earth only once a day (in UV-VIS) Geostationary satellites due to their rapid refresh rate, can obtain a more complete temporal coverage Advantage of aerosol observations from Geostationary Satellites Single snap shot of MODIS Composite image from multiple snap shots of GOES-12

17. No MODIS retrieval due to bright surface Aerosols close to the surface? MODIS treats thick smoke as cloud Combined use of MODIS-OMI observations for aerosol detection From Shohba Kondragunta