Measurable Aerosol Properties using Remote Sensing Techniques

1. 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) GE0-CAPE Workshop University of North Carolina-Chapel Hill 18-20 August 2008

2. Aerosol Types and Origin • Aerosol particles larger than about 1 mm 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. Aerosol Physical Properties Microphysical Size distribution Refractive index Shape Macrophysical properties Scattering function (P) Single scattering albedo (SSA) Ext. Optical thickness (AOT) Particle Scattering Theory Vertical distribution AOText = AOTsca + AOTabs AERONET Measurements SSA = AOTsca /(AOTsca + AOTabs) Scattering Phase Function (P):

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 Surface Albedo from GOME observations 630 nm

6. Handling of land reflectance issue in MODIS algorithm Aerosol reflectance at 2.1 microns is negligibly small so that TOA measurements are a direct measurement of surface albedo (R2.1). 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.

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 - AOTfine / AOTtotal - Effective radius (Based on spectral dependence of AOT) MODIS aerosol product over land: - AOT at 0.47 and 0.66 microns: - AOTfine / AOTtotal -Qualitative Information on particle size distribution via the Angstrom Exponent - AOT- SSA over deserts from Deep Blue Algorithm

8. September, 2000 0.0 0.2 0.4 0.6 0.8 Aerosol Optical Depth 0.0 0.5 1.0 1.5 2.0 Aerosol Effective Radius 0.0 0.25 0.50 0.75 1.0 Small Mode Fraction • Ocean product (10kmx10km): • Total Spectral Optical thickness • Effective radius • Optical thickness of small & large modes/ratio between the 2 modes • Dt ~ ±0.03±0.05t (dust excepted)

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 Rsfc is a Lambert Equivalent effective surface reflectvity value such that Rsfc is assumed wavelength independent Thus, the AAI definition reduces to:

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

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

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. Advantage of aerosol observations from Geostationary Satellites • 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 Composite image from multiple snap shots of GOES-12 Single snap shot of MODIS

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