SCATTERING OF RADIATION BY AEROSOLS Scattering efficiency is
SCATTERING OF RADIATION BY AEROSOLS Scattering efficiency is maximum when particle radius = l eparticles in 0. 1 -1 mm size range are efficient scatterers of solar radiation By scattering solar radiation, aerosols decrease visibility and increase the Earth’s albedo green light (λ = 0. 5 µm) 2 (diffraction limit)
HOW TO OBSERVE AEROSOLS FROM SPACE? Solar occultation (SAGE, POAM…) Active system (CALIOP…) Solar back-scatter (MODIS, MISR…) laser pulse EARTH Surface Pros: high S/N, vertical profiling Cons: sparse sampling, cloud interference, low horizontal resolution Pro: vertical profiling Con: sparse sampling, low S/N Surface Pro: horiz. resolution Con: daytime only, no vertical resolution
Aerosol observation from space by solar backscatter Relatively easy to do qualitatively for thick plumes over dark ocean… California fire plumes Pollution off U. S. east coast Dust off West Africa …but difficult quantitatively! Fundamental quantity is aerosol optical depth (AOD) Il ( ) aerosol scattering, absorption Il (0)=Il ( )exp[-AOD] Measured top-of-atmosphere reflectance = f (AOD, aerosol properties, surface reflectance, air scattering, gas absorption, Sun-satellite geometry)
MODIS AEROSOL RETRIEVAL OVER LAND 1. Use top-of-atmosphere (TOA) reflectance at 2. 13 mm (transparent atmosphere) to derive surface reflectance TOA reflectance 2. Assume fixed 0. 47/2. 13 and 0. 65/2. 13 surface reflectance ratios to derive atmospheric reflectances at 0. 47 and 0. 65 mm by subtraction 3. Assume generic aerosol optical properties to convert atmospheric reflectance to AOD 0. 47 mm 0. 65 mm 2. 13 mm SURFACE
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