Parasol retrieves aerosol optical and microphysical properties in a unique way thanks to its directional polarized measurements at 670 and 865 nm.
Daily products and monthly syntheses are produced at 20 km resolution (after cloud screening performed at 6 km).
With the best viewing geometries, measurements allow to discriminate between 3 classes of particles, namely small spherical particles, large spherical and non -spherical particles and to retrieve the following parameters:
- total and small particles optical thickness (aerosol load)
- non sphericity index
- angström coefficient
- effective radius and refractive index of the small particle mode
- refractive index of large spherical particles
A full retrieval of the aerosol model and load requires a large range of viewing geometries (scattering angle). When this large range is not available (on the upsun side of the orbit), or when the atmospheric aerosol load is too low, a limited set of aerosol parameters is retrieved by the algorithm. For other cases, the full set of parameters is retrieved from the data.
To date Parasol is the only sensor to provide an indicator of aerosol non sphericity.
The monthly maps suggest that non spherical aerosols are widespread over the Earth, extending largely outside the well known areas of dust transport.
It should be noted that presently AERONET retrievals are made under the spherical assumption which may cause a bias in the retrievals for some situations.
Parasol algorithms and products have been assessed by comparison with AERONET measurements.
Over 1 year, corresponding to 350 coincident measurements, Parasol and AERONET total optical thickness (@865nm) agree within 0.04 .
Parasol Angström coefficients show more dispersion but no bias with respect to AERONET measurements.
For the other parameters such as fine or coarse mode aerosol thicknesses, these are not direct measurements from sunphotometers; AERONET retrievals are also subject to hypotheses and the comparison should be performed with caution.
Comparison to MODIS has been performed at level 3 and also at level 2 over a variety of sites.
Besides the overall consistency of the two products work is in progress to understand/explain the differences in the products.
Retrieval of aerosol over land from total radiances is difficult because of the signal is generally dominated by the surface contribution. The use of polarized reflectances is an alternative approach to the dark dense vegetation method used by spectral sensors (MODIS, MERIS). The performance of the retrieval increases with more polarizing aerosols and least polarizing underlying surfaces.
This is why the method is sensitive to the small mode of the aerosol size distribution. aerosol with size less than 0.35 µm.
As a consequence Parasol aerosol parameters over land are:the optical thickness and the Angström coefficient of the small mode.
Validation against Aeronet observations shows an overall good consistency with a mean deviation from the 1:1 line of 0.025 over a large set of 700 coincident observations. Still, discreapancies appear, especially over urban areas which are known to be strong polarizing surfaces.
Parasol optical thickness, meaned on N clear pixels around
photometers, compared to AERONET thickness recomputed
for r < 0.35 µm (on the left the lines correspond to the
optical thickness dispersion on the studied area)
The comparison shows that the Parasol aerosol optical thickness matches the Aeronet inversion once restricted to aerosol with size less than 0.35 µm.
The very good consistency between the totally independent retrievals over land and ocean is another indicator of the quality of Parasol retrievals over land for small polarizing particles.
Comparison of April and September 2005 aerosol global maps (fine mode optical thickness, Angström cefficient and number of observations)
Fine mode Aerosol monitoring by Parasol in 2005