October 18, 2013



    The POLDER polarized radiances, versus the scattering angle (Dots) are compared to the retrieved radiances at the wavelengths 865 (black curve) and 670 nm (pink curve). The measurements were made over Russia (59.8°N, 94.5°E) on 06/10/2003.
    The aerosol parameters deduced from these measurements are the optical thickness at 865 nm (d = 0.16) and the Angstrom exponent (a = 2.44). The quality index (IQ = 0.98) indicates a good restitution. The polarized radiance of the ground and the spectral molecular contribution are shown on the picture.

    Continuity of the Aerosol Index (A.I.)

    Over land, only the accumulation mode (small spherical particles) is detected using the spectral polarized radiances. On the contrary, over the ocean where the total and polarized radiances are taken into account in the inversion scheme, the characteristics of the accumulation and coarse modes are derived.
    The picture shows the Aerosol Index A.I. over Mexico during a biomass burning event, which overflows over the Pacific Ocean and Caribbean Sea. Red and black colors indicate large aerosol loading. A good continuity is observed over coastal regions.

    Synthesis over May (left) and June (right) 2003 of the optical thickness at 865 nm.

    This picture shows the regions where the anthropogenic aerosol loadings are high. Extreme values are found over South-East Siberia, Korea and North China. High values appear on Central Africa, Mexico and Canada, corresponding to biomass burning or pollution events.

    The main regions with important anthropic aerosol loading are located in South-East Asia, Central Africa and North-East America.

    Angstrom exponent on May (left) and June (right) 2003

    The Angstrom exponent retrieval is difficult especially when the polarized aerosol radiances are low, so we only focus on the regions with high aerosol loadings. This coefficient increases from China, Mexico and Canada, to Central Africa.

    Optical thickness of the accumulation mode at 865 nm in May (left) and June (right) 2003

    The figure exhibits a rather good continuity over coastal regions even if this optical thickness is derived from two different algorithms: over ocean, polarized and total radiances are used while, over land, the inversion scheme only needs polarization measurements.
    The continuity is obvious where anthropogenic aerosols are present (East Asia, Mexico, Central Africa). Most of the time, the continuity is also found simultaneously over clear coastal regions.

    Number of days used in the synthesis on May (left) and June (right) 2003

    The cloud cover is important on the Earth (mean value close to 60%) and POLDER has a low spatial resolution that does not allow clear observations between fractional clouds. That explains that only few regions are observed with a high frequency (> 20 days) per month.
    On the contrary some regions, in white on the plate, are always cloudy (ITCZ, high latitudes)
    Or in the night (Antarctic).
    Most of the time, there is no continuity over the coasts because the cloud masks are different over land and over ocean.
    Over land, it is sometimes difficult to characterize the aerosols (weak loading, non polarizing aerosols) and the measurements are poorly simulated: in this case, the quality index is bad and the pixel is not considered within the monthly synthesis. This often happens over desert areas.

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