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Popular summary.Aerosols, tiny solid or liquid particles suspended in the atmosphere, were once only a side note in the Atmospheric Sciences. Today we realize the importance of aerosols in instigating or mitigating climate change, in modifling clouds and large-scale precipitation patterns and in affecting human health. Unlike greenhouse gases, which are well-mixed and long-lasting in the atmosphere, aerosols are temporally and spatially variable with lifetimes of a few days to a few weeks. Their transient natures make aerosols difficult to characterize and their effects on climate, hydrology and health difficult to model. Satellites provide the best means to observe the global aerosol system and narrow the uncertainties associated with aerosol characterization, but the satellite observations must be sufficiently accurate to be useful. The MODerate resolution Imaging Spectroradiometer (MODIS) aboard both NASA's Terra and Aqua satellites provides a unique tool to discern the global impact of aerosols. The products derived from MODIS data include aerosol optical thickness, which is a measure of aerosol amount, as well as products that describe the size of the aerosol particles. The MODIS aerosol retrievals are continuously evaluated against ground-truth of an existing global network of highly accurate instruments (AERONET). The results show an accuracy for the MODIS aerosol products that will sufficiently narrow the uncertainty of global aerosol characterization. Furthermore, the MODIS derivation of aerosol particle size aids in discriminating between man-made aerosol and naturally produced aerosols. This is a major step forward in narrowing the uncertainties associated with estimating the total anthropogenic effect on climate.

Remote sensing from satellite or airborne platforms of land or sea surfaces in the visible and near infrared is strongly affected by the presence of the atmosphere along the path from Sun to Target (surface) to Sensor. This paper presents 6S (Second Simulation of the Satellite Signal in the Solar Spectrum), a computer code which can accurately simulate the above problems. The 6S code is an improved version of 5S (Simulation of the Satellite Signal in the Solar Spectrum), developed by the Laboratoire d'Optique Atmospherique ten years ago. The new version now permits calculations of near-nadir (down-looking) aircraft observations, accounting for target elevation, non lambertian surface conditions, and new absorbing species (CH4, N2O, CO). The computational accuracy for Rayleigh and aerosol scattering effects has been improved by the use of state-of-theart approximations and implementation of the successive order of scattering (SOS) algorithm. The step size (resolution) used for spectral integration has been improved to 2.5 nm. The goal of this paper is not to provide a complete description of the methods used as that information is detailed in the 6S manual, but rather to illustrate the impact of the improvements between 5S and 6S by examining some typical remote sensing situations. Nevertheless, the 6S code has still limitations. It cannot handle spherical atmosphere and as a result, it cannot be used for limb observations. In addition, the decoupling we are using for absorption and scattering effects does not allow to use the code in presence of strong absorption bands.

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[1] Since first light in early 2000, operational global quantitative retrievals of aerosol properties over land have been made from Moderate Resolution Imaging Spectroradiometer (MODIS) observed spectral reflectance. These products have been continuously evaluated and validated, and opportunities for improvements have been noted. We have replaced the surface reflectance assumptions, the set of aerosol model optical properties, and the aerosol lookup table (LUT). This second-generation operational algorithm performs a simultaneous inversion of two visible (0.47 and 0.66 mm) and one shortwave-IR (2.12 mm) channel, making use of the coarse aerosol information content contained in the 2.12 mm channel. Inversion of the three channels yields three nearly independent parameters, the aerosol optical depth (t) at 0.55 mm, the nondust or fine weighting (h), and the surface reflectance at 2.12 mm. Retrievals of small-magnitude negative t values (down to À0.05) are considered valid, thus balancing the statistics of t in near zero t conditions. Preliminary validation of this algorithm shows much improved retrievals of t, where the MODIS/Aerosol Robotic Network t (at 0.55 mm) regression has an equation of: y = 1.01x + 0.03, R = 0.90. Global mean t for the test bed is reduced from $0.28 to $0.21.