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Desert dust is the dominant natural aerosol type. It can be transported over thousands of kilometres and affects atmospheric processes over a large part of the globe. Satellite observations are indispensable to see the global distribution and transport of dust. Elevated atmospheric desert dust can be detected very well in the ultraviolet (UV) range using the absorbing aerosol index (AAI or UVAI). This index uses the UV absorption of mineral dust to make a colour contrast with the dust-free atmosphere. Since the surface albedo of deserts is very low in the UV, dust detection is equally possible over land and ocean. Furthermore, the AAI is hardly affected by clouds. There exists a multi-decade time-series of satellite AAI data.The translation of AAI into aerosol optical thickness is however tricky, since the altitude of the aerosols is also affecting the AAI. Therefore, we have developed a method to retrieve aerosol height from the oxygen absorption band in the near-infrared (NIR) at 760 nm, the so-called O2 A-band. Here we use the fast FRESCO cloud retrieval algorithm, which produces an effective height for optically thick aerosol plumes. The resulting satellite product is called the Absorbing Aerosol Height (AAH). This product has been developed for GOME-2, flying on board the Metop satellite series of EUMETSAT. The AAH is currently being produced for GOME-2 on Metop-A and Metop-B. In November 2018 the Metop-C satellite was launched and first GOME-2C data are expected in Spring 2019. The distribution and the height of desert dust plumes is important information for the aviation sector. The EU H2020 project EUNADICS (www.eunadics.eu) aims at providing this information as a demonstration information system. GOME-2 satellite data of AAI and AAH are a key input to this system.The satellite instrument covering both the UV and NIR spectral ranges with the highest spatial resolution is TROPOMI onboard the ESA Sentinel-5p satellite. Its spatial resolution of 3.5 x 7 km2 is one to two orders of magnitude better than GOME-2 (10 x 40 km2 for the AAI, and 80 x 40 km2 for the AAH), and therefore provides much more details. TROPOMI was launched in October 2017, and the operational data of AAI and FRESCO are available since mid-2018. The Aerosol Layer Height product of TROPOMI, based on a sophisticated retrieval from the O2 A-band, will become available in the course of 2019. We will show the latest results of desert dust detection with GOME-2 and TROPOMI. [ABSTRACT FROM AUTHOR]

The Ozone Monitoring Instrument (OMI) flies on the National Aeronautics and Space Adminsitration's Earth Observing System Aura satellite launched in July 2004. OMI is a ultraviolet/visible (UV/VIS) nadir solar backscatter spectrometer, which provides nearly global coverage in one day with a spatial resolution of 13 km × 24 km. Trace gases measured include O3, NO2, SO2, HCHO, BrO, and OClO. In addition, OMI will measure aerosol characteristics, cloud top heights, and UV irradiance at the surface. OMI's unique capabilities for measuring important trace gases with a small footprint and daily global coverage will be a major contribution to our understanding of stratospheric and tropospheric chemistry and climate change. OMI's high spatial resolution is unprecedented and will enable detection of air pollution on urban scale resolution. In this paper, the instrument and its performance will be discussed. [ABSTRACT FROM AUTHOR]

We analyze the sensitivity of the degree of linear polarization in the Sun's principal plane as a function of aerosol microphysical parameters: the real and imaginary parts of the refractive index, the median radius and geometric standard deviation of the bimodal size distribution (both fine and coarse modes), and the relative number weight of the fine mode at a wavelength of 675 nm. We use Mie theory for single-scattering simulations and the doubling-adding method with the inclusion of polarization for multiple scattering. It is shown that the behavior of the degree of linear polarization is highly sensitive to both the small mode of the bimodal size distribution and the real part of the refractive index of aerosols, as well as to the aerosol optical thickness; whereas not all parameters influence the polarization equally. A classification of the importance of the input parameters is given. This sensitivity study is applied to an analysis of ground-based polarization measurements. For the passive remote sensing of microphysical and optical properties of aerosols, a ground-based spectral polarization measuring system was built, which aims to measure the Stokes parameters I, Q, and U in the visible (from 410 to 789 nm) and near-infrared (from 674 to 995 nm) spectral range with a spectral resolution of 7 nm in the visible and 2.4 nm in the near infrared. We compare polarization measurements taken with radiative transfer simulations under both clear- and hazy-sky conditions in an urban area (Cabauw, The Netherlands, 51.58 degrees N, 4.56 degrees E). Conclusions about the microphysical properties of aerosol are drawn from the comparison.

We compare the Earth reflectances of the spectrometers Global Ozone Monitoring Experiment (GOME) and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) over their overlapping wavelength range (240-800 nm). The goal is to investigate the quality of the radiometric calibration of SCIAMACHY using calibrated GOME data as a reference. However, severe degradation of the GOME instrument in the UV since 2001 prevents it from being a reliable reference below 500 nm. Above 500 nm, GOME is reliable and we find substantial disagreement between GOME and SCIAMACHY, of the order of 15%-20%, which we can attribute completely to the current calibration problems of SCIAMACHY. These numbers are supported by a previous study in which SCIAMACHY was compared with the imager Medium Resolution Imaging Spectrometer (MERIS) onboard the Envisat satellite.

Four ice-crystal models are tested by use of ice-cloud reflectances derived from Along Track Scanning Radiometer-2 (ATSR-2) and Polarization and Directionality of Earth's Reflectances (POLDER) radiance measurements. The analysis is based on dual-view ATSR-2 total reflectances of tropical cirrus and POLDER global-scale total and polarized reflectances of ice clouds at as many as 14 viewing directions. Adequate simulations of ATSR-2 total reflectances at 0.865 microm are obtained with model clouds consisting of moderately distorted imperfect hexagonal monocrystals (IMPs). The optically thickest clouds (tau > approximately 16) in the selected case tend to be better simulated by use of pure hexagonal monocrystals (PHMs). POLDER total reflectances at 0.670 microm are best simulated with columnar or platelike IMPs or columnar inhomogeneous hexagonal monocrystals (IHMs). Less-favorable simulations are obtained for platelike IHMs and polycrystals (POLYs). Inadequate simulations of POLDER total and polarized reflectances are obtained for model clouds consisting of PHMs. Better simulations of the POLDER polarized reflectances at 0.865 microm are obtained with IMPs, IHMs, or POLYs, although POLYs produce polarized reflectances that are systematically lower than most of the measurements. The best simulations of the polarized reflectance for the ice-crystal models assumed in this study are obtained for model clouds consisting of columnar IMPs or IHMs.

A method is presented for in-flight validation of space-based polarization measurements based on approximation of the direction of polarization of scattered sunlight by the Rayleigh single-scattering value. This approximation is verified by simulations of radiative transfer calculations for various atmospheric conditions. The simulations show locations along an orbit where the scattering geometries are such that the intensities of the parallel and orthogonal polarization components of the light are equal, regardless of the observed atmosphere and surface. The method can be applied to any space-based instrument that measures the polarization of reflected solar light. We successfully applied the method to validate the Global Ozone Monitoring Experiment (GOME) polarization measurements. The error in the GOME's three broadband polarization measurements appears to be approximately 1%.