Intercomparison of Sentinel-5P TROPOMI cloud products for tropospheric trace gas retrievals

Clouds have a strong impact on satellite measurements of tropospheric trace gases in the ultraviolet, visible, and near-infrared spectral ranges from space. Therefore, trace gas retrievals rely on information on cloud fraction, cloud albedo, and cloud height from cloud products. In this study, the cloud parameters from different cloud retrieval algorithms for the Sentinel-5 Precursor (S5P) TROPOspheric Monitoring Instrument (TROPOMI) are compared: the Optical Cloud Recognition Algorithm (OCRA) a priori cloud fraction, the Retrieval Of Cloud Information using Neural Networks (ROCINN) CAL (Clouds-As-Layers) cloud fraction and cloud top and base height, the ROCINN CRB (Clouds-as-Reflecting-Boundaries) cloud fraction and cloud height, the Fast Retrieval Scheme for Clouds from the Oxygen A-band (FRESCO) cloud fraction, the interpolated FRESCO cloud height from the TROPOMI NO2 product, the cloud fraction from the NO2 fitting window, the O2–O2 cloud fraction and cloud height, the Mainz Iterative Cloud Retrieval Utilities (MICRU) cloud fraction, and the Visible Infrared Imaging Radiometer Suite (VIIRS) cloud fraction. Two different versions of the TROPOMI cloud products OCRA/ROCINN, FRESCO, and the TROPOMI NO2 product are included in the comparisons (processor version 1.x and 2.x). Overall, the cloud parameters retrieved by the different algorithms show qualitative consistency in version 1.x and good agreement in version 2.x with the exception of the VIIRS cloud fraction, which cannot be directly compared to the other data. Differences between the cloud retrievals are found especially for small cloud heights with a cloud fraction threshold of 0.2, i.e. clouds that are particularly relevant for tropospheric trace gas retrievals. The cloud fractions of the different version 2 cloud products primarily differ over snow- and ice-covered pixels and scenes with sun glint, for which only MICRU includes an explicit treatment. All cloud parameters show some systematic problems related to the across-track dependence, where larger values are found at the edges of the satellite view. The consistency between the cloud parameters from different algorithms depends strongly on how the data are filtered for the comparison, for example, what quality value is used or whether snow- and ice-covered pixels are excluded from the analysis. In summary, clear differences were found between the results of various algorithms, but these differences are reduced in the most recent versions of the cloud data

Retrieval of Aerosol Optical Depth over Land using two-angle view Satellite Radiometry during TARFOX

A new aerosol optical depth retrieval algorithm is presented that uses the two-angle view capability of the Along Track Scanning Radiometer 2 (ATSR-2). By combining the two-angle view and the spectral information this so-called dual view algorithm separates between aerosol and surface contributions to the top of the atmosphere radiance. First validation of the dual view algorithm was performed during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX), which was conducted at the mid-Atlantic coast of the United States in July 1996. The satellite retrieved spectral aerosol optical depth is in good agreement with the aerosol optical depth from ground-based Sun/sky radiometers in three out of four cases. This shows the potential of aerosol retrieval over land using two-angle view satellite radiometry. Copyright 1998 by the American Geophysical Union

Aerosol Optical Depth Retrieval over Land from Two Angle View Satellite Radiometry

Atmospheric aerosol particles play an important role in the Earth’s radiation balance

Crystallisation of mixtures of ammonium nitrate, ammonium sulphate and soot

Crystallisation of laboratory aerosols of ammonium nitrate and of internal mixtures of this salt with ammonium sulphate were investigated using humidity controlled nephelometry. The aerosol was produced via nebulizing of solutions and then dried to 25% RH, which is a realistic minimum value for ambient air. Pure ammonium nitrate and aerosols consisting of internal mixtures of ammonium nitrate and ammonium sulphate with a mixing ratio of more than 1.2 did not crystallize, which is in contrast to assumptions that crystallisation of such mixtures occurs in the atmosphere. Addition of realistic amounts of 'soot' did not promote crystallisation. Aerosols with equal ratios of ammonium nitrate to ammonium sulphate crystallized and showed a (first) deliquescence point close to that of ammonium nitrate. Ambient aerosol with a similar mixing ratio behaved similarly. Because the laboratory tests demonstrated that the crystallisation of ammonium nitrate is not promoted by soot but only by the ammonium sulphate, it is concluded that the ambient aerosol particles consisted of internal mixtures of ammonium nitrate and ammonium sulphate

Aerosol optical depth over Europe in August 1997 derived from ATSR-2 data

Data from the Along Tract Scanning Radiometer 2 (ATSSR-2) on board the European ERS-2 satellite have been used to derive the spatial distribution of the aerosol optical depth (AOD) over Europe for August 1997. The AOD was retrieved in cloud free areas using the dual view algorithm. The results agree with co-located ground based sun-photometer data within 0.1. The AOD from ATSR-2 with a resolution of 1 x 1 km2 were averaged on a grid of 0.1° x 0.1°, to produce daily maps of the spatial aerosol distribution over Europe. A composite map of AOD over Europe was constructed by averaging all daily maps for August 1997. This composite map shows a large spatial AOD gradients with variations of a factor of 3 over a few hundreds of kilometers. The AOD at 0.555 μm for relatively clean areas is around 0.1 while in strong industrialised areas the AOD can be 0.5 or higher

Sensitivity of the OMI ozone profile retrieval (OMO3PR) to a priori assumptions

We have assessed the sensitivity of the operational OMI ozone profile retrieval (OMO3PR) algorithm to a number of a priori assumptions. We studied the effect of stray light correction, surface albedo assumptions and a priori ozone profiles on the retrieved ozone profile. Then, we studied how to modify the algorithm to improve the retrieval of tropospheric ozone. We found that stray light corrections have a significant effect on the retrieved ozone profile but mainly at high altitudes. Surface albedo assumptions, on the other hand, have the largest impact at the lowest layers. Selection of an ozone profile climatology which is used as a priori information has small effects on the retrievals at all altitudes. However, the usage of climatological a priori covariance matrix has a significant effect. Based on these sensitivity tests, we made several modifications to the OMO3PR algorithm: the a priori ozone climatology was replaced with a new climatology (TpO3), the a priori covariance matrix was calculated from the climatological ozone variance values, and the surface albedo was assumed to be linearly dependent on wavelength in the UV2 channel. We found that the a priori covariance matrix basically defines the vertical distribution of degrees of freedom for a retrieval. Moreover, all the studied versions of the OMO3PR algorithm were equally effective in reducing uncertainty in the retrieved ozone profile. This implies that the posterior error values depend mostly on the assumed a priori errors. Our case study over Europe showed that the new version produced over 10% smaller ozone abundances which reduced the systematic overestimation of ozone in the OMO3PR algorithm and improved correspondence with IASI retrievals.Geoscience & Remote SensingCivil Engineering and Geoscience

Radiative forcing due to sulfate aerosols in Europe

Climate effects due to the increasing burden of manmade aerosols are expected to countarct the warming effects due to increasing levels of greenhouse gases by a significant part. Here, we consider the direct effects of sulfate aerosols. The atmospheric lifetime of these aerosols is in the order of a week, depending on meteorological conditions. Hence, their concentratíons show a large variability in space and time. Sulfur emissions lead to the formation of sulfate aerosols. These particles are capable of reflecting solar radiation. In addition to this direct efiect, aerosols may alter the size distribution of cloud droplets and hence the reflectivity of clouds. The total aerosol efiect is not well-known. Uncertainties are largely associeted with estimates of the emissions of the precursors snd with the parameterizations of chemical processes as well as the modelled transports. In addition, uncertainties in the microphysical properties, such as seize distribution and the interactions of aerosols with water vapour and droplets, hamper accurate estimates of the radiative effect

Modelling of Aerosol Chemistry and Transport over Europe and Comparison with Measurements

Because of the short lifetime of aerosols (days-weeks), the radiative forcing associated with aerosols varies strongly spatially and temporally (see van Dorland et al., poster) Current measurement networks of sulfate and other aerosol species are unable to provide a representative picture of the aerosol distribition in time and space. Therefore, models are needed that describe the formation end transport of aerosols. Measurements, in return, are needed to validate these models. Satellite remote sensing potentiålly can provide measurements of column integreted aerosol optical properties on local, regional and global scales. In this work the joint KNMI and IMAU chemical tracer transport model (CTM ) TM3 that describes the global cycle of sulfate aerosols is compared to GOME retrievals of aerosol opticsl thickness. The possible role of nitrate is briefly discussed. This work is part ofthe NRP project: Aerosol; cycle and influence on the radiation balanc