[1] We review the standard nitrogen dioxide (NO 2 ) data product (Version 1.0.), which is based on measurements made in the spectral region 415-465 nm by the Ozone Monitoring Instrument (OMI) on the NASA Earth Observing System-Aura satellite. A number of ground-and aircraft-based measurements have been used to validate the data product's three principal quantities: stratospheric, tropospheric, and total NO 2 column densities under nearly or completely cloud-free conditions. The validation of OMI NO 2 is complicated by a number of factors, the greatest of which is that the OMI observations effectively average the NO 2 over its field of view (minimum 340 km 2 ), while a ground-based instrument samples at a single point. The tropospheric NO 2 field is often very inhomogeneous, varying significantly over tens to hundreds of meters, and ranges from <10 15 cm À2 over remote, rural areas to >10 16 cm À2 over urban and industrial areas. Because of OMI's areal averaging, when validation measurements are made near NO 2 sources the OMI measurements are expected to underestimate the ground-based, and this is indeed seen. Further, we use several different instruments, both new and mature, which might give inconsistent NO 2 amounts; the correlations between nearby instruments is 0.8-0.9. Finally, many of the validation data sets are quite small and span a very short length of time; this limits the statistical conclusions that can be drawn from them. Despite these factors, good agreement is generally seen between the OMI and ground-based measurements, with OMI stratospheric NO 2 underestimated by about 14% and total and tropospheric columns underestimated by 15-30%. Typical correlations between OMI NO 2 and ground-based measurements are generally >0.6.

Dynamic light scattering studies on the sol-gel transition of a suspension of anisotropic colloidal particles Kroon, M.; Wegdam, G.H.; Sprik, R. Published in:Physical Review E DOI:10.1103/PhysRevE.54.6541 Link to publicationCitation for published version (APA): Kroon, M., Wegdam, G. H., & Sprik, R. (1996). Dynamic light scattering studies on the sol-gel transition of a suspension of anisotropic colloidal particles. Physical Review E, 54, 6541-6550. https://doi.org/10.1103/PhysRevE.54.6541 General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We present a dynamic light scattering study on the sol-gel transition of a suspension of disk-shaped colloidal particles in water. We obtain the static and fluctuating part of the scattered intensity, the fraction of frozen-in density fluctuations, and the intermediate scattering function from a local time-averaged measurement of the intensity correlation function and the scattered intensity. The sol-gel transition is marked by a drastic change in the static part of the scattered intensity. The intermediate scattering function shows a stretching of the translational correlation time over more than five orders of magnitude. In the gel phase the function shows a power-law decay, with a concentration dependent scaling exponent. Our results show strong similarities with the scenarios given by the mode coupling theory of the structural glass transition. ͓S1063-651X͑96͒04212-2͔ PACS number͑s͒: 82.70. Dd, 78.35.ϩc, 82.70.Gg, 64.70.Pf I. INTRODUCTIONStructural relaxation in amorphous systems is an area of much current interest. Many studies have been devoted to the sol-gel transition in systems based on polymers ͓1-3͔, natural gelatine ͓4͔, and gels based on spherical colloids ͓5-7͔. Here we report on the sol-gel transition in a system of charged disks with an aspect ratio of 25, suspended in water. In such systems the orientational degrees of freedom play a crucial role not only in the dynamics but also in the static structure of the gel. Molecular dynamics simulations on hard disk systems reveal a rich phase diagram with nematic and cubatic liquid crystalline phases ͓8͔. However, before these liquid crystalline phases can be formed the system enters a glassy phase or gel. Current opinion favors the ''house of cards'' structure for the ge...

[1] In this paper we present validation results of the total ozone column data products of the Ozone Monitoring Instrument (OMI) on board the NASA EOS-AURA satellite through comparisons with ground-based observations by Dobson and Brewer spectrophotometer instruments. Quality-controlled and archived total ozone column data from these ground-based instruments located at stations worldwide have been used to validate more than 2 a of total ozone column observations from OMI. There are two operationally available satellite total ozone column data products, based on the OMI-TOMS and the OMI-DOAS retrieval algorithms, respectively. Validation with ground-based data focused on global comparisons and seasonal dependence and the possible dependence on latitude and solar zenith angle. Our results show a globally averaged agreement of better than 1% for OMI-TOMS data and better than 2% for OMI-DOAS data with the ground-based observations. The OMI-TOMS data product is shown to be of high overall quality with no significant dependence on solar zenith angle or latitude. The OMI-DOAS data product shows no significant dependence on latitude except for the high latitudes of the Southern Hemisphere where it systematically overestimates the total ozone value. In addition a significant dependence on solar zenith angle is found between OMI-DOAS and ground-based data. Comparisons of satellite and ground-based data tend to show a marginal seasonal dependence even though it remains unclear whether this dependence originates from the ground-based or spaceborne observations.

Abstract. In June 2009, 22 spectrometers from 14 institutes measured tropospheric and stratospheric NO 2 from the ground for more than 11 days during the Cabauw Intercomparison Campaign of Nitrogen Dioxide measuring Instruments (CINDI), at Cabauw, NL (51.97 • N, 4.93 • E). All visible instruments used a common wavelength range and set of cross sections for the spectral analysis. Most of the instruments were of the multi-axis design with analysis by differential spectroscopy software (MAX-DOAS), whose nonzenith slant columns were compared by examining slopes of their least-squares straight line fits to mean values of a selection of instruments, after taking 30-min averages. Zenith slant columns near twilight were compared by fits Correspondence to: H. K. Roscoe (h.roscoe@bas.ac.uk) to interpolated values of a reference instrument, then normalised by the mean of the slopes of the best instruments. For visible MAX-DOAS instruments, the means of the fitted slopes for NO 2 and O 4 of all except one instrument were within 10% of unity at almost all non-zenith elevations, and most were within 5%. Values for UV MAX-DOAS instruments were almost as good, being 12% and 7%, respectively. For visible instruments at zenith near twilight, the means of the fitted slopes of all instruments were within 5% of unity. This level of agreement is as good as that of previous intercomparisons, despite the site not being ideal for zenith twilight measurements. It bodes well for the future of measurements of tropospheric NO 2 , as previous intercomparisons were only for zenith instruments focussing on stratospheric NO 2 , with their longer heritage.Published by Copernicus Publications on behalf of the European Geosciences Union.

General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We have performed static scattering experiments on the transition in time from a fluidlike sol to a solidlike gel of a suspension of disk-shaped charged colloidal particles. The combination of static light scattering and small angle x-ray scattering probes more than three orders of magnitude in the scattering vector q. At the smallest q the static structure factor S(q) shows a q Ϫd dependence in both the sol and gel state. The algebraic exponent d evolves from 2.8 to 2.1 during the gelation. We find that the sol is not comparable to a simple liquid but rather to a low-viscosity precursor of the gel. At intermediate q a plateau connects this regime to the form factor F 2 (q) of the colloidal disks, which is observed at the largest q. On the plateau a small peak related to nearest-neighbor correlations is observed, which decays before gelation occurs. After application of shear on the suspensions we have observed the rapid formation of nematiclike order of the colloidal disks. This order decays in time due to reorientation of the colloidal disks while the final gel state is reached. The formation of the gel does not proceed via aggregation to form ever larger clusters. Based on our findings we propose that reorientation of the charged particles is the mechanism by which the gelation occurs.