Asielwetgeving en asielaanvragen: een ruimtelijke analyse van het Belgisch asielbeleid naar de landen van herkomst (1992-2003)

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On instrumental errors and related correction strategies of ozonesondes: possible effect on calculated ozone trends for the nearby sites Uccle and De Bilt

The ozonesonde stations at Uccle (Belgium) and De Bilt (the Netherlands) are separated by only 175 km but use different ozonesonde types (or different manufacturers for the same electrochemical concentration cell (ECC) type), operating procedures, and correction strategies. As such, these stations form a unique test bed for the Ozonesonde Data Quality Assessment (O3S-DQA) activity, which aims at providing a revised, homogeneous, consistent dataset with an altitude-dependent estimated uncertainty for each revised profile. For the ECC ozonesondes at Uccle mean relative uncertainties in the 4–6 % range are obtained. To study the impact of the corrections on the ozone profiles and trends, we compared the Uccle and De Bilt average ozone profiles and vertical ozone trends, calculated from the operational corrections at both stations and the O3S-DQA corrected profiles. In the common ECC 1997–2014 period, the O3S-DQA corrections effectively reduce the differences between the Uccle and De Bilt ozone partial pressure values with respect to the operational corrections only for the stratospheric layers below the ozone maximum. The upper-stratospheric ozone measurements at both sites are substantially different, regardless of the correction methodology used. The origin of this difference is not clear. The discrepancies in the tropospheric ozone concentrations between both sites can be ascribed to the problematic background measurement and correction at De Bilt, especially in the period before November 1998. The Uccle operational correction method, applicable to both ozonesonde types used, diminishes the relative stratospheric ozone differences of the Brewer–Mast sondes in the 1993–1996 period with De Bilt to less than 5 % and to less than 6 % in the free troposphere for the De Bilt operational corrections. Despite their large impact on the average ozone profiles, the different (sensible) correction strategies do not change the ozone trends significantly, usually only within their statistical uncertainty due to atmospheric noise. The O3S-DQA corrections bring the Uccle and De Bilt ozone trend estimates for 1997–2014 closer to each other in the lower stratosphere and lower troposphere. Throughout the whole vertical profile, these trend estimates are, however, not significantly different from each other, and only in the troposphere significantly positive. For the entire Uccle observation period (1969–2014), the operational corrections lead to height-independent and consistent ozone trends for both the troposphere and the stratosphere, with rates of +2 to +3 % decade−1 and −1 to −2 % decade−1, respectively

Vertical profiles and transport of ozone

A lidar system has been developed which is particularly suited for high resolution measurements of the ozone vertical distribution in the lower troposphere. This has been used for routine observations as well as for intensive measurement campaigns. An extensive dataset on the annual cycle of the ozone density in the lower troposphere has been collected, which confirms the persistently high ozone level in the free troposphere even during winter. The variability in this height regime is found rather small, which shows, that large scale processes are dominant here. The diurnal cycle of the boundary layer ozone in particular during episodes of enhanced ozone formation has been studied. The eddy correlation technique by remote sensing has been developed and applied successfully in a case study of the redistribution of ozone in the convective boundary layer. This method has been extended to estimate vertical profiles of ozone production rates in the boundary layer. Vertical transport terms were found important for the ozone budget over extended periods of time