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  • 1
    Publication Date: 2008-02-08
    Description: This paper presents extensive validation analyses of ozone observations from the Atmospheric Chemistry Experiment (ACE) satellite instruments: the ACE Fourier Transform Spectrometer (ACE-FTS) and the Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (ACE-MAESTRO) instrument. The ACE satellite instruments operate in the mid-infrared and ultraviolet-visible-near-infrared spectral regions using the solar occultation technique. In order to continue the long-standing record of solar occultation measurements from space, a detailed quality assessment is required to evaluate the ACE data and validate their use for scientific purposes. Here we compare the latest ozone data products from ACE-FTS and ACE-MAESTRO with coincident observations from satellite-borne, airborne, balloon-borne and ground-based instruments, by analysing volume mixing ratio profiles and partial column densities. The ACE-FTS version 2.2 Ozone Update product reports more ozone than most correlative measurements from the upper troposphere to the lower mesosphere. At altitude levels from 16 to 44 km, the mean differences range generally between 0 and +10% with a slight but systematic positive bias (typically +5%). At higher altitudes (45–60 km), the ACE-FTS ozone amounts are significantly larger than those of the comparison instruments by up to ~40% (typically +20%). For the ACE-MAESTRO version 1.2 ozone data product, agreement within ±10% (generally better than ±5%) is found between 18 and 40 km for the sunrise and sunset measurements. At higher altitudes (45–55 km), systematic biases of opposite sign are found between the ACE-MAESTRO sunrise and sunset observations. While ozone amounts derived from the ACE-MAESTRO sunrise occultation data are often smaller than the coincident observations (by as much as −10%), the sunset occultation profiles for ACE-MAESTRO show results that are qualitatively similar to ACE-FTS and indicate a large positive bias (+10 to +30%) in this altitude range. In contrast, there is no significant difference in bias found for the ACE-FTS sunrise and sunset measurements. These systematic effects in the ozone profiles retrieved from the measurements of ACE-FTS and ACE-MAESTRO are being investigated. This work shows that the ACE instruments provide reliable, high quality measurements from the tropopause to the upper stratosphere and can be used with confidence in this vertical domain.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 2
    Publication Date: 2008-01-11
    Description: The Odin satellite carries two instruments capable of determining stratospheric ozone profiles by limb sounding: the Sub-Millimetre Radiometer (SMR) and the UV-visible spectrograph of the OSIRIS (Optical Spectrograph and InfraRed Imager System) instrument. A large number of ozone profiles measurements were performed during six years from November 2001 to present. This ozone dataset is here used to make quantitative comparisons with satellite measurements in order to assess the quality of the Odin/SMR ozone measurements. In a first step, we compare Swedish SMR retrievals version 2.1, French SMR ozone retrievals version 222 (both from the 501.8 GHz band), and the OSIRIS retrievals version 3.0, with the operational version 4.0 ozone product from POAM III (Polar Ozone Atmospheric Measurement). In a second step, we refine the Odin/SMR validation by comparisons with ground-based instruments and balloon-borne observations. We use observations carried out within the framework of the Network for Detection of Atmospheric Composition Change (NDACC) and balloon flight missions conducted by the Canadian Space Agency (CSA), the Laboratoire de Physique et de Chimie de l'Environnement (LPCE, Orléans, France), and the Service d'Aéronomie (SA, Paris, France). Coincidence criteria were 5° in latitude x in 10° longitude, and 5 h in time in Odin/POAM III comparisons, 12 h in Odin/NDACC comparisons, and 72 h in Odin/balloons comparisons. An agreement is found with the POAM III experiment (10–60 km) within −0.3±0.2 ppmv (bias±standard deviation) for SMR (v222, v2.1) and within −0.5±0.2 ppmv for OSIRIS (v3.0). Odin ozone mixing ratio products are systematically slightly lower than the POAM III data and show an ozone maximum lower by 1–5 km in altitude. The comparisons with the NDACC data (10–34 km for ozonesonde, 10–50 km for lidar, 10–60 for microwave instruments) yield a good agreement within −0.15±0.3 ppmv for the SMR data and −0.3±0.3 ppmv for the OSIRIS data. Finally the comparisons with instruments on large balloons (10–31 km) show a good agreement, within −0.7±1 ppmv.
    Electronic ISSN: 1680-7375
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 3
    Publication Date: 2005-01-21
    Description: A total ozone depletion of 68±7 Dobson units between 380 and 525K from 10 December 2002 to 10 March 2003 is derived from ozone sonde data by the vortex-average method, taking into account both diabatic descent of the air masses and transport of air into the vortex. When the vortex is divided into three equal-area regions, the results are 85±9DU for the collar region (closest to the edge), 52±5DU for the vortex centre and 68±7DU for the middle region in between centre and collar. Our results compare well with other studies: We find good agreement with ozone loss deduced from SAOZ data, with results inferred from POAM III observations and with results from tracer-tracer correlations using HF as the long-lived tracer. We find a higher ozone loss than that deduced by tracer-tracer correlations using CH4. We have made a careful comparison with Match results: The results were recalculated using a common time period, vortex edge definition and height interval. The two methods generally compare very well, except at the 475K level which exhibits an unexplained discrepancy.
    Print ISSN: 1680-7316
    Electronic ISSN: 1680-7324
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 4
    Publication Date: 2009-01-16
    Description: This paper presents extensive {bias determination} analyses of ozone observations from the Atmospheric Chemistry Experiment (ACE) satellite instruments: the ACE Fourier Transform Spectrometer (ACE-FTS) and the Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (ACE-MAESTRO) instrument. Here we compare the latest ozone data products from ACE-FTS and ACE-MAESTRO with coincident observations from nearly 20 satellite-borne, airborne, balloon-borne and ground-based instruments, by analysing volume mixing ratio profiles and partial column densities. The ACE-FTS version 2.2 Ozone Update product reports more ozone than most correlative measurements from the upper troposphere to the lower mesosphere. At altitude levels from 16 to 44 km, the average values of the mean relative differences are nearly all within +1 to +8%. At higher altitudes (45–60 km), the ACE-FTS ozone amounts are significantly larger than those of the comparison instruments, with mean relative differences of up to +40% (about +20% on average). For the ACE-MAESTRO version 1.2 ozone data product, mean relative differences are within ±10% (average values within ±6%) between 18 and 40 km for both the sunrise and sunset measurements. At higher altitudes (~35–55 km), systematic biases of opposite sign are found between the ACE-MAESTRO sunrise and sunset observations. While ozone amounts derived from the ACE-MAESTRO sunrise occultation data are often smaller than the coincident observations (with mean relative differences down to −10%), the sunset occultation profiles for ACE-MAESTRO show results that are qualitatively similar to ACE-FTS, indicating a large positive bias (mean relative differences within +10 to +30%) in the 45–55 km altitude range. In contrast, there is no significant systematic difference in bias found for the ACE-FTS sunrise and sunset measurements.
    Print ISSN: 1680-7316
    Electronic ISSN: 1680-7324
    Topics: Geosciences
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  • 5
    Publication Date: 2012-09-25
    Description: Iodine oxide particles are known to nucleate in the marine boundary layer where gas phase molecular iodine and organoiodine species are produced by macroalgae. These ultra-fine particles may then grow through the condensation of other materials to sizes where they may serve as cloud condensation nuclei. There has been some debate over the chemical identity of the initially nucleated particles. In laboratory simulations, hygroscopic measurements have been used to infer that they are composed of insoluble I2O4, while elemental analysis of laboratory generated particles suggests soluble I2O5 or its hydrated form iodic acid, HIO3 (I2O5·H2O). In this paper we explore the response of super-micron sized aqueous iodic acid solution droplets to varying humidity using both Raman microscopy and single particle electrodynamic traps. These measurements reveal that the propensity of an iodic acid solution droplet to crystallise is negligible on drying to ~0% relative humidity (RH). On applying mechanical pressure to these droplets they shatter in a manner consistent with an ultra-viscous liquid or a brittle glass. Water retention in amorphous material at low RH is important for understanding the hygroscopic growth of aerosol particles and uptake of other condensable material. Subsequent water uptake between 10 and 20% RH causes their viscosity to reduce sufficiently that the cracked droplets flow and merge. The persistence of iodic acid solution in an amorphous state, rather than a crystalline state, suggests they will more readily accommodate other condensable material and are therefore more likely to grow to sizes where they may serve as cloud condensation nuclei. On increasing the humidity to ~90% the mass of the droplets only increases by ~20% with a corresponding increase in radius of only 6%, which is remarkably small for a highly soluble material. We suggest that the small growth factor of aqueous iodic acid solution droplets is consistent with the small aerosol growth factors observed in previous experiments.
    Print ISSN: 1680-7316
    Electronic ISSN: 1680-7324
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 6
    Publication Date: 2015-05-21
    Description: In Canada routine ozone soundings have been carried at Resolute Bay since 1966, making this record the longest in the world. Similar measurements started in the 1970s at three other sites, and the network was expanded in stages to 10 sites by 2003. This important record for understanding long-term changes in tropospheric and stratospheric ozone has been re-evaluated as part of the SPARC/IO3C/IGACO-O3/NDACC (SI2N) initiative. The Brewer–Mast sonde, used in the Canadian network until 1980, is different in construction from the ECC sonde, and the ECC sonde itself has also undergone a variety of minor design changes over the period 1980–2013. Corrections have been made for the estimated effects of these changes, to produce a more homogeneous dataset. The effect of the corrections is generally modest, and so should not invalidate past analyses that have used Canadian network data. However, the overall result is entirely positive: the comparison with co-located total ozone spectrometers is improved, in terms of both bias and SD, and trends in the bias have been reduced or eliminated. An uncertainty analysis (including the additional uncertainty from the corrections, where appropriate) has also been conducted, and the altitude-dependent estimated uncertainty is included with each revised profile. The resulting time series show negative trends in the lower stratosphere of up to 5% decade−1 for the period 1966–2013. Most of this decline occurred before 1997, and linear trends for the more recent period are generally not significant. The time series also show large variations from year to year. Some of these anomalies can be related to cold winters (in the Arctic stratosphere), or changes in the Brewer–Dobson circulation, which may thereby be influencing trends. In the troposphere trends for the 48 year period are small, and for the most part not significant. This suggests that ozone levels in the free troposphere over Canada have not changed significantly in nearly 50 years.
    Electronic ISSN: 1867-8610
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 7
    Publication Date: 2004-11-30
    Description: Ozone measurements from ozonesondes, AROTAL, DIAL, and POAM III instruments during the SOLVE-2/VINTERSOL period are composited in a time-varying, flow-following quasi-conservative (PV-θ) coordinate space; the resulting composites from each instrument are mapped onto the other instruments' locations and times. The mapped data are then used to intercompare data from the different instruments. Overall, the four ozone data sets are found to be in good agreement. AROTAL shows somewhat lower values below 16 km, and DIAL has a positive bias at the upper limits of its altitude range. These intercomparisons are consistent with those obtained from more conventional near-coincident profiles, where available. Although the PV-θ mapping technique entails larger uncertainties of individual profile differences compared to direct near-coincident comparisons, the ability to include much larger numbers of comparisons can make this technique advantageous.
    Print ISSN: 1680-7316
    Electronic ISSN: 1680-7324
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 8
    Publication Date: 2010-06-30
    Description: A multi-model study of the long-range transport of ozone and its precursors from major anthropogenic source regions was coordinated by the Task Force on Hemispheric Transport of Air Pollution (TF HTAP) under the Convention on Long-range Transboundary Air Pollution (LRTAP). Vertical profiles of ozone at 12-h intervals from 2001 are available from twelve of the models contributing to this study and are compared here with observed profiles from ozonesondes. The contributions from each major source region are analysed for selected sondes, and this analysis is supplemented by retroplume calculations using the FLEXPART Lagrangian particle dispersion model to provide insight into the origin of ozone transport events and the cause of differences between the models and observations. In the boundary layer ozone levels are in general strongly affected by regional sources and sinks. With a considerably longer lifetime in the free troposphere, ozone here is to a much larger extent affected by processes on a larger scale such as intercontinental transport and exchange with the stratosphere. Such individual events are difficult to trace over several days or weeks of transport. This may explain why statistical relationships between models and ozonesonde measurements are far less satisfactory than shown in previous studies for surface measurements at all seasons. The lowest bias between model-calculated ozone profiles and the ozonesonde measurements is seen in the winter and autumn months. Following the increase in photochemical activity in the spring and summer months, the spread in model results increases, and the agreement between ozonesonde measurements and the individual models deteriorates further. At selected sites calculated contributions to ozone levels in the free troposphere from intercontinental transport are shown. Intercontinental transport is identified based on differences in model calculations with unperturbed emissions and emissions reduced by 20% by region. Intercontinental transport of ozone is finally determined based on differences in model ensemble calculations. With emissions perturbed by 20% per region, calculated intercontinental contributions to ozone in the free troposphere range from less than 1 ppb to 3 ppb, with small contributions in winter. The results are corroborated by the retroplume calculations. At several locations the seasonal contributions to ozone in the free troposphere from intercontinental transport differ from what was shown earlier at the surface using the same dataset. The large spread in model results points to a need of further evaluation of the chemical and physical processes in order to improve the credibility of global model results.
    Print ISSN: 1680-7316
    Electronic ISSN: 1680-7324
    Topics: Geosciences
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  • 9
    Publication Date: 2012-03-13
    Description: A new global real-time Lagrangian diagnostic system for stratosphere-troposphere exchange (STE) developed for Environment Canada (EC) has been delivering daily archived data since July 2010. The STE calculations are performed following the Lagrangian approach proposed in Bourqui (2006) using medium-range, high-resolution operational global weather forecasts. Following every weather forecast, trajectories are started from a dense three-dimensional grid covering the globe, and are calculated forward in time for six days of the forecast. All trajectories crossing either the dynamical tropopause (±2 PVU) or the 380 K isentrope and having a residence time greater than 12 h are archived, and also used to calculate several diagnostics. This system provides daily global STE forecasts that can be used to guide field campaigns, among other applications. The archived data set offers unique high-resolution information on transport across the tropopause for both extra-tropical hemispheres and the tropics. This will be useful for improving our understanding of STE globally, and as a reference for the evaluation of lower-resolution models. This new data set is evaluated here against measurements taken during a balloon sonde campaign with daily launches from three stations in eastern Canada (Montreal, Egbert, and Walsingham) for the period 12 July to 4 August 2010. The campaign found an unexpectedly high number of observed stratospheric intrusions: 79% (38%) of the profiles appear to show the presence of stratospheric air below than 500 hPa (700 hPa). An objective identification algorithm developed for this study is used to identify layers in the balloon-sonde profiles affected by stratospheric air and to evaluate the Lagrangian STE forecasts. We find that the predictive skill for the overall intrusion depth is very good for intrusions penetrating down to 300 and 500 hPa, while it becomes negligible for intrusions penetrating below 700 hPa. Nevertheless, the statistical representation of these deep intrusions is reasonable, with an average bias of 24%. Evaluation of the skill at representing the detailed structures of the stratospheric intrusions shows good predictive skill down to 500 hPa, reduced predictive skill between 500 and 700 hPa, and none below. A significant low statistical bias of about 30% is found in the layer between 500 to 700 hPa. However, analysis of missed events at one site, Montreal, shows that 70% of them coincide with candidate clusters of trajectories that pass through Montreal, but that are too dispersed to be detected in the close neighbourhood of the station. Within the limits of this study, this allows us to expect a negligible bias throughout the troposphere in the spatially averaged STE frequency derived from this data set, for example in climatological maps of STE mass fluxes. This first evaluation is limited to eastern Canada in one summer month with a high frequency of stratospheric intrusions, and further work is needed to evaluate this STE data set in other months and locations.
    Print ISSN: 1680-7316
    Electronic ISSN: 1680-7324
    Topics: Geosciences
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  • 10
    Publication Date: 2011-12-07
    Description: Many important atmospheric aerosol processes depend on the chemical composition of the aerosol, e.g. water uptake and particle cloud interactions. Atmospheric ageing processes, such as oxidation reactions, significantly and continuously change the chemical composition of aerosol particles throughout their lifetime. These ageing processes are often poorly understood. In this study we utilize an aerosol flow tube set up and an ultra-high resolution mass spectrometer to explore the effect of relative humidity (RH) in the range of 50% there are about 15 oxidation products identified. This increased oxidation was observed even when the particles were exposed to high humidities long after a low RH ozonolysis reaction. This result might have negative implications for the use of water as an extraction solvent for the analysis of oxidized organic aerosols. These humidity-dependent differences in the composition of the ozonolyzed aerosol demonstrate that water is both a key reactant in the oxidation scheme and a determinant of particle phase and hence diffusivity. The measured chemical composition of the processed aerosol is used to model the hygroscopic growth, which compares favourably with water uptake results from the electrodynamic balance measurements. A reaction mechanism is presented which takes into account the RH dependent observations. This study emphasises the importance of studying the combined effects of several atmospheric parameters such as oxidants and RH to accurately describe the complex oxidation scheme of organic aerosols.
    Print ISSN: 1680-7316
    Electronic ISSN: 1680-7324
    Topics: Geosciences
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