Marcel Bartlome
Tropospheric ozone is a climate relevant greenhouse gas, as well as an atmospheric pollutant. Its abundance in the troposphere is mainly governed by the influx of stratospheric air masses and the photochemical production due to anthropogenic and biogenic ozone precursors. Precise model simulations and measurements are needed to assess the exact impact of these sources onto the total tropospheric ozone content. In the latter case, vertical tropospheric ozone profiles are commonly obtained from in-situ balloon-borne soundings. These routine observations are performed with a maximum frequency of several measurements per week. Therefore, they are not suited to resolve the rather fast changes in the tropospheric ozone concentration. However, accurate vertical ozone profiles with temporal and spatial resolutions suitable for studying those changes have been produced for decades using remote sensing by means of the ozone UV differential absorption lidar (DIAL) technique. The main goals of this thesis aimed to address some of the unresolved problems of the ozone origin and evolution in the upper troposphere and lower stratosphere and were: to develop an UV DIAL for the observation of the vertical ozone distribution in the free troposphere and at the tropopause region. to perform experimental measurements of vertical ozone profiles in the free troposphere and the tropopause in order to assess the ability of the ozone UV DIAL to observe fast variations in vertical ozone distribution caused by stratosphere-troposphere exchange (STE), long range transport, and advection from the boundary layer. to assess the feasibility of systematic ozone profile retrieval by the ozone UV DIAL technique from the High Altitude Research Station Jungfraujoch (HARSJ, 3580 m ASL, 7°59'2''E, 46°32'53''N). The HARSJ was chosen for the experiment because of its location, which allows ozone UV DIAL measurements of the tropopause region without perturbation of the lower troposphere. Furthermore, previous atmospheric studies have shown that because of the station's location, the upper tropospheric data taken at the HARSJ can be regarded as representative for central and western Europe. During the first two and a half years of the thesis project the ozone UV DIAL was designed and built. The ozone UV DIAL transmitter is based on a commercial, fourth harmonic Nd:YAG laser. The on- (284 nm) and off- (304 nm) ozone UV DIAL wavelengths are produced by stimulated Raman scattering in high-pressure nitrogen. The receiver uses the existing astronomical Cassegrain telescope (76 cm in diameter) at the HARSJ. Spectral separation of the backscattered ozone UV DIAL wavelengths is carried out by a polychromator based on an concave imaging diffraction grating. The entire ozone UV DIAL detection unit is integrated into the existing long-range multi-wavelength polychromator box optically coupled at the Cassegrain telescope's rear end. With the current design, the ozone UV DIAL system provides hourly averaged ozone profiles reaching from 6 km to 12 km ASL with a vertical resolution better than 400 m at 6 km ASL and 1000 m at 12 km ASL. The relative statistical error of the profiles is 10% at 12 km ASL. The experimental measurements started in the spring of 2008 but were interrupted several times because of serious laser failures. As a first step of the experiment, the ozone UV DIAL profiles were compared to balloon borne ozone profiles taken by electrochemical concentration cell (ECC) sondes. The sondes were launched by the Swiss Meteorological Institute – Payerne (SMI, 46°49'N, 6°56'E, 491 m ASL) situated 80 km in northwestern direction of the HARSJ. The ozone UV DIAL and the sonde measurements were taken quasi simultaneously. The relative differences between the ozone UV DIAL and the sonde profiles were found to be below ±25% in a horizontally homogeneous atmosphere. The intercomparison has shown that the ozone UV DIAL is capable to accurately reproduce the vertical ozone distribution. Intercomparison with vertical ozone profiles taken in the vicinity of the HARSJ by an airplane-borne UV-photometer confirmed the performance of the ozone UV DIAL. Time series with duration of up to 21 hours were taken in order to study the time evolution of tropospheric events characterized by elevated ozone concentrations. As a result of these measurements three events, demonstrating different processes have been identified. In the first case an air layer with elevated ozone concentration was observed in the altitude range between 6 and 7 km ASL. The lower than 10% relative humidity and the time evolution of this layer allowed to determine its stratospheric origin. The observations are in a good agreement with the EURopean Air Pollution Dispersion (EURAD) model. The model predictions also agree with the elevated ozone concentration measured by the UV-photometer of the National Air Pollution Monitoring Network (NABEL) operated at the HARSJ. The evolution of an ozone-enriched layer in the tropopause region was followed for 21 hours during the second case study. The time evolution, the ECC data, and the EURAD model predictions suggest the possibility of a short filamentation process developing at the tropopause region. The measurements of the third event were made according to a prediction of STE forecasted by a dynamical model developed at the ETHZ. A short living enhancement of the ozone concentration has been observed at an altitude of about 9.5 km ASL. There are two possible explanations for this event. The first one could be a short living reversible STE event. A second explanation is a possible long-range transport of ozone and ozone precursor rich air from the forest fires in the Athens region suggested by the Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) backward trajectories. However, the available information does not allow to make a definite conclusion. The ozone UV DIAL has been built successfully and has shown its ability to observe fast variation in vertical ozone distribution with high accuracy and precision. In combination with additional water vapor and temperature lidar observations and supported by model forecasting, this system could become a powerful tool for the improving of the understanding of upper troposphere and lower stratosphere ozone related processes. However, to allow future systematic observations the ozone UV DIAL will need an essential upgrade to resolve the problems related to the long preparation time, and the limited by technical reasons time of observation.
EPFL2010
