Lagrangian analysis of ozone trends at selected receptor sites in the Northern Hemisphere

Emissions of ozone (O3) precursors in Europe have substantially decreased over the last decades. However this reduction did not lead to the expected decline in tropospheric O3 peak values in summer in the planetary boundary layer air in many areas. It is commonly believed that O3 increase in the air advected from outside of Europe might have masked the O3 decrease resulting from the reduction in the O3 precursor emissions in Europe. Measurements at the high alpine site Jungfraujoch (JFJ, 3580 m above sea level) show a strong increase in O3 since 1990, particularly in winter (+0.63±0.22 ppb/year for 1990-2005) and in the 1990s (+0.97±0.49 ppb/year). In this thesis, a Lagrangian based analysis method has been used to study the O3 changes at selected sites in the North Hemisphere, with particular focus on O3 changes at JFJ. 10 days backward trajectories were calculated using Lagrangian trajectory tool LAGRANTO, and were applied to relate variability of O3 measurements to the past history of the air prior to sampling. The measurements were classified into the source regions of the air masses arriving at the receptor sites. The classification was successfully tested by carbon monoxide (CO) measurements at JFJ and free tropospheric CO measurements from regular aircraft-measurements of the program MOZAIC. Two Lagrangian approaches (LAGRANTO and the particle dispersion model FLEXPART) were compared in order to examine the models’ capability on predicting two typical atmospheric synoptical transport pathways: stratospheric intrusion (SI) and intercontinental transport (ICT), which occurred at JFJ in 2005. Simulation results of both FLEXPART and LAGRANTO were compared with trace gas measurements (O3, CO, water vapor and NOy) performed at JFJ. The results show that both FLEXPART and LAGRANTO are well capable of representing SI and ICT events if they last for 12 or more hours. Surface O3 measurements at JFJ of the period 1990-2005 were studied using trajectory analysis. Air masses arriving at JFJ in this period were split into European planetary boundary layer (PBL) air and background air. Positive trends in O3 are found for both European PBL air and background air in all seasons with maximum in winter and minimum in summer. Winter O3 increase in European air masses is most likely due to the reduction of NO emissions leading to less O3 titration, whereas summer O3 increase in European air masses is probably attributable to the combined effects of decreasing European O3 precursor emissions and increasing background O3 concentrations. A significant change in exposure time of JFJ to the European PBL condition is found in winter. Frequency study indicates that European PBL air and free tropospheric air are the two dominating air flow regimes, and the increase on European PBL air is balanced mainly by the free tropospheric air, whose frequency shows an evident decline from about 60% in the earlier 1990s to about 40% in the recent years. This needs to be considered in the interpretation of the long term composition changes determined by the measurements of the Global Atmosphere Watch (GAW) station JFJ. Further investigation on individual source regions of background air suggests a positive O3 trend in the air with various origins, among which O3 trend in the classes of the the stratospheric and free tropospheric air are larger than overall trend in all seasons, suggesting their important contribution to the overall O3 increase at JFJ. The same method of source regions’ classification was applied to ozonesonde measurements of two European ozonesonde stations (Hohenpeissenberg and Payerne, using Brewer Mast (BM) sondes) and six Canadian ozonesonde stations (Alert, Resolute, Eureka, Edmonton, Goose Bay and Churchill, using electrochemical concentration cell (ECC) sondes). Ozonesonde measurements have less frequent sampling rate but they have started O3 measurments earlier than surface measurements performed at JFJ. The used method provides plausible results. The two European ozonesonde stations provide conflicting results concerning tropospheric O3 long-term trends. With respect to the two dominating air classes, namely free tropospheric air and European PBL air, a large discrepancy is found in the O3 evolution between the two ozonesonde measurements and the surface O3 measurement at JFJ. This suggest that the