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Highlights of the tropospheric lidar studies at IFU within the TOR project

Abstract A summary of the ozone soundings with the tropospheric ozone lidar at IFU in the years 1991 and 1993 is given. The results cover vertical distributions obtained under a variety of meteorological conditions in different seasons such as during high pressure, before and after frontal passages and during stratospheric air intrusions. The lidar time series, carried out between typically 0.25 and 10 km and at intervals of about 1 h, are an excellent tool for transport studies. Quite frequently contributions of different processes may be observed even simultaneously which may yield insight on the troposphere as a whole. Although the time series were limited to single days during that phase information on a number of relevant transport processes could be extracted. In particular, the uplifting in the Alpine thermal wind system was investigated. The air in the valley is vented to heights in part even beyond 4 km a.s.l. during fair-weather summer days. The high efficiency of the underlying mechanism suggests a major contribution of the orographically induced transport in the Alps to the pollution export from the Central European boundary layer. A spectacular case of trans-Alpine ozone transport was examined which resulted in an ozone increase by about 40% after sunset. This case may, again, reflect the role of the Alps in the redistribution of air pollution in a larger area. In addition, episodes of long-range ozone and aerosol transport have been studied. In this paper, we present the example of intense Föhn with advection of dust-loaded air from the Sahara desert and beyond containing just 35 ppb of O3. A rather complex layering may be observed after cold-front passages associated with subsequent anticyclonic advection. The analysis of a two-day vertical-sounding series reveals that the air in different height ranges originated in the troposphere or stratosphere above rather different source regions, even in the lowermost 4 km above the United States. More recent studies have confirmed the reproducibility of the general layer pattern under such conditions. The in part considerable difference in ozone concentration makes the definition of a free-tropospheric background ozone level a difficult task.