Abstract
Abstract. The atmospheric composition is strongly influenced by a change in atmospheric dynamics, which is potentially related to climate change. A prominent example is the doubling of the stratospheric ozone component at the Zugspitze summit station (2962 m a.s.l., Garmisch-Partenkirchen, Germany) between the mid-seventies and 2005, roughly from 11 to 23 ppb (43 %). Systematic efforts for identifying and quantifying this influence have been made since the late 1990s. Meanwhile, routine lidar measurements of ozone and water vapour carried out at Garmisch-Partenkirchen (German Alps) since 2007, combined with in situ and radiosonde data and trajectory calculations, have revealed that stratospheric intrusion layers are present on 84 % of the yearly measurement days. At Alpine summit stations the frequency of intrusions exhibits a seasonal cycle with a pronounced summer minimum that is reproduced by the lidar measurements. The summer minimum disappears if one looks at the free troposphere as a whole. The mid- and upper-tropospheric intrusion layers seem to be dominated by very long descent on up to hemispheric scale in an altitude range starting at about 4.5 km a.s.l. Without interfering air flows, these layers remain very dry, typically with RH ≤5 % at the centre of the intrusion. Pronounced ozone maxima observed above Garmisch-Partenkirchen have been mostly related to a stratospheric origin rather than to long-range transport from remote boundary layers. Our findings and results for other latitudes seem to support the idea of a rather high contribution of ozone import from the stratosphere to tropospheric ozone.
Highlights
For many years the pronounced rise of tropospheric ozone due to growing anthropogenic air pollution has been the subject of intensive research
In 2007, routine measurements have been started with both differential-absorption lidar (DIAL) systems
Really strong efforts to make at least one measurement were limited to the EARLINET (European Aerosol Research Lidar Network) “climatology days” Monday and Thursday (EARLINET, 2003)
Summary
For many years the pronounced rise of tropospheric ozone due to growing anthropogenic air pollution has been the subject of intensive research. The background level of ozone has reached 50 ppb and more at some sites at the northern midlatitudes Layers of stratospheric air can be identified directly based on criteria like elevated ozone and low humidity. This direct import in deep stratospheric air intrusions has long resulted in estimates of the stratospheric influence on the tropospheric ozone level of about 10 % and less This would suggest a rather small relative importance of stratosphere-totroposphere transport (STT), with some uncertainty originating from the “indirect” stratospheric component that cannot
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