The underestimated role of stratosphere-to-troposphere transport on tropospheric ozone

Abstract. The atmospheric composition is strongly influenced by changing atmospheric dynamics, in potential relation to climate change. A prominent example is the doubling of the stratospheric ozone component at the summit station Zugspitze (2962 m a.s.l., Garmisch-Partenkirchen, Germany) between the mid-seventies and 2005, roughly from 11 ppb 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 since 2007, combined with in-situ and radiosonde data and trajectory calculations, have revealed the presence of stratospheric intrusion layers on 84 % of the yearly measurement days. The seasonal cycle for deep intrusions with a pronounced summer minimum seen at Alpine summit stations disappears if one looks at the entire free troposphere. The seasonal cycle previously obtained for the Zugspitze summit is rather well reproduced by the lidar data. The mid- and upper-tropospheric intrusion layers seem to be dominated by very long downward transport up to a full tour around the northern hemisphere in an altitude range starting at about 4.5 km a.s.l. Unless there is a strong perturbation, these layers remain considerably dry, typically with RH ≤ 5 % at the centre of the intrusion. It is interesting to note that, in recent years, most pronounced ozone maxima have been related to a stratospheric origin rather than to long-range transport from remote boundary layers. This fact could be caused by improving air quality in the most relevant source regions or changing transport patterns.