Zugspitze ozone 1978–2020: The role of stratosphere-troposphere transport

Abstract. The pronounced increase of ozone observed at the Alpine station Zugspitze (2962 m a.s.l.) since the 1970s has been ascribed to an increase of stratospheric air descending to the Alps. In this paper, we present a re-analysis of the data from 1978 to 2011 for both ozone and carbon monoxide, extended until 2020 by the data from the Global Atmosphere Watch site Schneefernerhaus (UFS, 2671 m a.s.l.) just below the Zugspitze summit. The analysis is based on data filtering utilizing the isotope ⁷Be (measured between 1970 and 2006) and relative humidity (1970 to 2011, UFS: 2002 to 2020). We estimate both the influence of stratospheric intrusions directly descending to the northern rim of the Alps from the full data filtering and the aged (“indirect”) intrusions from partial filtering with the ⁷Be data. The evaluated total stratospheric contribution to the annual-average ozone rises roughly from 12 ppb in 1970 to 24 ppb in 2003. It turns out that the increase of stratospheric influence is particularly strong in winter. A lowering in positive trend is seen afterwards, almost parallel to the beginning decrease of solar irradiation. The air masses hitting the Zugspitze summit have become drier in the percentile range up to at least 25 % until 2003, and we see the growing stratospheric contribution as an important factor to this drying. Both an increase of lower-stratospheric ozone and a growing width of the intrusion layer departing downward from just above the tropopause must be taken into consideration. Carbon monoxide in intrusions did not change much during the full measurement period 1990 to 2020, with perhaps a slight increase until 2005 and an almost constant behaviour afterwards. This is remarkable since outside intrusions a decrease by approximately 44 % was found, indicating a substantial improvement of the tropospheric air quality.