Ozone pollution puts forests in the Ore Mountains under stress even in winter

Year-round ozone stress and other environmental pollution damage the trees
As researchers found in a recent study, ozone pollution in the Ore Mountains (Erzgebirge) has not decreased in the winter months since the late 1990s. As a result, the forests suffer from ozone stress all year round, even though the air in Central Europe as a whole has become cleaner.

“Ozone is a form of oxygen. While the gas in the upper layers of the atmosphere acts as a protective jacket against UV radiation, high ozone concentrations near the ground are an air pollutant,” explains Prof. Jörg Matschullat from TU Bergakademie Freiberg. “Solar radiation, temperature and humidity, as well as the proportion of so-called ozone precursors from man-made sources influence the formation of ozone. The complex photochemical process has been better researched for the warm season than for the cold season,” adds the geoecologist.

At four Ore Mountains air quality measuring stations in Carlsfeld, Fichtelberg, Schwartenberg and Zinnwald, ozone has been measured continuously since 1981. Prof. Jörg Matschullat’s team has now been able to analyze the ozone values in detail with a focus on the winter half-year. “Most studies look at the annual mean values of ozone pollution. In a current scientific publication we looked specifically at winter ozone,” explains first author Hannah Gebhardt, who analyzed the measured values from the Ore Mountains as part of her bachelor thesis. When the researchers look at the data from 1981 to today, three trends can be identified: While ozone concentrations were low in winter in the 1980s (values around 35 µg m-3), there was a sharp increase in the 1990s (up to 60 µg m-3). The values have not decreased since 1997 and have settled at a high level. "That was a surprise because the air pollution with sulfur dioxide and dust decreased significantly in the same period, with nitrogen oxides from the 2000s," explains co-author Dr. Frank Zimmermann.

Ozone depletion weaker than expected despite reduced air pollution

The researchers provide an explanation for the phenomenon thanks to the newly collected data: Ozone depletion reactions often take place faster in cloud and fog droplets than in the air. In the Ore Mountains, the potential for this degradation has decreased significantly due to regional air pollution control measures. The decline in the degradation potential compensates for the lower ozone levels from long-distance atmospheric transport. As a result, winter ozone levels remain at the same level. The findings of the Freiberg researchers can also be transferred to other low mountain ranges in Europe.

Forest under stress

“The constant ozone concentrations are a stress factor for forest ecosystems. For several years, trees have not only suffered from drought (summer 2018, 2019, 2020), but also from ozone stress all year round, especially in mild winters. If the temperature rises further, winter ozone could become an additional stress factor in the future,” warns Prof. Jörg Matschullat. The Freiberg scientists therefore recommend including this burden in strategies for forest conversion in Saxony and throughout Europe: "The trees need more support than usual until they have reached the strength to have developed sufficient resistance to stand ‘on their own feet’," says Dr. Frank Zimmermann.

Background: Air quality measuring stations in the Ore Mountains

For the analysis of winter ozone values, the Freiberg researchers used hourly measured meteorological data and air quality data at the four stations Carlsfeld, Fichtelberg, Schwartenberg, Zinnwald near the German-Czech border. The data can explain the complex ozone formation and depletion behavior. All stations are operated by the Saxon State Office for Environment, Agriculture and Geology. In addition to ozone, nitrogen oxides (NOx), sulfur dioxide (SO2) and particulate matter are also measured.

Original publication: Hannah Gebhardt, Frank Zimmermann, Jörg Matschullat: 1981–2020 winter ozone trends, Erzgebirge, Central Europe. Geochemistry - Chemie der Erde 80, https://doi.org/10.1016/j.chemer.2020.125738

Prof. Dr. rer. nat. habil. Jörg Matschullat, joerg.matschullat@ioez.tu-freiberg.de, +49 3731 39-3399