Popular Science: A friend in the attic, an enemy on the ground floor – ozone: the challenge of man’s future?
Although it is the same molecule – O3 – the formation of ozone in the stratosphere and in the troposphere is different. In the stratosphere, ozone is formed by shortwave solar radiation with the co-action of a catalyst, especially N2. However, solar radiation with wavelengths below 290 nm is absent in the troposphere. Under normal conditions, ozone is formed in the troposphere as well, but this happens on the basis of a completely different mechanism, and due to its high reactivity, its molecule decays very rapidly in the atmosphere.
Overall, ground-level ozone concentration increases with increasing solar radiation. Other factors are higher temperature and lower relative humidity. Therefore, the highest O3 content is observed more often in the summer months and in the mountains. Another key input parameter is required, though, and that is the precursors (mainly nitrogen oxides and volatile organic compounds), and both their absolute amount and their relative representation in the atmosphere are essential.
As ozone is among the most powerful oxidizing agents known (and as such it is often used for disinfecting indoor spaces and swimming pools), it has a highly adverse effect on living organisms. Breathing ozone causes breathing problems, inflammatory diseases, impaired lung development and reduced lung function, eye irritation, headaches and insomnia, and is also associated with increased mortality. In Europe, critical levels are currently permanently exceeded, and although various countries have enacted different limits (EU: 120 μg.m−3, USA: 140 μg.m−3, China: 160 μg.m−3), the WHO states that a concentration as low as 100 μg.m−3 represents a health risk.
Another problem is the harmful effect on vegetation. Ozone damages plants by entering leaf openings called stomata and oxidizing (burning) plant tissue during respiration. It is currently assumed that ozone, together with an excessive amount of nitrogen, plays a crucial role in increased forest death, especially in mountain areas. In addition to higher solar radiation, air humidity is also higher there. During more humid conditions, stomata remain open as opposed to dry periods when they close to protect plants from water loss but as well as ozone ingress.
To include all types of environments in our territory, a total of 12 measuring stations from different environments, geographical regions and altitudes were selected for the study, representing the main three groups of areas – urban, rural and mountain sites. The stations selected were those that had been conducting measurements for the longest period of time, since 1993. An important advantage was that the equipment and procedures did not change over the entire period, which otherwise often causes problems in the evaluation of results. To evaluate the trend, seasonal changes and changes in the seasonal profile over the years, researchers used a generalized additive model (GAM).
At the beginning of the observed period, in the mid-1990s, ozone concentrations at the individual stations were very different and inconsistent behaviour was observed between stations until 1998. This was mainly influenced by very different emissions of precursors in the individual regions of the Czech Republic before the implementation of the strict emission limits in the 1990s, which led to a substantial decrease in ozone precursor emissions. At some stations, there was a significant decrease in relatively high concentrations of ozone (urban and rural stations). One of the mountain stations, which was located in the area referred to as the Black Triangle (i.e., the border area between the former GDR, Poland and Ore Mountains), more specifically in the Ore Mountains, the concentrations were the lowest initially and then increased most rapidly. Researchers report that, with a few exceptions, there has been a tendency to trend shape homogenisation across stations after 2005. Around 2003 and 2006 it was possible to observe local maxima and around 2013 a local minimum. These fluctuations are mainly related to climatic conditions. After 2014, however, there has been a constant increase in ozone concentrations occurring at all monitored stations despite a significant reduction in the emissions of ozone precursors in the Czech Republic as well as virtually throughout Europe, which is quite worrying and is probably related to the ongoing climate change. After all, the last five years have been extremely hot and dry in the Czech Republic.
Seasonal changes for the entire period were similar at all stations, with the maximum ozone concentrations always observed in May or June, primarily in connection with higher solar radiation and air temperature. Very interesting results were evident in the evaluation of changes in the shape of the seasonal profile, which showed that at some stations there has been a shift in maximum ozone concentrations, with one of them showing a shift of a full month later over the entire period of twenty-three years (one station in the Ore Mountains). By contrast, the station in Prague-Libuš only observed minor changes.
The results of the current study show that the occurrence of tropospheric ozone in our territory is currently increasing. Due to its harmful effects on human health and vegetation, reducing its concentration in the air is more than desirable. However, this is not an easy task since the formation of ground-level ozone is a very complicated process affected by many factors. In addition to the influence of precursors, especially nitrogen emissions and volatile organic compounds, climate change also contributes to its occurrence. This is a difficult yet very urgent task that will have to be solved.
Hůnová, I., Brabec, M., Malý, M. (2020): Trends in ambient O3 concentrations at twelve sites in the Czech Republic over the past three decades: Close inspection of development. Science of the Total Environment 746, 141038. https://doi.org/10.1016/j.scitotenv.2020.141038.