How high is the ambient ozone?
Ambient ozone (O3) is part of the so-called photochemical smog, which is currently caused not only by automobile traffic and fossil fuel combustion (as the most important sources of nitrogen oxides - NOx - ed.), but also by volatile organic compounds (VOCs) emitted naturally, e.g. from forest stands, under the influence of sunlight. Its elevated concentrations are of great concern as they have a very negative effect on human health - from respiratory problems, burning eyes, headaches, or dizziness to a general reduction in the body's immunity and increased mortality. In addition to damaging animal tissues, it also causes significant damage to plant tissue and is thus another stressor affecting forestry or agricultural crops.
Due to its hazardous impacts, ozone concentrations are monitored over a long period of time, but mainly spatially, in the so-called respirable zone, i.e. 2 metres above the ground. Research led by Iva Hůnová has now taken a closer look at the ozone situation at different hights near the Earth's surface. For their research, the experts chose data from the measurement observatory in Košetice, as it is a typical Central European rural area, and in particular, a tall measuring tower has been used for measurements for some time. Ozone concentrations measured continuously at a total of four hight levels - namely 2, 8, 50, and 230 m above ground level - were analysed between 2015 and 2021. In general, the overall average daily ozone value increased from 64.7 μg.m-3 at 2 m to 79.3 μg.m-3 at 230 m above the ground (the EU limit value is 120 μg.m-3; according to the World Health Organisation, a concentration of 100 μg.m-3 is already a health risk - ed.), but the results were not so simple.
For the calculations, the researchers used a semiparametric GAM (generalized additive model) that focused on both the annual trend and seasonality. The year-to-year trend was asymmetric, as the highest average ozone values were measured in 2018 with a decrease in ozone concentrations towards the end of the study period compared to the beginning. Considering the time of year, there was a significant peak of maximum ozone values in the summer months, which is mainly due to higher solar radiation intensity and higher temperature.
The results of the vertical ozone concentration gradient (expressed as the change in ozone concentration per 1 m of height) were not uniform for the entire air column (2-230 m) studied. The O3 concentration gradient appeared to vary significantly within and between years. Although there was a mainly positive gradient of ozone concentrations between different levels, the pair of levels at 2 and 8 m above the ground surprisingly showed higher O3 concentrations closer to the ground, especially during the winter months when ozone concentration is generally lower due to lower temperatures and less solar radiation. This may be because the rural environment is farther away from other sources of pollution such as nitric oxide (NO), which contributes to ozone re-decomposition. In addition, snow cover also increases albedo, so higher solar reflectance may contribute to photochemical reactions.
Although there has been a gradual decline in maximum ozone concentrations since the beginning of ozone measurements in the Czech Republic in 1993, which is related to a decline in ozone precursors emissions, since 2014 a steady increase in daily average concentrations can be observed. Given the increasing traffic and ongoing climate change, the future perspective is not much brighter. Continuous monitoring and new approaches are thus more than needed.
The results of this study show an innovative approach to measuring ozone at different levels, allowing for better characterization and providing deeper insight into the 3D structure of the atmosphere and its pollution. As the gradient of ozone concentrations varied systematically over the year, the researchers warn of possible inaccurate conclusions when assessing ozone concentrations based on averages over longer periods of time or at only one high. In a future study, the researchers also want to focus on an even more detailed assessment of the changes in the O3 concentration gradient depending on different atmospheric stability conditions, which could also aid in better understanding and predicting ozone concentrations in the Central European region.
Hůnová, I., Brabec, M. & Malý, M. Ambient ozone at a rural Central European site and its vertical concentration gradient close to the ground. Environ Sci Pollut Res (2023). https://doi.org/10.1007/s11356-023-28016-8