What's raining not only in our backyards?

If we look at the chemical composition of precipitation in Europe, the most abundant ions, apart from sodium and chlorine from seawater, are sulphates (SO₄^2-), nitrates (NO₃^-), and ammonium ions (NH₄⁺). These compounds, which reach the ground via liquid precipitation, are formed in the atmosphere by the chemical reaction of water and the individual precursors, i.e. sulphur dioxide (SO₂), nitrogen oxides (NO_x), and ammonia (NH₃). They come from both natural and anthropogenic emission sources, in particular from the combustion of fossil fuels (which produces mainly SO₄^2- and NO₃^-) and agricultural activities (mainly NH₄⁺).

Over the last few decades, significant changes have occurred in the ionic composition of precipitation, particularly due to the transformation of the structure and quantity of individual pollution sources, but also partly due to atmospheric changes and climate change. In the current research, a team of experts has therefore looked at the composition of precipitation from a slightly different perspective. Not just the development of absolute values, but in particular the changes in the ratios of major ions in precipitation. This innovative approach has thus provided new information that also reflects the changing atmospheric chemistry.

Season has been shown to have a much greater influence on the precipitation chemistry behaviour than geographical location. One of the investigated urban meteorological stations was Prague - Libuš. Photo: František Nyklíček.

The researchers used a unique data set of the above-mentioned ions in precipitation (SO₄^2-, NO₃^-, NH₄⁺) covering the time series from 1980 to 2020. The data were analysed not only in terms of trends but also in relation to seasonal changes. For the analyses, the researchers used innovative Bayesian inference with the Integrated Nested Laplace Approximation, a highly flexible and general approach suitable for the analysis of large and complicated datasets. This method also allows for the decomposition of long-term temporal trends and seasonal characteristics with correction for certain irregularities in the measurements (e.g., different exposure times of sampling devices at different stations and within one station). Several sampling sites with the longest and most complete time series representing three different environments - urban, rural, and mountain – were used to compare spatial changes.

The ratios of the main Iic Ions have changed significantly over the study period. There was a significant increase in the NO₃^-/SO₄^2- and NH₄⁺/SO₄^2- ratios. These results reflect a significant decrease in the amount of sulphur emitted to the atmosphere due to, for example, flue gas desulphurisation of thermal power plants, but still a significant source of both oxidised and reduced forms of nitrogen, mainly from fossil fuel combustion and increasing transport, but also from agriculture. In this context, however, scientists also point to changes in chemical reactions in relation to overall changes in atmospheric chemistry. With regard to seasonality, the opposite phenomenon was evident for these ratios, with higher values for NO₃^-/SO₄^2- during winter and, conversely, higher values for NH₄⁺/SO₄^2- overall during summer, consistently throughout the period studied. For both substance ratios, the type of environment, geographical position, or altitude did not play a role. Surprising was the evolution of the NH₄⁺/NO₃^- ratio, which, contrary to the expected increase, was relatively stable throughout the measurement period, with only a slight increase in the last period, with a marked seasonal behaviour with higher values in summer and lower values in winter.

Liquid precipitation is one of the important water sources for the entire ecosystem. As it passes through the atmosphere, it also changes its composition significantly due to chemical reactions with surrounding substances, thus further influencing the various natural components. They bring both nutrients and pollutants into ecosystems. The innovative analytical approach presented in this study has revealed fundamental changes in the long-term evolution of Czechia, including seasonal behaviour. It also highlighted the influence not only of overall changes in emissions but also of ongoing climate change and various chemical reactions in the atmosphere.

Mountain, rural or urban regions? It turns out that the chemical composition of atmospheric precipitation is much more influenced by season or long-term trend. Illustrative photo: K. Fraindová.

Kateřina Fraindová

Hůnová, I., Brabec, M., Malý, M. (2024): Major ions in Central European precipitation – Insight into changes in NO₃^-/SO₄^2-, NH₄⁺/NO₃^-and NH₄⁺/SO₄^2-ratios over the last four decades. Chemosphere 349, 140986. https://doi.org/10.1016/j.chemosphere.2023.140986