The effective quantum yield of photochemical energy conversion in photosystem II (Φ2) was shown to be reduced in the terricolous lichens Cetraria aculeata and Cetraria islandica by short-term exposure to aqueous SO2 at pH values occurring in the precipitation of areas with high SO2 pollution. Significant reduction of Φ2 was found at pH <= 3.3. At pH 2.8, Φ2 was close to zero and did not recover within 24 h. This suggests that sensitivity to SO2 (primarily associated with epiphytic lichens in the past) has contributed to the decline of both species in central Europe. In C. islandica, but not in C. aculeata, thalli with the natural content of lichen substances were more tolerant to SO2 than thalli where the extracellular lichen substances were extracted before the experiment. This supports published results that the depsidone fumarprotocetraric acid, a major lichen substance of C. islandica, increases the pollution tolerance in lichens. Artificial acidic precipitation with aqueous sulphur dioxide at pH 2.8-3.5 affects terricolous Cetraria species.

To assess the effect of changes in traffic density and fuels used for heating at the beginning of the 1990s, 1992-2005 monthly averages of PM₁₀, SO₂, NO₂, NO, CO and O₃ from Prague, the Czech capital, were analyzed together with long term trends in emissions of major pollutants, fuel consumption and number of vehicles registered in Prague. The data from all monitoring stations were retrieved from the database of the state automated monitoring system. Correlation coefficients between ambient monthly averaged temperature and all pollutants of concern showed distinct seasonal trends. The results showed that while SO₂ and to some extent also CO concentrations dropped namely in the first half of the analyzed period (1992-1997) as a result decreased fossil fuel consumption for local heating, the behaviour of other pollutant concentrations followed a different pattern. PM₁₀ concentrations decreased during the beginning of the 1990s but showed a sign of increase after 2000. Concentrations of ozone and NO₂ did not reveal any significant change throughout the whole studied period. It can be concluded that during the studied period traditional urban sources of pollution, such as coal and oil combustion, lost their importance but were simultaneously substituted by pollutants from automotive transport (namely PM and NO₂) making the problem of air quality even worse.