4.1. Spatial and Temporal Trends of Meteorological and Pollution Data (O3, NO2 and NO)
The analysis of a 22-year long time series of weather and air pollution data recorded at five forest sites in Western Germany revealed differences in air quality and meteorological conditions between these sites, with some of them being significant. These differences can partly be explained by different altitude and distance to human settlements with increased traffic and industrial activity.
The study revealed a clear decreasing trend in NO
2 and NO concentrations between 1998 and 2019, which is in agreement with findings by the German Environment Agency (UBA) [
86], Junk et al. (2003) [
87] and Georgoulias and Stammes (2019) [
88]. The sites in the present study located above 500 m a.s.l. (Hortenkopf, Leisel, and Wascheid) are good examples for sites affected by long-range O
3 transport, while the lower altitude sites are more affected by local O
3 formation due to local precursor emissions [
89].
Figure 4b,c show high monthly mean NO
2 and NO concentrations during the winter period due to increased emissions from the use of fossil fuels for heating [
90,
91]. NO
2 (17.4 µg/m
3) and NO (2.67 µg/m
3) concentrations were highest in winter months, possibly reflecting increased residential heating. Junk et al. (2003) [
87] stated high monthly NO
2 concentrations in winter and low monthly NO
2 concentrations in summer in Trier, Germany.
Surprisingly the present study revealed no temporal trend in mean 24 h or daytime O
3 concentrations during the time series. It seems that that decreasing O
3 extremes (95% Quantile) and increasing low O
3 concentrations (5% Quantile) held the high varying O
3 concentration on unmodified level (
Table 3). High O
3 concentrations recorded during the 22 years at some high-altitude sites might be the result of long-range transport from nearby urban agglomerations, as the concentrations of O
3 precursors (e.g., nitrogen oxides) at these sites are too low to explain the high O
3 concentrations. These precursors are transported into clean air forest regions, where reduction agents in the air are either missing or are rare, hence O
3 has a higher life expectancy as in densely populated urban areas. Differences in O
3 concentrations depending on different altitudes are not as distinct as expected. That may have two reasons, at first the altitudinal gradient in the present study is small (200–300 m) compared to other studies and second the long-range transport of O
3 and the distances of the stations to urban agglomerations could be different to other studies. This is in agreement with Sicard et al. (2009) [
27], who reported a slightly increasing trend in tropospheric O
3 between 1995 and 2003 at French rural monitoring network stations, which was strongly influenced by the altitude. Also, Klingberg et al. (2009) [
92] in Sweden and Wehner and Wiedensohler (2003) [
93] in Germany showed that high global radiation at high altitudes is contributing to high O
3 concentrations, which supports the findings of the present study. As shown in
Figure 2 and
Table S1, 2003 was exceptionally hot with the highest global radiation and the highest O
3 concentrations as compared to the other years. Baumgarten et al. (2000) [
94] and Treffeisen and Hald (2000) [
95] argued that the cumulative hourly O
3 concentrations were enhanced at higher altitude stations in Germany as compared to lower altitude stations most likely due to transport from urban agglomerations.
Figure 4a shows a higher mean O
3 concentration observed in summer as compared to the winter due to high temperature, low humidity and high light intensity, all of which promote O
3 formation [
96]. Similar annual trends in Germany were observed by Treffeisen and Halder (2000) [
95] and Meleux et al. (2007) [
97].
Surface O
3 showed negative correlations with NO and NO
2 at all stations (
Table S3). Therefore, a rise in O
3 concentrations is associated with a reduction in NO and NO
2 concentrations. Correlations among the concentrations of different pollutants (O
3, NO
2, and NO) were stronger at Herdorf than in other forest stations, because it was the lowest altitude station of this study with a higher density of traffic and human activity. Mavroidis and Ilia (2012) [
98], Latif et al. (2014) [
99], and Minkos et al. (2020) [
86] found corroborating results, i.e., that there is are close correlations between concentrations of different pollutants (O
3, NO
2 and NO).
Figure 5 and
Table S3 show correlations of meteorological parameters (G, VPD, T, P and W) with O
3 concentrations, reflecting photochemical processes in the atmosphere which are responsible for O
3 formation.
Figure 5 illustrates the highest O
3 concentrations during periods with low precipitation. Lower precipitation (lower cloud cover) usually corresponds with higher global radiation, higher temperatures, and higher O
3 formation rates as reported by Tarasova and Karpetchko (2003) [
100] and Kovač-Andrić, et al. (2009) [
79]. Dawson et al. (2007) [
101] indicated that temperature exerted the largest influence on O
3 concentrations. Singla et al. (2012) [
102] revealed a strong correlation between global radiation intensity and O
3 concentration. The analysis of Camalier et al. (2007) [
103] confirmed that O
3 increases generally with increasing temperature while O
3 decreases with increasing relative humidity (because relative humidity usually decreases with increasing temperature). All these investigations confirm the results obtained by this study, which showed that in higher altitudes the air is cooler and more humid, and the vegetation periods are shorter, thereby representing a montane climate. Furthermore, the VPD is lower but wind speed is higher as compared to lower altitudes (
Table S1).
The present investigations also display increasing temporal trends in global radiation at all sites probably as result of less cloud cover, decreasing temporal trend in precipitation and wind speed with exception of one site (Leisel), probably due to a decrease in thunderstorms during summer months. These meteorological trends could at least be partly linked to a changing climate, but might be partly modified by the altitude, slope, and aspect (orientation) of the forests sites and measuring stations.