Spatio-Temporal Variation of Ozone Concentrations and Ozone Uptake Conditions in Forests in Western Germany

Spatio-temporal variation of ozone pollution risk and its influencing factors in China based on Geodetector and Geospatial models

Ozone (O₃) has become the primary pollutant in many cities, and high concentrations of O₃ cause significant harm to the ecological environment and human health. This study investigated the spatiotemporal distribution of surface concentrations of ozone over entire China and analyzed the influencing factors based on the geographical detector technique. Moreover, the Pearson correlation analysis was used to analyze the influence of various meteorological factors on ozone concentrations. The results showed that, on the national scale, the daily average O₃ concentration in the cities of China in 2019 was 92.441 μg/m³ and the nonattainment rate of daily average ozone was 7.98%. However, the ozone nonattainment rate was 33.33% in heavily polluted regions. The highest O₃ concentration was observed in summer, and the lowest was observed in spring. The O₃ concentrations in cities across the country showed significant spatial distribution characteristics. Among the five pollutants, the highest correlation was observed between O₃ and PM_2.5 and the lowest was observed between O₃ and SO₂. Among the metrological factors, wind speed and solar radiation are the most influencing factors, and showed positive correlation. Moreover, the annual precipitation is negatively correlated with O₃-8h concentrations. The methods and findings of this paper can be used as an aid for air pollution control programs in different regions for diminishing the risk of exposure to various air pollutants.

Influence of Ozone and Drought on Tree Growth under Field Conditions in a 22 Year Time Series

Studying the effect of surface ozone (O3) and water stress on tree growth is important for planning sustainable forest management and forest ecology. In the present study, a 22-year long time series (1998–2019) on basal area increment (BAI) and fructification severity of European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) H.Karst.) at five forest sites in Western Germany (Rhineland Palatinate) was investigated to evaluate how it correlates with drought and stomatal O3 fluxes (PODY) with an hourly threshold of uptake (Y) to represent the detoxification capacity of trees (POD1, with Y = 1 nmol O3 m−2 s−1). Between 1998 and 2019, POD1 declined over time by on average 0.31 mmol m−2 year−1. The BAI showed no significant trend at all sites, except in Leisel where a slight decline was observed over time (−0.37 cm2 per year, p < 0.05). A random forest analysis showed that the soil water content and daytime O3 mean concentration were the best predictors of BAI at all sites. The highest mean score of fructification was observed during the dry years, while low level or no fructification was observed in most humid years. Combined effects of drought and O3 pollution mostly influence tree growth decline for European beech and Norway spruce.

Assessment of ozone risk to Central European forests: Time series indicates perennial exceedance of ozone critical levels

In this study, the stomatal ozone (O₃) fluxes were investigated at five low-elevation forest sites in Western Germany (Rhineland Palatinate) over the time period 1998-2019. The Phytotoxic Ozone Dose with an hourly threshold of uptake (Y), to represent the detoxification capacity of trees (POD1 in mmol m^-2 per leaf area, with Y = 1 nmol O₃ m^-2 s^-1), and the number of exceedances of the O₃ critical level of 5.2 mmol O₃ m^-2 per leaf area for European beech and 9.2 mmol O₃ m^-2 per leaf area for Norway spruce were calculated by using the DO₃SE model. A Principal Component Analysis revealed strong correlations between daily O₃ concentrations, daytime O₃ (for hours with global radiation exceeding 50 W m^-2), POD1, global radiation, vapor pressure deficit and air temperature. Moreover, a significant correlation was obtained between POD1 and soil water content (SWC) at all sites (r = 0.51-0.74). The Random Forests Analysis confirmed that the SWC is the most important predictor of stomatal O₃ fluxes. The soil water supply is very important for POD1 estimation, because drought decreases stomatal conductance, leading to a reduction of transpiration, as well as to lower O₃ uptake through stomata. Between 1998 and 2019, the drier and warmer climate induced a soil drought (on average, SWC - 0.15 % per year) leading to lower stomatal O₃ uptake by forests (- 0.36 mmol O₃ m^-2 per year). Hence, during growing seasons with sufficient water supply and often lower O₃ levels compared to hot and dry periods, forests are at higher O₃ risk than during hot and dry periods when the drought stress is more significant than O₃ stress despite relatively higher O₃ levels. Irrespective of these differences in O₃ uptake between relatively cool and humid as compared to relatively hot and dry years in the study region, the Critical Level for O₃ was exceeded in late spring/early summer (May/June) during all 22 years. Risk assessment for the protection of European forests, which is urgently needed due to the forests current critical state after several successive years of drought and exceedance of the O₃ critical level in large areas of Europe, should therefore become flux-based to account for the inter-twined effects of drought and O₃ on the physiology and health of forest trees in the region. For stomatal O₃ fluxes estimation, a better soil water and leaf parameterization is needed e.g., by taking into account both O₃- and drought-induced effects.