Trends in tropospheric ozone concentrations and forest impact metrics in Europe over the time period 2000–2014

In Europe, tropospheric ozone pollution appears as a major air quality issue, and ozone concentrations remain potentially harmful to vegetation. In this study we compared the trends of two ozone metrics widely used for forests protection in Europe, the AOT40 (Accumulated Ozone over Threshold of 40 ppb) which only depends on surface air ozone concentrations, and the Phytotoxic Ozone Dose which is the accumulated ozone uptake through stomata over the growing season, and above a threshold Y of uptake (PODY). By using a chemistry transport model, we found that European-averaged ground-level ozone concentrations (− 2%) and AOT40 metric (− 26.5%) significantly declined from 2000 to 2014, due to successful control strategies to reduce the emission of ozone precursors in Europe since the early 1990s. In contrast, the stomatal ozone uptake by forests increased from 17.5 to 26.6 mmol O3 m−2 despite the reduction in ozone concentrations, leading to an increase of potential ozone damage on plants in Europe. In a climate change context, a biologically-sound stomatal flux-based standard (PODY) as new European legislative standard is needed.


Introduction
Tropospheric ozone (O 3 ) is a secondary short-lived climate pollutant (Shindell et al. 2012), formed by the photochemical oxidation of NO x in the presence of carbon monoxide (CO), methane (CH 4 ) and volatile organic compounds (VOCs) (Chameides et al. 1988). It is also the third most important greenhouse gas in terms of radiative forcing (Mickley et al. 2001). Despite the implementation of legislative standards to control the emission of O 3 precursors worldwide (Cooper et al. 2014;Monks et al. 2015;Simon et al. 2015;Sicard et al. 2016a), O 3 concentrations remain potentially harmful to vegetation over some regions around the world (Sicard et al. 2016a(Sicard et al. , 2017Cailleret et al. 2018;Mills et al. 2018). In Europe, surface O 3 pollution appears as a major air quality issue (Sicard et al. 2013(Sicard et al. , 2018(Sicard et al. , 2020aEEA 2018), particularly in Southern Europe where road traffic and industrial emissions, combined with higher solar radiation, enhance O 3 formation (Millán et al. 2000), and causes threat to vegetation (e.g. Sanz et al. 2000;Paoletti 2006;Wittig et al. 2009;Anav et al. 2011;Mills et al. 2011;Sicard et al. 2016b). Currently, the European standard used to protect vegetation against negative impacts of O 3 is the Accumulated Ozone over a Threshold of 40 ppb (AOT40), i.e. the cumulative exposure to hourly O 3 concentrations above 40 ppb over the daylight hours of the growing season (Directive 2008/50/EC). In Europe, a target value of 9,000 ppb h, Abstract In Europe, tropospheric ozone pollution appears as a major air quality issue, and ozone concentrations remain potentially harmful to vegetation. In this study we compared the trends of two ozone metrics widely used for forests protection in Europe, the AOT40 (Accumulated Ozone over Threshold of 40 ppb) which only depends on surface air ozone concentrations, and the Phytotoxic Ozone Dose which is the accumulated ozone uptake through stomata over the growing season, and above a threshold Y of uptake (PODY). By using a chemistry transport model, we found that European-averaged ground-level ozone concentrations (− 2%) and AOT40 metric (− 26.5%) significantly declined from 2000 to 2014, due to successful control strategies to reduce the emission of ozone precursors in Europe since the early 1990s. In contrast, the stomatal ozone uptake by forests increased from 17.5 to 26.6 mmol O 3 m −2 despite the reduction in ozone concentrations, leading to an increase of averaged over 5 years, is recommended by the Directive 2008/50/CE for the vegetation protection whilst a critical level of 5,000 ppb h is recommended by UNECE (2017) for forest protection. Although AOT40 metric is widely used, the O 3 uptake through stomata is a better metric to assess plant damage because it estimates the quantity of O 3 entering in the leaf tissues (Musselman et al. 2006;De Marco et al. 2015;Sicard et al. 2016c). The Phytotoxic Ozone Dose above a threshold Y of uptake (PODY) is the accumulated stomatal O 3 flux over the growing season and can be modelled using the Deposition of Ozone and Stomatal Exchange (DO3SE) model (UNECE 2017). The threshold Y represents a detoxification threshold below which any O 3 molecule absorbed by the plant is detoxified (CLRTAP 2017). High ambient O 3 levels may not damage plants when stomata are closed (Ronan et al. 2020). Conversely, high PODY and resulting damages can occur at low O 3 levels when stomata are open under favourable environmental conditions such as optimal air temperature and soil moisture (Ronan et al. 2020). For these reasons, the stomatal flux-based approach is recommended as more realistic compared to the exposure-based approach (Paoletti and Manning 2007;Sicard et al. 2016c;Agathokleous et al. 2018).
The evaluation of temporal trends in air pollutant levels in European Union (EU) countries is an essential tool to assess the improvement of air quality due to emissions control strategies (Guerreiro et al. 2014). To date, many studies have investigated O 3 trends for a small number of monitoring stations, in particular at rural sites representative of background O 3 conditions (De Leeuw 2000). In 2016, a report was published by the co-operative programme for monitoring and evaluation of the long-range transport of air pollutants in Europe

Environmental data: the WRF-CHIMERE modelling system
Hourly air temperature data and O 3 concentrations were obtained, respectively, from the Weather Research and Forecasting (WRF), a mesoscale meteorological model (Skamarock and Klemp 2008) and CHIMERE, an Eulerian offline chemistry-transport model developed to analyse the gas-phase chemistry, aerosol formation, transport and deposition at regional scale. Data were provided at 1-h temporal resolution and 12 km × 12 km of spatial resolution over the time period 2000-2014. The O 3 concentrations at 20-25 m of height from the ground (top of the canopy) provided by the CHIMERE model were used to calculate AOT40 and PODY. Further information about the validation of data obtained by the WRF-CHIMERE modelling system can be found in Menut et al. (2013), Martin et al. (2014) and Anav et al. (2016).

AOT40 calculation
The O 3 exposure index AOT40 (in ppb hours, abbreviated to ppb h) was calculated as sum of the hourly exceedances above 40 ppb, for daylight hours (8 am-8 pm) during the growing season, i.e. 1st April-30th September for the protection of forest trees (UNECE 2010), according to the methodology for O 3 risk assessment in Europe.
where [O 3 ] is hourly O 3 concentration (ppb), n denotes the number of hours to be included in the calculation period and dt is time step (1-h). The function "maximum" ensures that only values exceeding 40 ppb are taken into account. In Europe, the critical values for the protection of forests is 5000 ppb h as recommended by UNECE (2010).
However, Klingberg et al. (2014) showed that AOT40 does not consider the influence of climate change on the growing season duration. By consequence, we used in this study the AOT40 formula proposed by Anav et al. (2016) that is more plausible from a physiological point of view as the revised AOT40 was calculated from 1st January to 31st December for hours with stomatal conductance (g sto ) higher than 0: [O 3 ] is the hourly O 3 concentration (ppb), dt is the time step (1-h) and g sto is the stomatal conductance computed according to Eq. (3). However, AOT40 does not provide any information on the O 3 uptake by leaves .

Phytotoxic ozone dose calculation
The PODY was calculated using the DO 3 SE model, based on the multiplicative Jarvis' algorithm (Jarvis 1976) for estimation of g sto (mmol O 3 m −2 s −1 ). The g sto is calculated as a species-specific function where the maximum value of stomatal conductance (g max ) is reduced by limiting functions, scaled from 0 to 1 as described in Eq. 3.
where g max is the maximum stomatal conductance of a plant species to O 3 (mmol O 3 m −2 s −1 per leaf area). The functions f phen , f light , f temp , f VPD and f SWC stand for the g max variation with leaf age, photosynthetically flux density at the leaf surface (PPFD, μmol photons m −2 s −1 ), surface air temperature, (T, °C), vapor pressure deficit (VPD, kPa) estimated through the surface air humidity, and volumetric soil water content (SWC, m 3 m −3 ), respectively. The function f min is the minimum g sto expressed as a fraction of g max . We assumed that f phen was 1 throughout the growing season. The following formulas were applied: where light a is an adimensional constant; PPFD is hourly photosynthetic photon flux density estimated through the solar radiation; T opt , T min , and T max , represent the optimum, minimum, and maximum temperature for stomatal conductance, respectively; VPD min and VPD max are minimum and maximum vapor pressure deficit for stomatal conductance, respectively; WP is SWC at wilting point and FC is SWC at field capacity. These two parameters are constant and depend on the soil type obtained from a module included into the WRF-CHIMERE model. (3) The dominant vegetation data, required to estimate g sto , were retrieved from the spatial tree distribution, based on the European Forest Institute database (Brus et al. 2011). Species-specific values of DO3SE parameters were derived from UNECE (2017) for each dominant plant species.
Once the stomatal conductance was computed, the stomatal O 3 flux was calculated over the growing season and expressed as PODY (nmol O 3 m −2 s −1 per leaf area). PODY (mmol m −2 ) was calculated from hourly data as: where PODY is the accumulated stomatal O 3 flux above a threshold Y over the growing, g sto represents the hourly stomatal conductance values, [O 3 ] is the hourly O 3 concentrations (ppb) and dt is the time step (1-h). PODY was calculated with Y = 0 nmol O 3 m −2 s −1 per leaf area, by considering that any O 3 molecule is harmful for plants (Musselman et al. 2006), and Y = 1 nmol O 3 m −2 s −1 per leaf area, as recommended by CLRTAP (2017).

Estimation of annual trends
A 10-year time-series of O 3 data is considered long enough to assess short-term changes as reported in Sicard et al. (2016a). The non-parametric tests are robust and suitable for non-normally distributed data with missing and extreme values (Sicard et al. 2009). The non-parametric Mann-Kendall test was used to assess whether there is a monotonic upward or downward trend of O 3 over time (Sicard et al. 2013;Guerreiro et al. 2014). To quantify linear trends, the non-parametric Sen's slope estimator was used (Sicard et al. 2013(Sicard et al. , 2016aAraminienė et al. 2019). Annual trends were calculated for O 3 metrics over the time period 2000-2014 for each European country as well as for four European regions as classified in UNECE (2010). Table 1 shows the regional classification of countries. The results are considered statistically significant at p < 0.05.

European distribution of air temperature and ozone metrics
Over the time period 2000-2014, the minimum annual air temperatures (7.0 ± 0.3 °C) occurred in Northern Europe while the maximum values (12.7 ± 0.5 °C) were found in Mediterranean Europe (Fig. 1). Atlantic and Continental central Europe showed mean annual temperature ranging from 8.2 to 10.2 °C (Fig. 1). Taking into account mean O 3 concentrations, Mediterranean Europe showed values  (Fig. 2). Similarly, the highest AOT40 values (38,359 ppb h) were observed in Mediterranean Europe whilst the lowest AOT40 values (5094 ppb h) were found in Northern Europe (Fig. 3). In Continental central Europe, AOT40 ranged from 13,636 to 23,515 ppb h while in Atlantic central Europe AOT40 varied from 8207 to 13,751 ppb h (Fig. 3). Taking into account POD0 values, the minimum value (14.0 mmol O 3 m −2 ) was found in 2006 in Northern Europe while the maximum values, 29.7 and 32.1 mmol O 3 m −2 were observed in Atlantic and Mediterranean Europe, respectively (Fig. 4). Similar results were found for POD1 (Fig. 5).

Trends in ground-level ozone metrics at European level
The annual trend magnitudes for O 3 concentrations, AOT40, POD0 and POD1 over the time period 2000-2014 are shown in Tables 2, 3, 4 and 5. The O 3 mean concentrations decreased significantly (p < 0.05) by 0.4 ppb per decade in Continental Central Europe and by 1.1 ppb per decade in Mediterranean Europe (Table 2). In Atlantic Central Europe and Northern Europe, the trends for O 3 mean concentration were not statistically significant ( Table 2). The exposure index AOT40 significantly declined in Atlantic, Continental and Mediterranean Europe with a magnitude of 2124, 5532 and 7161 ppb h per decade, respectively (Table 3). POD0 and POD1 increased significantly over the time period 2000-2014 all over Europe, except in Northern Europe showing a positive but not significant (p > 0.5)

Trends in ground-level ozone metrics at country-level
Over the time period 2000-2014, the exposure AOT40 index significantly declined in most of European countries (Fig. 6a) (Fig. 6b). In particular, only six countries showed positive trends (Denmark, United Kingdom, the Netherlands, Germany, Sweden and Svalbard). On the contrary, trends of POD0 and POD1 were positive in all countries ( Fig. 7a and  b) with minimum values of + 0.03 and + 0.04 mmol m −2 per year and maximum values of + 1.06 and + 0.93 mmol m −2 per year for POD0 and POD1, respectively.  (Liu et al. 2016a, b;Fu et al. 2017;Anav et al. 2019). They reported earlier green-up dates and delayed dormancy dates then a longer growing season due to changing climate. Moreover, climate change increases the stomatal conductance thanks to the positive effects of higher air temperature and solar radiation on stomata opening (Hoshika et al. 2015). Even if the O 3 mean concentrations decreased, higher PODY levels were observed over time leading to higher O 3 risk to European forests Anav et al. 2019). Anav et al. (2019) hypothesized that the positive feedback between climate change and PODY will increase in the near future and the efforts in controlling emissions of O 3 precursors could be significantly offset by climate change, thus increasing the O 3 risk for forests. A primary goal is to define a metric for O 3 -risk assessment, which can identify ecosystems at O 3 risk to protect them using standards and policies. In Europe, AOT40 has been widely used, under the assumption that plant injury and exposure to O 3 concentrations are correlated (EPA 2007;UNECE 2011;Fares et al. 2013).

Discussion and conclusions
To date, several studies report a general growing consensus for moving toward a biologically-sound stomatal flux-based standard (PODY) as new European legislative standard (Mills et al. 2011;Fares et al. 2013;Sicard et al. 2016b,c;Anav et al. 2016Anav et al. , 2019 although critical levels for vegetation protection still need to be validated (Sicard et al. 2016c The question about deriving new critical levels is still a challenge for the scientific community (De Marco and Sicard 2019), even because some advantage of AOT40 are still present. Indeed the simplicity and fast applicability of AOT40 could be an advantage for the use of the concentration-based metric . But on the other side AOT40 is not taking into consideration more biological processes linked to the stomatal aperture and can have spatially and temporally different patterns (De Marco et al. 2015). These consideration highlights the role of climate into determination of the impacts of ozone on forests. Consequently, strategies integrating both climate and air quality policies are urgently needed for forest protection against the negative impacts of O 3 (Ainsworth et al. 2012).
Funding Open access funding provided by Ente per le Nuove Tecnologie, l'Energia e l'Ambiente within the CRUI-CARE Agreement.
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