Surface ozone risk to human health and vegetation in tropical region: The case of Thailand.

Tropospheric ozone (O3) is a threat to vegetation and human health over the world, in particular in Asia. Knowledge on O3 impacts on tropical ecosystems is still very limited. An O3 risk assessment to crops, forests, and people from 25 monitoring stations across the tropical and subtropical Thailand during 2005-2018 showed that 44% of sites exceeded the critical levels (CLs) of SOMO35 (i.e., the annual Sum Of daily maximum 8-h Means Over 35 ppb) for human health protection. The concentration-based AOT40 CL (i.e., sum of the hourly exceedances above 40 ppb for daylight hours during the assumed growing season) was exceeded at 52% and 48% of the sites where the main crops rice and maize are present, respectively, and at 88% and 12% of the sites where evergreen or deciduous forests are present, respectively. The flux-based metric PODY (i.e., Phytotoxic Ozone Dose above a threshold Y of uptake) was calculated and was found to exceed the CLs at 1.0%, 1.5%, 20.0%, 1.5%, 0% and 68.0% of the sites where early rice, late rice, early maize, late maize, evergreen forests, and deciduous forests can grow, respectively. Trend analysis indicated that AOT40 increased over the study period (+5.9% year-1), while POD1 decreased (- 5.3% year-1), suggesting that the role of climate change in affecting the environmental factors that control stomatal uptake cannot be neglected. These results contribute novel knowledge on O3 threat to human health, forest productivity, and food security in tropical and subtropical areas.

Estimates of biomass reductions of ozone sensitive herbaceous plants in California.

Exposure to tropospheric ozone pollution impairs photosynthesis and growth in plants and this can have consequences for ecosystems. However, exposure-response research in the United States (U.S.) has historically focused on trees and crops, and less attention has been given to non-crop herbaceous species. We combined U.S. Environmental Protection Agency ozone monitoring data from the entirety of 2016 with published exposure-response relationships from controlled exposure experiments for twenty herbaceous plant species occurring in California. The U.S. Department of Agriculture PLANTS database was used to identify county-level occurrence data of these plant species. A kriged ozone exposure surface for 2016 was generated using data from monitoring stations in California and surrounding states, using Accumulated Ozone exposure over a Threshold of 40 ppb (AOT40) as an exposure metric. County-wide ozone exposure estimations were then combined with published exposure response functions for focal plants, and maps were created to estimate ozone-induced growth losses in the counties where the plants occur. Plant species had estimated annual growth losses from <1 % to >20 % based on exposure levels and sensitivity. Of the 20 species, 17 had predicted biomass loss >5 % in at least one county, emphasizing the vulnerability of herbaceous species at recent ozone concentrations. Butte, Nevada, Plumas, San Luis Obispo, and Shasta Counties, an area of about 31,652 km2, had the highest number of species (6) with >10 % estimated biomass loss, the loss threshold for European critical levels. White clover (Trifolium repens L.) was one of the most affected species with more than an estimated 10 % annual estimated growth loss over 59 % of the state. Overall, these estimated growth losses demonstrate potential for shifts in plant communities and negative effects on ecosystems. This study addresses critical policy needs for risk assessments on herbaceous species in a single year of ozone exposure.

Reductions in crop yields across China from elevated ozone.

Exposure of crops to high concentrations of ozone can cause substantial reductions in yield that pose a serious threat to global food security. Here we provide comprehensive estimates of yield losses for key crops across China between 2014 and 2017 attributed to ozone using a number of new approaches. We use an air quality model at 5 km resolution and crop-specific dose-response functions developed for both concentration- and flux-based metrics. We bias correct modelled ozone concentrations and metrics using observations from more than 1000 locations. We find that on a 4-year average basis, production losses of key crops are 34-91 million metric tonnes (Mt/yr), dependent on the approach used, with highest losses in Henan province. At a national level, loss of winter wheat production derived using a China-specific dose-response function increased by 82% from 2014 to 2017, with large interannual variations in the North China Plain and in eastern China. Winter wheat losses estimated using flux-based functions, which require robust simulation of stomatal conductance and underlying vegetation physiology, are significantly lower, at 30 Mt/yr. We show that the definition of the growing season may have a greater impact on estimated losses than small biases in ozone surface concentrations. Although uncertainties remain, our findings demonstrate that increasing ozone concentrations have substantial adverse impacts on crop yields and threaten food security in China. It is important to control ozone concentrations to mitigate these negative impacts.

Human, Forest and vegetation health metrics of ground-level ozone (SOMO35, AOT40f and AOT40v) in Tehran.

PURPOSE: We aimed to investigate the spatial O3 indices (SOMO35: annual sum of maximum daily 8-h ozone means over 35 ppb, AOT40: the accumulated exposure over an hourly threshold of 40 ppb during daylight hours between 8:00 and 20:00 in the growing seasons of plants) in Tehran (2019-2020). METHODS: The data of ambient O3 concentrations, measured at twenty-three regulatory ambient air quality monitoring stations (AQMSs) in Tehran, were obtained. RESULTS: The annual mean O3 concentrations were found to be 15.8-25.7 ppb; the highest and lowest annual mean concentration of ambient O3 were observed in Shahrdari 22 and Shahr-e-Rey stations, respectively. Spatial distribution of exposure to O3 across Tehran was in the range of 1.36-1.64; the highest O3 concentrations were observed in the northern, west and south-western parts of Tehran, while the central and south areas of Tehran city experienced low to moderate concentrations. The indices of SOMO35, AOT40f and AOT40v across AQMSs in Tehran was in the range of 1830-6437 ppb. Days, 10,613-39,505 ppb.h and 4979-16,804 ppb.h, respectively. For Tehran city, the indices of SOMO35 and AOT40f were 4138 ppb. days and 27,556 ppb.h respectively. Our results revealed that the value of SOMO35 across AQMSs of Tehran was higher than the recommended target value of 3000 ppb. days. CONCLUSIONS: To reduce O3 pollution and its effects on both human and plants health, the governmental organizations should take appropriate sustainable control policies.

Yield and economic losses of winter wheat and rice due to ozone in the Yangtze River Delta during 2014-2019.

Ground-level ozone (O3) is the main phytotoxic air pollutant causing crop yield reduction in China. As the main grain producing area in China, the Yangtze River Delta (YRD) is facing serious O3 pollution. This study analyzed the hourly ground-level O3 observation data of 158 stations from 2014 to 2019 in YRD, and grain production data of 193 districts and counties. The exposure-response relationships based on AOT40 (accumulated hourly O3 concentration above 40 ppb) was used to estimate the yield loss and economic loss of two food crops (winter wheat and rice). This study used spatial interpolation and calculated the specific data values of each district and county in order to improve the assessment reliability. For years 2014-2019, averaged O3 concentration during the 75 days growing period of rice and wheat were 33.1-50.6 ppb and 32.2-48.0 ppb, AOT40 value were 5.2-12.0 ppm h and 4.6-9.4 ppm h, and the averaged relative yield losses were 4.9%-11.4% and 9.4%-19.3%, respectively. The trend of O3 in the YRD in a six-year period peaked in 2016 and 2017 for rice and winter wheat, respectively. During 2014-2017, the average estimated yield loss of rice was 2445 Mt. accounting for about 9.1% of the actual production, and the average estimated economic loss was about 1037 million USD; for winter wheat, it was 2025 Mt, 20.4% and 736 million USD, respectively. These results urge governments to provide effective policies and measures to control O3 pollution.

First observation-based study on surface O3 trend in Indo-Gangetic Plain: Assessment of its impact on crop yield.

Tropospheric ozone (O3) is an important air pollutant which causes substantial losses in crop production. Increasing O3 levels in India particularly in Indo-Gangetic Plain (IGP) is a major issue as it is reducing the crop yield. The present study is an attempt to determine the O3 and its precursor trend using continuous ground-based observations at a suburban site in IGP. The study focuses on the overall characteristics of annual, monthly, diurnal and hourly measurements of O3. Annual mean values of O3 have shown an increment of 19.2% from 2010 to 2015. Similarly, nitrogen oxide (NOx) levels increased by 30.2%. O3 levels at the study site showed a significant increasing trend of 0.7 ppb/yr. The observed O3 trend was analyzed in terms of changes in NOx levels and meteorological parameters. No significant difference in meteorological parameters was observed during 2010-15, however, NOx levels have shown an increasing trend of 0.9 ppb/yr. Further to quantify the impact of increasing O3 on crops, ozone-related crop yield losses for rice and wheat crop were determined for the period 2010-15. AOT40 (accumulated ozone exposure over a threshold of 40 ppb) and M7 (mean 7-h O3 mixing ratio from 09:00 to 15:59 LT) O3 exposure metrics were used to calculate the reduction in crop yield during major crop growing seasons: Rabi and Kharif.

Assessment of growth, physiological, and yield attributes of wheat cultivar HD 2967 under elevated ozone exposure adopting timely and delayed sowing conditions.

The present study was conducted to assess the impact of elevated levels of O3 and shifting of crop calendar practice, singly, and in combination on Triticum aestivum cv. HD 2967 on its growth, gas exchange parameters, and yield attributes in open-top chambers (OTCs). Two sowing dates were considered: timely sown and late sown. Late sowing was delayed by 20 days from the timely sowing date. The result revealed that wheat plants under elevated O3 and timely sown conditions (ET) showed reductions in growth parameters, while such effects were synergistic when plants were exposed to elevated O3 under late sown conditions (EL). Photosynthetic rate, stomatal conductance, and water use efficiency reduced significantly under EL followed by ET and AL as compared with AT (ambient O3 + timely sown) whereas transpiration rate showed maximum increment under EL. Grain yield reduced by 45.3% in EL as compared with AT and 16.2% in ET as compared with AT. The growth parameters and yield attributes obtained from the present experiment revealed that (i) O3 is affecting the growth and productivity of the wheat and (ii) late sowing practice has not proved to be a feasible adaptation strategy for the wheat cultivation against O3-induced production losses under the prevailing conditions of Indo-Gangetic Plain. This is the first report documenting the shifting of crop calendar practice at the present and future scenario of O3 concentration under agro-ecological conditions in the tropical region of India.

Effect of water deficit stress on an Indian wheat cultivar (Triticum aestivum L. HD 2967) under ambient and elevated level of ozone.

The response of a wheat cultivar (HD 2967) under the combination of elevated ozone (O3) and water deficit stress (WS) was evaluated in terms of morphological, physiological and yield parameters along with nutrient uptake and their redistribution to different plant parts. An open-top chamber experiment has been conducted under O3 exposures (ambient (A) and ambient +20 ppb O3 (E)) along with two different water regimes (well-watered; WW and water deficit with 50% of soil capacity; WS). Most of the growth parameters showed significant reductions due to elevated O3 under both WW and WS conditions. Stomatal conductance and assimilation rate reduced significantly under the combined stress as compared to their controls (AWW). The maximum decrease in grain yield was observed under the additive effect of both the stresses of water deficit and elevated O3 (-43.6%), followed by water deficit stress (-19.8%) and elevated O3 (-17.9%) as compared to the control (AWW). Furthermore, the study displayed that reduced water availability has checked the uptake of nutrients in the roots, shoot and leaves, while, a higher carbon accumulation has been observed with subsequent increases in C: N and C: K ratios in the leaves. Such limitation of nutrients uptake and photosynthates availability weakened the antioxidative defense system of the test cultivar, making it more sensitive against combined stresses. Besides, the study displayed that the defense system has been remarkably suppressed under the presence of interactive stress factors, which allowed us to predict that the distribution of limited carbon pool has inverse relationship between the plant's defense system and growth.

Evaluating the effects of surface O3 on three main food crops across China during 2015-2018.

In order to tackle China's severe air pollution issue, the government has released the "Air Pollution Prevention and Control Action Plan" (known simply as the "Action Plan") since 2013. A recent study reported a decreased trend in PM2.5 concentrations over 2013-2017, but O3 pollution has become more serious. However, the effects of surface O3 on crops are unclear after the implementation of the "Action Plan". Here, we evaluated the potential negative effects of surface O3 on three main food crops (winter wheat, maize and rice) across China during 2015-2018 using nationwide O3 monitoring data and AOT40-yield response functions. Results suggested that mean O3 concentration, AOT40 and relative yield loss in China showed an overall upward trend from 2015 to 2018. During winter wheat, maize, single rice, double-early rice, and double-late rice growing seasons, mean O3 concentration in recent years ranged from 38.6 to 46.9 ppb, 40.2-43.9 ppb, 39.3-42.2 ppb, 33.8-40.0 ppb, and 35.9-39.1 ppb, respectively, and AOT40 mean values ranged from 8.5 to 14.3 ppm h, 10.5-13.4 ppm h, 9.8-11.9 ppm h, 5.2-9.2 ppm h, and 8.0-9.5 ppm h, respectively. O3-induced yield reductions were estimated to range from 20.1 to 33.3% for winter wheat, 5.0-6.3% for maize, 7.3-8.8% for single rice, 3.9-6.8% for double-early rice and 5.9-7.1% for double-late rice. O3-induced production losses for winter wheat, maize, single rice, double-early rice, and double-late rice totaled 39.5-88.2 million metric tons, 12.6-21.0 million metric tons, 9.5-11.3 million metric tons, 1.2-1.8 million metric tons, and 2.2-2.7 million metric tons, respectively, and the corresponding economic losses totaled 14.3-32.0 billion US$, 3.9-6.5 billion US$, 3.9-4.6 billion US$, 0.5-0.7 billion US$, and 0.9-1.1 billion US$, respectively. Our results suggested that the government should take effective measures to reduce O3 pollution and its effects on agricultural production.

Growing season extension affects ozone uptake by European forests.

Climate change significantly modifies terrestrial ecosystems and vegetation activity, yet little is known about how climate change and ozone pollution interact to affect forest health. Here we compared the trends of two metrics widely used to protect forests against negative impacts of ozone pollution, the AOT40 (Accumulated Ozone over Threshold of 40 ppb) which only depends on surface air ozone concentrations, and the POD (Phytotoxic Ozone Dose) which relies on the amount of ozone uptaken by plants through stomata. Using a chemistry transport model, driven by anthropogenic emission inventories, we found that European-averaged ground-level ozone concentrations significantly declined (-1.6%) over the time period 2000-2014, following successful control strategies to reduce the ozone precursors emission; as a consequence, the AOT40 metric declined (-22%). In contrast, climate change increased both growing season length (~7 days/decade) and stomatal conductance and thus enhanced the stomatal ozone uptake by forests (5.9%), leading to an overall increase of potential ozone damage on plants, despite the reduction in ozone concentrations. Our results suggest that stomatal-flux based strategies of forest protection against ozone in a changing climate require a proper consideration of the duration of the growing season with a better estimation of start and end of the growing season.

Ozone impact on wheat in Europe, Asia and North America - A comparison.

Data from experiments where field-grown wheat was exposed to ozone were collated in order to compare the effects in Europe, Asia and North America using dose-response regression. In addition to grain yield, average grain mass and harvest index were included to reflect the influence of ozone on the crop growth pattern. In order to include as many experiments as possible, daytime average ozone concentration was used as the ozone exposure index, but AOT40, estimated from average ozone concentrations, was also used to compare the performance of the two exposure metrics. The response to ozone differed significantly between the continents only for grain yield when using AOT40 as the exposure index. North American wheat was less sensitive than European and Asian that responded similarly. The variation in responses across all three continents was smallest for harvest index, followed by grain mass and grain yield. The highly consistent effect on harvest index shows that not only effects on biomass accumulation, but also on the partitioning of biomass, are important for the ozone-induced grain yield loss in wheat. The average duration of daily ozone exposure was longer in European experiments compared to North American and Asian. It cannot be excluded that this contributed to the indicated higher ozone sensitivity in European wheat in relation to North American. The main conclusions from this study are that on the average the response of wheat to ozone was lower for the older North American experiments and that the ozone response of the growth pattern reflected by grain mass and harvest index did not differ between continents.

Current ambient and elevated ozone effects on poplar: A global meta-analysis and response relationships.

The effects of current and future elevated O3 concentrations (e[O3]) were investigated by a meta-analysis for poplar, a widely distributed genus in the Northern Hemisphere with global economic importance. Current [O3] has significantly reduced CO2 assimilation rate (Pn) by 33% and total biomass by 4% in comparison with low O3 level (charcoal-filtered air, CF). Relative to CF, an increase in future [O3] would further enhance the reduction in total biomass by 24%, plant height by 17% and plant leaf area by 19%. Isoprene emissions could decline by 34% under e[O3], with feedback implications in reducing the formation of secondary air pollutants including O3. Reduced stomatal conductance and lower foliar area might increase runoff and freshwater availability in O3 polluted areas. Higher cumulated O3 exposure over a threshold of 40 ppb (AOT40) induced larger reductions in Pn, total biomass and isoprene emission. Relationships of light-saturated photosynthesis rates (Asat), total biomass and chlorophyll content with AOT40 using a global dataset are provided. These relationships are expected to improve O3 risk assessment and also to support the inclusion of the effect of O3 in models addressing plantation productivity and carbon sink capacity.

[Seasonal Variation in Surface Ozone and Its Effect on the Winter Wheat and Rice in Nanjing, China].

In recent years, surface ozone concentration has been increasing. A high concentration of ozone can affect the growth of crops, and reduce crop yields. In this paper, based on hourly ozone concentration data in the Nanjing area obtained from 2014 to 2016, we analyzed the variation characteristic of ozone concentration and its effect on the production and economic loss of winter wheat and rice. The results shows that the mean concentrations of ozone in 2014, 2015, and 2016 were 62.9, 68.6 and 69.1 µg·m-3, respectively; the ozone concentration and the number of days exceeding the standard gradually increased each year. The order (high to low) of seasonal average ozone concentrations was summer, spring, autumn, and winter. The diurnal variation of ozone concentration in the four seasons showed a single-peak curve, with the peak and valley ozone values appearing at 15:00-16:00 and 07:00-08:00, respectively. The accumulated ozone exposure over a threshold of 40 ppb (AOT40) values during the growing season of winter wheat in 2014, 2015, and 2016 were 10.5, 14.4, and 9.4 µL·L-1·h, respectively. The ozone AOT40 values during the growing season of rice in 2014, 2015, and 2016 were 8.5, 20.0, and 25.6 µL·L-1·h, respectively. At the current ozone level, the impact of surface ozone on winter wheat is higher than that on rice; the range of ozone effect on the yield loss rate of winter wheat was 21.4%-32.8%, and the annual economic loss amounted to 150.766-277.996 million Yuan RMB. The range of ozone effect on the yield loss rate of rice was 8.1%-24.3%, and the annual economic loss amounted to 197.472-680.757 million Yuan RMB.

Five-year volume growth of European beech does not respond to ozone pollution in Italy.

A unique database of stand volume growth, estimated as periodic annual volume increment (in m3 ha-1 per year over the period 2001-2005) from 728 European beech (Fagus sylvatica L.) sites distributed across Italy, was used to assess the effects of ambient ozone (O3), expressed as annual average (M24), accumulated exposure above a 40 ppb hourly threshold (AOT40), and total stomatal ozone flux (POD0). Growth data were from the National forest inventory of Italy, while climate data and ozone concentrations were computed by the WRF and CHIMERE models, respectively. Results show that the growth increased with increasing solar radiation and air temperature and decreased with increasing number of cold days, while effects of soil water content and O3 were not significant. In contrast, the literature results suggest that European beech is sensitive to both drought and O3. Ozone levels resulted to be very high (48 ppb M24, 51,200 ppb h AOT40, 21.08 mmol m-2 POD0, on average) and thus able to potentially affect European beech growth. We hypothesize that the high-frequency signals of soil water and O3 got lost when averaged over 5 years and recommended finer time-resolution investigations and inclusion of other factors of variability, e.g., thinning, tree age, and size.

Concentration- and flux-based dose-responses of isoprene emission from poplar leaves and plants exposed to an ozone concentration gradient.

Concentration- and flux-based O3 dose-responses of isoprene emission from single leaves and whole plants were developed. Two poplar clones differing in O3 sensitivity were exposed to five O3 levels in open-top chambers for 97 d: charcoal-filtered ambient air (CF), non-filtered ambient air (NF) and NF plus 20 ppb (NF + 20), 40 ppb (NF + 40) and 60 ppb (NF + 60). At both leaf and plant level, isoprene emission was significantly decreased by NF + 40 and NF + 60 for both clones. Although intra-specific variability was found when the emissions were up-scaled to the whole plant, both leaf- and plant-level emissions decreased linearly with increasing concentration-based (AOT40, cumulative exposure to hourly O3 concentrations >40 ppb) and flux-based indices (PODY , cumulative stomatal uptake of O3  > Y nmol O3 m-2 PLA s-1 ). AOT40- and POD7 -based dose-responses performed equally well. The two clones responded differently to AOT40 and similarly to PODY (with a slightly higher R2 for POD7 ) when the emission was expressed as change relative to clean air. We thus recommend POD7 as a large-scale risk assessment metric to estimate isoprene emission responses to O3 in poplar.

Ozone risk assessment for an Alpine larch forest in two vegetative seasons with different approaches: comparison of POD1 and AOT40.

The upper vegetation belts like larch forests are supposed to be under great pressure because of climate change in the next decades. For this reason, the evaluation of the risks due to abiotic stressors like ozone is a key step. Two different approaches were used here: mapping AOT40 index by means of passive samplers and direct measurements of ozone deposition.Measurements of ozone fluxes using the eddy-correlation technique were carried out for the first time over a larch forest in Paspardo (I) at 1750 m a.s.l. Two field campaigns were run: the first one in 2010 from July to October and the second one in the following year from June to September. Vertical exchange of ozone, energy, and momentum were measured on a tower platform at 26 m above ground level to study fluxes dynamics over this ecosystem. Since the tower was located on a gentle slope, an "ad hoc" methodology was developed to minimize the effects of the terrain inclination. The larch forest uptake was estimated by means of a two-layer model to separate the understorey uptake from the larch one. Even if the total ozone fluxes were generally high, up to 30-40 nmol O3 m-2 s-1 in both years, the stomatal uptake by the larch forest was relatively low (around 15% of the total deposition).Ozone risk was assessed considering the POD1 received by the larch forest and the exposure index AOT40 estimated with both local data and data from the map obtained by the passive samplers monitoring.

Validation of ozone response functions for annual Mediterranean pasture species using close-to-field-conditions experiments.

Ozone (O3) critical levels have been established under the Long-Range Transboundary Air Pollution Convention to assess the risk of O3 effects in European vegetation. A recent review study has led to the development of O3 critical levels for annual Mediterranean pasture species using plants growing in well-watered pots at a coastal site and under low levels of competition. However, uncertainties remain in the extrapolation of the O3 sensitivity of these species under natural conditions. The response of two O3-sensitive annual Mediterranean pasture Trifolium species at the coastal site was compared with the response of the same species growing at a continental site, in natural soil and subject to water-stress and inter-specific competition, representing more closely their natural habitat. The slopes of exposure- and dose-response relationships derived for the two sites showed differences in the response to O3 between sites attributed to differences in environmental growing conditions, growing medium and the level of inter-specific competition, but the effect of the individual factors could not be assessed separately. Dose-based O3 indices partially explained differences due to environmental growing conditions between sites. The slopes showed that plants were more sensitive to O3 at the continental site, but homogeneity of slopes tests revealed that results from both experimental sites may be combined. Although more experimental data considering complex inter-specific competition situations and the effect of important interactive factors such as nitrogen would be needed, these results confirm the validity of applying the current flux-based O3 critical level under close to natural growing conditions. The AOT40-based O3 critical level derived at the coastal site was also considered a suitable risk indicator in close to natural growing conditions in the absence of soil moisture limitations on plant growth.

Developing ozone critical levels for multi-species canopies of Mediterranean annual pastures.

Ozone (O3) critical levels (CLe) are still poorly developed for herbaceous vegetation. They are currently based on single species responses which do not reflect the multi-species nature of semi-natural vegetation communities. Also, the potential effects of other factors like the nitrogen (N) input are not considered in their derivation, making their use uncertain under natural conditions. Exposure- and dose-response relationships were derived from two open-top chamber experiments exposing a mixture of 6 representative annual Mediterranean pasture species growing in natural soil to 4 O3 fumigation levels and 3 N inputs. The Deposition of O3 and Stomatal Exchange model (DO3SE) was modified to account for the multi-species nature of the canopy following a big-leaf approach. This new approach was used for estimating a multi-species phytotoxic O3 dose (PODy-MS). Response relationships were derived based on O3 exposure (AOT40) and flux (PODy-MS) indices. The treatment effects were similar in the two seasons: O3 reduced the aboveground biomass growth and N modulated this response. Gas exchange rates presented a high inter-specific variability and important inter-annual fluctuations as a result of varying growing conditions during the two years. The AOT40-based relationships were not statistically significant except when the highest N input was considered alone. In contrast, PODy-MS relationships were all significant but for the lowest N input level. The influence of the N input on the exposure- and dose-response relationships implies that N can modify the O3 CLe. However, this is an aspect that has not been considered so far in the methodologies for establishing O3 CLe. Averaging across N input levels, a multi-species O3 CLe (CLef-MS) is proposed POD1-MS = 7.9 mmol m-2, accumulated over 1.5 month with a 95% confidence interval of (5.9, 9.8). Further efforts will be needed for comparing the CLef-MS with current O3 CLef based on single species responses.

Surface ozone levels in the forest and vegetation areas of the Biga Peninsula, Turkey.

Spatial and temporal variability of surface ozone in the rural, mountainous and suburban sites of Biga Peninsula, at the northwest of Turkey which is about 300km southwest of Istanbul was investigated using passive samplers and continuous analyzers. A total 10 passive samplers and two continuous analyzers were used between 1.1.2013 and 31.12.2014. OX levels in the study region were examined to understand NOx dependent or independent contribution to ozone. The influences of the meteorological parameters on ozone levels were also examined by wind speed and ambient temperature. The results clearly show that mountainous areas have higher cumulative exposure to ozone than suburban locations. In order to understand the long range transport sources contributing to the high ozone levels in the region backward trajectories were computed using HYSPLIT model and then clustering of trajectories are performed. The results clearly show the characteristics of pollutant transport from north to Biga Peninsula. Additionally, AOT40 (Accumulated hourly O3 concentrations Over a Threshold of 40ppb) cumulative index was calculated using daytime hourly measurements. The results indicate that the ozone values in the study area are much higher than the critical levels for forest and vegetation based on EU Directive 2008/50/EC.