Increasing risk for negative ozone impacts on vegetation in northern Sweden

Tropospheric ozone assessment report: Global ozone metrics for climate change, human health, and crop/ecosystem research

Assessment of spatial and temporal variation in the impacts of ozone on human health, vegetation, and climate requires appropriate metrics. A key component of the Tropospheric Ozone Assessment Report (TOAR) is the consistent calculation of these metrics at thousands of monitoring sites globally. Investigating temporal trends in these metrics required that the same statistical methods be applied across these ozone monitoring sites. The nonparametric Mann-Kendall test (for significant trends) and the Theil-Sen estimator (for estimating the magnitude of trend) were selected to provide robust methods across all sites. This paper provides the scientific underpinnings necessary to better understand the implications of and rationale for selecting a specific TOAR metric for assessing spatial and temporal variation in ozone for a particular impact. The rationale and underlying research evidence that influence the derivation of specific metrics are given. The form of 25 metrics (4 for model-measurement comparison, 5 for characterization of ozone in the free troposphere, 11 for human health impacts, and 5 for vegetation impacts) are described. Finally, this study categorizes health and vegetation exposure metrics based on the extent to which they are determined only by the highest hourly ozone levels, or by a wider range of values. The magnitude of the metrics is influenced by both the distribution of hourly average ozone concentrations at a site location, and the extent to which a particular metric is determined by relatively low, moderate, and high hourly ozone levels. Hence, for the same ozone time series, changes in the distribution of ozone concentrations can result in different changes in the magnitude and direction of trends for different metrics. Thus, dissimilar conclusions about the effect of changes in the drivers of ozone variability (e.g., precursor emissions) on health and vegetation exposure can result from the selection of different metrics.

Past, present and future concentrations of ground-level ozone and potential impacts on ecosystems and human health in northern Europe

This review summarizes new information on the current status of ground-level ozone in Europe north of the Alps. There has been a re-distribution in the hourly ozone concentrations in northern Europe during 1990-2015. The highest concentrations during summer daytime hours have decreased while the summer night-time and winter day- and night-time concentrations have increased. The yearly maximum 8-h mean concentrations ([O₃]_8h,max), a metric used to assess ozone impacts on human health, have decreased significantly during 1990-2015 at four out of eight studied sites in Fennoscandia and northern UK. Also the annual number of days when the yearly [O₃]_8h,max exceeded the EU Environmental Quality Standard (EQS) target value of 60ppb has decreased. In contrast, the number of days per year when the yearly [O₃]_8h,max exceeded 35ppb has increased significantly at two sites, while it decreased at one far northern site. [O₃]_8h,max is predicted not to exceed 60ppb in northern UK and Fennoscandia after 2020. However, the WHO EQS target value of 50ppb will still be exceeded. The AOT40 May-July and AOT40 April-September metrics, used for the protection of vegetation, have decreased significantly at three and four sites, respectively. The EQS for the protection of forests, AOT40 April-September 5000ppbh, is projected to no longer be exceeded for most of northern Europe sometime before the time period 2040-2059. However, if the EQS is based on Phytotoxic Ozone Dose (POD), POD₁, it may still be exceeded by 2050. The increasing trend for low and medium range ozone concentrations in combination with a decrease in high concentrations indicate that a new control strategy, with a larger geographical scale than Europe and including methane, is needed for ozone abatement in northern Europe.

Effects of atmospheric pollutants (CO, O3, SO2) on the allergenicity of Betula pendula, Ostrya carpinifolia, and Carpinus betulus pollen

Pollen of Betula pendula, Ostrya carpinifolia, and Carpinus betulus was exposed in vitro to relatively low levels of the air pollutants, namely carbon monoxide, ozone, and sulfur dioxide. The allergenicity of the exposed pollen was compared with that of non-exposed pollen samples to assess if air pollution exposition affects the allergenicity potential of pollen. The immunodetection assays indicated higher IgE recognition by all sera of allergic patients to the pollen protein extracts in all exposed samples in comparison to the non-exposed samples. These results show that the pollen exposition to low pollutants' levels induces increased allergic reaction to sensitized individuals.

Insights into ozone deposition patterns from decade-long ozone flux measurements over a mixed temperate forest

Long-term fluxes of ozone (O(3)) were measured over a mixed temperate forest using the aerodynamic gradient method. The long-term average O(3) flux (F) was -366 ng m(-2) s(-1) for the period 2000-2010, corresponding to an average O(3) concentration of 48 μg m(-3) and a deposition velocity v(d) of 9 mm s(-1). Average nocturnal ozone deposition amounted to -190 ng m(-2) s(-1), which was about one third of the daytime flux. Also during the winter period substantial O(3) deposition was measured. In addition, total O(3) fluxes were found to differ significantly among canopy wetness categories. During the day, highest deposition fluxes were generally measured for a dry canopy, whereas a rain-wetted canopy constituted the best sink at night. Flux partitioning calculations revealed that the stomatal flux (F(s)) contributed 20% to the total F but the F(s)/F fraction was subject to seasonal and diurnal changes. The annual concentration-based index AOT40 (accumulated dose over a threshold of 40 ppb) and the Phytotoxic Ozone Dose (POD(1) or accumulated stomatal flux above a threshold of 1 nmol m(-2) s(-1)) were related in a curvilinear way. The O(3) deposition was found to be largely controlled by non-stomatal sinks, whose strength was enhanced by high friction velocities (u(*)), optimizing the mechanical mixing of O(3) into the canopy and the trunk space. The long-term geometrical mean of the non-stomatal resistance (R(ns)) was 136 s m(-1) but lower R(ns) values were encountered during the winter half-year due to higher u(*). The R(ns) was also subject to a marked diurnal variability, with low R(ns) in the morning hours, when turbulence took off. We speculate that non-stomatal deposition was largely driven by scavenging of ozone by biogenic volatile organic compounds (BVOCs) and especially NO emitted from the crown or the forest floor.

Responses of native broadleaved woody species to elevated ozone in subtropical China

To assess ozone sensitivity of subtropical broadleaved tree species and explore possible underlying mechanisms, six evergreen and two deciduous native species were exposed to either charcoal-filtered air or elevated O(3) (E-O(3), ∼150ppb) for one growing season. Initial visible symptoms in deciduous species appeared much earlier than those in evergreen species. The species which first showed visible symptoms also had the largest reductions in biomass. E-O(3) induced significant decreases in photosynthesis rate, chlorophyll content and antioxidant capacity but a significant increase in malondialdehyde content in two deciduous species and two evergreen species (Cinnamomum camphora and Cyclobalanopsis glauca). Except C. glauca, however, E-O(3) had no significant effects on stomatal conductance (g(s)), total phenols and ascorbate contents. Difference in O(3) sensitivity among all species was strongly attributed to specific leaf mass rather than g(s). It suggests that some subtropical tree species will be threatened by rising O(3) concentrations in the near future.

Ethylenediurea (EDU): a research tool for assessment and verification of the effects of ground level ozone on plants under natural conditions

Ethylenediurea (EDU) has been widely used to prevent ozone (O(3)) injury and crop losses in crop plants and growth reductions in forest trees. Successful use requires establishing a dose/response curve for EDU and the proposed plant in the absence of O(3) and in the presence of O(3) before initiating multiple applications to prevent O(3) injury. EDU can be used to verify foliar O(3) symptoms in the field, and to screen plants for sensitivity to O(3) under ambient conditions. Despite considerable research, the mode of action of EDU remains elusive. Additional research on the mode of action of EDU in suppressing O(3) injury in plants may also be helpful in understanding the mode of action of O(3) in causing injury in plants.

Inter- and intra-specific responses to elevated ozone and chamber climate in northern birches

We studied the responses of micropropagated, northern provenances of downy, mountain and silver birches to elevated ozone (O(3)) and changing climate using open-top chambers (OTCs). Contrary to our hypothesis, northern birches were sensitive to O(3), i.e. O(3) levels of 31-36 ppb reduced the leaf and root biomasses by -10%, whereas wood biomass was affected to a lesser extent. The warmer and drier OTC climate enhanced growth in general, though there were differences among the species and clones, e.g. in bud burst and biomass production. Inter- and intra-specific responses to O(3) and changing climate relate to traits such as allocation patterns between the above- and belowground parts (i.e. root/shoot ratio), which further relate to nutrient and water economy. Our experiments may have mimicked future conditions quite well, but only long-term field studies can yield the information needed to forecast responses at both tree and ecosystem levels.