Tropospheric Ozone Assessment Report: Present-day tropospheric ozone distribution and trends relevant to vegetation

Abstract

This Tropospheric Ozone Assessment Report (TOAR) on the current state of knowledge of ozone metrics of relevance to vegetation (TOAR-Vegetation) reports on present-day global distribution of ozone at over 3300 vegetated sites and the long-term trends at nearly 1200 sites.TOAR-Vegetationfocusses on three metrics over vegetation-relevant time-periods across major world climatic zones: M12, the mean ozone during 08:00–19:59; AOT40, the accumulation of hourly mean ozone values over 40 ppb during daylight hours, and W126 with stronger weighting to higher hourly mean values, accumulated during 08:00–19:59.Although the density of measurement stations is highly variable across regions, in general, the highest ozone values (mean, 2010–14) are in mid-latitudes of the northern hemisphere, including southern USA, the Mediterranean basin, northern India, north, north-west and east China, the Republic of Korea and Japan. The lowest metric values reported are in Australia, New Zealand, southern parts of South America and some northern parts of Europe, Canada and the USA. Regional-scale assessments showed, for example, significantly higher AOT40 and W126 values in East Asia (EAS) than Europe (EUR) in wheat growing areas (p< 0.05), but not in rice growing areas. In NAM, the dominant trend during 1995–2014 was a significant decrease in ozone, whilst in EUR it was no change and in EAS it was a significant increase.TOAR-Vegetation provides recommendations to facilitate a more complete global assessment of ozone impacts on vegetation in the future, including: an increase in monitoring of ozone and collation of field evidence of the damaging effects on vegetation; an investigation of the effects on peri-urban agriculture and in mountain/upland areas; inclusion of additional pollutant, meteorological and inlet height data in the TOAR dataset; where not already in existence, establishing new region-specific thresholds for vegetation damage and an innovative integration of observations and modelling including stomatal uptake of the pollutant.

Closing the global ozone yield gap: Quantification and cobenefits for multistress tolerance

“…At the national scale, the countries identified as having the largest potential effects of ozone (e.g. the United States, India and China) match those with the highest monitored ozone concentrations (Mills, Pleijel et al., ) as well as those predicted using concentration‐based approaches to have the highest potential yield losses (Avnery et al., ,b; Van Dingenen et al., ). At the subnational scale, however, there were some differences in areas predicted to be at risk, where our stomatal uptake modelling method took into account the modifying effects of climate and soil moisture on ozone uptake rather than simply predicting the largest effects in the areas with the highest ozone concentrations.…”

“…40% for major ozone precursors such as NOx, VOC and CO being made between 1990 and 2013 (Maas & Grennfelt, ). These cuts have been associated with significant decreasing trends in the concentration‐based metric AOT40 at 26% and 11% of monitoring sites in wheat‐growing areas of NAM and EUR, respectively, over the period 1995–2014 (Mills, Pleijel et al., ), although the dominant trend for EUR remains “no change.” Over the same time period, increases in precursor emissions of 20%–30% in other areas of the world, including by 50% in India and China, have led to increases in ozone concentration in these regions (Maas & Grennfelt, ). For example, there has been a significant increase in ozone concentration at nearly 50% of wheat‐growing monitoring sites in EAS, with average annual increases in AOT40 at these sites being in the range 300–700 ppb h/y over the period 1995–2014 (Mills, Pleijel et al., ).…”

“…To target the regions of the world where ozone‐tolerant crop varieties are most required, we need to understand which crops are most at risk and where they are growing in relation to current high‐risk areas for ozone. We know from a recent analysis of ozone concentrations at over 3,000 rural sites that the highest ozone values are in many of the world's important crop‐growing regions, including parts of the United States, Europe, India and China (Mills, Pleijel et al., ). Overall, the latter study showed that the global mean cumulative ozone exposure is double the critical level set by the United Nations as a target for ozone pollution control, above which direct adverse effects on sensitive vegetation may occur according to present knowledge (CLRTAP, ).…”

Water use strategy affects avoidance of ozone stress by stomatal closure in Mediterranean trees—A modelling analysis

“…Tropospheric ozone (O 3 ) is recognized as a significant phytotoxic air pollutant (Grulke & Heath, ). The background level of O 3 concentration is constantly increasing due to increased human activity since the pre‐industrial age (Mills et al, ). Once O 3 has entered the leaf, it is dissolved in the apoplast and converted into reactive oxygen species (ROS), which cause a damage to physiological and biochemical processes in the plant leaves (Paoletti, ).…”

Tropospheric ozone pollution reduces the yield of African crops

“…Analysis of compiled data sets from many experiments has shown a wide range in ozone sensitivity between different crops, based on ozone concentration (Mills et al, ; Mills & Harmens, ; Mills, Sharps, Simpson, Pleijel, Broberg, et al, ). Model‐based studies using dose‐response functions from such data sets have indicated potential crop yield reductions due to ozone across wide regions of the world (Avnery, Mauzerall, Liu, & Horowitz, ; Mills, Pleijel, et al, ; Mills, Sharps, Simpson, Pleijel, Broberg, et al, ; Mills, Sharps, Simpson, Pleijel, Frei, et al, ; Van Dingenen et al, ). Experimental investigations have shown impacts of ambient ozone concentrations on a wide range of crop species in Europe and the USA by comparing responses of plants in filtered air to those in non‐filtered air (De Temmerman, Legrand, & Vandermeiren, ; Marzuoli, Finco, Chiesa, & Gerosa, ; Pleijel, Broberg, Uddling, & Mills, ).…”