Legislative and functional aspects of different metrics used for ozone risk assessment to forests

Surface ozone (O3) is a threat to forests by decreasing photosynthesis and, consequently, influencingthe strength of land carbon sink. However, due to the lack of continuous surface O3 measurements,observational-based assessments of O3 impacts on forests are largely missing at hemispheric to globalscales.Currently, some metrics are used for regulatory purposes by governments or national agencies toprotect forests against the negative impacts of ozone: in particular, both Europe and United States (US)makes use of two different exposure-based metrics, i.e. AOT40 and W126, respectively. However,because of some limitations in these metrics, a new standard is under consideration by the EuropeanUnion (EU) to replace the current exposure metric.We analyse here the different air quality standards set or proposed for use in Europe and in the US toprotect forests from O3 and to evaluate their spatial and temporal consistency while assessing theireffectiveness in protecting northern-hemisphere forests. Then, we compare their results with theinformation obtained from a complex land surface model (ORCHIDEE).We find that present O3 uptake decreases gross primary production (GPP) in 37.7% of the NH forestedarea of northern hemisphere with a mean loss of 2.4% year-1. We show how the proposed US (W126)and the currently used European (AOT40) air quality standards substantially overestimate theextension of potential vulnerable regions, predicting that 46% and 61% of the Northern Hemisphere(NH) forested area are at risk of O3 pollution. Conversely, the new proposed European standard(POD1) identifies lower extension of vulnerability regions (39.6%).