资源环境科技发展态势分析平台

Abstract. At high concentration, tropospheric O₃ deteriorates air quality, inducing adverse effects on human and ecosystem health. Meteorological conditions are key to understand the variability of O₃ concentration, especially during extreme weather events. They modify the photochemistry activity and the vegetation state. An important source of uncertainties and inaccuracy in simulating surface O₃ during droughts and heatwaves is the lack of interactions between the biosphere and the troposphere. Based on the biogenic emission model MEGAN v2.1 and the chemistry-transport model CHIMERE v2020r1, the first objective of this study is to assess the sensitivity of biogenic emissions, O₃ dry deposition and surface O₃ to biomass decrease and soil dryness effect (using several configurations) during the extremely dry summer 2012. Secondly, this research aims at quantifying the variation of observed (EEA’s air quality database, 2000–2016) and simulated (CHIMERE, 2012–2014) surface O₃ during summer heatwaves and agricultural droughts that have been identified using the Percentile Limit Anomalies (PLA) method. Our sensitivity analysis shows that soil dryness is a key factor during drought events, decreasing considerably the C5H8 emissions and O₃ dry deposition velocity. This effect has a larger impact than the biomass decrease. However, the resulting effect on surface O₃ remains limited.

Based on a cluster approach using the PLA indicator, we show that observed O₃ concentration is on average significantly higher during heatwaves (+18 μg/m³ in daily maximum) and droughts (+9 μg/m³) compared to normal conditions. Despite a difference of several μg/m³, CHIMERE correctly simulates the variations of O₃ concentration between the clusters of extreme events. The overall increase of surface O₃ during both heatwaves and droughts would be explained by O₃ precursor emission enhancement (in agreement with HCHO satellite observations), O₃ dry deposition decrease and favourable weather conditions. However, we simulated a decrease of C₅H₈ emissions (in agreement with HCHO observations) during droughts not accompanied by a heatwave, resulting in a non-significant difference of surface O₃ compared to normal conditions (from both observations and simulations).

Finally, we stress that considerable uncertainties characterize our simulated surface-troposphere interactions. Multi-year flux measurements would contribute to better assess the model performance. Nevertheless, we emphasize the need for a more dynamical representation of interactions between vegetation, hydrology, meteorology and atmospheric chemistry in models in order to improve the simulation of summer O₃.