Interactions between the terrestrial biosphere and atmosphere during droughts and heatwaves: impact on surface ozone over Southwestern Europe

International audienceAt 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 MEGANv2.1 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 (+18µg/m3 in daily maximum) and droughts (+9µg/m 3) 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)