Reply on RC1 & RC3

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> At high concentration, tropospheric <em>O</em>₃ deteriorates air quality, inducing adverse effects on human and ecosystem health. Meteorological conditions are key to understand the variability of <em>O</em>₃ 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 <em>O</em>₃ 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, <em>O</em>₃ dry deposition and surface <em>O</em>₃ 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&rsquo;s air quality database, 2000&ndash;2016) and simulated (CHIMERE, 2012&ndash;2014) surface <em>O</em>₃ 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 <em>O</em>₃ dry deposition velocity. This effect has a larger impact than the biomass decrease. However, the resulting effect on surface <em>O</em>₃ remains limited. Based on a cluster approach using the PLA indicator, we show that observed <em>O</em>₃ concentration is on average significantly higher during heatwaves (+18 <em>&mu;g</em>/<em>m</em>³ in daily maximum) and droughts (+9 <em>&mu;g</em>/<em>m</em>³) compared to normal conditions. Despite a difference of several <em>&mu;g</em>/<em>m</em>³, CHIMERE correctly simulates the variations of <em>O</em>₃ concentration between the clusters of extreme events. The overall increase of surface <em>O</em>₃ during both heatwaves and droughts would be explained by <em>O</em>₃ precursor emission enhancement (in agreement with <em>HCHO</em> satellite observations), <em>O</em>₃ dry deposition decrease and favourable weather conditions. However, we simulated a decrease of <em>C</em>₅<em>H</em>₈ emissions (in agreement with <em>HCHO</em> observations) during droughts not accompanied by a heatwave, resulting in a non-significant difference of surface <em>O</em>₃ 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 <em>O</em>₃.