Impact of droughts and heatwaves on surface ozone over Southwestern Europe
Authors: Guion Antoine , Turquety Solène , Cholakian Arineh , Polcher Jan , Ehret Antoine , Lathière Juliette 
Published: March 2022

&lt;p&gt;Tropospheric ozone (O&lt;sub&gt;3&lt;/sub&gt;) plays a critical role in maintaining the oxidative capacity of the troposphere. However, as a high oxidant, it also deteriorates air quality at high concentration, inducing adverse effects on human and ecosystem health. Meteorological conditions are key to understand the variability of many surface atmospheric pollutants and of the vegetation state. The variability of O&lt;sub&gt;3&lt;/sub&gt; concentration is generally well represented in chemistry-transport models (CTM) compared to observations, but the amplitude of the variations are more difficult to simulate (peaks and minima). One factor that has been identified as a possible cause of these uncertainties is the lack of interactions between the biosphere and the atmosphere.&lt;/p&gt;&lt;p&gt;The aim of this study is to quantify the variation of surface O&lt;sub&gt;3&lt;/sub&gt; over the Southwestern Europe during agricultural droughts, combined or not with heatwaves. Therefore, we analyze both emissions of biogenic volatile organic compounds (BVOCs) and O&lt;sub&gt;3&lt;/sub&gt; dry deposition velocity during these extreme events, based on the available observations (O&lt;sub&gt;3 &lt;/sub&gt;from the EEA surface network and formaldehyde (HCHO) from OMI satellite instrument) and regional CTM simulations (CHIMERE model), which have been clustered depending on the underlying meteorological conditions. To better understand the observed variations, sensitivity studies are performed implementing the effect of soil dryness and biomass decrease in CHIMERE CTM simulations using online calculation of BVOC emissions from the MEGAN model, during three selected summers: 2012, 2013 and 2014.&lt;/p&gt;&lt;p&gt;Our results show that observed O&lt;sub&gt;3 &lt;/sub&gt;concentration is on average significantly higher during heatwaves (+10&amp;#181;g/m&lt;sup&gt;3&lt;/sup&gt; in daily mean and +18&amp;#181;g/m&lt;sup&gt;3&lt;/sup&gt; in daily maximum) and droughts (+5&amp;#181;g/m&lt;sup&gt;3&lt;/sup&gt; and +9&amp;#181;g/m&lt;sup&gt;3&lt;/sup&gt;), due to an overall O&lt;sub&gt;3&lt;/sub&gt; precursor emissions enhancement (in agreement with HCHO observations) and O&lt;sub&gt;3 &lt;/sub&gt;dry deposition decrease. However, isolated droughts are characterized by reduced O&lt;sub&gt;3&lt;/sub&gt; precursor emissions (in agreement with HCHO observations) and reduced O&lt;sub&gt;3&lt;/sub&gt; dry deposition, compared to normal conditions. Both effects compensate each other with a slight dominance of the latter one, leading to a small but significant increase of observed O&lt;sub&gt;3&lt;/sub&gt; concentration for the daily maximum only (+4 &amp;#956;g/m&lt;sup&gt;3&lt;/sup&gt;). However, important uncertainties appear to be related to BVOC concentrations, especially about the land cover classification, and to NO&lt;sub&gt;X&lt;/sub&gt;&lt;sub&gt;&amp;#160;&lt;/sub&gt;concentrations for which CHIMERE presents limited performance scores of validation. Nevertheless, we emphasize the need for a more dynamical interaction between surface vegetation and hydrology, meteorology and atmospheric chemistry for the simulation of O&lt;sub&gt;3&lt;/sub&gt; during summers in Southwestern Europe.&lt;/p&gt;&lt;p&gt;Finally, almost half of summer days exceeding the EU standard of O&lt;sub&gt;3&lt;/sub&gt; for air quality in Southwestern Europe occurred during droughts or heatwaves, on average for the time period 2000-2016. However, this percentage can increase (up to 80%) for exceptionally dry and hot summers, like in 2012. Only 14% of the exceedance days occurred during isolated droughts (summers 2000-2016).&lt;/p&gt;