Biogenic isoprene emissions, dry deposition velocity, and surface ozone concentration during summer droughts, heatwaves, and normal conditions in southwestern Europe

International audienceAt high concentrations, tropospheric ozone (O 3) deteriorates air quality, inducing adverse effects on human and ecosystem health. Meteorological conditions are key to understanding the variability in O 3 concentration, especially during extreme weather events. In addition to modifying photochemistry and atmospheric transport, droughts and heatwaves affect the state of vegetation and thus the biosphere-troposphere interactions that control atmospheric chemistry, namely biogenic emissions of precursors and gas dry deposition. A major source of uncertainty and inaccuracy in the simulation of surface O 3 during droughts and heatwaves is the poor representation of such interactions. This publication aims at quantifying the isolated and combined impacts of both extremes on biogenic isoprene (C 5 H 8) emissions, O 3 dry deposition, and surface O 3 in southwestern Europe. First, the sensitivity of biogenic C 5 H 8 emissions, O 3 dry deposition, and surface O 3 to two specific effects of droughts, the decrease in soil moisture and in biomass, is analysed for the extremely dry summer 2012 using the biogenic emission model MEGANv2.1 and the chemistry transport model CHIMEREv2020r1. Despite a significant decrease in biogenic C 5 H 8 emissions and O 3 dry deposition velocity, characterized by a large spatial variability, the combined effect on surface O 3 concentration remains limited (between +0.5 % and +3 % over the continent). The variations in simulated biogenic C 5 H 8 emissions, O 3 dry deposition, and surface O 3 during the heatwaves and agricultural droughts are then analysed for summer 2012 (warm and dry), 2013 (warm), and 2014 (relatively wet and cool). We compare the results with large observational data sets, namely O 3 concentrations from Air Quality (AQ) e-Reporting (2000-2016) and total columns of formaldehyde (HCHO, which is used as a proxy for biogenic emissions of volatile organic compounds) from the Ozone Monitoring Instrument (OMI) of the Aura satellite (2005-2016). Based on a cluster approach using the percentile limit anomalies indicator, we find that C 5 H 8 emissions increase by +33 % during heatwaves compared to normal conditions, do not vary significantly during all droughts (either accompanied or not by a heatwave), and decrease by −16 % during isolated droughts. OMI data confirm an average increase in HCHO during heatwaves (between +15 % and +31 % depending on the product used) and Published by Copernicus Publications on behalf of the European Geosciences Union