Scorched Earth: Ecosystem-Atmosphere Interactions Exacerbate Climate Penalty on Ozone Pollution Extremes in Europe

Reducing surface ozone to the European Union target level for protection of human health has proven to be challenging, despite stringent controls on ozone precursor emissions over the past few decades. The highest ozone pollution episodes are strongly linked to heatwaves and droughts, which are increasing in frequency and intensity over Europe. While detrimental to society, the dry conditions accompanying heatwaves often have severe additional impacts. Under drought stress, plants close their stomata to reduce water loss, consequently limiting the ozone uptake by vegetation (dry deposition), leading to increased surface ozone concentrations. The feedbacks from such ecosystem-atmosphere interactions are often overlooked in air quality projections, owing to a lack of process-based model formulations. Using a suite of observations and earth system model hindcast simulations with an interactive dry deposition scheme, we quantify the drought-related climate penalty on the observed surface ozone trends and extreme events over Europe during the late twentieth and early twenty-first centuries (1960-2018). Incorporated into a combined stomatal-photosynthesis model, this deposition scheme mechanistically describes the response of ozone deposition to phenology, CO₂ concentration, temperature, canopy air vapor pressure deficit, and soil water availability. We show that the slow progress towards cleaning up ozone air quality in Europe can be partly explained by declining ozone removal by vegetation in a drying and warming climate over the past few decades. The vegetation feedbacks exacerbate climate penalty on ozone air pollution in densely populated regions of western Europe, worsening surface ozone peak episodes during the mega-heatwaves such as the historic 2003 event, offsetting much of the air quality improvements gained from regional emission controls. Our analysis suggests it is unlikely for Europe with the current legislation to achieve compliance with the existing ozone air quality standard by 2020 - the goal set by the European Union in a Clean Air Policy Package adopted in 2013. With extremely hot and dry summers set to increase over the coming decades, the climate penalty could be devastating and therefore needs to be considered when designing the European Union clean air policy.