Understanding Drivers of Extreme Ozone Events: A Sensitivity Study with WRF-Chem

Extreme weather events, such as stagnation conditions and heatwaves, are known to exacerbate hazardous air quality situations by promoting the accumulation and persistence of pollutants like ozone (O₃) in the near-surface environment. In particular, during the summer over the Iberian Peninsula (IP), extreme O₃ values often exceed the 180 µg/m³ threshold, significantly impacting air quality and public health. While meteorological factors like high temperatures and radiation are important drivers of these events, classifying them based solely on synoptic weather patterns fails to capture the full scope of the risks involved. Even when events are classified under the same synoptic category, O₃ concentration can vary greatly. This implies that variability is not only due to direct meteorological influences, but also other factors related to transport, previous concentrations or processes linked to surface conditions can modify emissions of compouds that affect O₃ formation. In this regard, biogenic volatile organic compound (BVOC) emissions from vegetation can significantly influence O₃ formation in a complex and non-linear way. The amount and type of vegetation, as well as the available soil moisture, modulate these emissions.

In this study, we conducted sensitivity experiments using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem, version 4.6.0) to explore how factors like soil moisture and vegetation amount influence extreme O₃ events over the IP. We first performed simulations varying leaf area index (LAI) and soil moisture (SM) by multiplying the original fields of SM (all layers) and LAI by a factor ranging from 0.25 to 2. The results indicate that decreasing  the available soil water (-50%) increases BVOCs emission rates (20% spatial and temporal average), which is reflected in an increase in daily maximum O₃ concentrations (10%). On the other hand, increasing the vegetation (50 %) leads to an increase in BVOC emission rates (10%), as well as in O₃ concentrations (up to 10 %). The combined experiments exacerbated the changes, although most of the time they are smaller than the sum of the isolated experiments. 

Regional Climate Models (RCMs) often use climatological variables to characterize vegetation. Some studies show that differences in vegetation fraction with respect to climatological values can reach up to 40% over the IP. We performed a series of simulations of extreme O₃ occurrences, employing both observed and climatological values of vegetation. Preliminary results indicate that such differences in vegetation moderately modify final O₃ concentrations, in most cases obtaining better agreement with observations. 

Acknowledgements: Project PID2020-115693RB-I00 funded by MCIN/ AEI /10.13039/501100011033