Ozone-induced gross primary productivity reductions over European forests inferred from satellite observations

Abstract. Tropospheric O₃ damages leaves and directly inhibits photosynthesis, posing a threat to terrestrial carbon sinks. Previous investigations have mostly relied on sparse in-situ data or simulations using land surface models. This work is the first to use satellite data to quantify the effect of O₃ exposure on gross primary productivity (GPP). O₃-induced GPP reductions were estimated to vary between 0.36–9.55% across European forests along a North-South transect between 2003–2015, in line with prior estimates. No significant temporal trend could be determined over most of Europe, while Random Forest analysis (RFA) shows that soil moisture is a significant variable governing GPP reductions over the Mediterranean. Comparisons between this work and GPP reductions simulated by the Yale Interactive Biosphere (YIBs) model suggest that satellite-based estimates over the Mediterranean region may be biased by +12%, potentially because of differences in modelling stomatal sensitivity to soil moisture and prior O₃ exposure. This work has demonstrated for the first time that satellite-based datasets can be leveraged to assess the impact of O₃ on the terrestrial carbon sink, which are comparable with in-situ or model-based analyses.