Effects of Rising CO2 Concentration on Global Ozone Air Quality

Surface ozone is a major air pollutant that harms not only human health but also crop and vegetation productivity. The continuously rising atmospheric CO₂concentration can affect surface ozone levels through various vegetation-mediated ecophysiological pathways. These pathways include higher leaf densityfollowing CO₂ fertilization, which can lead to increased biogenic volatile organic compound (BVOC) emissions and dry deposition, inhibition of BVOC emissions due to competitive biochemical effects in leaves, and lower stomatal conductance resulting in lower dry deposition. In this study, we implemented an ecophysiology module that explicitly links the computation of dry deposition velocity and isoprene emission to photosynthesis calculation in the GEOS-Chem global 3-D chemical transport model, and conducted model experiments to simulate the effects of rising CO₂ levels on surface ozone pollution via CO₂ fertilization, isoprene inhibition and stomatal closure under an atmospheric CO₂ level projected by Representative Concentration Pathway (RCP) 8.5 scenarios in 2050, with 2010 as the base year for comparison. The effects of rising CO₂ on enhancement in leaf area index (LAI) were simulated separately using a land surface model (Community Land Model, CLM). The simulated results indicate that the CO₂-induced ecophysiological effects depend largely on the environmental regime and plant traits. The combination of all the CO₂-induced ecophysiological responses lead to –3 to +5 ppbv of changes in surface ozone. While for the simulated results with the ecophysiology module implemented, the effects of all ecophysiological pathways result in an overall decrease in ozone levels by –0.047% in global tropospheric ozone burden, which corresponds to –2 to +2 ppbv of changes in surface ozone. In regions with low-NO_x environment and dense vegetation, the effect of CO₂ fertilization outweighs that of isoprene inhibition and stomatal closure, giving an overall decrease in ozone. In high-NO_x environment like North America and Europe, the effect of isoprene inhibition offsets that of CO₂ fertilization or stomatal closure. By comparing with the results, the impact of rising CO₂ on ozone levels is found to beoverall modest due to the counteracting effects of different pathways, but can be regionally important for specific pathways, underscoring the necessity of comprehensively analysing the interplay between the atmosphere and biosphere when examining the influence of increasing CO₂ on global atmospheric chemistry.