Effects of Rising CO2 Concentration on Global Ozone Air Quality&#160;
Authors: Cheng Kylie W. K. , Tai Amos P. K. , Wong Anthony Y. H. 
Published: March 2024

Surface ozone is a major&#160;air pollutant that harms not only human health but also crop and vegetation productivity.&#160;The continuously rising&#160;atmospheric CO2concentration&#160;can affect surface ozone levels through various&#160;vegetation-mediated&#160;ecophysiological&#160;pathways. These pathways include higher leaf densityfollowing CO2&#160;fertilization, which can&#160;lead to increased biogenic&#160;volatile organic compound&#160;(BVOC)&#160;emissions and dry deposition, inhibition of&#160;BVOC&#160;emissions due to competitive&#160;biochemical&#160;effects in leaves, and lower stomatal conductance resulting in lower dry deposition.&#160;In this&#160;study,&#160;we&#160;implemented an ecophysiology module that explicitly links the computation of dry deposition velocity and isoprene emission to photosynthesis calculation in the GEOS-Chem&#160;global 3-D chemical transport model, and&#160;conducted&#160;model&#160;experiments&#160;to simulate the effects of rising CO2&#160;levels on surface ozone pollution via CO2&#160;fertilization, isoprene inhibition and stomatal closure under an atmospheric CO2&#160;level projected by Representative Concentration Pathway (RCP) 8.5 scenarios in 2050, with 2010 as the base year for comparison.&#160;The effects&#160;of rising CO2&#160;on enhancement in&#160;leaf area index (LAI) were simulated separately using&#160;a land surface model (Community Land Model,&#160;CLM).&#160;The&#160;simulated results&#160;indicate&#160;that the CO2-induced&#160;ecophysiological&#160;effects depend&#160;largely on&#160;the environmental regime and plant traits.&#160;The combination of all the CO2-induced&#160;ecophysiological&#160;responses lead to &#8211;3 to +5&#160;ppbv&#160;of changes in surface ozone.&#160;While for the&#160;simulated&#160;results&#160;with the ecophysiology module implemented,&#160;the&#160;effects of all&#160;ecophysiological&#160;pathways&#160;result&#160;in an overall&#160;decrease in ozone levels&#160;by &#8211;0.047% in&#160;global&#160;tropospheric&#160;ozone&#160;burden,&#160;which corresponds to&#160;&#8211;2 to&#160;+2&#160;ppbv&#160;of changes in surface ozone.&#160;In&#160;regions with low-NOx&#160;environment and dense vegetation, the effect of&#160;CO2&#160;fertilization outweighs that of isoprene inhibition and stomatal closure,&#160;giving an overall decrease in ozone.&#160;In high-NOx&#160;environment like North America and Europe, the effect of isoprene inhibition&#160;offsets that of&#160;CO2&#160;fertilization&#160;or&#160;stomatal closure.&#160;By comparing with the&#160;results,&#160;the impact of rising&#160;CO2&#160;on ozone levels is&#160;found&#160;to beoverall&#160;modest&#160;due to the counteracting effects&#160;of different pathways, but&#160;can be&#160;regionally important for&#160;specific pathways,&#160;underscoring the necessity of comprehensively analysing the interplay between the atmosphere and biosphere when examining the influence of increasing&#160;CO2&#160;on global&#160;atmospheric chemistry.&#160;