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Zusammenfassung:
The overestimation of surface ozone concentration in low-resolution global atmospheric chemistry and climate models has been a long-standing issue. We first update the ozone dry deposition scheme in both high- (0.25°) and low-resolution (1°) Community Earth System Model (CESM) version 1.3 runs, by adding the effects of leaf area index and correcting the sunlit and shaded fractions of stomatal resistances. With this update, 5-year-long summer simulations (2015–2019) using the low-resolution CESM still exhibit substantial ozone overestimation (by 6.0–16.2 ppbv) over the U.S., Europe, eastern China, and ozone pollution hotspots. The ozone dry deposition scheme is further improved by adjusting the leaf cuticle conductance, reducing the mean ozone bias by 19%, and increasing the model resolution further reduces the ozone overestimation by 43%. We elucidate the mechanism by which model grid spacing influences simulated ozone, revealing distinctive pathways in urban versus rural areas. In rural areas, grid spacing mainly affects daytime ozone levels, where additional NOx emissions from nearby urban areas result in an ozone boost and overestimation in low-resolution simulations. In contrast, over urban areas, daytime ozone overestimation follows a similar mechanism due to the influence of volatile organic compounds from surrounding rural areas. However, nighttime ozone overestimation is closely linked to weakened NO titration owing to the redistribution of urban NOx to rural areas. Additionally, stratosphere-troposphere exchange may also contribute to reducing ozone bias in high-resolution simulations, warranting further investigation. This optimized high-resolution CESM may enhance understanding of ozone formation mechanisms, sources, and changes in a warming climate.
