Ryoo et al. used a combination of modeling and observations to investigate trends driving background ozone over Western North America, specifically the lower tail (e.g., observations below the 33rd percentile) of the O3 concentrations, in the free troposphere. Using the combination of models and observations, the authors found that changing emissions patterns have influenced this lower percentile of observations. A combination of lowering NOx emissions in Western North America and increasing surface emissions from shipping and Southeast Asia has led to the increasing O3 concentrations. Further, they investigate the upper troposphere/lower stratosphere and find that there is an increasing influence from high-altitude sources, specifically transport across the Pacific, as well as increasing aircraft emissions. Overall, the study provides a narrative about the global influence towards background O3.