Abstract

Long-term atmospheric NO$_x$/CO enhancement ratios in megacities provide evaluations of emission inventories. A fuel-based emission inventory approach that diverges from conventional bottom-up inventory methods explains 1970–2015 trends in NO$_x$/CO enhancement ratios in Los Angeles. Combining this comparison with similar measurements in other U.S. cities demonstrates that motor vehicle emissions controls were largely responsible for U.S. urban NO$_x$/CO trends in the past half century. Differing NO$_x$/CO enhancement ratio trends in U.S. and European cities over the past 25 years highlights alternative strategies for mitigating transportation emissions, reflecting Europe’s increased use of light-duty diesel vehicles and correspondingly slower decreases in NO$_x$ emissions compared to the U.S. A global inventory widely used by global chemistry models fails to capture these long-term trends and regional differences in U.S. and Europe megacity NO$_x$/CO enhancement ratios, possibly contributing to these models’ inability to accurately reproduce observed long-term changes in tropospheric ozone.