Analysis of Long-term Observations of NOx and CO in Megacities and Application to Constraining Emissions Inventories

Long-term atmospheric NOx/CO enhancement ratios in megacities provide evaluations of emission inventories. A fuel-based emissions inventory approach that diverges from traditional bottom-up inventory methods explains 1970 - 2015 trends in NOx/CO enhancement ratios in Los Angeles. Combining this comparison with similar measurements in other US cities demonstrates that motor vehicle emissions controls were largely responsible for US urban NOx/CO trends in the past half-century. Differing NOx/CO enhancement ratio trends in US and European cities over the past 25 years highlight alternative strategies for mitigating transportation emissions, reflecting Europe's increased use of light-duty diesel vehicles and correspondingly slower decreases in NOx emissions compared to the US. A global inventory widely used by chemistry-climate models fails to capture long-term trends and regional differences in US and Europe megacity NOx/CO enhancement ratios, possibly contributing to these models' inability to accurately reproduce observed long-term changes in tropospheric ozone.