Further reducing ozone pollution in metropolitan areas will therefore require improved quantification of NOx emissions. Exposure to NOx is also directly associated with asthma exacerbation in vulnerable groups and premature death . One major limitation of our current observing network is the inability to accurately quantify NOx emissions on a sector-by-sector basis in a timely fashion, with the exception of continuous emissions monitoring systems (CEMS) on electricity generating units. Additionally, many nonroad sources of NOx emissions, such as industrial or construction emissions, have large uncertainties .