Direct observation of changing NO x lifetime in North American cities.

NO x lifetime relates nonlinearly to its own concentration; therefore, by observing how NO x lifetime changes with changes in its concentration, inferences can be made about the dominant chemistry occurring in an urban plume. We used satellite observations of NO2 from a new high-resolution product to show that NO x lifetime in approximately 30 North American cities has changed between 2005 and 2014 in a manner consistent with our understanding of NO x chemistry.