LAUSR

A shifting lifetime for NOx Nitrogen oxides (NOx) have a central role in controlling air quality, so designing effective strategies to combat air pollution, especially by ozone and particulate matter, depends on knowing their atmospheric abundances. Those abundances depend not only on NOx production rates but also on their lifetimes. Laughner and Cohen report that NOx lifetimes can be measured directly from satellite observations of nitrogen dioxide. They used data on 49 North American cities to show how NOx lifetimes there changed between 2005 and 2014. Accounting for this change in lifetime should help to reconcile conflicting trends between bottom-up and top-down emissions estimates. Science, this issue p. 723 The lifetimes of atmospheric nitrogen oxides can be determined by remote satellite measurements. NOx lifetime relates nonlinearly to its own concentration; therefore, by observing how NOx 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 NOx lifetime in approximately 30 North American cities has changed between 2005 and 2014 in a manner consistent with our understanding of NOx chemistry.