A shifting lifetime for NO x Nitrogen oxides (NO x ) 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 NO x production rates but also on their lifetimes. Laughner and Cohen report that NO x lifetimes can be measured directly from satellite observations of nitrogen dioxide. They used data on 49 North American cities to show how NO x 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

Direct observation of changing NO x lifetime in North American cities