The new ability to measure NOx lifetime alongside concentration is crucial to understanding the relationship between the two nonlinear factors as well as in gauging how air quality in urban areas will change in response to emission reductions. Thus, say the authors, their analysis holds implications for future efforts to control pollution. Its results may also help to reconcile conflicting trends in NOx emissions estimates; accounting for changes in NOx lifetime is necessary to correctly interpret the relationship between changing NOx emissions and concentrations. Nitrogen oxides - namely nitric oxide and nitrogen dioxide - are powerful pollutants and play an important role in controlling air quality. The highly reactive gases often belched from motor vehicle engines and industrial processes are largely responsible for acid rain and intense air pollution, including the formation of atmospheric particulate matter and ground-level ozone, all of which poorly affect the health of plants and humans. As a result, many nations worldwide have implemented NOx emission standards and technologies to combat air pollution. However, the critical variables involved - NOx concentration and its lifetime in the urban air - share a difficult-to-interpret nonlinear relationship, which can be made more complicated by the chemistry occurring within the plume. Joshua Laughner and Ronald Cohen used daily satellite observations from the Berkeley High Resolution NO2 product to directly measure changes in NOx lifetimes in 34 cities from 2005-2014. Laughner and Cohen suggest that all sampled cities entered what is known as a "NOx -limited chemical regime" - in which NOx lifetime decreases even as NOx concentrations increase - by 2013. The results show that NOx lifetime changes need to be accounted for when relating NOx emissions and concentrations; ignoring these changes could lead to under- or over- estimates of NOx emissions. Fully understanding the factors driving NOx lifetime, though, will require new techniques, the authors say.