Nitrogen oxides, or NOx, are widespread pollutants that are emitted by vehicle and industrial combustion and help produce harmful surface-level ozone. However, addressing NOx pollution is complicated by the fact that the pollutants’ lifetimes in the atmosphere depend on factors other than their concentration, such as the levels of other pollutants. Now, scientists have directly observed changes in NOx lifetimes across North America between 2005 and 2014 (Science 2019, DOI: 10.1126/science.aax6832).
The study used high-resolution ultraviolet–visible spectra collected by satellites to determine the amount of NOx in the atmosphere at a given location. By comparing the NOx concentrations in adjacent pixels of the images collected and incorporating wind speed data, the researchers could come up with a lifetime for NOx in each city studied.
Although other studies have used similar methods to estimate NOx lifetimes, this is the first to look at changes in these lifetimes. While NOx emissions dropped in each city over the period studied, NOx lifetimes increased in some cities. University of California, Berkeley, atmospheric chemist Ronald Cohen, who led the study, says the differences are likely attributable to varying concentrations of volatile organic compounds, which react with NOx to form ozone.
The authors also found that by 2013, NOx were the limiting reagents in ozone production in all the cities studied, suggesting that continuing to reduce NOx emissions will lead to further improvements in ozone pollution. Lyatt Jaeglé, an atmospheric chemist at the University of Washington, says the study provides important observational evidence to back up theoretical predictions of NOx behavior in the atmosphere.