Because of the short lifetime of NO_x — hours to days depending on season, latitude and altitude — high concentrations are found close to the source of the emission. At higher altitudes NO_x has a longer lifespan and can be carried significant distances from its source. As a result of the larger wind speeds found at higher altitudes, NO_x transported or emitted in the free troposphere can have an impact on regions far downwind from the source of the emission.

Fig 1.1.2.2.1: NO_x emissions from anthropogenic sources in 2000.

Image after: Netherlands Environmental Assessment Agency: Edgar32FT2000