Globally, increasing NOx emissions

Globally, increasing NOx emissions in the period 1960 to 2019
result in an increase in simulated tropospheric ozone (Fig. 1b, green
line), which is also apparent from observations, e.g. at Hohenpeissenberg
(Fig. 1b, blue line). The red line in Fig. 1 shows only that
part of tropospheric ozone that originates from tropospheric
photochemistry. The stratospheric contribution is subtracted, (Note
that this already requires some form of tagging). This tropospheric
ozone vs. NOx emission curve also gives a clear indication of the
saturation in ozone production rates. A 10 TgN/year increase in NOx
emissions from 55 to 65 TgN/year gives little or no increase in
ozone (red line). At lower emission levels, however, e.g. from 10 to
20 TgN/year, the same 10 TgN/year increase results in substantial
ozone increase. This saturation effect is important. It leads to
important differences from a linear chemistry, which would follow
e.g. the black line in Fig. 1b. Ultimately, this deviation from linearity results in a difference between the contributions assigned to
a specific emission sector, like traffic, by the “perturbation method”
and by the “tagging method” (Grewe et al., 2010).