We have applied both methods (tagging and perturbation) in
numerical simulations with our climate-chemistry model. Results
for 1990 are shown in Fig. 3. Both methods use the exact same simulations:
A base case with 1990 emissions, and a “zero road” simulation, where all emissions due to road traffic have been turned off. When using the perturbation method, the difference of the tropospheric ozone columns between the two simulations
gives the net ozone column added due to road traffic emissions.
There is no additional information. This is symbolized by the two
uniform columns on the left of Fig. 3.