Ground-level reactive nitrogen oxides and ozone due to road traffic

Road transport emissions are one of the largest sources of nitrogen oxides in urban areas and thus influencing air quality. Besides the direct influence on air quality, tropospheric ozone is formed by nitrogen oxides, together with carbon monoxide, methane and volatile organic compounds. As tropospheric ozone is an irritant gas it influences air quality, further it is a greenhouse gas and impacts the climate.

For the first time, the effects of land traffic emissions on ozone and its precursors in Europe have been quantified in detail by scientists from the DLR Institute of Atmospheric Physics in cooperation with the University of Bonn. For the study an innovative modelling approach was used, in which the newly developed global-regional chemistry-climate model MECO(n) was equipped with a diagnostic labelling technique of trace gases.

The results of the study underline the large contributions of land transport emissions to the budget of reactive nitrogen over Europe. In detail, the study quantifies that 50 – 70% of the reactive nitrogen near ground level can be attributed to land transport emissions. The largest absolute contributions of land transport emissions to reactive nitrogen have been found near large urban areas such as the Paris region, southern England, western Germany and the Po Valley.

As tropospheric ozone has numerous sources besides land transport emissions, the relative contribution of land transport emissions to ground level ozone is lower as for reactive nitrogen and ranges from 8-16% during summer in Europe. Most striking are the differences between the geographical distributions of the maxima of ozone compared to those of reactive nitrogen. While the contributions of land transport emission to reactive nitrogen are largest in regions with high traffic volumes, their largest contributions to ozone are located in the Po Valley. This is mainly due to the complex, non-linear photochemistry involved in the formation of tropospheric ozone. Besides the ratio of the different ozone precursors, ozone production depends on the meteorological conditions. For a very efficient ozone production, large temperatures, stable weather conditions, and a lot of sunshine are needed, which are prevailing conditions especially in the Po Valley.