Traffic emissions form a large part of EU air pollution and are the subject of climate change policy1. Ozone pollution in the troposphere adversely affects human health and damages vegetation including crops, but it also behaves as a potent greenhouse gas (GHG). It occurs naturally but is also formed by the interaction of sunlight with pollutants emitted by transport and industry.
This recent study was conducted as part of the EU Integrated Project QUANTIFY2. Six different atmospheric chemistry models were applied in order to estimate the impact of emissions from road transport, aviation and shipping on ozone. The impact on the hydroxyl radical OH was also estimated. OH is a natural constituent of the atmosphere which has been referred to as the 'detergent' of the troposphere because it removes pollutants and GHGs such as methane and carbon monoxide.
The results indicate that the largest impact from total traffic emissions on total ozone occurs in the summer in the northern hemisphere. The greatest impact extends from the eastern US over the Atlantic to western Europe. In the southern hemisphere, changes are about 50 per cent lower than in the northern hemisphere.
Ship emissions have the greatest effect on the lower troposphere, causing over half of transport-induced changes in ozone in some regions. Notably, the effect of aircraft emissions does not dominate globally at the upper troposphere, but dominates the effect of traffic on ozone in the tropopause region (the boundary between the troposphere and the stratosphere) north of 30°N. Road traffic also strongly affects the northern upper troposphere, especially during the northern summer. During northern winter, the relative contributions from each traffic sector are all about the same.
Ship emissions have the largest impact on global OH at the lower troposphere and, therefore, the largest impact on reducing methane lifetime as they are released in relatively clean regions over the sub-tropical and tropical oceans where OH is highly sensitive to traffic emissions.
The study also considered the possible impact of ozone and methane on climate change by quantifying the associated radiative forcing (RF). RF is a measure of the imbalance between incoming radiation and outgoing radiation caused by a disruption of the atmosphere’s composition. Positive RF leads to a warming and negative RF causes a cooling.
Road emissions and aircraft emissions both caused positive forcing overall, with road emissions having the largest impact. Shipping's effect on OH and methane caused negative RF, indicating that its climate forcing impact through additional ozone formation was more than compensated for by its impact on OH and the resulting destruction of atmospheric methane. However, shipping emissions have several adverse effects such as the impact of NOx, NO2 and SO2 emissions on eutrophication, health and acidification. The EU strategy to reduce emissions from ships3 sets out actions to reduce these negative impacts.