Nitrogen Oxides from ships are cooling atmosphere

Until now, the role of emissions from international shipping has largely been neglected in policy considerations regarding atmospheric change. However, because pollutants from ships are released to the atmosphere mainly in coastal and open ocean regions, the ship emissions have a strong potential for influencing the atmosphere in marine regions.

In the November issue of Nature, researchers Mark G. Lawrence and Paul J. Crutzen of the Max Planck Institute for Chemistry in Mainz, Germany, have calculated that ship emissions of nitrogen oxides can indeed have an important influence on the atmosphere, which should be taken into account in future international policy decisions.

For their study, Lawrence and Crutzen employed a global computer model of the atmosphere, known as MATCH-MPIC (Model of Atmospheric Transport and Chemistry at the Max Planck Institute for Chemistry), developed in collaboration with the National Center for Atmospheric Research in Boulder, CO, USA.

The model simulates the sources, transport, reactions and losses of key atmospheric gases, such as ozone (O3), carbon monoxide (CO), the hydroxyl radical (OH), and the nitrogen oxides (NOx = NO + NO2), in order to calculate how their atmospheric concentrations will vary in time and space.

The researchers considered two model runs, one without any NOx emissions from ships, and one with ship NOx emissions based on recent literature data; otherwise all parameters are identical for the two model runs.

The model computations indicate that during July, the emissions from ships can be responsible for increasing the NOx concentrations over the North Atlantic by 100 times more than their background levels.

Since NOx is important for the formation of O3, this results in a twofold increase in O3 concentrations in the region. More importantly, the NOx emissions lead to OH concentrations five times higher than normal over the ocean.

The levels of OH provide a key link to the climate system. First, OH is the main gas responsible for removal of numerous gases, in particular methane (CH4), which can act as greenhouse gases. More OH results in lower levels of greenhouse gases, which translates to a cooling of the atmosphere.

The reaction of OH with sulphur-containing gases such as sulphur dioxide (SO2) is also the main pathway for forming new particles in the atmosphere. These small particles are necessary for the formation of cloud droplets. Both the particles themselves as well as cloud droplets reflect sunlight, resulting in a cooling of the atmosphere, which will also be enhanced due to the ship NOx emissions.

Future studies will be needed to assess the effects of ships on the environment. In addition to NOx, ships also emit SO2; this has been found in an independent study recently published in Nature to also have a cooling effect on the climate. An important next step will be to combine these two effects in a global climate model.

These effects on the atmosphere are expected to increase over the coming decades, since ship traffic is growing by about 3% per year.