Emissions of nitrogen oxides from oceangoing ships perturb atmospheric chemistry and climate

November 09, 1999

The emissions of nitrogen oxides from oceangoing ships can significantly change levels of gases such as ozone and hydroxyl radical, resulting in a cooling effect, especially in the North Atlantic region, has been computed by researchers from the Max Planck Institute for Chemistry in Mainz, Germany (Nature, 11 November, 1999).

The atmosphere is strongly affected by emissions due to human activities. This can be seen, for instance, in the urban smog which forms in the summer in many cities worldwide. Transport of this pollution away from the cities can affect larger regions of the globe as well. This influences not only air quality, but also the climate, for instance, through the greenhouse effect of ozone and carbon dioxide. Until now, the role of emissions from one major industrial activity - 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, far away from the majority of industrial emissions which occur over land, 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 computed 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 employ a global computer model of the atmosphere, known as MATCH-MPIC (Model of Atmospheric Transport and Chemistry at the Max Planck Institute for Chemistry), which has been under development at their institute in collaboration with researchers at the National Center for Atmospheric Research in Boulder, CO, USA for the last 5 years. This 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 compute how their atmospheric concentrations will vary in time and space. They consider 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 over their "background" levels. Since NOx is important for the formation of O3, this results in a twofold increase in O3 concentrations in this region. More importantly, the NOx emissions lead to OH concentrations that are 5 times higher than normal over the ocean. This provides 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. Furthermore, the reaction of OH with sulfur-containing gases such as sulfur dioxide (SO2) is 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 accurately assess the effects of ships on the environment. In particular, 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. Thus ship emissions will need to be considered in future international policy measures. However, since ship emissions result in a cooling of the climate, in contrast to the greenhouse warming scenarios for the next century computed by many current climate models, policy makers are presented with a dilemma which is particularly striking in this case.
Published: 10-11-99
Contact: Mark G. Lawrence
Max Planck Institute for Chemistry, Mainz/Germany
E-Mail: lawrence@mpch-mainz.mpg.de
Phone: +49 6131 3045-67
Fax: +49 6131 3045-36


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