We have undertaken three brief surveys with the Lidar at Dungeness, in an attempt to detect ship plumes and to measure their detailed dispersion.
Although global shipping is a larger contributor to human CO2 emissions than the aviation industry, it is other shipping emissions that are perhaps of greater significance (and concern) in regions where shipping densities are high. For instance, while emissions of many pollutants from stationary and mobile terrestrial sources in Europe have been falling for many years, emissions from the marine transport sector have continued to rise. The most notable emissions are of SO2, NOx and PM (both directly emitted aerosol and secondary formation). Emissions of nitrogen oxides (NOx) and sulphur dioxide (SO2) from shipping in the European Union, indeed, are expected soon to exceed those from all land based combustion sources. Recent estimates of the global mortality arising from the emissions of particulate matter (PM) from shipping, for example, suggest an annual toll of 60,000 cardiopulmonary and lung cancer deaths, with one of the highest rates being in SE England. In order to improve the representation of this clearly significant pollution source in global models, we need a better understanding of the detailed processes occurring in ship plumes: this was the main goal of this project.
The key to modelling the production of secondary pollutants (e.g. acidic aerosol) from the primary shipping emissions lies in being able to construct a realistic model of the plume dispersion, internal mixing, and entrainment of air from outside of the plume. A typical scheme in existing models might use Briggs-type Gaussian parameters for the cross-sectional area of the dispersing and then to assume that the primary emissions are well mixed with ambient air within this volume. A difficulty with such schemes, however, is that the Gaussian plume strictly only arises as a result of time averaging over an ensemble of random dispersion events. Plume spread parameters are thus strictly only relevant to time averaged behaviour, typically over 30 min. Much (if not most) of the plume spread over such a period arises from looping or meandering of the short term plume, which is clearly irrelevant to the intimate mixing required for in-plume chemistry to take place. There was thus a clear need to make some high resolution measurements of the dispersion of marine plumes at some point relatively close to the point of emission, and to observe how they evolve.
The Dover Strait is the busiest international shipping lane in the world, being the principal route to most of the major container ports in Northern Europe and the UK; several hundred vessels transit each day. Our field site was at Dungeness Point on the Kent coast, where the major sea lane reaches its closest approach to the channel coast on the UK side (about 7 km offshore). With the permission of the Romney Marsh Countryside Project, we were able to site the Lidar on the foreshore, with a clear view from SW to E, and observe the vessels passing.
Sadly, the winds were adverse for the whole of the time during all of these visits. We could observe the vessels in the main shipping lane and determine their location and velocity using the Lidar, but their smoke was too distant to be detected. Nevertheless, we detected smoke plumes from the Dover area and made useful observations of the marine boundary layer.