Year: 2012
Type: article
Abstract: . A unique methodology to measure gas fluxes of SO2 and NO2 from ships has been developed in a Swedish national project using optical remote sensing. The measurement system is based on Differential Optical Absorption Spectroscopy using reflected skylight from the water surface as light source. A grating spectrometer records spectra around 311 nm and 440 nm, respectively, with the telescope pointed downward at a 30° angle from the horizon. The mass column values of SO2 and NO2 are retrieved from each spectrum and integrated across the plume. To obtain the total emission in kg h−1 the resulting total mass across the plume is multiplied with the apparent wind, i.e. a dilution factor corresponding to the vector between the wind and the ship speed. The system was tested in two feasibility studies in the Baltic Sea and Kattegat, from a CASA-212 airplane in 2008 and in the North Sea outside Rotterdam from a Dauphin helicopter in an EU campaign in 2009. In the Baltic Sea the average SO2 emission out of 22 ships was (54 ± 13) kg h−1, and the average NO2 emission was (33 ± 8) kg h−1, out of 13 ships. In the North Sea the average SO2 emission out of 21 ships was (42 ± 11) kg h−1, NO2 was not measured here. The system was able to detect plumes of SO2 in 60% of the measurements when the described method was used. The optical measurement carried out on a passenger ferry on two consecutive days was compared to onboard emission data obtained from analysed fuel content and power consumption. The comparison shows agreement of (−30 ± 14) % and (−41 ± 11) %, respectively, for two days, with equal measurement precision of about 20%, this indicates the presence of systematic error sources that are yet unaccounted for when deriving the flux. Two such error sources are the difficulty in estimating the optical path of the ocean scattered light due to waves, and direct and multiple scattering in the exhaust plume. Rough estimates of these sources have been accounted for in the total uncertainty, 30–45 %. A ship emission model, FMI-STEAM, has been compared to the optical measurements showing a 18% overestimation and a correlation coefficient (R2) of 0.6. It is shown that a combination of the optical method with modelled power consumption can estimate the sulphur fuel content within 40%, which would be sufficient to detect the difference between ships running at 1% and at 0.1%, limits applicable within the IMO regulated areas.
Authors: N. Berg, J. Mellqvist, J.-P. Jalkanen, J. Balzani
Language: English
Subfield: Environmental Engineering
Field: Environmental Science
Domain: Physical Sciences
SDG: Climate action
Open Access status: gold
Estimated APC paid: $1,585
OpenAlex ID: W2108149285