Ship emissions of SO<sub>2</sub> and NO<sub>2</sub>: DOAS measurements from airborne platforms

Abstract

Abstract.A unique methodology to measure gas fluxes of SO 2 and NO 2 from ships using optical remote sensing is described and demonstrated in a feasibility study. 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 SO 2 and NO 2 are retrieved from each spectrum and integrated across the plume. A simple geometric approximation is used to calculate the optical path. 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 SO 2 emission out of 22 ships was (54 ± 13) kg h −1 , and the average NO 2 emission was (33 ± 8) kg h −1 , out of 13 ships. In the North Sea the average SO 2 emission out of 21 ships was (42 ± 11) kg h −1 , NO 2 was not measured here. The detection limit of the system made it possible to detect SO 2 in the ship plumes in 60 % of the measurements when the described method was used.A comparison exercise was carried out by conducting airborne optical measurements on a passenger ferry in parallel with onboard measurements. The comparison shows agreement of (−30 ± 14) % and (−41 ± 11) %, respectively, for two days, with equal measurement precision of about 20 %. This gives an idea of the measurement uncertainty caused by errors in the simple geometric approximation for the optical light path neglecting scattering of the light in ocean waves and direct and multiple scattering in the exhaust plume under various conditions. A tentative error budget indicates uncertainties within 30-45 % but for a reliable error analysis the optical light path needs to be modelled.A ship emission model, FMI-STEAM, has been compared to the optical measurements showing an 18 % overestimation and a correlation coefficient (R 2 ) 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.

Field test of available methods to measure remotely SO<sub>x</sub> and NO<sub>x</sub> emissions from ships

“…A detailed discussion of this comparison can be found in Berg et al (2012). The comparison showed differences of (−30 ± 14) % and (−41 ± 11) %, respectively, between the measurements and the certified fuel sulphur content for 2 days, with equal measurement precision of about 20 %.…”

“…The ship exhaust follows the apparent wind as shown in Fig. 2 (Berg et al, 2012). Direction as well as speed of the apparent wind can be significantly changed by changing the ship speed, in the figure the apparent wind changes by 90 • for a ship with opposite orientation.…”

“…During the campaign a DOAS unit was operated by Chalmers from a Dauphin helicopter (Berg et al, 2012;Beecken et al, 2014).…”

“…An upper limit to the overall uncertainty has been roughly estimated as 30-45 % while the repeatability was about 20 % during sequential measurements (Berg et al, 2012).…”

“…Main contributions to the uncertainties on the DOAS measurements stem from the evaluation of 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, which is estimated to be 30-45 % (Berg et al, 2012).…”