Towards operational monitoring of ship emissions using Long Path Differential Optical Absorption Spectroscopy

In contrast to land-based sources of air pollution, which have been regulated and reduced since several decades, NO_x and SO_x emissions from ships were only recently identified as significant sources of air pollution. As one consequence the sulphur content of ship fuel used within the so-called Sulphur Emission Control Areas (SECA) was recently regulated to a maximum of 0.1% (m/m) (MARPOL Annex VI). Therefore, especially monitoring the emission of sulphur compounds is of particular interest.

Within a 6-week measurement campaign in July and August of 2016, ship emissions were measured at the river Elbe in Germany, near Hamburg using the Long Path (LP)-DOAS technique. The measurements were carried out within the framework of the project MeSMarT (MEasurements of Shipping emissions in the MARine Troposphere), which investigates the influence of ship emissions on chemical processes in the atmosphere. Currently, monitoring of ship emission plumes is typically achieved by a combination of in situ trace gas monitors and meteorological sensors. In contrast to that the LP-DOAS technique is capable of simultaneously measuring signatures of multiple trace gases along an absorption path across a well-frequented waterway close to the ship exhaust-pipes and thus directly in the emission plume at a time resolution of a few seconds.

For our study, a LP-DOAS instrument was set up side by side to an in situ MeSMarT measurement station at the river Elbe at Wedel (15 km downriver of Hamburg harbour) where NO₂ and SO₂ emission signatures of a total of 5037 ship passes (of 1044 individual ships) were monitored. While the in situ method detected 16% of the ships, the LP-DOAS was able to assign emission plumes to 41% of all passing ships. With meteorology mainly limiting the in situ detection yield, the major limitation for the LP-DOAS was found to be due to the high traffic density and thus the difficulty to unambiguously assign recorded plumes to particular vessels, rather than to the sensitivity to the emission plume itself.

Based on the results of this feasibility study, we present a newly designed LP-DOAS system fulfilling the requirements for operational ship emission monitoring (robust mechanical setup, broad-band long-lifetime light source, compact sealed housing, automized alignment and data acquisition). This new system is now operated continuously to measure the ship emissions on the river Elbe.