DOAS applied to shipping emission monitoring: compliance assessment and comparison to satellite measurements

While shipping is generally the most energy-efficient freight transportation mode (in terms of gCO2 t^-1 km^-1), its intensive use (80 % to 90 % of global merchandise trade volume), coupled with high pollutant emission factors, leads to serious impact on the environment and the human health. The primary pollutants emitted by ships, nitrogen oxides (NO_x = NO₂ + NO), sulphur oxides (SO_x, mainly SO₂) and particulate matter (PM), degrade the air quality and are involved in the formation of secondary pollutants as tropospheric ozone (O₃). As 70 % of ship emissions occur within 400 km of coastlines, ship emissions have strong impact in harbour cities and coastal areas situated along high traffic shipping lanes.

Compliance monitoring for fuel sulphur content (FSC) is usually done by the collection and the analysis of fuel samples by competent authorities from ships at berth. The complexity of the method results in very few ships being formally controlled. In consequence, various remote compliance monitoring techniques of FSC have been developed in the last years, mainly involving sniffer techniques (extractive methods coupled with analyser instruments) performed from fixed or moving (e.g., manned and unmanned aircrafts, ships) platforms. Optical remote sensing techniques such as differential optical absorption spectroscopy (DOAS) have also been applied to shipping emission monitoring in the past years.

Here we present results of several ship-based campaigns conducted in the last years (in the Mediterranean Sea in 2019 and 2021, and in the English Channel in May 2022), with a focus on the DOAS technique. While the performed DOAS zenith measurements are not fully suitable to conduct a quantitative monitoring of the ship compliance to the FSC regulations, we propose a method to qualitatively identify potential non-compliant ships. Comparisons are made with state-of-the-art sniffer measurements. Despite the larger uncertainty yielded by this technique in comparison to sniffer systems, it may be applied for the guidance of formal controls (e. g., authorities at port).

During the English Channel campaign in May 2022, we modified our DOAS setup in order to sequentially scan various elevation angles from the horizon to the zenith (i.e., multi axis DOAS, MAX-DOAS) and, thus, to retrieve vertical tropospheric columns of NO₂. Here we compare our shipborne MAX-DOAS NO₂ columns to the ones retrieved from TROPOMI (TROPOspheric Monitoring Instrument). TROPOMI could be further involved in the monitoring of shipping emissions as it has been recently used to identify the emission plumes of single (or group of) ships. Therefore, its validation with independent datasets at sea is needed to strengthen the monitoring of shipping emissions.