NOx emissions from inland shipping using plume cross-sections obtained from MAX-DOAS measurements

Nitrogen Oxides (NO­_x, i.e., NO and NO₂) are major contributors to local air pollution. They negatively affect human health and play an essential role in tropospheric chemistry. While air quality concerns are often focusing on heavy road traffic and seagoing ships, long-lasting diesel engines of inland waterway vessels can also be strong NO_x emitters and might represent a significant local pollution source. The Rhine River, Europe’s most important and busiest inland waterway, connects key seaports, industrial hubs, and densely populated cities, highlighting its importance for emission monitoring. Emissions from inland ships are concentrated near waterways, making their effect on air quality particularly relevant in residential areas located along intensively used waterways. Understanding and quantifying these emissions is important to assess inland shipping’s impact on local air quality.

 

In this work, we analyse NO_x emissions from inland ship exhaust plumes based on measurements performed in cooperation with the Federal Institute of Hydrology at the Rhine River in Koblenz, Germany. Over the course of more than one year, NO₂ measurements were taken using two MAX-DOAS (Multi-AXis Differential Optical Absorption Spectroscopy) instruments, providing high temporal resolution (6-8 seconds) and a large dataset for good statistical analysis. This remote sensing technique captures ship exhaust plumes from a riverbank while the ships pass by the line of sight of the instrument, making the detection of ship emissions less dependent on wind direction compared to using in-situ measurements. By measuring NO₂ column densities at different elevation angles, MAX-DOAS provides not just a single average value for the entire plume, but information about vertical NO₂ distribution within the plume. Here, we estimate the emission flux through the cross-section of the plume (in grams per second) based on measured column densities, ship position information, and wind data. Retrieved emission rates then can be converted to units in grams per kilowatt-hour, allowing for a direct comparison with European emission standards.