Measurement report: Inland ship emissions and their contribution to NO_<i>x</i> and ultrafine particle concentrations at the Rhine

Abstract

Emission plumes of around 4700 ship passages were detected between March 2021 and June 2022 in the Upper Rhine Valley in Worms, Germany. In combination with ship-related data recorded via the automatic identification system (AIS), the plume composition of individuals ships was analyzed, and it was possible to quantify their contribution to the overall emission load. To obtain an integral picture of inland ship emissions, nitrogen oxide (NOx = NO + NO2) and carbon dioxide (CO2) measurements in the gas phase were combined with detailed particle-phase measurements including particle number concentration (PNC), particle size distribution (PSD) from 5 nm to 10 µm, particulate matter (PM1 and PM2.5), ultrafine particle fraction (UFP, diameter < 100 nm) and aerosol black carbon (BC). One measuring station was located inside a bridge directly above the navigation channel and was especially helpful in deriving emission factors under real-world driving conditions for the fleet on the Upper Rhine. The other station was situated on a riverbank at about 40 m distance to the shipping lane and was thus representative of the exposure of people working or living close to the Rhine. Inland ships contributed 1.2 µg m−3 or 7 % on average to the local nitrogen dioxide (NO2) concentration at the bridge above the shipping lane. NOx concentrations were increased by 10.5 µg m−3 (50 %), PNC by 800 cm−3 (10 %), PM1 by 0.4 µg m−3 (4 %) and BC by 0.15 µg m−3 (15 %). On the riverbank a NOx increase of 1.6 µg m−3 (8 %) and an NO2 increase of 0.4 µg m−3 (3 %) were observed. More than 75 % of emitted particles were found in the UFP range with a geometric mean particle diameter of 52±23 nm. Calculated emission factors (25–75 percentiles) were 26–44 g kg−1 of fuel for NOx, 1.9–3.2 g kg−1 for NO2, 0.3–0.7 g kg−1 for BC, 0.9–2.3 g kg−1 for PM1 and (1–3) × 1015 kg−1 for PNC, with a large variability observed from ship to ship. Relating these values to ship-specific parameters revealed the importance of engine characteristics, i.e., vessels using old motors with low revolutions per minute (rpm) caused comparably high emission factors for both NOx and PNC. A comparison with emission regulation limits set by the Central Commission for the Navigation of the Rhine (CCNR) and the European Union (EU) showed that – within the uncertainty of our calculation method – mean energy-dependent emission factors under real-driving conditions were slightly exceeding those under controlled laboratory conditions. The results from this study underline the importance of long-term measurements with high temporal resolution to reliably estimate the contribution of inland shipping to air pollution in cities along heavy traffic waterways and to monitor a potential future emission reduction when modernizing the fleet.