Measurement report: Characterization of uncertainties in fluxes and fuel sulfur content from ship emissions in the Baltic Sea

Jari Walden,Juha Hatakka,Toivo Truuts,Tuomas Laurila,Heidi Pettersson,Harri Nordlund,Miika Meretoja,Liisa Pirjola,Jukka-Pekka Jalkanen,Tuomas Walden,Kimmo K Kahma

doi:10.5194/acp-21-18175-2021

Abstract

Abstract. Fluxes of gaseous compounds and nanoparticles were studied using micrometeorological methods at Harmaja in the Baltic Sea. The measurement site was situated beside the ship route to and from the city of Helsinki. The gradient (GR) method was used to measure fluxes of SO2, NO, NO2, O3, CO2, and Ntot (the number concentration of nanoparticles). In addition, the flux of CO2 was also measured using the eddy-covariance (EC) method. Distortion of the flow field caused by obstacles around the measurement mast was studied by applying a computation fluid dynamic (CFD) model. This was used to establish the corresponding heights in the undisturbed stream. The wind speed and the turbulent parameters at each of the established heights were then recalculated for the gradient model. The effect of waves on the boundary layer was taken into consideration, as the Monin–Obukhov theory used to calculate the fluxes is not valid in the presence of swell. Uncertainty budgets for the measurement systems were constructed to judge the reliability of the results. No clear fluxes across the air–sea nor the sea–air interface were observed for SO2, NO, NO2, NOx (= NO + NO2), O3, or CO2 using the GR method. A negative flux was observed for Ntot, with a median value of -0.23×109 m−2 s−1 and an uncertainty range of 31 %–41 %. For CO2, while both positive and negative fluxes were observed, the median value was −0.081 µmol m−2 s−1 with an uncertainty range of 30 %–60 % for the EC methods. Ship emissions were responsible for the deposition of Ntot, while they had a minor effect on CO2 deposition. The fuel sulfur content (FSC) of the marine fuel used in ships passing the site was determined from the observed ratio of the SO2 and CO2 concentrations. A typical value of 0.40±0.06 % was obtained for the FSC, which is in compliance with the contemporary FSC limit value of 1 % in the Baltic Sea area at the time of measurements. The method to estimate the uncertainty in the FSC was found to be accurate enough for use with the latest regulations, 0.1 % (Baltic Sea area) and 0.5 % (global oceans).

Highlights

The Baltic Sea, owing to its nature as a relatively small inland sea that experiences heavy ship traffic and is surrounded by populated areas, is very sensitive to pollutants
Ship emissions of most air pollutants except CO decreased during the period from 2006 to 2018, but greenhouse gas emissions from ships remained stable throughout the abovementioned period, regardless of the growth of ship transport reported in tonne kilometers (HELCOM, 2019)
In the GR method, the wind speed should be measured at different heights, usually with conventional cup anemometers, whereas in the EC method, the fluctuation in the threedimensional wind speed is measured by a sonic anemometer

Summary

Introduction

The Baltic Sea, owing to its nature as a relatively small inland sea that experiences heavy ship traffic and is surrounded by populated areas, is very sensitive to pollutants. Due to the very narrow and shallow strait of Kattegat in Denmark, the exchange of seawater between the North Sea and the Baltic Sea is limited. Ship emissions enter the sea mostly via indirect deposition of sulfur and nitrogen compounds through chemical conversion in the atmosphere (de Leeuw et al, 2003; Hongisto and Joffre, 2005; Hongisto, 2014), or via direct deposition from the gas phase. The latest revision includes more stringent emission limits for NOx and SO2. In spite of these abatement regulations, the ship emissions of IMO-registered vessels and non-IMO-registered vessels show constant or slightly increasing trends in NOx, SO2, and PM2.5 compounds as well as a clearly increasing trend in CO. Once the stringent regulation of the fuel sulfur content (FSC) in marine fuel came into power on 1 January 2015, the emissions of SO2 and PM2.5 decreased rapidly at both regional and global levels (Johansson and Jalkanen, 2016; Jonson et al, 2020; Seppälä et al, 2021)

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