Review status: this preprint is currently under review for the journal ACP.
Measurement report: Characterization of uncertainties of fluxes and
fuel sulfur content from ship emissions at the Baltic Sea
Jari Walden1,Liisa Pirjola2,3,Tuomas Laurila1,Juha Hatakka1,Heidi Pettersson4,Tuomas Walden1,Jukka-Pekka Jalkanen1,Harri Nordlund2,Toivo Truuts5,Miika Meretoja6,and Kimmo K. Kahma4Jari Walden et al.Jari Walden1,Liisa Pirjola2,3,Tuomas Laurila1,Juha Hatakka1,Heidi Pettersson4,Tuomas Walden1,Jukka-Pekka Jalkanen1,Harri Nordlund2,Toivo Truuts5,Miika Meretoja6,and Kimmo K. Kahma4
Received: 18 Oct 2020 – Accepted for review: 06 Jan 2021 – Discussion started: 15 Jan 2021
Abstract. Deposition of gaseous compounds and nanoparticles from ship emissions was studied by micrometeorological methods at Harmaja in the Baltic Sea. The gradient method was used to measure fluxes of SO2, NO, NO2, O3, CO2, and Ntot (number concentration of nanoparticles). In addition, the fluxes of CO2 were measured by the eddy covariance 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, and the wind speed as well as the turbulent parameters at each of the established heights were recalculated for the gradient model. The effect of waves on the boundary layer was taken into consideration, because 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 sea-air interface were observed for SO2, NO, NO2, NOx (= NO + NO2) or O3, while 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 −0.0036 mg m−2 s−1 with uncertainty ranges of 25–36 % and 30–60 % for the GR and EC methods, respectively. Ship emissions were responsible for 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 SO2 and CO2 concentrations. A typical value of 0.40 ± 0.06 %, was obtained for FSC, which is in compliance with the contemporary FSC limit value of 1 % in the Baltic Sea Area. The method to estimate the uncertainty of FSC was found to be accurate enough for use with the latest regulations, 0.1 % (Baltic Sea Area) and 0.5 % (Global Oceans).
Walden et al use a gradient method to investigate sea-atmosphere fluxes of various species. The detection limit of the gradient method is not sufficient to observe exchange fluxes for most gases. The authors report particle deposition fluxes, likely originating from ship emissions. Additionally FSC is assessed. The FSC in my opinion is the most interesting part of the manuscript, why weren’t concomitant NOx and particle plumes tracked, as this would seem a natural extension of the experiment. As for the flux analysis a number of major issues arise when reading the paper. While presenting CFD calculations to support the measurement site setup, it is not clear whether stationarity criteria were fulfilled due to passing ships and associated plumes being advected over the site. I disagree that stationarity is primarily characterized by concentration trends, which are part of the longer wave spectrum, often filtered out by turbulence averaging intervals. There are better ways to investigate stationarity (see standard textbooks on micrometeorological data pre-processing). As such the interpretation of fluxes needs to be evaluated carefully, because many fundamental criteria often implied for flux measurements might not be fulfilled. This might also relate to different footprints of individual levels of the gradient tower (e.g. is the lowest level even seeing the water surface or partially also influenced by the island?)
The fact that ship plumes on the order of a few seconds were observed suggests that homogeneity and stationarity was largely not fulfilled for quantifying fluxes from ships.
A comparison between CO2 eddy covariance fluxes and gradient measurements is shown, but it is not indicated what QAQC criteria (e.g. u*, ICT, stationarity,??) were used to filter data, and how much of the original data was used for the analysis after QAQC filtering. Were storage fluxes considered? To that end it would also be good to quantitatively compare the two flux methods for CO2, as it could help validating the gradient method. In this context I would expect to see a scatter plot and regression of both fluxes, - how well did the two methods really compare?
Measurement report: Characterization of uncertainties of fluxes and fuel sulfur content from ship emissions at the Baltic SeaJari Walden, Liisa Pirjola, Tuomas Laurila, Juha Hatakka, Heidi Pettersson, Tuomas Walden, Jukka-Pekka Jalkanen, Harri Nordlund, Toivo Truuts, Miika Meretoja, and Kimmo K. Kahma https://doi.org/10.5281/zenodo.4439605
Jari Walden et al.
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Ship emissions play an important role in deposition of gaseous compounds and nanoparticles affecting climate, human health especially at coastal areas, and eutrophication. Results by micrometeorological methods showed that ship emissions were mainly responsible for deposition of Ntot, whereas they accounted a minor proportion for CO2 deposition. Uncertainty analysis applied to the results of the fluxes and fuel sulfur contents of the ships demonstrate reliability of the results.
Ship emissions play an important role in deposition of gaseous compounds and nanoparticles...
