ARTICLE 
COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-023-01050-7 
50. Explicit ApS. Airborne Monitoring of Sulphur Emissions from Ships in Danish 70. Jacob, D. J. Introduction to Atmospheric Chemistry (Princeton University 
Waters: 2018 Campaign Results. https://www2.mst.dk/Udgiv/publications/ Press, 1999). 
(2019). 
Explicit ApS. Airborne Monitoring of Sulphur Emissions from Ships in Danish Waters: 
2021 Campaign Results (Danish Environmental Protection Agency, 2022). 
Explicit ApS. Airborne Monitoring of Sulphur Emissions from Ships in Danish 
Waters: 2017 Campaign Results. https://www2.mst.dk/Udgiv/publications/ 
(2018). 
Mellqvist, J. & Conde, V. Surveillance of Sulfur Fuel Content in Ships at the 
Great Belt Bridge 2018. https://www2.mst.dk/Udgiv/publications/ (2019). 
Mellqvist, J. & Conde, V. Surveillance of Sulfur Fuel Content in Ships at the 
Great Belt Bridge 2019. https://www2.mst.dk/Udgiv/publications/ (2020). 
Weigelt, A., Thomsen, C. & Bliemeister, J. German Ship Emission Monitoring 
Network Remote Emission Measurements to Support MARPOL Annex VT 
Compliance Monitoring. https://www.bsh.de/EN/TOPICS/Monitoring_ 
systems/Ship_emission_monitoring_network/ship_emission_monitoring 
network_node.html (2021). 
Mellqvist, J., Beecken, J., Conde, V. & Ekholm, J. Surveillance of Sulfur 
Emissions from Ships in Danish Waters. Report to the Danish Environmental 
Protection Agency. https://research.chalmers.se/publication/500251/file/ 
500251_Fulltext.pdf (2017). 
Van Roy, W. & Scheldeman, K. CompMon Report: Compliance Monitoring 
Pilot for MARPOL Annex VI, Results MARPOL Annex VI Monitoring Report 
Belgian Sniffer Campaign 2016. https://trimis.ec,europa.eu/project/ 
compliance-monitoring-pilot-marpol-annex-vi (2016). 
IMO. Guidelines for On Board Sampling for the Verification of the Sulphur 
Content of the Fuel Oil Used On Board Ships, Circular MEPC.1/Circ.864/Rev.1 
MO, 2019). 
European Maritime Safety Agency. Inspection Database to support EU 
Legislation other than PSC-THETIS EU. https://www.emsa.europra.eu/thetis- 
eu.html (2022). 
60. IMO. https://gisis.imo.org/Public/Default.aspx (2020). 
61. Sprinthall, R. C. Basic Statistical Analysis 9th edn (Pearson, 2011). 
62. MEPC. Effective date of implementation of the Fuel Oil Standard in Regulation 
14.1.3 of MARPOL Annex VI, Resolution MEPC.280(70), Adopted on 28 
October 2016, Entered into Force on 1 January 2020. 
Theys, N. et al. A sulfur dioxide covariance-based retrieval algorithm 
(COBRA): application to TROPOMI reveals new emission sources. Afmos. 
Chem. Phys. 21, 16727-16744 (2021). 
van Geffen, J. H. G. M., Eskes, H. J., Boersma, K. F., & Veefkind, J. P. 
TROPOMI ATBD of the Total and Tropospheric NO, Data Products. Report 
S5P-KNMI-L2-0005-RP, version 2.2.0, 2021-06-16, KNMI. http://www. 
tropomi.eu/data-products/nitrogen-dioxide/ (2022). 
Kimbrough, S., Chris Owen, R., Snyder, M. & Richmond-Bryant, J. NO to 
NO, conversion rate analysis and implications for dispersion model chemistry 
methods using Las Vegas, Nevada near-road field measurements. Atmos. 
Environ. 165, 23-34 (2017). 
Atkinson, R. Atmospheric chemistry of VOCs and NO,. Atmos. Environ. 34, 
2063-2101 (2000). 
Levelt, P. F. et al. Air quality impacts of COVID-19 lockdown measures 
detected from space using high spatial resolution observations of multiple 
race gases from Sentinel-5P/TROPOMI. Atmos. Chem. Phys. 22, 
L0319-10351 (2022). 
Tack, F. et al. Assessment of the TROPOMI tropospheric NO, product based 
on airborne APEX observations. Afmos. Meas. Tech. 14, 615-646 (2021). 
Riess, T. C. V. W. et al. Improved monitoring of shipping NO, with 
TROPOMI: decreasing NO, emissions in European seas during the COVID- 
19 pandemic. Atmos. Meas. Tech. 15, 1415-1438 (2022). 
Acknowledgements 
The European Maritime Safety Agency (EMSA) is gratefully acknowledged for providing 
‚he KLD data and the number of executed fuel samples in the EU. The authors wish to 
hank Christophe Swolfs and Bart Colaers from the Federal Public Service for Mobility 
and Transport for their help in obtaining the Thetis-EU data. Acknowledgement to the 
1ational competent authorities for their approval to use their RPAS monitoring and port 
nspection data: the Danish Environmental Protection Agency (EPA), the Danish Mar- 
time Agency (DMA), the French General Directorate of Maritime Affairs, Fishery and 
Aquaculture (DGAMPA), the German Federal Maritime and Hydrographic Agency 
(BSH), the Human Environment and Transport Inspectorate of the Ministry of Infra 
structure and Water Management of the Netherlands (ILT), the Swedish Transport 
Agency, the Spanish Ministry of Transport, Mobility and Urban Agenda (MITMA). 
Finally, we wish to thank Rebecca Gualandi for proofreading the manuscript. 
Author contributions 
W.V.R. designed the research and wrote the paper; W.V.R., B.V.R., L.V., A.V.N., K.S. and 
‚.-B.M. performed the analysis; A.W., J.M., J.v.V., D.v.D., J.B., F.T. and N.T. contributed 
data; A.V.N., K.S., J.-B.M., A.W., J.M., J.v.V. and F.M. read and commented on the paper. 
1/ 
Correspondence and requests for materials should be addressed to Ward Van Roy. 
Peer review information Communications Earth & Environment thanks the anonymous 
‚eviewers for their contribution to the peer review of this work. Primary handling editor: 
Martina Grecequet. A peer review file is available. 
Reprints and permission information is available at http://www.nature.com/reprints 
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in 
published maps and institutional affıliations. 
© ® | Open Access This article is licensed under a Creative Commons 
Attribution 4.0 International License, which permits use, sharing, 
adaptation, distribution and reproduction in any medium or format, as long as you give 
appropriate credit to the original author(s) and the source, provide a link to the Creative 
Commons license, and indicate if changes were made. The images or other third party 
naterial in this article are included in the article’s Creative Commons license, unless 
indicated otherwise in a credit line to the material. If material is not included in the 
article’s Creative Commons license and your intended use is not permitted by statutory 
regulation or exceeds the permitted use, you will need to obtain permission directly from 
he copyright holder. To view a copy of this license, visit http://creativecommons.org/ 
censes/by/4.0/ 
OMMUNICATIONS EARTH & ENVIRONMENT | (2023)4:391 | https: //doi.0rg/10.1038/s43247-023-01050-7 | www.nature.com/commsenv