Air pollutants emitted by ocean-going vessels (OGVs) cause numerous environmental and human health problems. In 2016, the Belgian Coastguard aircraft was equipped with a sniffer sensor to monitor compliance with MARPOL Annex VI Regulation 14. However, the sensor was susceptible to NO and Volatile Organic Compounds (VOCs), which had a negative impact on the measurement uncertainty. The elimination of measurement errors was achieved by modifying the sensor, including among others the addition of a NOx sensor and a custom-designed hydrocarbon kicker. This resulted in a substantial improvement in the measurement quality and uncertainty of the derived Fuel Sulfur Content (FSC). As a direct result of this, the reporting thresholds for non-compliance drastically improved. The data analysis of sampled OGVs showed that compliance levels notably improved between 2019 and 2020 (from 95.9% to 97.3%), coinciding with the implementation of the Global Sulfur Cap. Findings in this study have also demonstrated that OGVs equipped with emission abatement technology (scrubbers) are more susceptible to non-compliance with Regulation 14 of MARPOL Annex VI. Given these results, this article provides an answer to the question of how to monitor effective implementation of NO emissions from OGVs.