Sensors
- Sentinel-5P TROPOMI: The primary workhorse for this application. NO2 tropospheric column at 3.5 × 5.5 km (upgraded from 7 × 3.5 km after August 2019 processor update), SO2 at 3.5 × 5.5 km, daily global coverage. Corridor signals become statistically robust in 30-to-90-day composites; individual overpasses rarely isolate a single vessel.
- Aura OMI: NASA instrument operational since 2004, providing the longest continuous tropospheric NO2 and SO2 record at roughly 13 × 24 km nadir resolution. Row anomaly reduces coverage from 2009 onward, but the archive is indispensable for pre-2017 trend analysis and IMO cap baseline construction.
- GOME-2 (MetOp-A/B/C): EUMETSAT instrument with 40 × 80 km pixels in standard mode, insufficient to resolve individual corridors but useful for long-term trend cross-validation and SO2 background characterisation over open ocean. Three platforms extend the record and reduce single-satellite sampling gaps.
- Sentinel-4 (forthcoming, Meteosat Third Generation): Geostationary UV-VIS-NIR sounder planned for European coverage at roughly 8 km spatial sampling and hourly revisit. When operational, it will allow diurnal NO2 cycle analysis over the North Sea and Mediterranean corridors, which is currently impossible with polar-orbiting sensors.
What a shipping lane looks like from 824 km up
Heavy fuel oil combustion produces both nitrogen oxides and sulphur dioxide in quantities that, summed across thousands of transits per month, create column enhancements detectable in multi-week TROPOMI composites. The English Channel, one of the world's busiest straits, shows a persistent NO2 ridge aligned precisely with the traffic separation scheme. The Malacca Strait and the Red Sea exhibit similar linear features. These are not artefacts: their orientation matches vessel routing, their intensity correlates with traffic density, and they weaken markedly over anchorage areas where ships idle at reduced load.
A single TROPOMI overpass at 3.5 × 5.5 km cannot attribute a column enhancement to one vessel or even one day's traffic. The physics of vertical mixing and the instrument's detection floor mean that ship-scale signals only emerge statistically. Thirty-day composites over high-density corridors typically show enhancements of 0.5 to 2 × 10¹⁵ molecules cm⁻² above the local marine background, depending on season and wind regime. That figure is consistent with published literature using OMI and early TROPOMI data.
The 2020 IMO sulphur cap as an accidental experiment
From 1 January 2020, IMO Annex VI reduced the global sulphur limit in marine fuels from 3.5% to 0.5% by mass. This was one of the largest deliberate changes to a mobile emission source ever attempted, and it produced a detectable signal in the satellite record. Studies using TROPOMI SO2 retrievals published in peer-reviewed journals documented SO2 column reductions of 50 to 80% over major shipping corridors in the months following implementation, after controlling for meteorological variability. The English Channel and the South China Sea showed the clearest responses because traffic density is high enough to produce a strong pre-cap baseline.
This episode illustrates both the capability and the limit of the method. TROPOMI can confirm fleet-wide compliance with a sulphur regulation at the corridor level. It cannot identify which specific vessels are burning non-compliant fuel on a given day. Port-state control inspections and fuel sampling remain the enforcement mechanism for individual ships; satellite data provides the statistical context that justifies deploying those inspectors.
Separating ship exhaust from continental outflow
The hardest analytical problem in this domain is not sensitivity but specificity. Continental NO2 from power plants, road traffic and industry is advected offshore by prevailing winds and can overlay shipping corridors entirely. The North Sea is particularly awkward: industrial emissions from the Rhine-Ruhr region and the English Midlands regularly produce background columns that exceed the ship-lane enhancement. The Red Sea has the opposite problem, with relatively clean marine air making corridor signals easier to isolate.
Several published approaches address this. Wind-rotation compositing aligns retrievals by wind direction before averaging, which smears continental plumes while reinforcing the fixed-geometry corridor signal. Trajectory back-analysis using meteorological reanalysis fields (ERA5 is the standard) can tag pixels by air-mass origin. Comparing NO2 to SO2 ratios also helps: the high sulphur content of heavy fuel oil produces an SO2/NOx ratio that differs from most terrestrial combustion sources, providing a chemical fingerprint. None of these methods is clean in regions of dense continental outflow; honest attribution there requires stating a confidence range rather than a point estimate.
What the archive can and cannot tell a regulator
The OMI archive from 2004 and the TROPOMI record from 2017 together span a period that includes two major IMO regulatory changes and the COVID-19 traffic collapse of 2020, which provided an unplanned baseline for separating shipping from other sources. A regulator asking whether a corridor's air quality has improved over a decade can get a defensible answer from this archive. A regulator asking whether Vessel X was burning non-compliant fuel on Tuesday cannot.
Cloud cover is a genuine constraint. Tropical corridors such as the Malacca Strait have persistent convective cloud that reduces valid retrieval frequency, lengthening the compositing period needed to achieve statistical significance. High solar zenith angles in winter reduce retrieval quality at high latitudes, which affects North Atlantic and Norwegian Sea corridor analysis. TROPOMI's aerosol index product partially compensates by flagging scenes where cloud or aerosol interference is likely, but flagged pixels must simply be excluded rather than corrected.
Satellize runs TROPOMI-based corridor attribution as part of its open-constellation analytics stack, applying wind-rotation compositing and ERA5 trajectory tagging to produce quarterly compliance-context reports. The methodology is the same published approach used in peer-reviewed corridor studies; what Satellize adds is operational delivery and integration with AIS traffic data to weight retrievals by vessel density.
Sentinel-4 and what changes when a sensor goes geostationary
All current operational sensors for this application are in polar low Earth orbit, which means one overpass per day per location at best. Diurnal variation in NO2 is significant: photolysis destroys NO2 through the day, and ship emissions accumulate differently in morning and afternoon boundary layers. A polar sensor always samples at the same local time, so it captures a snapshot rather than a daily budget. Sentinel-4, once operational over Europe and North Africa, will provide hourly retrievals at roughly 8 km spatial sampling. That is not fine enough to track individual ships, but it will allow researchers to observe how corridor plumes evolve through the day and to separate freshly emitted NO2 from chemically aged background. For the English Channel and the Mediterranean, this will be a material improvement in attribution confidence.
Typical figures
| TROPOMI NO2/SO2 spatial resolution | 3.5 × 5.5 km (post-August 2019 processor); 7 × 3.5 km for earlier data |
| TROPOMI revisit | Daily global coverage; ~14 orbits per day |
| OMI spatial resolution | 13 × 24 km at nadir; degraded by row anomaly from 2009 |
| OMI archive depth | October 2004 to present |
| TROPOMI archive depth | May 2018 to present (Sentinel-5P launched October 2017) |
| Typical corridor detection threshold (NO2) | ~0.5 × 10¹⁵ molecules cm⁻² above marine background in 30-day composites over high-density lanes |
| Spectral bands used | UV (312–405 nm) for SO2 DOAS; visible (405–465 nm) for NO2 DOAS |
| Cloud/aerosol constraint | Retrievals flagged at cloud radiance fraction >0.5; tropical corridors may lose 30–60% of overpasses |
| Sentinel-4 planned resolution (geostationary) | ~8 km spatial sampling, hourly revisit, European/North African coverage |
| Data latency (TROPOMI near-real-time product) | 3 hours after overpass; offline reprocessed product within 5 days |
Analytics Satellize can run
| Corridor NO2 enhancement map | Multi-week TROPOMI tropospheric NO2 column compositing with quality flag filtering and marine background subtraction | GeoTIFF or GIS layer showing mean column enhancement above local background, updated monthly |
| SO2 compliance-context report | Pre/post IMO cap SO2 column trend analysis using TROPOMI and OMI records, with meteorological normalisation via ERA5 | PDF quarterly report with corridor-level SO2 time series and statistical significance assessment |
| Wind-rotation composite for source attribution | Retrievals binned by ERA5 wind direction before averaging, isolating fixed-geometry corridor signal from advected continental outflow | Annotated composite raster with confidence intervals; delivered as GIS layer with accompanying methodology note |
| SO2/NOx ratio fingerprinting | Pixel-level ratio of coincident SO2 and NO2 retrievals to distinguish heavy fuel oil combustion signature from terrestrial sources | Tabular output per corridor segment, flagging anomalous ratio zones for follow-up inspection prioritisation |
| Traffic-weighted column analysis | AIS vessel density grids co-registered with TROPOMI retrievals; column values weighted inversely by vessel density to identify per-vessel emission anomalies at fleet level | Monthly time series per named corridor with traffic-normalised SO2 and NO2 indices |
| Diurnal profile (Sentinel-4, forthcoming) | Hourly geostationary NO2 retrievals stacked to reconstruct daily emission and photolysis cycle over European corridors | Hourly GIS layer stack; daily NO2 budget estimate per corridor segment |
Who does the work
We can get this done for you. Satellize runs its own analyst desk and a strong science team. You do not buy a data feed and work out what it means; our people source the imagery, run the analysis described on this page, and hand you the answer with its confidence limits stated. Discuss this requirement.