Spatiotemporal Coastal Air-Quality Contrasts under Transboundary Influence: Sentinel-5P CO and NO₂ over Prince Edward Island (Canada) and Massachusetts (U.S.)

Coastal and island airsheds experience compound pressures from local emissions and long-range transport, yet multi-year comparisons between lower-emission island receptors and densely urbanized coastal jurisdictions remain limited. Using Prince Edward Island (PE), Canada, and Massachusetts (MA), U.S., a lower-emission island and a densely urbanized coastal state, we analyse Sentinel-5P TROPOMI observations from 2019 to 2024 to compare column behaviour of CO and NO₂, pollutants with contrasting atmospheric lifetimes. The results show that (1) CO exhibits closely synchronized jurisdiction-scale behaviour consistent with a strong regional-background influence, including elevations in 2023, with closely similar province/state means and overlapping 95% confidence intervals; (2) NO₂ exhibits opposing seasonal regimes, with PE summer-elevated (mean summer ≈ 7.5 × 10⁻⁵ mol m⁻²) and MA winter-elevated (≈4.2 × 10⁻⁵ mol m⁻²), producing a counterintuitive PE > MA annual mean despite higher localized urban peaks in MA; (3) spatial maps show compact corridor-associated NO₂ hotspots in MA versus a comparatively more spatially diffuse NO₂ pattern in PE; and (4) exploratory concordance with independent external evidence shows construct-dependent correspondence across pollutants and jurisdictions. The six-year temporal and spatial patterns support a screening-versus-compliance framework in which satellite columns can help prioritize season-specific ground monitoring, identify candidate corridors for follow-up, and support smoke-event screening, while exposure assessment and regulatory compliance remain based on validated surface observations or appropriately validated fused products. The findings therefore support summer NO₂ monitoring priorities in PE and winter corridor monitoring in MA and provide a transferable screening framework for other coastal and island jurisdictions.