Grid-based screening of fine particulate matter and water-optical conditions in Southern Benin: the Cotonou, Lake Nokoué, and Porto-Novo Corridor.

Air and surface-water conditions are commonly monitored separately in West African coastal cities, leaving limited evidence on where environmental pressures overlap. This study developed a 1-km spatial screening framework for the Cotonou, Lake Nokoué, and Porto-Novo corridor in southern Benin. Annual fine particulate matter (PM2.5) concentrations were analyzed for 2001-2022, and a temporally aligned air-water assessment was conducted for 2018-2022. Sentinel-2 water-optical indicators were evaluated against field turbidity and chlorophyll-a measurements from 19 monitoring stations. Unlike a composite vulnerability index, the framework separated environmental classification from population, built-up land, and hydrological context. Across 2589 grid cells, mean PM2.5 was 32.17 µg/m3 for 2001-2022 and 32.70 µg/m3 for 2018-2022. No corridor-wide linear trend was statistically supported, and no cell remained significant after false-discovery-rate correction. Water analysis was restricted to 205 cells meeting the primary mapped-water and valid-pixel criteria. The Normalized Difference Turbidity Index was positively associated with turbidity (Spearman ρ = 0.488; station-cluster 95% confidence interval, 0.369-0.590), while the red-edge or red ratio was associated with chlorophyll-a (ρ = 0.432; 0.295-0.544). Under the primary 80th-percentile criterion, six cells showed co-occurring elevated PM2.5 and water-optical signals: four in Adjara and two in Porto-Novo. Sensitivity analysis showed that the count varied from 16 cells at the 75th percentile to none at the 90th percentile. The framework identifies relative monitoring priorities rather than confirmed pollution or complete environmental vulnerability.