A CASE STUDY OF 2019–2024

This study examined the concentration trends and spatial distribution of tropospheric formaldehyde (HCHO) and ozone (O₃) across Lagos State, Southwestern Nigeria, from 2019 to 2024 using Sentinel-5P data processed on the Google Earth Engine (GEE) platform. These pollutants were selected due to their relevance to urban photochemical processes and respiratory health risks in rapidly developing megacities. Formaldehyde exhibited a marked seasonal cycle, with elevated concentrations during the dry Harmattan season (January–March) and lower values during the rainy months (June–September). High HCHO levels were consistently observed in dense and industrialized local government areas (LGAs) such as Ikeja, Kosofe, Shomolu, Oshodi-Isolo, and Lagos Island. Ozone demonstrated an opposite seasonal pattern, peaking in mid-year months (July–August), particularly across coastal LGAs including Eti-Osa, Ibeju-Lekki, and Epe, due to increased solar radiation and marine airflow dynamics favourable for photochemical ozone formation. Paired sample t-test results revealed no statistically significant differences in formaldehyde concentrations across all consecutive years (p > 0.05), indicating emission stability during the study period. Conversely, ozone concentrations showed a highly significant increase between 2021 and 2022 (p < 0.01), suggesting enhanced photochemical activity or intensified precursor emissions during that period. Overall findings highlight the role of anthropogenic emissions, seasonal meteorology, and policy interventions in shaping Lagos air quality. The study recommends strengthened emission controls, improved transportation systems, and continuous satellite-based air-quality monitoring to mitigate pollution impact.