Seasonal Structure Outweighs Daily Meteorological Associations in Tropospheric Formaldehyde over Abidjan, West Africa

Abstract

Tropospheric formaldehyde (HCHO) is a short-lived oxidation product of methane and non-methane volatile organic compounds and is widely used as a satellite indicator of VOC oxidation. In single-year satellite records, HCHO and meteorological variables may covary because both follow the annual cycle, potentially confounding interpretation. We analysed 205 Sentinel-5P/TROPOMI HCHO observations over Abidjan, Côte d’Ivoire, in 2024 and matched them to daily maximum temperature, relative humidity, precipitation, and wind speed from ERA5, MERRA-2, CHIRPS, and NASA/POWER. Mean HCHO was 1.783 × 10⁻⁴ mol m⁻² (95% CI: 1.641–1.925 × 10⁻⁴), with a coefficient of variation of 57.8%. HCHO differed strongly among climatic seasons (Kruskal–Wallis H = 61.27, p = 3.14 × 10⁻¹³, ε² = 0.290) and months (H = 82.60, p = 4.63 × 10⁻¹³, ε² = 0.371). In a meteorology-only model, ERA5 maximum temperature remained significant and the model explained 16.1% of daily variance. After adjustment for season, however, the temperature coefficient decreased to 0.026 × 10⁻⁴ mol m⁻² °C⁻¹ (p = 0.595), while meteorology increased R² by only 0.013 beyond the season-only model. Adjustment for month yielded the same interpretation. These results show that shared temporal regime structure can dominate apparently significant daily meteorological associations. Over Abidjan, the 2024 TROPOMI HCHO record is therefore best interpreted as a seasonally structured indicator of VOC oxidation rather than as a direct isoprene or source-specific proxy.