Interannual Recurrence of Satellite-Observed Formaldehyde Transitions and Seasonally Structured, Coordinated Multipollutant Shifts at a Tropical Urban Forest–city Interface in West Africa (2024–2026)

Forest–city interfaces combine biogenic, urban and regional combustion influences under strong seasonality. We test whether recurrent 2024–2026 tropospheric HCHO transitions at the Banco forest–metropolis interface in Abidjan are accompanied by coordinated multipollutant shifts, and whether such shifts can be separated from the seasonal cycle. The change points fall on 9 March 2024, 13 April 2025 and 13 April 2026, all within the long rainy season. The primary PCA uses the HCHO–NO₂–tropospheric-O₃ block on 441 complete-case dates; a five-species PCA adding CO and SO₂ is complementary because their extraction chain cannot be fully certified. Primary PC1 explains 54.1 % of variance, with loadings +0.861 (HCHO), +0.821 (NO₂) and −0.456 (O₃), and its raw transition contrasts are −1.416, −1.737 and −1.368 in 2024–2026. After categorical seasonal adjustment, without refitting the PCA, these contrasts become +0.026, −0.409 and −0.188; only the 2025 residual decrease remains robust across block lengths. Under a pre-specified harmonic adjustment, no primary residual contrast is robust in any year on either the 441-date or the 487-date support, while raw contrasts remain robust. HCHO–NO₂ covariance remains robust across block lengths after categorical seasonal adjustment (ρ = 0.267; within-year block bootstrap). The evidence supports recurrent, strongly seasonally structured coordinated multipollutant shifts. Categorical and harmonic seasonal representations diverge, consistent with transitions recurring at similar calendar dates; a season-independent component therefore cannot be robustly isolated from three years of data, which does not establish that none exists. No source attribution is made, and the primary inference does not rely on CO/SO₂.