Unveiling the spatial and temporal effects of urban vegetation coverage on atmospheric formaldehyde pollution in Chinese megacities over recent decades.

Urban vegetation coverage is widely promoted to improve livability and reduce pollutant exposure, yet its air-quality effects remain debated. Formaldehyde (HCHO), formed largely via oxidation of anthropogenic and biogenic VOCs, poses significant health risks and is a precursor of O3 and PM2.5. Here we examined how urban vegetation coverage was associated with HCHO across urbanization stages in Chinese megacities during 2005-2017. We integrated monthly OMI/Aura HCHO columns, MODIS-based Normalized Difference Vegetation Index (NDVI), and Landsat-derived landscape metrics, harmonized on a common 0.1 grid. Cities were grouped into five urbanization classes via k-means on z-scored indicators (GDP per capita, population density, built-up ratio, and nightlights), with k = 5 supported by elbow, silhouette, and Calinski-Harabasz criteria. Associations between NDVI and HCHO were assessed using SRCC and spatial heterogeneity was evaluated with Geographically Weighted Regression (GWR). Across stages, the NDVI-HCHO association shifted from weakly negative in Class V (SRCC = -0.09, p < 0.001) to strongly positive in Class III (SRCC = 0.81, p < 0.001), then weakened in Classes II and I (0.63 and 0.72). Regionally, BTH and YRD showed concurrent increases in NDVI (3.29 % and 5.94 %) and HCHO (22.53 % and 8.7 %), whereas CY experienced a 10.18 % NDVI rise but a 9.74 % HCHO decline. GWR highlighted LPI as a robust structural predictor of HCHO variability across stages. These patterns are consistent with context-dependent interactions among BVOC potential, anthropogenic VOC/NOx, and meteorology. Our results caution against one-size-fits-all greening: the air-quality outcome of urban vegetation coverage depends on urbanization stage and landscape configuration. Differentiated planning and controlling anthropogenic precursors in more urbanized settings and optimizing vegetation type/patch structure around the Class III "peak association" can better align greening with HCHO mitigation.