Hyperspectral Vertical Remote Sensing Bridges the Satellite-to-Surface Gap: Unraveling Height-Resolved Evolution Mechanisms of Atmospheric Formaldehyde in China.

Formaldehyde (HCHO) is a hazardous atmospheric pollutant; however, spatiotemporal discrepancies between tropospheric column density (CHCHO) and surface concentration (SHCHO) hinder the accurate derivation of surface concentrations from satellite observations. Based on 2.5 years of vertical profile measurements from 18 sites within China's hyperspectral remote sensing network, this study demonstrates that HCHO is predominantly enriched below 0.2 km. Its diurnal variation exhibits a midday-evening bimodal pattern driven by boundary layer dynamics, photochemistry, and anthropogenic emissions. Using Generalized Additive Models (GAMs), we identify the dominant drivers of CHCHO and SHCHO across urban, suburban, and background environments. CHCHO is consistently governed by regional photochemistry (31-52% contribution), whereas SHCHO is primarily controlled by anthropogenic emissions in urban areas (29%) and shifts toward photochemical dominance in suburban (42%) and background (51%) regions. Discrepancies in the response magnitudes of CHCHO and SHCHO to strong emissions and photochemical activity substantially weaken their correlation. By integrating vertical profiles with height-resolved GAMs, we further characterize altitude-dependent mechanisms governing column-to-surface conversion. HCHO below 0.2 km is regulated by local emissions and photochemistry, transitions to photochemical dominance in the middle layer, and becomes increasingly influenced by seasonal variability, large-scale meteorology, and biogenic activity in the upper layer (>1.0 km).