Beyond water solubility: physicochemical controls on VOC dry deposition in a mixed temperate forest

Volatile organic compounds (VOCs) strongly influence tropospheric ozone, secondary organic aerosol formation, and methane’s atmospheric lifetime. Although most VOCs are biogenically emitted, dry deposition to ecosystems represents an important but poorly constrained sink, particularly beyond a few highly water-soluble compounds.
We measured VOC concentrations and fluxes above and below the canopy at the mixed temperate ICOS forest site of Vielsalm (Belgium) over three growing seasons (2022–2024) using a PTR-ToF-MS instrument. Minimum deposition velocities were derived from negative net fluxes and interpreted with a two-layer (canopy–soil) big-leaf resistive model.
Significant deposition was observed for 47 VOC groups spanning a wide range of physicochemical properties. Median deposition velocities ranged from 0.4 cm s⁻¹ (formic acid) to 1.5 cm s⁻¹ (C₈H₈O₂H⁺). Below-canopy uptake accounted for ~10% of total deposition, indicating that canopy processes dominate VOC removal.
The widely used Wesely scheme reproduced deposition velocities only for highly water-soluble compounds (e.g. formaldehyde, formic and acetic acids), for which deposition increased with relative humidity and peaked in autumn. For many other VOCs, the model underestimated deposition by up to three orders of magnitude.
Under dry conditions, deposition velocities were positively correlated with the octanol–air partition coefficient, suggesting uptake into lipid phases such as the leaf cuticle. This complementary aqueous and lipid uptake was corroborated by an independent VOC flux dataset over a winter wheat field in the Paris region. Overall, our results call for deposition schemes that account for the deposition of both hydrophilic and hydrophobic VOCs to ecosystems.