Humidity and light control methacrolein dry deposition to vegetation: measurements and model implications

Introduction Volatile organic compounds (VOC), especially isoprene oxidation products such as methacrolein (MACR), contribute significantly to tropospheric ozone and secondary organic aerosol (SOA) formation. While VOC emissions are well studied, their dry deposition to vegetation remains poorly understood and is often misrepresented in atmospheric models, owing to the scarcity of dry deposition data. Methods We conducted controlled twin-chamber experiments to quantify the dry deposition velocity (Vd) of MACR to coniferous (Scots pine, white spruce), deciduous (birch) and evergreen (English ivy) species, each in three biological replicates, under varying temperature, relative humidity (RH) and light, with concentrations measured by PTR-ToF-MS. The resulting vegetation-specific, RH- and light-dependent parameterisations were implemented in two independent chemistry-transport models, GEOS-Chem and EMEP, within the Wesely (1989) resistance-in-series scheme. Results MACR dry deposition was significantly higher under light, with Vd showing strong RH dependence during illuminated periods and much lower, RH-independent values in the dark. The response varied by species: conifers displayed higher deposition rates and greater RH sensitivity than the deciduous broadleaf birch. Compensation points remained near zero, consistent with active uptake involving stomatal control and/or metabolic processing of MACR within the leaves. Deposition to water was also non-negligible and stable across RH, challenging model assumptions that assign zero deposition to water bodies. In the models, the new scheme altered simulated MACR Vd by up to ~30% in GEOS-Chem and ~200% in EMEP, changing SOA by only 0.04% in GEOS-Chem and up to 12% in EMEP across the boreal and temperate forest biomes. Discussion MACR serves as a tractable test case: its moderate solubility and clear stomatal signal suit it to controlled-chamber quantification, whereas extending this approach to other biogenic VOC oxidation products will require overcoming bidirectional exchange, lower solubility and measurement interferences. Accurately representing VOC dry deposition is essential to better predict SOA formation, regional air quality and their policy implications. By resolving how stomatal and non-stomatal pathways jointly govern the uptake of a reactive gas across contrasting plant functional types, this work provides a mechanistic basis for scaling leaf-level gas exchange to canopy- and ecosystem-scale deposition, linking plant physiological control of surface conductance to atmospheric composition.