Improved parameterization for dry deposition of methacrolein, an oxidation product of isoprene

Volatile organic compounds (VOCs) and nitrogen oxides are significant contributors to ground-level ozone and PM_2.5 pollution via secondary organic aerosols (SOA), with detrimental effects on human health and the environment. While previous research has predominantly focused on quantifying VOC emissions from plant canopies and their drivers, less attention has been given to measuring the dry deposition of primary and secondary VOCs onto surfaces. As a result, dry deposition processes are associated with large uncertainties in atmospheric models. To address this gap, we conducted a measurement-based investigation into VOC dry deposition under the ‘Dry Deposition Processes of VOCs’ project funded by the Natural Environment Research Council. Our primary aim was to refine atmospheric chemistry models by improving parameterisations for the VOC dry deposition. Here, we present the findings of our laboratory study on plant exposure to methacrolein (MACR), among other selected VOCs.

An automated dynamic gas-exchange chamber system was developed to fumigate test plants with a range of VOCs concentrations. Five plant species were subjected to each experiment spanning four days: one day to observe background emissions and three days of VOC exposure at 20, 15 and 10 °C. Three levels of relative humidity (RH) were maintained over light and dark conditions, being fumigated with four concentrations of VOCs within each RH level. In total, eleven VOCs were chosen for exposure: water-insoluble (isoprene, benzene, toluene, xylene, a-pinene) and water-soluble (methanol, acetonitrile, acetaldehyde, acetone, acetic acid and MACR). VOCs were monitored using a proton transfer reaction instrument equipped with time-of-flight mass spectrometer (PTR-Qi-TOF). Fluxes were computed based on the concentration difference between reference and measurement chambers, subsequently normalized by the corresponding plant leaf area indices.

MACR, together with methyl-vinyl ketone, is the main product of isoprene oxidation (under NOx conditions) and ozonolysis, playing an important role in air pollution through SOA formation. The observed MACR deposition rate was larger at higher fumigating concentrations. We revealed that MACR deposition velocity (Vd) increased with increasing RH under light conditions; no similar pattern was observed for dark periods with much lower Vd magnitudes. This humidity-dependent Vd variation indicates a strong stomatal control (see Morumoto et al., 2015) and a possible contribution of on-leaf heterogenic chemistry under daylight conditions. At all times, MACR compensation points were found to be negligible, suggesting minor or no impact on deposition rates.

These findings will benefit atmospheric chemistry models, providing an improved parameterization for MACR dry deposition to typical coniferous, deciduous, and evergreen temperate climate plant species.