Humidity-dependent dry deposition of methacrolein to plant species

Volatile organic compounds (VOCs) together with nitrogen oxides contribute to the formation of ground-level ozone as well as PM_2.5 pollution through secondary organic aerosol formation, with adverse effects on human health and environment. Researchers have mainly focused on quantifying VOC emissions from plant canopies and their controls, leading to improvements in atmospheric chemistry models (Jimenez et al., 2009). However, much less attention has been spent on quantifying dry deposition of primary and secondary VOCs to surfaces, with most models often using deposition rates extrapolated from SO₂ (as a proxy of a water-soluble gas of limited reactivity) and O₃ (as a proxy of an insoluble reactive gas), making uncertain assumptions on the relative behaviour of key VOCs (Wesely, 2007). To address this, we conducted the first systematic, measurement-based investigation into VOC dry deposition as part of the ‘Dry Deposition Processes of VOCs’ project funded by Natural Environment Research Council. The overarching aim of the study was to reduce uncertainty in atmospheric chemistry models by developing parameterisations for the dry deposition of VOCs. The preliminary results of our laboratory study on plant fumigation with methacrolein (MACR), among other selected VOCs, are presented here.

An automated dynamic gas-exchange chamber system was developed to expose test plants to specific VOCs at various concentrations under controlled conditions. Overall, six plant species (see below) were tested with each experiment lasting four days: one day to observe background emissions and three days with VOC fumigation at 20, 15 and 10 °C. Three levels of relative humidity (RH) were applied during day and night times, being fumigated with five concentrations of VOCs within each RH level. In total, eleven VOCs were selected for fumigation: water-insoluble (isoprene, benzene, toluene, xylene, a-pinene) and water-soluble (methanol, acetonitrile, acetaldehyde, acetone, acetic acid and MACR). VOCs were measured using a proton transfer reaction instrument equipped with time-of-flight mass spectrometer (PTR-Qi-TOF). Fluxes were calculated based on concentration difference between blank and measurement chambers and then normalized by the corresponding plant leaf area indices.

MACR appears to be ‘valuable’ VOC to study dry deposition as it is not typically emitted by plants but is an important first-order product of isoprene oxidation in the atmosphere. Nevertheless, minor MACR emissions have been reported, suggesting that oxidation may also take place within leaves (Fares et al., 2015).   

The deposition velocity of MACR was found to increase with RH, and larger deposition velocities were consistently observed during the daytime compared to the night. This diurnal dependence indicates either stomatal control or photochemical processes, or a combination of the two, were present under daylight conditions. However, this varied substantially across tested plants being ranked in the following order Pinus sylvestris > Hedera sp. > Picea glauca > Betula sp. > Tsuga heterophylla > Ilex aquifolium. At all times, MACR compensation points were found to be negligible (near zero) or even negative, suggesting minor or no impact on deposition rates.

These findings are enhancing our understanding of VOC deposition and will inform the development of new parameterizations for atmospheric chemistry models.