Modelling bidirectional fluxes of methanol and acetaldehyde with the FORCAsT canopy exchange model
- 1Climate and Space Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA
- 2Department of Civil and Environmental Engineering, Washington State University, Pullman, WA 99164, USA
- 3Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA
- 4School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA
- anow at: Royal Society Dorothy Hodgkin Research Fellow at Lancaster Environment Centre, Lancaster University, LA1 4YQ, UK
- bformerly at: of Department of Chemistry, Amherst College, Amherst, MA 01002, USA
- cnow at: Peking University, Beijing 100000, China
Abstract. The FORCAsT canopy exchange model was used to investigate the underlying mechanisms governing foliage emissions of methanol and acetaldehyde, two short chain oxygenated volatile organic compounds ubiquitous in the troposphere and known to have strong biogenic sources, at a northern mid-latitude forest site. The explicit representation of the vegetation canopy within the model allowed us to test the hypothesis that stomatal conductance regulates emissions of these compounds to an extent that its influence is observable at the ecosystem scale, a process not currently considered in regional- or global-scale atmospheric chemistry models.
We found that FORCAsT could only reproduce the magnitude and diurnal profiles of methanol and acetaldehyde fluxes measured at the top of the forest canopy at Harvard Forest if light-dependent emissions were introduced to the model. With the inclusion of such emissions, FORCAsT was able to successfully simulate the observed bidirectional exchange of methanol and acetaldehyde. Although we found evidence that stomatal conductance influences methanol fluxes and concentrations at scales beyond the leaf level, particularly at dawn and dusk, we were able to adequately capture ecosystem exchange without the addition of stomatal control to the standard parameterisations of foliage emissions, suggesting that ecosystem fluxes can be well enough represented by the emissions models currently used.