Modelling bidirectional fluxes of methanol and acetaldehyde with the FORCAsT canopy exchange model

Kirsti Ashworth,Karena A Mckinney,J William Munger,Scot T Martin,Allison L Steiner,Serena H Chung,Ying Liu

doi:10.5194/acp-16-15461-2016

Abstract

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.

Highlights

The exchange of many oxygenated volatile organic compounds from forest canopies has recently been observed to be bidirectional, with periods of strongly positive and negative fluxes (Park et al, 2013; Karl et al, 2005; McKinney et al, 2011)
While most incorporate online calculations of biogenic emissions of isoprene and monoterpenes, based on the light and temperature-dependence algorithms developed by Guenther et al (1995, 2006, 2012), methanol emissions have only been recently included in some chemistry and transport models (CTMs) (e.g. GEOS-Chem, Millet et al, 2010; Laboratoire de Météorologie Dynamique zoom (LMDz), Folberth et al, 2006), and most still rely on nondynamic emissions inventories for methanol and acetaldehyde if primary biogenic emissions of these species are included (e.g. UKCA; O’Connor et al, 2014)
When light-dependent emissions of methanol and acetaldehyde were included, the FORCAsT canopy–atmosphere exchange model successfully simulated the bidirectional exchange of methanol and acetaldehyde at Harvard Forest, a northern mid-latitude mixed deciduous woodland

Summary

Introduction

The exchange of many oxygenated volatile organic compounds (oVOCs) from forest canopies has recently been observed to be bidirectional, with periods of strongly positive (i.e. up out of the canopy to the atmosphere above) and negative (i.e. downward) fluxes (Park et al, 2013; Karl et al, 2005; McKinney et al, 2011) Several of these compounds, e.g. acetone, acetaldehyde, and methanol, are present in the atmosphere in large quantities (Singh et al, 1995; Heikes et al, 2002; Millet et al, 2010; Jacob et al, 2002). Dry deposition schemes in CTMs are usually based on fixed deposition velocities (Wohlfahrt et al, 2015) or calculated from roughness lengths and leaf area index values assigned to generic land cover types (e.g. FRSGC-UCI, Wild et al, 2014; LMDz, Folberth et al, 2006) This simplistic approach to biogenic sources and sinks may be a critical omission limiting their capability of accurately simulating atmospheric composition in many world regions

Methods

Results

Conclusion