2.30. Biogenic Volatile Organic Compounds (BVOCs)
This section briefly describes the biogenic volatile organic compound (BVOC) emissions model implemented in CLM. The CLM3 version (Levis et al. 2003; Oleson et al. 2004) was based on Guenther et al. (1995). Heald et al. (2008) updated this scheme in CLM4 based on Guenther et al (2006). The current version was first implemented in CLM4.5 and is currently based on MEGAN2.1 discussed in detail in Guenther et al. (2012). As of CLM5, CLM-MEGAN has included these features: 1) expansion to 147 chemical compounds, 2) the treatment of the light-dependent fraction (LDF) for each compound, 3) inclusion of the inhibition of isoprene emission by atmospheric CO2, 4) emission factors mapped to the specific PFTs of the CLM. As of CLM6, CLM-MEGAN includes two new features: 5) the impact of drought, and 6) high-latitude specific isoprene emissions.
MEGAN2.1 describes the emissions of speciated monoterpenes, sesquiterpenes, oxygenated VOCs as well as isoprene. A flexible scheme has been implemented in the CLM to specify a subset of emissions. This allows for additional flexibility in grouping chemical compounds to form the lumped species frequently used in atmospheric chemistry. The mapping or grouping is therefore defined through a namelist parameter in drv_flds_in, e.g. megan_specifier = 'ISOP = isoprene', 'BIGALK pentane + hexane + heptane + tricyclene'.
Terrestrial BVOC emissions from plants to the atmosphere are expressed as a flux, \(F_{i}\) (\(\mu\) g C m-2 ground area h-1), for emission of chemical compound \(i\)
where \(\gamma _{i}\) is the emission activity factor accounting for responses to meteorological and phenological conditions, \(\rho\) is the canopy loss and production factor also known as escape efficiency (set to 1), and \(\varepsilon _{i,\, j}\) (\(\mu\) g C m-2 ground area h-1) is the emission factor at standard conditions of light, temperature, and leaf area for plant functional type j with fractional coverage \(\left(wt\right)_{j}\) (Guenther et al. 2012). The emission activity factor \(\gamma _{i}\) depends on plant functional type, temperature, LAI, leaf age, and soil moisture (Guenther et al. 2012) For isoprene only, the effect of CO2 inhibition is now included as described by Heald et al. (2009). Previously, only isoprene was treated as a light-dependent emission. In MEGAN2.1, each chemical compound is assigned a LDF (ranging from 1.0 for isoprene to 0.2 for some monoterpenes, VOCs and acetone). The activity factor for the light response of emissions is therefore estimated as:
where the LDF activity factor (\(\gamma _{P\_ LDF}\) ) is specified as a function of PAR as in previous versions of MEGAN.
The values for each emission factor \(\epsilon _{i,\, j}\) are now available for each of the plant functional types in the CLM and each chemical compound. This information is provided in an external file, allowing for more frequent and easier updates.
The impact of drought on isoprene emissions is based on the theory proposed by Potosnak et al. (2014). Specifically, isoprene emissions are expected to increase under mild to moderate drought because drought raises leaf temperature, which stimulates isoprene emissions. Under severe drought, however, isoprene emissions are inhibited because substrate supply becomes constrained. Because the effect of leaf temperature is already represented by the leaf temperature activity factor \(\gamma _{T}\) and its influence on isoprene emissions, only the inhibitory effect of severe drought (substrate supply impact, \(\gamma _{sub}\) ) is parameterized as:
where \(a_1=-7.4463\) and \(b_1=3.2552\) are empirical parameters (described in Wang et al., 2022).
Compared with Guenther et al. (2012), updates have been made to represent isoprene emissions from high-latitude plants, specifically boreal broadleaf deciduous shrubs (BBDS) and C3 Arctic grass (C3AG), in order to account for acclimation processes. These updates are based on leaf-enclosure and in situ measurements conducted at Toolik Field Station in Alaska, USA (Wang et al., 2024a, 2024b). For BBDS, the isoprene emission factor is adjusted according to the mean temperature of the previous day as:
where \(T_{24}\) denotes the mean air temperature of the preceding day (Wang et al., 2024a). For C3AG, the isoprene emission factor responds over a longer timescale of 10 days (Wang et al., 2024b) and is parameterized as a function of the mean air temperature over the preceding 10 days (\(T_{240}\)):
In addition, a dynamic temperature response curve for C3AG depends on recent temperature history as:
where \(T_{leaf}\) denotes the leaf temperature, \(R\) is the gas constant (ct3 in code, 0.00831 kJ/mol) and \(C_{g}\) is the parameter controlling the isoprene temperature response of C3AG and changes varies with \(T_{240}\) as: