.. _rst_Biogenic Volatile Organic Compounds (BVOCs):

Biogenic Volatile Organic Compounds (BVOCs)
===============================================

This chapter 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 implemented in CLM4.5 and is based on MEGAN2.1 discussed in
detail in Guenther et al. (2012). This update of MEGAN incorporates four
main 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 CO\ :sub:`2` and
4) emission factors mapped to the specific PFTs of the CLM.

MEGAN2.1 now 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, :math:`F_{i}` (:math:`\mu` \ g C m\ :sup:`-2` ground area h\ :sup:`-1`), for emission of chemical compound
:math:`i`

.. math::
   :label: ZEqnNum964222 

   F_{i} =\gamma _{i} \rho \sum _{j}\varepsilon _{i,j}  \left(wt\right)_{j}

where :math:`\gamma _{i}`  is the emission activity factor accounting
for responses to meteorological and phenological conditions,
:math:`\rho`  is the canopy loss and production factor also known as
escape efficiency (set to 1), and :math:`\varepsilon _{i,\, j}` 
(:math:`\mu` \ g C m\ :sup:`-2` ground area h\ :sup:`-1`) is
the emission factor at standard conditions of light, temperature, and
leaf area for plant functional type *j* with fractional coverage
:math:`\left(wt\right)_{j}`  (Guenther et al. 2012). The emission
activity factor :math:`\gamma _{i}`  depends on plant functional type,
temperature, LAI, leaf age, and soil moisture (Guenther et al. 2012).
For isoprene only, the effect of CO\ :sub:`2` 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:

.. math::
   :label: 28.2) 

   \gamma _{P,\, i} =\left(1-LDF_{i} \right)+\gamma _{P\_ LDF} LDF_{i}

where the LDF activity factor (:math:`\gamma _{P\_ LDF}` ) is specified
as a function of PAR as in previous versions of MEGAN.

The values for each emission factor :math:`\epsilon _{i,\, j}`  are
now available for each of the plant functional types in the CLM and
each chemical compound. This information is distributed through an
external file, allowing for more frequent and easier updates.