Surface (Heterogeneous) Reaction
================================

Surface reactions transform gas-phase species into gas-phase products according to a rate that is calculated based on the total exposed surface area of a condensed phase.

For surface reactions of the gas-phase species X, the reaction rate is calculated assuming large particles (continuum regime) as:

.. math::

   r_{surface} = k_{surface}[X]

where ``[X]`` is the gas-phase concentration of species ``X`` :math:`[\mathrm{mol}\,\mathrm{m}^{-3}]`, and the rate constant :math:`(k_{\mathrm{surface}}) [\mathrm{s}^{-1}]` is calculated as:

.. math::

   k_{surface} = \frac{4N_a \pi r_e^2}{\left(\frac{r_e}{D_g} + \frac{4}{v(T)\gamma}\right)}

where ``N_a`` is the number concentration of particles :math:`[\mathrm{particles}\,\mathrm{m}^{-3}]`, :math:`r_e` is the effective particle radius :math:`[\mathrm{m}]`, 
``D_g`` is the gas-phase diffusion coefficient of the reactant :math:`[\mathrm{m}^2\mathrm{s}^{-1}]`, ``gamma`` is the reaction probability [unitless], and ``v`` is the mean free speed of the gas-phase reactant:

.. math::

   v = \sqrt{\frac{8RT}{\pi MW}}

where ``R`` is the ideal gas constant :math:`[\mathrm{J} \mathrm{K}^{-1} \mathrm{mol}^{-1}]`, ``T`` is temperature :math:`[\mathrm{K}]`, and ``MW`` is the molecular weight of the gas-phase reactant :math:`[\mathrm{kg}\ \mathrm{mol}^{-1}]`.

A conforming implementation of version 1.0 of the configuration will provide ways to specify the particle effective radius and number concentration.

Input data for surface reactions have the following format:

.. tab-set::

   .. tab-item:: YAML

      .. code-block:: yaml

         type : SURFACE
         name : foo-surface
         gas phase : gas
         gas-phase species: foo
         gas-phase products :
            - species name: bar
              coefficient: 0.5
            - species name: baz
              coefficient: 0.3
         reaction probability : 0.2


   .. tab-item:: JSON

      .. code-block:: json

         {
            "type": "SURFACE",
            "name": "foo-surface",
            "gas phase": "gas",
            "gas-phase species": "foo",
            "gas-phase products": [
               {
                  "species name": "bar",
                  "coefficient": 0.5
               },
               {
                  "species name": "baz",
                  "coefficient": 0.3
               }
            ],
            "reaction probability": 0.2
         }

The ``gas-phase species`` is required and should be name of the species colliding with the surface.

The ``gas-phase products`` is a list of gas-phase species that are produced by the surface reaction.
Each product must have a ``species name``. The the ``coefficient`` is not specified, it defaults to 1.0.

The ``gas phase`` key is required and must be set to the name of the phase the reaction takes place
in. The gas-phase species and each gas-phase product must be present in the specified phase.

The gas-phase reactant species must include the property ``molecular weight [kg mol-1]``,
which specifies the molecular weight of the species in :math:`\mathrm{kg}\ \mathrm{mol}^{-1}`.

The phase configuration must include the property ``diffusion coeff [m^2 s-1]`` for the
gas-phase species, which specifies the diffusion coefficient for the reacting species in
that phase in :math:`\mathrm{m}^2\ \mathrm{s}^{-1}`.

Rate constants are in units of :math:`\mathrm{s}^{-1}`.