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:

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

where \([\mbox{X}]\) is the gas-phase concentration of species X [ppm] and the rate constant \(k_{surface}\) [1/s] is calculated as:

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

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

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

where R is the ideal gas constant [ \(\mbox{J}\, \mbox{K}^{-1}\, \mbox{mol}^{-1}\)], T is temperature [K], and MW is the molecular weight of the gas-phase reactant [ \(\mbox{kg}\, \mbox{mol}^{-1}\)]

Input data for surface reactions have the following format :

{
  "type" : "SURFACE",
  "gas-phase reactant" : "my gas species",
  "reaction probability" : 0.2,
  "gas-phase products" : {
     "my other gas species" : { },
     "another gas species" : { "yield" : 0.3 }
  },
  "areosol phase" : "my aqueous phase"
}

The key-value pairs gas-phase reactant, reaction probability, and aerosol-phase are required. Only one gas-phase reactant is allowed, but multiple products can be present. The key-value pair yield for product species is optional and defaults to 1.0.

The gas-phase reactant species must include properties diffusion coeff [m2 s-1], which specifies the diffusion coefficient in \(\mbox{m}^2\,\mbox{s}^{-1}\), and molecular weight [kg mol-1], which specifies the molecular weight of the species in \(\mbox{kg}\,\mbox{mol}^{-1}\).