Hello!
I am trying to add a gaseous phase mechanism into the CMAQ (version 5.4), and the new reaction is as follows,
<Reaction 1> Phenol + OH = C6H5O + H2O # 7.4E-12;
<Reaction 2> C6H5O + NO2 = C6H5NO3 # 2.12E-18;
The Reaction 2 proceeds with the constant rate of “2.12E-18 cm3·molecule-1·s-1” with the presence of two-molecule water cluster (i.e., consisting of two water molecules bonded together through hydrogen bonds) in the atmosphere. The reaction rate of Reaction 2 should be obtained by multiplying the concentration of C6H5O with the concentration of two-molecule water cluster ((H2O)2) to represent the effect of water cluster on increasing production rate of C6H5NO3.
My question is: are there any measures to incorporate calculation of the concentrations of (H2O)2 in the atmosphere during the simulation process? Now I have a Fortran90 script to calculate the concentrations of (H2O)2 based on the surface temperature and relative humidity of each grid, but i don’t know how to incorporate this script into the CMAQ model and obtain the (H2O)2 concentration for each time step during the simulation. Could you please give me some advice on which file(s) I can modify?
Any advice would be appreciated!
Yi SU
Hi Yi SU,
We recently added a reaction of a CMAQ species with water trimers (H2O)3 for a different application. Our approach was to add H2O as a reactant in the reaction to incorporate some RH dependence. We then assumed the reaction rate constant was constant with temperature based on our review of the temperature-dependence of water trimers versus monomers. This was a simplification though.
If you wanted to incorporate temperature more explicitly, then I would recommend trying one of the available forms of the reaction rate constant with temperature dependence like the following:
- k = A * exp(-Ea/RT) or
- k = A * exp(-Ea/RT) * (T/T0)^a)
and fitting the parameters to achieve the overall temperature and RH dependence you are looking for in the water dimer concentrations from the product of the rate constant and the water monomer: k(T)*[H2O].
A more complicated alternative would be to add water dimer as a species and explicitly add its formation with a new reaction (of two water monomers) as well as its destruction. You would also need the temperature-dependent rate constant for that process.
Good luck!
Ben Murphy
Hi Ben,
Thank you for your prompt response and I’m now more clear about considering the impact of temperature and RH on reaction rate constant explicitly. But I’m still a bit confused about the meaning of “fitting the parameters to achieve the overall temperature and RH dependence in the water dimer concentrations from the product of the rate constant and the water monomer” in your first suggestion. Did you suggest that I should fit the relationship between reaction rate constant with water dimer concentrations, temperature and RH, and then incorporate the fitted coefficients into the model to better account for the impact of dimer concentrations, temperature, and RH on reaction rate constant (instead of regarding water dimer as a reactant, as in your second suggestion). Please correct me if I misunderstood your suggestion.
If I take your second suggestion and add water dimer as a species, could I add the formation of water dimer based on RH and temperature (implemented by a Fortran90 function shared by a previous research) into the model? If possible, which file could I modify to calculate the water dimer formation.
Best wishes.
Yi SU