# Add a new reaction related to dry deposition **URL:** \ **Category:** CMAQ\ **Created:** [October 14, 2024, 9:18am UTC](https://forum.cmascenter.org/t/add-a-new-reaction-related-to-dry-deposition/5284 "2024-10-14T09:18:07Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Fortuna](https://avatars.discourse-cdn.com/v4/letter/f/ea666f/32.png) [@Fortuna](https://forum.cmascenter.org/u/Fortuna)\ **Post date:** [October 14, 2024, 9:18am UTC](https://forum.cmascenter.org/t/add-a-new-reaction-related-to-dry-deposition/5284/1 "2024-10-14T09:18:07Z")
Hi all, I’m using CMAQv5.3.2 with m3dry now. And I’d like to add a new reaction which reactant is dry deposited NO3. My plan is to take the output dry deposition NO3 and HNO3 as a global variable and then go back to the CHEMMECH tool to add the new reaction. But I have some problems: 1. What’s the relationship between variable `VDEP_AE` in `aero_depv.F` and `pvd` in `m3dry.F`? Does `pvd` or `VDEP_AE` represent the dry deposition rate for all species of aerosol? 2. Which module calculates dry deposition value (kg/ha) for gas and aerosol species? 3. I notice that the unit of gas depo velocity `depvel_gas` is cm/s, aerosol depo velocity `pvd` is ppm\*m/s, `VDEP_AE` is m/s, and the output file `CCTM_DRYDEP` is kg/ha/hr. On the other hand, reaction rate `RKI` in `RXNS_FUNC_MODULE.F90` is ppm/min. If I wanted to add a new species `dNO3` to `AE.nml` and `mech.def` to represent dry deposition NO3, what units do I ultimately need to ensure `dNO3` is in for adding new reaction? 4. If I calculate the variable `dNO3` in the corresponding module, is it directly recognized by `AE.nml` and `mech.def`? Or I need to set it as a global variable? If you have some suggestions on this, please let me know. Thanks for your kind help. Liu ---
**Author:** ![cgnolte](https://yyz1.discourse-cdn.com/flex027/user_avatar/forum.cmascenter.org/cgnolte/32/53_2.png) [@cgnolte](https://forum.cmascenter.org/u/cgnolte)\ **Post date:** [October 15, 2024, 12:07pm UTC](https://forum.cmascenter.org/t/add-a-new-reaction-related-to-dry-deposition/5284/2 "2024-10-15T12:07:32Z")
I do not understand the process you are modeling. Does the deposited NO3 that does not react accumulate on the surface, or is it only the NO3 that is being deposited in a given time step that has an opportunity to react chemically? What will you do for layers above layer 1? Will you set this dry deposited NO3 to 0? ---
**Author:** ![Fortuna](https://avatars.discourse-cdn.com/v4/letter/f/ea666f/32.png) [@Fortuna](https://forum.cmascenter.org/u/Fortuna)\ **Post date:** [October 16, 2024, 3:33am UTC](https://forum.cmascenter.org/t/add-a-new-reaction-related-to-dry-deposition/5284/3 "2024-10-16T03:33:38Z")
Hi Chris, I refer to the description from Sarwar et al. (2008, Atmospheric Environment): ![image](https://canada1.discourse-cdn.com/flex027/uploads/cmas/original/2X/f/f07ef93cb1138da76acdd40ed4768155991bea47.png) My assumption is that the part that deposits to the surface and accumulates will participate in the reaction before precipitation, and the dry settlement above the layer 1 will not be modified. Given that I do not care about the final dry deposition value of NO3, can I simply add a chemical equation with only products but no reactants to the CHEMMECH tool? The reaction rate is the HNO3 photolysis rate calculated online in the model multiplied by a known coefficient. ---
**Author:** ![cgnolte](https://yyz1.discourse-cdn.com/flex027/user_avatar/forum.cmascenter.org/cgnolte/32/53_2.png) [@cgnolte](https://forum.cmascenter.org/u/cgnolte)\ **Post date:** [October 16, 2024, 1:45pm UTC](https://forum.cmascenter.org/t/add-a-new-reaction-related-to-dry-deposition/5284/4 "2024-10-16T13:45:38Z")
Thanks for the clarifications. This is a complex set of questions, and several staff are at conferences this week and next. It may take a while for a response. As an aside, if you’re starting a new coding project I would recommend updating to CMAQv5.5. There were significant updates to the aerosol dry deposition routines in CMAQv5.4, affecting both M3DRY and STAGE deposition options. ---
**Author:** ![Fortuna](https://avatars.discourse-cdn.com/v4/letter/f/ea666f/32.png) [@Fortuna](https://forum.cmascenter.org/u/Fortuna)\ **Post date:** [October 17, 2024, 2:56am UTC](https://forum.cmascenter.org/t/add-a-new-reaction-related-to-dry-deposition/5284/5 "2024-10-17T02:56:10Z")
Thanks for your suggestion. I will give a try on CMAQv5.5. In addition, I hope that CMAQv5.5 can do WRF-CMAQ coupling. ---
**Author:** ![gsarwar](https://avatars.discourse-cdn.com/v4/letter/g/5f9b8f/32.png) [@gsarwar](https://forum.cmascenter.org/u/gsarwar)\ **Post date:** [October 24, 2024, 12:45pm UTC](https://forum.cmascenter.org/t/add-a-new-reaction-related-to-dry-deposition/5284/6 "2024-10-24T12:45:51Z")
Hi Liu, It is possible to add such a reaction. If you do not want to change the deposited HNO3, then the implementation is somewhat easier. The reaction has, however, high uncertainty. One issue I encountered is the duration for which you want to account for the photolysis of deposited HNO3. If you want to use all HNO3 deposited since the last precipitation, then the amount deposited can be large in areas with low precipitation. I have seen a recent article where authors used one hour which is probably too short of a duration for any impact on model results. The choice of the duration will make an impact on the results. I am assuming your NOx levels are too low and want to examine such a pathway. In short, it is possible to include such a pathway but it has high uncertainty. Please contact me after running CMAQv5.5, I can help you in implementing such a reaction. Thanks, Sarwar