[QUESTION] CH4 simulation

[atmrszh]

Hi I am a new user of GEOS-Chem, and I have two questions: (My version of GEOS-Chem is 13.1.2)

1. Why only the emission inventory of methane is used in the “CH4 simulation”, and the emission inventory of other species is not used? For example, the NO2 emission inventory. I think the concentrations of other species also affect methane's concentrations, so I have doubts about that.

2. Whether or not the downloaddata.py script can only download specific sample GEOS-Chem restart files. For example, in the CH4 simulation, the initial time is set to February 1, 2019, and the download result is "initial" GEOSChem rst.2x25 CH4.nc”. This file actually corresponds to the initial conditions on January 1, 2019, so should I manually download the initial data of February 2019 from the 10-year full-chemistry benchmark for 13.0.0.

Thanks for your reply！

[msulprizio]

Why only the emission inventory of methane is used in the “CH4 simulation”, and the emission inventory of other species is not used?

The CH4 simulation is a simplified simulation for the CH4 species only, so only advection, chemistry, and emissions of only that species are simulated. For other species that impact CH4 chemistry, we read in fields from file (e.g. OH and Cl). The current form of the simulation is described by Maasakkers et al. [2019] (as indicated in the GEOS-Chem narrative).

• Maasakkers, J.D., D.J. Jacob, M.P. Sulprizio, T. Scarpelli, H. Nesser, J.-X. Sheng, Y. Zhang, M. Hersher, A.A. Bloom, K.W. Bowman, J.R. Worden, G. Janssens-Maenhout, and R.J. Parker, Global distribution of methane emissions, emission trends, and OH concentrations and trends inferred from an inversion of GOSAT satellite data for 2010-2015, Atmos. Chem. Phys., 19, 7859-7881, 2019. [LINK]

Whether or not the download_data.py script can only download specific sample GEOS-Chem restart files.

The download_data.py script can only download the sample GEOS-Chem restart files that we provide, and we do not have the resources to provide sample restart files for all dates. Instead, we recommend using the sample restart files to spin-up a restart file for your production simulation. See our GEOS-Chem restart files wiki page for more details.

Please also note that CH4 in full-chemistry simulations is chemically active and advected, but its surface concentrations are prescribed and no CH4 emissions are used. From the GEOS-Chem narrative:

Methane is prescribed as a surface boundary condition from monthly mean maps of spatially-interpolated NOAA flask data, and subsequently allowed to advect and react [Murray et al., 2016].

[atmrszh]

@msulprizio Thanks a lot for the reply! I just think that the atmosphere is integrated, that all species interact with each other. For example, hydroxyl radical is the main sink of methane, but NO2 is also a key precursor of hydroxyl radical. So I think the accuracy of methane simulation will be limited.

[yantosca]

@zhuhaors, thanks for your comment. While it would be optimal to do an integrated CH4 chemistry simulation, we often have to make tradeoffs due to computational constraints. For example, to construct a Jacobian matrix for inverse modeling takes many independent CH4 simulations. If the simulations contained the full chemistry mechanism it would an inordinate amount of time before they all completed.

[atmrszh]

@yantosca, thanks a lot for the reply! I get it.

[msulprizio]

I'm going to close this issue for now, but please feel free to reply here with additional questions and we will reopen. You may also choose to open a new issue.