BWma1850 compset question

[ntandon]

This concerns the BWma1850 compset under CESM 2.1.3. Does the chemistry in this compset follow the earlier version of WACCM described in Marsh et al. (2013)? I.e. there is no tropospheric chemistry other than CO and methane oxidation?

 

[mmills]

Yes, that is correct. The "ma" the compset name stands for "middle atmosphere" chemistry. So it is an updated version of the chemistry in CESM1(WACCM4) (Marsh et al., 2013), and a subset of the full TSMLT chemistry that is generally used in CESM2(WACCM) (Gettelman et al., 2019).

Once you create a case with the BWma1850 compset, you can find the chemical mechanism under the CaseDocs folder, in the files chem_mech.in and chem_mech.doc.

 

[ntandon]

Ok thanks. So if I understand correctly, there were some updates to the middle atmosphere chemistry since CESM1(WACCM). But as for tropospheric chemistry in BWma1850 in CESM2(WACCM), that still only includes CO and methane oxidation. Is that right?

 

[mmills]

I attach the chemical mechanism text file. Unlike CESM1(WACCM4), CESM2(WACCM6) needs to include the tropospheric chemistry that is in CAM6, which generates tropospheric aerosols (sulfate and secondary organics).

 

[ntandon]

Hmm, I had seen that file as well in my BWma1850/CaseDocs folder, and I noted the following lines:

d(so4_a1)/dt = 0
d(so4_a2)/dt = 0
d(so4_a3)/dt = 0
d(soa_a1)/dt = 0
d(soa_a2)/dt = 0
d(SOAG)/dt = 0

which I (naively?) interpreted to mean that sulfate aerosols and SOAs are prescribed rather than calculated internally. By way of comparison, for the BW1850 compset, I do not see zero on the RHS of the SOA tendency equations in chem_mech.doc. Am I misunderstanding the contents of chem_mech.doc?

 

[ntandon]

Would you be able to send me chem_mech.doc for CESM1(WACCM) as described in Marsh et al. (2013)?

 

[mmills]

Attached. Also available here:

chem_mech.doc

 

[ntandon]

Comparing the chem_mech files for BW1850 and BWma1850, it appears that the difference in the troposphere relates to SOA precursors: BWma1850 prescribes emissions of a lumped gas-phase SOA precursor "SOAG." In contrast, BW1850 prescribes emissions of individual precursors derived from the MEGAN dataset and includes all of the reactions they undergo to form tropospheric aerosols. I assume that the SOAG scheme used in BWma1850 follows what is described in Tilmes et al. (2019): "The preprocessed surface emissions of SOAG are derived from emission data sets of five primary VOCs, using the following mass yields: 5% BIGALK (lumped butanes and larger alkanes), 5% BIGENE (lumped butenes and larger alkenes), 15% aromatics, 4% isoprene, 25% monoterpenes (Liu et al., 2012)."

Please confirm that everything above is correct. Many thanks!