the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Intercomparison of GEOS-Chem and CAM-chem tropospheric oxidant chemistry within the Community Earth System Model version 2 (CESM2)
Abstract. Tropospheric ozone is a major air pollutant and greenhouse gas. It is also the primary precursor of OH, the main tropospheric oxidant. Global atmospheric chemistry models show large differences in their simulations of tropospheric ozone budgets. Here we implement the widely used GEOS-Chem atmospheric chemistry module as an alternative to CAM-chem within the Community Earth System Model version 2 (CESM2). We compare the resulting simulations of tropospheric ozone and related species to observations from ozonesondes, the ATom-1 aircraft campaign over the Pacific and Atlantic, and the KORUS-AQ aircraft campaign over the Seoul Metropolitan Area. We find that GEOS-Chem and CAM-chem within CESM2 have similar tropospheric ozone budgets and concentrations usually within 5 ppb but important differences in the underlying processes including (1) photolysis scheme (no aerosol effects in CAM-chem), (2) aerosol nitrate photolysis, (3) N2O5 cloud uptake, (4) tropospheric halogen chemistry, and (5) ozone deposition to the oceans. Global tropospheric OH concentrations are the same in both models but there are large regional differences reflecting the above processes. Carbon monoxide is lower in CAM-chem (and lower than observations) because of higher OH concentrations in the northern hemisphere and insufficient production from isoprene oxidation in the southern hemisphere. CESM2 does not scavenge water-soluble gases in convective updrafts leading to some upper tropospheric biases. Comparison to KORUS-AQ observations shows successful simulation of oxidants under polluted conditions in both models but suggests insufficient boundary layer mixing in CESM2. The implementation and evaluation of GEOS-Chem in CESM2 contributes to the MUSICA vision of modularizing tropospheric chemistry in Earth system models.
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RC1: 'Comment on egusphere-2024-470', Anonymous Referee #1, 08 Apr 2024
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This study compares tropospheric ozone simulations using GEOS-Chem and CAM-chem in CESM2. While both models show similar ozone budgets and concentrations within 5 ppb, they differ in key processes like photolysis schemes, aerosol effects, and halogen chemistry. Evaluation against observations suggests successful simulation of oxidants under polluted conditions but highlights potential biases in boundary layer mixing in CESM2. This integration supports the MUSICA vision of modularizing tropospheric chemistry in Earth system models.
Main comments
- There are numerous global-scale atmospheric chemistry transport models available today. Could you provide more context on why selected these two particular models for comparison? Giving a more in-depth discussion on the scientific significance behind this choice would be helpful.
- This study has extensively compared vertical profiles, but what about comparisons of surface observational elements? My suggestion would be to include data from ground monitoring stations to assess ozone and nitrogen oxides.
- In figure 4, I am wondering why surface ozone were rather high in western China (especially in regions like Tibet, almost the highest around the world) where anthropogenic emissions, i.e, NOx, were relatively low. Did you compare the surface simulations with surface observations?
- Have these two models taken into account the impact of halogen chemistry mechanisms on the formation of photochemical ozone? If they have, please provide some discussion.
Minor suggestions
- Line 155-160 “Fast-JX includes aerosol extinction but TUV does not, which explains the larger differences over polluted and open fire regions” Please give some examples to indicate these regions
- Line 200-205, Please extend more about the recent update in isoprene oxidation chemistry why isoprene does not titrate OH in GEOS-Chem
- Why GEOS-Chem simulated NOx in oceans were notably higher than CAM-chem?
- Figure 5 uses pressure while figure 6 and 7 use altitude (km) to show height, it is suggested to use the same unit of height, for instance, kilometers.
Citation: https://doi.org/10.5194/egusphere-2024-470-RC1 -
RC2: 'Comment on egusphere-2024-470', Anonymous Referee #2, 23 Apr 2024
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Li et al. implemented the GEOS-Chem atmospheric chemistry module into the CAM-Chem and evaluated the difference in O3 and OH chemistry between model simulations. They also conducted the model sensitivity analysis and discussed the difference in O3, NOx, CO, and OH due to underlying processes including the photolysis scheme, aerosol nitrate photolysis, N2O5 cloud uptake, halogen chemistry, and ozone deposition to the oceans. I appreciate the tremendous technical work involved in implementing the CESM-GC capacity. I suggest minor revisions before this paper is accepted in ACP.
- The authors provided some high-level explanations of the underlying processes that lead to the difference across the model simulations. However, more detail is recommended. For instance, what is the process of scavenging water-soluble gases in convective updrafts? Are these gases NOx and less reactive VOCs? Do they mainly affect O3 formation in the upper troposphere?
- Could you provide some discussion on the halogen chemistry mechanism implemented in the model? How large is the uncertainty in this mechanism? Is the uncertainty introduced through the chemical mechanism smaller than the difference observed here with and without halogen chemistry?
- For the calculation of OH in Table 2, is it air mass-weighted column OH?
- In Table 2, both models generate the same OH. However, from Figure 3, the difference in OH is considerably large, with -3.2% at the surface and -10.1% at 500 hPa. Could you explain this discrepancy?
- The spatial resolutions of both models are coarse, how does the model’s spatial resolution affect the model comparison against observations, especially those from ATOM1 and KORUS-AQ observations?
- Both models consistently show a high bias in O3 compared to observations, except for GEOS-Chem without the PNO3 photolysis. Could you discuss more on the possible causes of the high O3 bias? For instance, how do stratospheric O3 and lightning NOx emissions contribute to O3 in the upper troposphere?
- The figures are vague, please update them to a higher resolution.
Citation: https://doi.org/10.5194/egusphere-2024-470-RC2
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