|The paper by Schneising et al. has improved compared to the previous submission. While I rate the new version publishable in ACP with only minor modifications, I would like to note that the conclusions changed substantially between the initial submission and the revision, both qualitatively and quantitatively. |
The new estimates for the urban CO burden are about a factor 2 (or more) lower than in the initial submission, which is largely due to the fact that the heights of the planetary boundary layer (PBL) were severely underestimated (wrong time of day). So, while air quality standards for “some neighborhoods […] are likely exceeded” (initial submission, abstract) in the initial submission, “even the most polluted city scenes likely comply with the national ambient air quality standards” (revision, abstract) in the revised assessment.
Recalling the sensational wording and the lack of methodological information in the initial submission, I would like to emphasize that methodological rigor should have priority over swiftness and sensation in scientific publications.
- I am still not convinced that the PBL heights are as accurate as claimed by the manuscript. The error estimate is based on the ensemble spread of the ECMWF ERA-5 model, which is actually quite small. But there is no discussion of potential systematic model errors (e.g. one could compare various models, not only runs of the same model). Is there any validation study showing how ERA-5 PBL heights compare to measurements? How does CAMS PBL heights compare to ERA-5?
- P3, l14: I guess, one should also mention the required topography input.
- P10, l4; Fig. 7: “As can be seen in Figure 7, the IS4FIRES injection heights corresponding to the top of the plume are equal or smaller than the respective maximum boundary layer height at the location of the fires, with the exception of the first day of the Camp Fire.” Is this statement true? If I get Figure 7 correctly, the injection heights of the Camp Fire (orange horizontal bars) would need to be compared to the PBL heights in the vicinity of Sacramento (dark purple). So, actually, the Camp Fire injection height always exceed the PBL heights substantially. For the Woolsey fire (pink horizontal bars), one would need to compare to Los Angeles (dark blue). There, the statement appears kind of true although injection heights seem to reach always up to the top of the PBL. In consequence, the rationale for the Camp Fire needs to be changed.
- P14, l10: Are the boundary layer concentrations quoted for CAMS calculated by using CAMS PBL heights or the ERA-5 PBL heights? If the latter, how would the numbers change if one uses CAMS PBL heights?