|To the authors of the manuscript, |
Thank you for the responses to the comments. I (Referee #2) was not really questioning the results from OSSEs (because it is OSSE). However, I had a hard time to assess the scientific significance of this study and thus I was asking the questions and requesting more information. I think the revised manuscript carries fair statements regarding limitations that have been pointed out. Here, I provide some responses and suggestions to mainly further examine the scientific significance of this study. I also ask for some clarifications.
General comments 3: This study is addressing a component (plume detection) of the CO2M based CO2 monitoring system (= measurement technique). It is important to show this study's relevance and significance in the large picture of CO2 monitoring, especially because the authors have highlighted their relevance to CO2 monitoring. Thus, I still think the authors need to provide how CO2M will be contributing to CO2 monitoring to show this study's significance. As CO2M has been a big focus in this study, introducing the satellite with a few sentences by citing other studies does not seem to be sufficient. This is an OSSE study, but we need to understand which instrument/measurement technique the authors studied.
General comments 5: I am not here questioning the feasibility of detection, but the step you attribute a plume to a certain source. I thought emission attribution is an important component of the CO2 monitoring (that's why I keep asking a question like above...). Yes, what the authors mention is fair and probably theoretically possible. However, I don't think it is practical and the authors haven't show the ability to maintaining policy-relevant quality. The authors should check how the Google Earth image look like over China... the long term average can be used, but detection is not the last step... anyway.
P2, P10: Again, even before having a monitoring system, many of cities have no idea of their emissions as they don't have emission inventories.
P2, CO2 observation system -> Emission Verification Support System (?)
P2, L28: detecting? If you want to stick to detection, Nassar et al. (2017), who sticked to a few cases that worked out (later, the author acknowledged one of the demonstrated power plants was not correct) and Reuter et al. (2019), who used NO2 not just CO2 alone. This sentence is misleading.
P4, L4: This is not just for weak anthropogenic plumes. For example, Nassar et al. (2017) simply assumed the subset of the OCO2 stripe as a background w/o any investigation. How do you distinguish large and weak CO2 emitters? Is 10Mt CO2/yr the threshold?
P5, L8: If it's only available in German, I would still request a few sentences summarizing this inventory for many non-German speaking audience including myself. W/o knowing this inventory, we don't get a good sense of what the authors discuss at L5-L9.
P10, L28: simplified from?
P32, L19: point sources -> coal-fired power plants? Coal-fired power plants are not the only kind for CO2 point sources.