Comment on amt-2020-512

The study focuses on estimating the potential impact of ALTIUS ozone observations in improving model short-term forecasts and assimilation analysis of the upper troposphere to middle atmosphere ozone field over the globe using an OSSE. The applied approach and evaluation are scientifically sound. Generally, the content is clearly presented, concise, and well organized. On occasion, it would benefit from additional information as indicated below.

There are minor issues in the grammar in some sections, some of which were considered not worth pointing out. There is the issue of choosing the singular vs plural with 'bias', standard deviation', and 'correlation' especially. The plural would be better in many cases and the choice between the two could be more subjective in others. It would need to be plural when referring to quantities (potentially multiple values).
While the suggested revisions are not very major, the paper would benefit from another review.
Title: 'On the capability of UV-VIS limb sounders to …' is too general in part considering that the content and text of the paper referring essentially to the ALTIUS mission and instrument specifications -even though some implications would admittedly hold anyways to other limb sounders. Many previous studies have already provided some insight on different aspects of this topic. I recommend having the title not refer to 'the capability of UV-VIS limb sounders' and focus on the capability of ALTIUS itself. Section 1. Introduction L19: Providing a reference would be good. L31: Please provide a related reference for the first sentence if possible.
L38-41: Any reference or related document? L48: Include also the mention and references of OSSES regarding stratospheric chemical composition (including ozone). This is not the first OSSE involving stratospheric ozone measurements. As well conclusions from previous assimilation studies involving actual satellite ozone data (including profile sources such as MLS and others) would also be relevant to this work. It would be important to summarize/mention relevant conclusions from earlier assimilation studies involving simulated and actual ozone measurements from satellites in relation to the objectives of this study. What conclusions from earlier OSSEs and OSEs with satellite ozone data are pertinent to this study and what might all of these lack in answering questions regarding the impact of ALTIUS (this relates to one or two statements in Section 2)? L55-56: Specifying BASCOE without COPCAT can be misleading as most already familiar with BASCOE may/will assume use of the full chemisty package at this point. Maybe best to mention either both or neither here.
L56: Starting with the mention of BASCOE in the sentence lends to confusion in reading the remainder of the sentence. It is recommend to instead have a paragraph or sentence before L55 to describe/mention the need for a simulation process to provide the measurements used for investigating both questions.
L55-65: Then again, some/much of the content here would fit better in an introduction of a methodology section (e.g. Section 4) instead of the introduction. Some of the content summarizes the methodology as opposed to introducing the subject. Removing some of the methodology details in these paragraphs (if not most these entirety of these two paragraphs) is recommended.
Section 2. The ALTIUS Mission Section 3. The BASCOE System L116-117: 'As well, the effective vertical resolution stemming from the averaging effect of the averaging kernels -… -so that their use' Is this what is meant? (If so, does this actually apply to the ALTIUS ozone product -as this is dependent on the applied retrieval constraints, including the relative effects of the measurement and a priori/constraint error covariances/weights.) Otherwise, the last part of the sentence does not seem to work. Section 4. The OSSE setup L120: Please state the assimilation window period and/or interval (e.g. every 6 hours covering +/-3 hours about each synoptic time?) L128: 'using a minimum of three' L131: In OSSEs, one could potentially or often simulate many or all observation sourcesthis depends on the intent and the setup. So the control run could technically, depending on the setup and what is intended, use simulated observations except for one or more target sets. Maybe some re-phrasing is needed.
L134-135: 'This ensures .. only …' -maybe not likely. All the 'old instruments' and the common aspect of model physics, etc, would contribute some similarity in results. Please re-phrase. Maybe the intent was to say that the 'increased similarity or agreement between the NR and AR results "as compared to that between the NR and CR" is most likely due to …' L137: Saying it is 'solved' may be too strong. Maybe something like 'the concern of the identical twin issue is largely removed' or attenuated (maybe not entirely removed).
L139: If the CR or another AR does not perform assimilation of other ozone profile sources such MLS or OMPS-LP (NPP) for example, then the target AR will not show the value added benefit of including ALTIUS to one or more other ozone profile sources. Some mention of not doing so would be relevant here -if not also in the conclusions sections.
Section 4.1. The nature run Figures 1, 3, and 4: Having AR and CR results in the NR section is not ideal. Maybe the text in section 4.1 should indicate that the AR and CR results included in the figureswill be discussed in later sections. Additionally, comparing AR and CR to actual measurements, other than MLS maybe as it was assimilated for the NR, is not as meaningful or clean as comparing to the NR itself -since the NR is the truth for the AR and the CR (even MLS itself is not the truth here -it is the NR). It would be worth discussing this. It is good to compare the NR with these observations though to evaluate the realism of the NR and the effectiveness of the MLS assimilation as is done in this section.  L166: Specify vertical ranges of applicability and temper the comment with 'typically', 'mostly' or .,. when these limits are not satisfied for all latitude bands. The sentence 'These are, however, …' would not be needed in that case -and it is better for the previous sentence to be precise in its statements. Section 4.3.2. Ozone profiles I did not notice any mention of the spatial sampling (for the daytime limb measurements) and especially the vertical resolution(s) of the ALTIUS profiles. If not mentioned, it is important to do so.
L218-220: There was no mention of accounting for the geometry of the measurements (limb viewing) and consideration of scattering not just within a vertical column.
L228: This is done to generate the covarirance matrix. It needs to be stated before this sentence, saying that covariance matrices are defined first before simulating the final observation profiles. L290 or so: Is the COPCAT more chemically fast acting in the upper troposphere than in the stratosphere (as it would/might also be at even higher vertical levels)?
L305-314: This discussion is rather late in the text considering that this pertains to Figs. 3 and 4. As a reader, I was a bit puzzled not seeing this near the beginning of the section. See also earlier points regarding Figs. 3 and 4 in comparing to actual observations in this case.
L310-314: Please discuss cases where CR has smaller standard deviations than for AR in Figure 4. L317: Maybe not so evident (if not that likely). Any demonstrable proof from other work (e.g. from CAMS itself)? If so, a reference would be good. If not, maybe better to exclude that statement. Section 6. Added value of the different ALTIUS modes of observation Section 7. Conclusions L334-335: Suggest removing the commas or re-writing to refer specifically to ALTIUS, e.g. 'from the ALTIUS UV-VIS-NIR limb sounder' which would be better.