Please see review in attached PDF.

Review of: Measurement report: Ammonia in Paris derived from ground-based open-path and satellite observations (Viatte et al., 2023)

The manuscript discusses relatively long-term NH3 measurements from the IASI instrument and a miniDOAS instrument located in the center of Paris. Correlation, seasonal and weekly cycles are discussed as well as the potential sources of the NH3 emissions. The manuscript is well-written and well structured. The findings do not introduce substantial new discoveries. The finding that traffic is a significant source of NH3 in the Paris region would be more convincing with a better placement of the miniDOAS or when NH3 measurements from more stations would have been included. Such measurements seem to be available in the Paris region. Although I find the study a bit lightweight for ACP, I still recommend its publication. However, revisions (detailed below) need to be conducted in the paper before publication. Please note that all questions/comments below should not only be addressed in the author’s reply, but also in the manuscript.

General comments

p.4 l. 103: One of the main comments is that a clear motivation is needed why you put the instrument at an altitude of 40 m above ground level. You are far less sensitive to the traffic related emissions you want to monitor as well? In the introduction (p.3 l.59) it is stated that this is badly monitored.

Fig.1 top panel: It is difficult to see the NH3 distribution in the Paris area on this map. It would be helpful to also provide a zoom for the red, blue and green box (maybe in appendix).

Fig.1 lower panel: I'm not sure if applying these different spatial criteria make much sense. Also it would be expected that when applying more strict spatial constraints (30 km) that the agreement would be better, due to better co-location, which is not the case. Please explain.

Fig.1 lower panel: Moreover, why don't you make use of the maximum spatial resolution of IASI, i.e. comparing with the closest pixel or smaller radius especially as you mention city-scale studies earlier on.

Fig.1 lower panel: Please explain the spread of data points over the seasons. Why do you have significantly more points in spring than in summer and other seasons?

p.7 l.173: The satellite retrievals are strongly impacted by thermal contrast. It is highly surprising and difficult to believe that there is no impact of temperature on the correlation as it affects the sensitivity of the satellite data. On the other hand near-surface measurements are highly impacted by PBL height, while this is less the case for the column observations. Please explain.

p.7 l.177: If I understand it correctly, in the further analysis you average the IASI retrievals within a box of 50 km e.g. to compare with miniDOAS? In that case you cannot speak about a city-scale study neither about studying the spatiotemporal variability in Paris. Temporal yes, but not spatial!

p.7 l.186: Please describe here or provide a reference how the error budget is done.

p.8 l.199: Agreed, but vertically you are also at large distance from the potential traffic emission source that you want to assess in this study, when not putting it at the surface. Official monitoring stations usually sample at 3 m altitude.  This might also explain why you measure lower concentrations than in other urban areas / other studies.

p.15 l. 376: If the data from this traffic station (and maybe others) is available, why isn’t it used in this study, even if it isn’t a miniDOAS? It would make your results far more convincing, e.g. to check if the results are consistent for different stations and to assess the effect of having the miniDOAS at 40 m.

Minor comments

p.4 l.88: Would be good to include as well https://amt.copernicus.org/articles/9/2721/2016/amt-9-2721-2016.pdf

p.8 l.202: I assume you provide first averages and after min max ranges? Please specify the numbers that are provided here.

p.8 l.212: Please elaborate shortly on these high pollution events.

p.10 L.250: It is not really clear why this is then only the case in 2021, and for example not seen in 2020?

p.12 l.296: Please elaborate on why this is observed in agricultural areas. Is it related to reduced agricultural activity over the weekend?

Technical corrections

p.1 l.20: in other urban areas

p.3 l.62: replace ‘monitored’ by ‘detected’?

p.7 l.156: do you mean quantitatively?

p.11 l.278: ‘raw’ should be ‘row’

p.15 l.380: I suggest replacing ‘Atmospheric’ by ’Temporal’