General comments

This paper describes a source-receptor relationships (SRRs) database using Flexpart dispersion model with ERA5 reanalysis data. The objective is to provide a data base usable for ozone observations studies over Western North America.

The methodology is pertinent and very well described in chapter 3, with suitable product illustration in chapter 4, with sensitivity illustrations with altitude and seasonal. The chapter 5 shows a possible application to NOx emissions influence on O3 photochemical production.

The text and the figures are excellent quality and I have no additional comment before publication.

Minor comments (typos)

Line 53: replace “western” by “Western”

Line 76: add “.” after “ECMWF model”

The paper submitted by Cui et al. describe a data base of Source-Receptor-Relationships (SSR) corresponding to tropospheric ozone observations in Western North America. SSR retrieval is based on FLEXPART model V.10 following an approach similar to Cooper et al. 2010 but using more up to date meteorological analysis (ERA 5) and a better seasonal coverage. A data fusion technique has been implemented using the work of Chang et al. (2023) but applied to a more comprehensive ozone data base including new vertical levels and all observations available in Western North America (ozonesonde, lidar, aircraft).

The paper is a  brief presentation of the SRR data base with some illustrations of its potential usefulness. More elaborated results will be described in future papers. The main results of the present paper are (i) a description of the FLEXPART model configuration in section 3, (ii) two illustrations of the spatial distributions of SRR for low or high ozone data set in section 4, (iii) while section 5 describes how SRR can be combined with source distributions of ozone precursor. In section 5 the example of the sensitivity to NOx tropical lightning source has been chosen.

I recommend publication of this paper as it is a useful addition to future papers using this SRR data base. However I believe that a short paragraph is missing in section 3 in order to describe differences between this new SSR and the Cooper et  al. (2010) 1994-2008 spring time analysis. Such a new short paragraph would be a nice way to discuss the benefits of using ERA 5 and a more comprehensive ozone observation data set.

Since long range transport analysis is often viewed as a key component to understand how stratosphere-troposphere exchanges (STE) control ozone increase in the free troposphere, the authors might consider an additional illustration to show how the new SRR could help to address this issue.

Detailed remarks

line 78. CTMs are indeed more complex tools but we can address different question related, especially the complex inetraction between dynamical an photochemical processes

line 130 Indeed the measurement error might be minor considering the large data set considered in this work, however Fig. 1 shows also a change an significant increase of  data sampling after 2004. I wonder how this feature will be accounted for in the SRR data analysis.

line 236 Fig. 2 only show that long range transport change when considering different altitude levels in WNA. Such a very genral picture is difficult to discuss detailed questions as the sensitivity to aircraft emission, biomass burning or STE.

line 250 Fig. 3 Is indeed a good illustration to describe the potential of the new data base.

Fig. 4:  I believe Fig. 4 shows the seasonal sensitivity for the whole tropospheric column, this should be clarified

Fig. 5:  I am not sure how to interpret the trend in the sum of the residence time. More residence time in the 3-13 km vertical range during the 21th century for the air masses with high ozone ?