Comments on the manuscript of “Assessing the Sources of Submicron Airborne Elements at two sites in the Fos-Marseille Basin through Rolling Positive Matrix Factorization” by Mathilde et al.

General comments

This manuscript investigates the sources of submicron airborne elements at an urban background site and an industrial site in the Fos-Marseille Basin using one-year high-time-resolution Xact measurements and rolling PMF. The study is well-designed, data-rich, and methodologically sound, with a clear novelty in applying the rolling PMF approach to long-term elemental XRF data. The dual-site setting enables the identification of regional and industrial contributions, and the major sources (secondary aerosols, shipping, biomass burning, steel industry, etc.) are reasonably interpreted. The manuscript is generally suitable for publication in Aerosol Research after minor revision, as several key scientific clarifications, data validation, and presentation improvements are required to strengthen the robustness of source apportionment and readability.

Specific comments

1. Line 195: Are there any references indicating the shortcomings of static PMF in capturing the seasonal variations in long-term data? If so, please suggest the relevant literature.
2. When comparing the static and Rolling PMF, only the variability of the factor profiles was analyzed, without comparing the differences in the quantitative source contribution concentrations between the two methods (such as the annual average contribution and seasonal contribution differences of each source). It is suggested to supplement the relevant quantitative comparisons to more comprehensively demonstrate the advantages of Rolling PMF.
3. Does the selection method for high BDL (below MDLs) element data affect the construction of the error matrix, thereby influencing the source apportionment results of PMF? Is it necessary to compare the PMF factor results after removing or retaining the high BDL elements?
4. In Section 4.2, the analysis of each PMF factor reveals that the Zn-rich factor (MRS-LCP site) has excluded traffic and industrial sources, and it is hypothesized that solid fuel combustion is the main source. However, there is a lack of quantitative coupling analysis with biomass burning organic aerosols (BBOA), such as concentration correlation and ratio characteristics. It is recommended to supplement relevant statistical analysis to strengthen the basis for attributing the source of this factor.
5. It is suggested to compare the correlation between the HOA factors of TOF-ACSM OA PMF and the Steel Industry and Zn-rich factors at MRS-LCP, in order to further determine whether road transport may contribute to the Steel Industry and Zn-rich factors at MRS-LCP.
6. The Cl‑rich factor from Xact and Cl^- from ACSM represent the same submicron particulate chlorine. A correlation analysis is suggested to validate the factor.
7. The extremely high contribution of the S-rich factor (>50%) is reasonable given strong photochemical sulfate formation in this photochemically active, industrial, and coastal Mediterranean basin, but the authors should further quantify SO₂/sulfate sources (shipping, industry, etc.) to improve interpretation.

Technical corrections

1. Line 114 and 119: Please ensure the consistency of punctuation and formatting in the writing of latitude and longitude values (43°27’32.092’’N;120 4°56’4.412’’E).
2. Is the “X” in Table 1 indicating that the instrument was equipped at this site?
3. Line 157-158: Excessive commas in the “The lack of quantitative assessment … on marine aerosols,” and the citation format of the literature needs to be adjusted in “(e.g. fresh and aged sea salt, Pey et al., 2013a)”.
4. Line 214: Fig. S3 & S4 should be corrected to Figs. S3 & S4. All similar situations in the text should be uniformly revised.
5. Line 311: “eg.” should be “e.g.”.
6. In the sentence “PM₂.₅ in Sylvestre et al., 2017 versus PM₁ in the present study”, “Sylvestre et al., 2017” should be “Sylvestre et al. (2017)”.

Regarding the manuscript entitled “Assessing the Sources of Submicron Airborne Elements at two sites in the Fos-Marseille Basin through Rolling Positive Matrix Factorization,” I find the study to be very well written and scientifically interesting. The manuscript focuses on a highly relevant and complex region, and the dual-site design provides valuable insight into the spatial variability of submicron elemental sources. In particular, the work is noteworthy because it applies a methodological approach that has not been implemented before on a year-long, high-time-resolution elemental dataset. Overall, the study makes a valuable contribution to the understanding of airborne particulate sources in this important coastal-industrial region and can be published after minor revisions.

Lines 214–223: The selection of elements retained in the PMF input matrix deserves a more detailed discussion in relation to factor identification. Since several elements were kept despite very high fractions below detection limit, the authors should clarify how their inclusion affected the robustness and interpretability of the resolved factors, especially for industrial factors driven by low-abundance tracers.

Lines 230–233: The exclusion of short-lived events may have important consequences for factor identification. Since episodic events such as Sirocco outbreaks, fireworks, or local construction emissions can carry distinct elemental fingerprints, removing them may simplify the factor structure but may also suppress potentially meaningful sources or alter mixed factors. This effect should be discussed more explicitly. Why have the authors not reintroduced these points at the end of the analysis and add extra factors?

Lines 254–264: The fact that the Shipping and Cl-rich factors could not always be identified in unconstrained seasonal runs raises an important question about factor stability. The manuscript would benefit from a clearer explanation of how confidently these factors can be considered physically distinct sources rather than partially resolved or constraint-dependent solutions.

Lines 265–269: The decision to retain a Biomass Burning factor throughout the year is understandable, but the manuscript should discuss more carefully whether this reflects a true year-round source or whether, during summer, potassium may partly be redistributed from other sources into this factor. This is especially important for factor identification in periods when independent biomass-burning tracers are weaker.

Lines 355–356: The observation that the Zn-rich factor may reflect multiple sources is important, but the discussion should go further. If this factor is not chemically homogeneous, the authors should clarify whether it should really be interpreted as a single source, or rather as a mixed factor grouping several Zn-containing emissions.