Dear Dr Zhang,

Thank you very much for submitting the revised version of your manuscript entitled "Characteristics, main sources, health risks of PM2.5-bound perfluoroalkyl acids in Zhengzhou, central China: From seasonal variation perspective". We appreciate the time and effort you have invested in addressing the reviewer comments.

Although your revision has improved the manuscript in several aspects, one of the reviewers still feels that some major concerns remain unaddressed (please see below). After careful consideration, I agree that these comments are substantial and must be adequately addressed before the manuscript can be considered for publication in ACP.

We therefore invite you to revise your manuscript again, taking into account the reviewer’s remaining concerns in a detail. Please include a point-by-point response that clearly explains how each issue has been handled.

Thank you once again for your interest in ACP. I look forward to receiving your revised submission.

Kind regards,

Ivan Kourtchev

Reviewer 2 comments:
Thank you for addressing my previous comments (Reviewer 2) in the revised manuscript. While I appreciate the efforts made, I believe there is still room for improvement in terms of the overall readability and clarity of the manuscript. Below, I provide further comments in response to the authors' answers to my earlier questions.

Response to comment 2 - specific comments
The author’s response to the comment on positive sampling artefacts is somewhat unclear. Specifically, I don’t understand the link between positive sampling artefacts and practices like filter baking or the observation that PFAA levels were below MDL in process blanks, as this does not directly address the issue. As previously noted, certain PFAS compounds are known to partition between gas and particle phases. Given that several ionic PFAS are semi-volatile, they may exist in the gas phase and can adsorb onto quartz fiber filters during PM sampling. While the authors briefly mention the adsorption of gaseous PFAS in the revised manuscript, this discussion remains insufficient. It should be also noted that positive sampling artefacts such as adsorption of gaseous PFAS along with PFAS in particulate phase are not only due to increased flow rate but also could be due to the physicochemical properties of PFAS and filter material. I think the discussion on positive sampling artefacts is not adequate and a slightly better discussion should be given.

Response to comment 5 - specific comments
Matrix effects are about suppression/enhancement of ionisation of analytes due to the presence of matrix, which can still occur even if your blank is “clean”. A blank below MDL values could mean the matrix doesn’t contain detectable levels of your analyte, but it doesn’t confirm that the matrix won’t affect the analyte’s signal when it is present.
ESI is shown to be prone to matrix effects, leading to ion suppression or enhancement due to the interference of certain inorganic salts and compounds (Chekmeneva et al., 2017 – https://doi.org/10.1021/acs.jproteome.6b01003; Silva et al., 2016 - https://doi.org/10.5935/0103-5053.20150296). Atmospheric PM constituents such as sulfates, nitrates, and ammonium salts could cause ion suppression in ESI (Kourtchev et al., 2020 - https://doi.org/10.1021/acs.analchem.0c00971). Therefore, it is important to assess the matrix effect even if you have various blanks.

Response to comment 11 - specific comments
Thank you for clarifying that the concentrations (0.26–1.90 pg·m⁻³) refer to PFAA levels in the Middle–Lower Yangtze River plains. Since these values are sourced from original studies, it would be more appropriate to cite the original research article rather than the critical review by Faust et al. (2023), as previously suggested.

Dear Zhang et al.,

Thank you for your revised manuscript and responses to the reviewers' comments.

One of the reviewers still feels that the issue regarding PFAS adsorption has not been fully addressed, and I agree with their assessment. You can find their detailed comment below.

Before the manuscript can be considered for publication, we kindly ask you to revise it to address the following points:

""Thank you for addressing my previous comments carefully. I understand the authors tested for gaseous-phase PFAS adsorption using a Teflon–Quartz double filter system, following Turpin et al. (1994). The authors reported that PFAS were below MDL level in the second (quartz) filter and thus suggest that adsorption artefacts from gaseous PFAS are negligible. While I appreciate these efforts, I have some follow-up questions and suggestions that I believe should be addressed before final acceptance:

1. Where was the double filter system experiment conducted? Was it carried out at the same location and under similar environmental conditions as the main PM sampling campaign? The modified manuscript text does not provide this important contextual information.
2. The added text (Lines 184–189) could be misinterpreted. It gives the impression that the double filter system was employed throughout the study, rather than being used in a one-time supplementary experiment.
3. From the response, it appears that only a single experiment with the double filter setup was performed. Could the detection of PFAS in the quartz filter below MDL levels simply be due to low atmospheric PFAS levels on that specific day?
4. Since the double filter system was not employed during the main sampling campaign, the potential for gaseous PFAS adsorption onto quartz fiber filters remains a valid concern. While your additional test is useful, it does not fully rule out the occurrence of positive sampling artefacts throughout the study period."

Kind regards,

Ivan