Shipping emissions play important roles in coastal air quality, influencing the ambient concentrations of SOx, NOx, PM2.5 and their components in atmospheric environment. Recent years, ship fuel oils have experiencing multi-stages transitions due to low sulfur fuel oil policies implemented in national scale and global scales. This study presents comprehensive impacts of changes in ship emissions on PM2.5 as well as its gas precursors and primary and secondary components in China. The emission inventory updating based on field and on-board measurements is original and crucial for estimating atmospheric influence under different policy stages. Generally, this paper was structured and written well. The study results could add inputs for providing good reference for air pollution prevention in coastal cities. 

There are also some places need to be clarified clearer. I just post my suggestions and my questions as the below: 

(1)    Section Introduction: the introduction should be updated to include more recently published research results, especially introduce some new research results after IMO 2020 policy.

(2)    Section Methodology: The authors stated that the emission updating was based on previous measurement study in Shanghai. Since the following simulation work cover the national scale, how about the components changing situation of PM2.5 in other coastal cities? Have any results been reported that could be compared with each other?

(3)    Model Performance: Figue2 (b), there was not the evident reduction in the PM2.5 concentration contributed by ship in 2019 compared with 2017 and 2018. Since the policy is step by step as stated in Line84-88 in Introduction, why didn’t PM2.5 response well with the staged policy?

 

(4)    Results: As authors stated as “NH3 is sufficiently consumed by SO2 and NOx from land-based emissions, and thus the formation of secondary aerosols related to shipping is inhibited, which called the competitive mechanism by land-based sources.”, what is the change in the competitive relationship between SO2 and NOx in marine atmosphere after low sulfur fuel oil policy? 

(5)    Minor suggestions: maybe move some figures and text into the supplementary material to make the full length shorten.

This study investigates the impacts of low-sulfur fuel policies on PM_2.5, its gaseous precursors (SO₂ and NO₂), and its components (V, Ni, SO₄²⁻, NO₃⁻, NH₄⁺, etc.) using WRF-CMAQ model simulations with an updated shipping emission inventory for China from 2017 to 2021. The results effectively quantify the effects on each component, compare air pollution variations in port cities across China, and examine potential meteorological influences. These findings provide timely and valuable insights for air quality studies in coastal regions. Additionally, the manuscript is well-structured and detailed. However, several issues need to be clarified before publication. Below are specific comments:

Specific Comments

1. Lines 80 – 105: In response to IMO regulations, China has implemented various regional strategies in recent years. To improve clarity, the authors should include a table summarizing key aspects of China’s IMO regulations, such as revisions and objectives. Additionally, the full names of “IMO,” “Tier II,” and “Tier III” should be provided for clarity.
2. Section 2 (Methods): The authors cite previous studies to describe updates to the emissions inventory and CMAQ model. However, all details directly relevant to this manuscript’s results should be explicitly presented, even if they have been previously published.
3. Lines 215 – 225: It is recommended to mark recurring areas (e.g., Yangtze River Delta, Pearl River Delta) on a map. For the 21 port cities (DL, YK, CFD, BH, YT, QD, RZ, LYG, SH, NB, ZS, HZ, NT, ZJG, NJ, FZ, XM, SZ, GZ, ZH, and QZ), abbreviations should be used consistently in both the text and figures.
4. Line 235:

(a) Table A1 is not found in the manuscript.

(b) The values of IoA and RMSE were used to assess the simulation performance of V and Ni, while different statistical indicators (e.g., r and NMB) were used for SO₂, NO₂, MDA8 O₃, PM2.5, SO₄²⁻, NO₃⁻, and NH₄⁺ (Tables S8 & S9). Is there a specific reason for this distinction?

1. Lines 250 – 260: Some data in the manuscript differs from Table 2. For example: “The monthly average V emissions from shipping dropped by 90.8%, from 118.8 t in 2017 to 43.9 t in 2021.” “The monthly average Ni emissions decreased from 11.0 t in 2017 to 24.1 t in 2021, a reduction of 42.0%.” Please verify and ensure consistency.
2. Section 3.2 (Model Performance): Some content could be moved to the supplementary material for conciseness.
3. Sections 3.3 and 3.4: Since policy changes were implemented gradually, the differences in air pollution trends due to policy shifts should be highlighted more clearly. Meanwhile, location-specific results could be streamlined to shorten the manuscript.
4. Lines 370 – 375:

(a) The maximum SO₂ concentration in 2018 reached 30.1 μg/m³, significantly higher than in other years. What could be the underlying cause?

(b) A high-pressure system typically leads to stable weather with low wind speeds, facilitating pollutant accumulation.

1. Line 415: The discussion appears unclear, as contributions from sea salt are presumably excluded in Figure 5.
2. Lines 430 – 435: Regarding the seasonal shift in high PM₅ concentrations from southern to northern regions, the impact of the Asian Summer Monsoon should be considered.
3. Lines 475 – 480: Distributions of NH₃ emissions from 2017 to 2019 should be provided.
4. Section 3.5: Only PM₅ components in Shanghai were verified. Are similar data available for other port cities?
5. Figure 13:
6. a) Are the data averaged across all ports? If so, confidence intervals should be provided in the text.
7. b) Previous discussions indicated an increase in NOₓ from shipping. However, Figure 13b shows that the average NO₂ level in 2021 was lower than in 2017, which appears inconsistent. Please clarify.
8. Conclusion: The conclusion should be more concise.

Technical Corrections

1. Lines 125–130: Clarify whether the time resolution is “hourly” or “6-hourly.”
2. Lines 160–165: Modify to: “sulfur dioxide (SO₂), carbon monoxide (CO), nonmethane volatile organic compounds (NMVOCs)…” The abbreviation should follow its full name upon first mention.
3. Line 760: "Fig. 15" should be "Figure 15."
4. The manuscript contains minor spelling and grammatical errors that should be corrected.