This study focuses on the phenomenon of nocturnal ozone increase (NOI), which has garnered significant attention in recent years. Using Xinxiang, a typical city on the North China Plain, as a case study, the research examines the characteristics of NOI events and analyzes the formation mechanisms of NOI under different weather conditions. However, despite the interesting topic, I consider this manuscript to have significant flaws in its scientific reasoning, and several key conclusions are not sufficiently supported by evidence.  I cannot recommend this study for publication in the present form.

Major comments:

1. The authors selected only four stations in Xinxiang City for their study and further identified 197 days during which NOI events occurred simultaneously at all stations as a representative sample.Why do these four stations, which are located relatively close to one another, have more days with non-overlapping NOI events than days with overlapping events?If the NOI is primarily driven by large-scale atmospheric circulation, why did the four sites not experience simultaneous events? This strongly suggests the dominant role of local emissions or mesoscale processes, making the authors’ subsequent focus on the analysis of large-scale atmospheric circulation appear to lack a solid foundation.
2. The authors did not specify whether they used meteorological fields covering the entire day for Lamb-Jenkinson classification. The fundamental flaw in this approach is that the classification is based on average atmospheric circulation throughout the day or at specific times, whereas NOI is a short-term phenomenon occurring during specific hours at night.
3. Lines 87–92: If I understand correctly, the author defines an NOI event as a rise in hourly O₃concentration during the night exceeding 10 μg m^–³ compared to the previous hour. This definition does not account for the diurnal cycle of O₃concentrations. Between 00:00 and 06:00, as sunrise approaches and temperatures rise, O₃ concentrations naturally tend to increase. If the diurnal cycle is not accounted for, a significant portion of the detected so-called NOI events may simply be part of normal daily variations.
4. The grouping of weather types is overly subjective and lacks quantitative justification, resulting in ambiguity regarding the physical significance of the categories. The authors subjectively grouped the 23 weather types generated by the Lamb-Jenkinson method into four major categories: A-type, C-type, S-type, and WNE-type. The rationale for this grouping is stated as being “based on the position of central system and different meteorological elements of each classification”, but this has not been quantified or clearly explained in the manuscript.
5. The logic of the entire Section 3.3 is inverted. Rather than analyzing how the specific meteorological conditions of each weather type led to NOI, the author devotes a significant amount of space to explaining how the meteorological conditions of each type contribute to the maintenance of atmospheric circulation. Of greater interest is which local meteorological variables (such as temperature, wind, and boundary layer height) directly drive the formation of NOI within the context of already classified atmospheric circulation.
6. In the analysis of NOI formation mechanisms, it is unreasonable to study the mechanisms of NOI occurring in different seasons together. For example, the S-type events in spring and those in autumn have vastly different precursor substances and meteorological conditions, which naturally leads to differences in the formation mechanisms of NOI.

Specific comments:

1. The content within the parentheses in the caption of Figure S5 is not specified.
2. The reference to Figure 1 on line 160 does not support the explanation of the preceding sentence; please correct this. After the correction, the entire content of Section 3.1.2 is cited from the Appendix; consider incorporating some of this content from the Appendix into the main text.
3. Lines 190–195: The authors merely state facts (e.g., Type A events occur more frequently in autumn) without linking them to the subsequent analysis of NOI mechanisms. How does this information contribute to understanding the causes of NOI?
4. Lines 205–210 and Table S2: The analysis of meteorological conditions for each weather type here lists only averages but does not include any significance tests.
5. Line 220: The authors infer that the 850 hPa wind field “likely carried substantial moisture” based solely on the wind field itself. We recommend providing more direct evidence, such as calculating and presenting the 850 hPa water vapor flux and its divergence.

Review of the paper “Impacts of synoptic circulation types on nocturnal ozone increase in the North China Plain: Meteorological drivers and formation mechanisms” submitted to Atmospheric Chemistry and Physics by Wang et al.

The manuscript investigates the meteorological drivers and transport features of nocturnal ozone increase (NOI) events in Xinxiang, North China Plain. The authors utilize the Lamb-Jenkinson circulation classification, gridded meteorological datasets, and HYSPLIT back-trajectories to characterize these episodes.

While the topic is scientifically relevant and falls within the scope of ACP, the manuscript currently suffers from significant methodological inconsistencies and a lack of deep scientific interpretation. In its current state, the evidence provided does not fully support the conclusions. Therefore, I recommend a rejection with an encouragement to resubmit after a substantial revision that addresses the fundamental concerns outlined below.

General comments:

• Scientific Significance of NOI: The introduction does not sufficiently justify the atmospheric importance of NOI episodes. A nighttime increase of 10 µg m^-3 may be numerically detectable, but its impact on overall air quality or subsequent daytime peaks is not clearly established, particularly when background levels are low. The authors should better articulate the scientific or environmental relevance of studying these specific increments.
• The NOI classification in Section 2.2 appears somewhat arbitrary. The unequal duration of the defined periods makes frequency comparisons statistically difficult. Furthermore, the transition from a time-based categorization to a weather-type categorization is abrupt and creates a disjointed narrative. The authors should harmonize these approaches or more clearly explain how the two frameworks intersect.
• Station Typology: A critical omission is the lack of information regarding monitoring site classification (e.g., urban, rural, industrial). Since ozone concentrations are heavily influenced by local NO titration and precursor availability, the interpretation of NOI events is incomplete without considering the specific typology of the stations used in the study.
• Vertical Transport Analysis: The back-trajectory analysis is currently limited to the surface level. Given that the downward transport of ozone from the residual layer or upper troposphere is a well-documented driver of ozone increases, it is essential to include trajectories at higher altitudes to capture the full transport dynamics.
• Causal Connections: While the manuscript provides a detailed description of meteorological features, it fails to sufficiently link these features to the underlying ozone phenomenology. The discussion often focuses on meteorological variables that may not be the primary drivers of ozone variability, leaving the physical connection between transport types and NOI poorly resolved.
• The occurrence of NOI cannot be related solely with long-range transport ignoring local/regional features.

Specific Comments

• Line 48: Please expand on the specific negative impacts of NOI to underscore the environmental importance of the study.
• Line 64: Provide more comprehensive metadata for the study area, including population density in Xinxiang, primary precursor sources, and general climatological characteristics.
• Section 2.1: It is essential to specify the spatial and temporal resolutions of the MERRA-2 and ERA5 datasets used.
• Section 2.3: The use of Sea Level Pressure (SLP) for the Lamb-Jenkinson classification may overlook transport occurring at higher altitudes. I suggest incorporating data from higher geopotential heights to ensure the synoptic classification is representative of the layers where ozone transport is most active.
• Line 119: Please cite the HYSPLIT model formally (e.g., Stein et al., 2015).
• Lines 131–136: Figure 1 indicates a secondary peak in June that is currently unaddressed. This feature should be interpreted in the context of seasonal ozone precursors or meteorological shifts.
• Section 3.4.1: When identifying geographical source areas via back-trajectories, the authors should discuss the known precursor emission profiles of those regions to provide a more robust chemical context.

The topic of this manuscript is scientifically relevant. Isolating nocturnal ozone increases from photochemical effects offers a useful window into transport-driven dynamics, and the results could contribute to a better understanding of ozone transport phenomena. However, the manuscript has substantial methodological weaknesses, and the analysis of results lacks depth. Many of the insights presented are predictable and do not meaningfully advance the field. In its current form, I cannot recommend this manuscript for publication.

Methodological Issues

• NOI significance is not addressed. The definition of a NOI event does not account for baseline ozone levels. An increase of 10 ug/m3 over a baseline of 70 ug/m3 is not equivalent to the same increase over 120 ug/m3. Similarly, the temporal trajectory matters: a 10 ug/m3 rise followed by an immediate decline is fundamentally different from a sustained or accelerating increase. These dimensions must be incorporated into the event detection framework.
• The separation of night and morning periods is not adequately justified. While the intent is presumably to identify isolated days, this artificial split breaks the continuity of nocturnal dynamics. An increase beginning at night may well persist or evolve into the morning hours, and this continuity is lost under the current approach.
• The HYSPLIT methodology is described too briefly to be reproducible. As written, it would not allow an independent researcher to replicate the analysis. Furthermore, the Concentration-Weighted Trajectory (CWT) approach receives no methodological explanation whatsoever.
• The weather type classification lacks transparency. The identification and classification methodology appears highly subjective in its current description.
• The measurement stations are not described. When presenting NOI statistics by station, the surrounding environment of each site is essential context. If a station is located near NOx sources, the ozone signal will be titrated, and any observed increase will be attenuated or masked. Without characterizing the local emission environment, it is impossible to assess whether the detected NOIs are genuinely representative of mesoscale transport or simply artefacts of local source variability from the preceding hours.
• The rationale for selecting 850 hPa in Figure 3 is not provided in the methodology, and the wind scale is missing from the figure.
• Vertical profile data sources are not identified. When vertical profiles are discussed, no information is given about the data origin. The wind legend in Figure 5 is also incomplete.

Results and Interpretation

• The finding that most NOI events occur between March and September adds no new knowledge. This is simply when background ozone concentrations are highest, and the result follows trivially from that fact.
• The role of calm conditions versus high wind speeds is not sufficiently interrogated. While the results suggest a higher frequency of NOI under calm conditions, events are also identified under elevated wind speeds. These cases could reflect fumigation from elevated layers, or transport associated with pre-frontal air masses carrying accumulated pollution that is subsequently scavenged after frontal passage — a mechanism already documented in the literature. This alternative explanation is not discussed.
• The WNW weather type results are likely of limited ozone relevance. High wind speeds under this type are typically associated with enhanced dispersion, making it doubtful that these events represent meaningful ozone loading.
• Line 249 mentions a thermal low-pressure system without discussing its potential influence on ozone dynamics, which is a notable omission given the likely relevance of this feature to the observed transport patterns.
• The decoupling of weather type frequencies, ozone concentrations, and Figure 3 meteorology makes it impossible to assess the actual severity of the identified increases. A summary table integrating these dimensions would greatly improve readability and analytical clarity. As currently presented, the results section is difficult to follow.
• Supplementary figures are referenced repeatedly in the main text without adequate explanation.
• The absence of summary tables makes the results section hard to navigate. Given the number of weather types and stations discussed, tabulated summaries are necessary.

Conceptual and Terminological Issues

• Line 50: Discussing "formation mechanisms" in the context of nocturnal ozone is conceptually inconsistent. At night, photochemical ozone production is absent; the relevant process is titration by NO, not formation. The framing needs to be corrected.
• "Meteorological activities" (Line 49) is not standard scientific terminology and should be revised.