the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Feb 2026
Wintertime photochemistry of acyl peroxynitrates and ozone in South Korea during the ASIA-AQ campaign
Abstract. Wintertime photochemical air pollution in East Asia remains poorly constrained despite its impact on regional air quality. Sources and formation pathways of acyl peroxynitrates (PANs) and ozone (O3), key photochemical products, are not well understood, hindering effective mitigation strategies. We investigate PANs and O3 over South Korea using observations from the ASIA-AQ campaign (February–March 2024). PANs reached 5.5 ppbv, strongly correlating with formaldehyde and particulate matter, indicating active winter photochemistry. Median PANs were higher in the mid-southern peninsula (MS; 987 pptv) and Yellow Sea (1197 pptv) than the Seoul Metropolitan Area (842 pptv). Elevated homologue-to-acetyl peroxynitrate ratios over the MS, with enhanced acryloyl peroxynitrate, acrolein, and ethylene oxide, provided tracers for petrochemical emissions and their impacts. Acetaldehyde contributed 53–80 % of PAN production. Ethanol was a major precursor of acetaldehyde (~50 %). Strong correlations (r² > 0.8) of ethanol and halocarbons indicate industrial and solvent sources under-represented in inventories. Formaldehyde and C2+ aldehydes contributed ~30 % to ozone production. Low ozone production efficiency (< 10) and radical termination dominated by nitric acid and PANs (> 80 %) indicate VOC-limited conditions. The fractional PANs contribution to NOx loss increased with decreasing OH reactivity ratio of NO2 to aldehydes, suggesting spatial increases in ozone production following NOx reductions. These findings demonstrate that a comprehensive understanding of VOC oxidation, particularly oxygenates from industrial sources, is essential for representing winter photochemistry. PANs measurements provide critical constraints on oxidation processes and their implications for emission control.
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Status: open (until 23 Mar 2026)
- RC1: 'Comment on egusphere-2025-6264', Anonymous Referee #1, 04 Mar 2026 reply
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RC2: 'Comment on egusphere-2025-6264', Anonymous Referee #2, 06 Mar 2026
reply
The paper describes measurements of PAN and its homologues over South Korea in wintertime during the ASIA-AQ campaign. It shows how the measurements of PAN homologues can be used as tracers for industrial emissions, which do not have long atmospheric lifetimes and it provides further understanding of wintertime oxidation of complex VOC mixtures. I would recommend publishing the paper after minor corrections as suggested below.
General comment:
A lot of the figures contain a lot of information (and a lot of panels and sub-panels) and for some of them I wonder if it would be easier to follow the story if the figures were split into smaller ones when the different panels are not directly linked. However, I’m also aware that it would bring the total amount of figures to a quite high amount.
Minor comments:
Line 142: How much isotopically labelled PAN do you calibrate with? Could there be any problems with overlaps between PAN at m/z 59 and your calibration at m/z 61 when adding isotopic PAN continuously?
Line 312-314: You observed higher APAN towards the boundaries of your investigation area and explain the increase to the southeast by local influence from the Gumi Industrial Complex through northeasterly winds. However, when looking at figure 2, the Gumi Complex looks like it is to the southeast of your southeast measurements. Does that mean the emissions from the Gumi Complex is transported north and then west or does Figure 2 not show all the data? Could it be other industrial areas further to the east that causes the increase in the southeast?
Line 486-487: You write “The general increase in P(O3) is due to increasing OH reactivity with NOx, as the calculation does not account for suppression of OH at high NOx levels.” Does that mean the calculated increase is significantly overestimated?
Line 488-494: If ketones and alcohols are important contributors, how come alkanes, alkenes and aromatics are not, when they all have similar contributions according to the pie charts when over South Korea (SMA and MS) and alkanes and alkenes are at least double that of ketones and alcohols over the yellow sea?
Line 508: Does “episodic enhancements of alkenes” refer to the one “spike” observed in Figure 7d at around 55 data points? Or are there more episodes in data not shown here since it is attributed to petrochemical plumes? I also think that some clarification on what is used for the x-axis in this figure would be good.
Line 523: The percentage of the radical loss due to reactions with NOx sounds like it only includes the production of HNO3 and PANs, however, it also includes the small contributions leading to the formation of alkyl nitrates and aromatic nitrates. Could you make that clear in the text? Although later these pathways are mentioned as minor radical termination pathways (line 525), so I would suggest choosing one place to include them.
Line 530: Is the median of 0.9 a median across all the measurements over land or are each data point a median of a group of measurements? If the median of all the measurements is 0.9, then I would suggest writing “Figure 8a shows a median Ln/Q value of 0.9 over South Korea, …”.
Figure 8, panel b-d: What do the numbers in the legend after base describe in these three panels?
Figure S6: It looks like there are multiple very different population densities plotted for some of the longitudes, can you explain that? Is that from flying over the same longitudes, but different latitudes and then averaging the aromatics across the latitudes? And across the five flights used for this analysis?
Technical comments:
Abstract: O3 and NO2 are written without the numbers in subscript
Figure S1: The orange square is very difficult to see under the red flight tracks. Could you make it a different color?
Section 2.3.1: There are some inconsistencies whether things are written in italics or not. For example, in line 185 you write RXi, however, in equation (1) it says RXi. OH is written in italics in equation (3). In line 203 OVOC is written both in italics and normal. I would suggest reading through the section to check for these inconsistencies.
Line 171: The Seoul Metropolitan Area has been defined as SMA in the introduction, but here SMA is given as Seoul and Suburban, which is probably the same as the Metropolitan area, but I would suggest choosing one definition.
Line 193: In equation (3) RVOCi represent the OH reactivity for VOCs i, but in line 193 it is defined as RVOCs.
Line 199: I would add commas or brackets around P(HNO3) and P(PANs).
Line 200: Add an s after radical.
Line 201 and equation 5: Do you want to call it αHA or αHydrogen Abstraction?
Line 277: A punctum is missing after environments.
Line 282: add “the” to write “and the sum of…”
Line 314: Do you mean the PANs distribution in Figure 1a?
Line 339-340: “During ASIA-AQ” is repeated in the sentence.
Line 381: Add an s to contribution
Line 391: I think you mean assess instead of access
Line 485: insert “an” before OH concentration
Line 499: add “the” in front of “VOC classes”
Line 558: Is the Romer et al. reference the same as the one mentioned in line 545 or is it actually a different study?
Line 620: Maybe add “other” in front of precursors
Equation S1-S6: Molecules are usually not written in italics.
Citation: https://doi.org/10.5194/egusphere-2025-6264-RC2
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This is a well written and well-cited paper describing data of PAN and a range of different PAN-type compounds taken over aircraft flights over S. Korea in the winter of 2024. It contains novel data that will contribute to the literature of these compounds, especially photochemical production during the wintertime. The collection of measurements allows model calculations to be well constrained and to address questions of precursors in ways that were hard to imagine in the recent past. I think that the paper mostly stands on its own without need for significant revisions, although I have some concerns and suggestions.
As is common with these papers, there are a lot of abbreviations and acronyms used. I would urge the authors to clearly define all terms.
For example, What exactly does “OH exposure” (first mentioned on Line 320) mean? It is described later in the text of Section 4, but without sufficient explanation. Also, how is the term 𝜒 being used in this manuscript? And is “zero-out sensitivity analysis (Line 434) explained somewhere?
Perhaps the strongest comment I have is about the density of information crammed into the figures in this manuscript. There are sub-figures within sub-figures within sub-figures that sometimes serve more to obscure than enlighten the significant story the authors are telling. In some cases (e.g. Figures 4 & 6) legends actually cover up some of the data because the figures are so crammed. It’s also not always apparent how the sub-figures are grouped together. I understand the desire to convey as much information as possible, but I wonder if the authors would consider simplifying the figures either by separating them into more individual figures or summarizing their points in the text (for example by creating a table with correlation parameters in lieu of all the scatterplots) and/or putting some of the figures into the supplemental materials. I would encourage the authors to consider which graphics are essential for conveying their story and which are not. This would have the additional benefit of simplifying the captions to be more readable.
Specific comments:
Line 304 – I would delete the first sentence of this paragraph as it seems redundant.
Line 307 - What is the reference for the short atmospheric lifetime of APAN?
Line 326 – Also, give a reference for the attribution of biomass burning with HCN and CH3CN.
Line 370 – It would be good to explain why Figure 4a is on a log scale
Line 399 – I would replace the word “increased” with “higher”
Line 540 - I might add at the end of the paragraph something along the lines of “…especially given the importance of anthropogenic VOC like aromatics, as shown above.”
Line 542 - Why is the heterogeneity so “marked” given the geographic range of the flights? Is heterogeneity the best term to describe what the authors are saying?
Line 550 – Can the authors address the deviation of the green and purple traces at low RNO2/RALD?
At the end of the paper I was left wanting to have more closure on the comparison of wintertime chemistry to summertime chemistry and how that can be applied to management strategies.