the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Jul 2026
Emission control redirects aerosol formation toward nocturnal oxidant chemistry: Observations at a Korean petrochemical complex
Abstract. Industrial emission-control programs limit primary emissions to reduce aerosols, yet certain chemical and meteorological conditions can paradoxically enhance secondary formation. We report wintertime measurements of submicron aerosol composition at the Daesan Petrochemical Industrial Complex, South Korea (2023–2024), using a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) with trace-gas and meteorological data. Although mean aerosol concentrations were modest (14.9 ± 16.9 μg m⁻³), episodic pollution was dominated by secondary species (86 % of total mass). The atmosphere exhibited a titration-suppressed, oxidant-rich regime, with elevated ozone (30–40 ppb) maintained by reduced NO titration. This sustained nighttime oxidation via NO₃ and N₂O₅ chemistry, driving nitrate formation through heterogeneous hydrolysis and secondary organic aerosol (SOA) production after sunset. Comparable day–night correlations between Ox (= O₃ + NO₂) and the more-oxidized organic aerosol fraction (r ≈ 0.5) indicate that nocturnal oxidation of industrial VOCs drives SOA formation. Meteorological stagnation amplified these processes: nitrate and MO-OOA were enhanced 6.6-fold and 3.1-fold relative to general conditions while O₃ rose only ~10 %, demonstrating that the enhancement was driven by chemical conversion rather than accumulation, also supported by elevated ∆SOA/∆CO ratios during stagnation. NOx-only emission control can therefore shift industrial atmospheres toward nocturnal, titration-suppressed regimes; effective mitigation requires coordinated management of reactive VOCs.
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Status: open (until 12 Aug 2026)
- RC1: 'Comment on egusphere-2026-2795', Anonymous Referee #2, 19 Jul 2026 reply
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RC2: 'Comment on egusphere-2026-2795', Anonymous Referee #1, 19 Jul 2026
reply
Kim et al. present an interesting study on the transition from daytime photochemistry to nighttime chemistry based on comprehensive field observations conducted at the Daesan Petrochemical Industrial Complex. The authors demonstrate that under the emission-control strategy, the daytime NO titration effect is weakened, allowing O3 to accumulate and thereby enhancing nighttime nitrate and SOA formation. The study provides valuable insights into the atmospheric chemical response to NOx emission reductions and has important implications for future air pollution control strategies. Overall, the manuscript is well organized and suitable for publication in ACP after minor revision. The following comments should be addressed before publication.
Specific comments
- Lines 14–15: Does the reported concentration refer to PM1 or PM2.5? Please clarify.
- Lines 19–20: The reported correlation appears to result from the co-production of gas- and particle-phase species. However, the underlying mechanisms during daytime and nighttime may differ substantially. In particular, nighttime O3 and NO2 chemistry is driven by different oxidation pathways than daytime photochemistry. Although similar correlations are observed, the associated chemical mechanisms may not be the same. Please clarify this point.
- Line 25: Please provide the full name of VOCs when it first appears. The same should be done for other abbreviations throughout the Abstract.
- Line 59: It would be helpful to provide additional background information on the emission-control strategy, including its implementation and achivement.
- Lines 83–86: The final paragraph of the Introduction does not appear to follow the standard format recommended by ACP. Please revise it to comply with the journal style.
- Line 100: Please specify the time resolution for each measurement mode.
- Line 145: Please use a consistent abbreviation for HR-ToF-AMS throughout the manuscript.
- Line 190: Does the reported aerosol concentration include black carbon (BC)?
- Line 193: Please provide a brief description of the emission-control strategy and its expected impacts so that readers can better understand the background of this study.
- Line 194: Please provide appropriate references to support this statement.
- Lines 200–201: If this PMF factor is strongly associated with waste incineration emissions, its current name may not accurately represent its source. Please provide additional evidence and references supporting the current source attribution.
- Section 3.2: This is the core section of the manuscript and provides a clear explanation of the transition from daytime photochemistry to nighttime chemistry. The discussion of qualitative contribution of nighttime chemistry to nitrate formation may be useful.
- Line 282: PMF.
- Section 3.2: The relationship between VOCs and MO-OOA is interesting and provides useful information on the precursors of oxidized organic aerosol. However, oxidation of VOCs is generally expected to proceed from freshly formed LO-OOA to more aged MO-OOA. Why is no comparable relationship observed between VOCs and LO-OOA? Additional discussion would help clarify this result.
- Line 320: The reference format is incorrect.
- Line 325: Please provide more details about the stagnant periods discussed here. Did these periods mainly occur during nighttime? In addition, it would be useful to compare the concentrations of the major species between daytime and nighttime during these periods.
- Figure 2: For the time series shown in panels (i–l), several data points appear to be missing while continuous lines are still plotted. This presentation may give a misleading impression of continuous concentration changes. The missing values should be masked (e.g., as NaNs) so that discontinuities are correctly displayed.
Citation: https://doi.org/10.5194/egusphere-2026-2795-RC2
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- 1
This manuscript presents a study investigating the aerosol formation due to nocturnal oxidant chemistry based on observations at an emission-controlled petrochemical complex in South Korea. The topic of this study falls within the scope of the journal Atmospheric Chemistry and Physics. This manuscript is generally laid out well and shows its academic value. This manuscript is recommended to be published after addressing the following minor concerns.
Line 98: What does NR stand for? Does it mean non-refractory? Please provide the full form first before using the acronym by itself.
Line 138: Please provide the name and definition of m/z even though this is a common parameter in the field of mass spectrometry.
Line 304: What does MLH stand for? Does it stand for mixing layer height? Please provide the full form first before using the acronym by itself.
Line 323: Please provide the full form of AGL. Does it stand for above ground level?
Figure 1: Please provide the orientation in the map for clarity and to ensure academic rigor.
Figure 3: It seems that mixing layer height is also provided but not mentioned in the caption for Panel (b).
Figure 3: Total Particulate Matter in the literature is usually referred to as TSP (total suspended particulate) rather than PM1 (Particulate Matter with aerodynamic diameters less than or equal to 1 micrometer). Please revise the caption for Panel (e) to avoid confusion.
Figure 4: It seems that Panel (b) shows not only from 01:00 to 07:00 but from 00:00 to 24:00. Please revise either the caption or the figure.
Figure 4: Panel (c) seems to show NO3 production on the left vertical axis and [NO2]*radiation on the right vertical axis, which are not N2O5 concentration and not consistent with the caption. Please revise the manuscript to ensure consistency.
Figure 5: What Ox concentration range is used for each box plot? Please provide the description in the caption for readability.
Figure 6: What is the range of binned VOC levels for each box plot? Please provide the description in the caption for readability.
Table 1: What does the bold font mean for each parameter shown in Panel (c)?