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https://doi.org/10.5194/egusphere-2025-6434
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2025-6434
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
08 Jan 2026
| 08 Jan 2026
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).
Insights on Ozone Formation Sensitivity in Southeast and East Asian Megacities during ASIA-AQ
Abstract. Controlling ozone (O3) in rapidly urbanizing megacities in Southeast and East Asia remains a challenge. O3 is a secondary pollutant formed through nonlinear photochemical reactions with its precursors: nitrogen oxides (NOx) and volatile organic compounds (VOCs). Observation-based quantification of precursor sensitivity remains scarce, limiting actionable O3 control. To address this, we leverage airborne observations from the NASA DC-8 during the ASIA-AQ campaign conducted in February and March 2024 across four Asian megacities: Metro Manila, the Seoul Metropolitan Area, the Tainan-Kaohsiung Metropolitan Area, and the Bangkok Metropolitan Region. These extensive measurements of various trace gases were used to constrain a zero-dimensional box model and estimate the net production rates of Ox (POx, Ox = O3 + NO2). Precursor sensitivity regimes were characterized for each megacity by generating isopleths of POx across varying levels of NOx and VOCs. The analysis revealed that Manila and Tainan-Kaohsiung exhibited predominantly NOx-sensitive conditions, favoring NOx reduction as an effective O3 mitigation strategy, while Bangkok showed a more mixed sensitivity, suggesting combined NOₓ and VOC reductions. In contrast, Seoul, under colder and low solar irradiance conditions, exhibited a primarily VOC-sensitive regime, underscoring the importance of VOC-focused strategies. In addition, to quantitatively assess sensitivity transitions, we computed orthogonal distances from the isopleth transition boundaries for all four study areas. Diurnal analyses of these distances revealed a shift from more VOC-sensitive conditions in the morning toward more NOx-sensitive regimes in the afternoon. These findings provide critical insights for formulating effective, city-specific O3 control policies in urban environments.
How to cite. Cho, C., Franchin, A., Flocke, F., Lesko, K., Owen, C., Hall, S. R., Ullmann, K., Apel, E. C., Hills, A. J., Hornbrook, R. S., Roozitalab, B., Jeong, D., Diskin, G. S., Choi, Y., DiGangi, J. P., Miech, J., Wolfe, G. M., Hanisco, T. F., St. Clair, J. M., Liao, J., Delaria, E. R., Sebol, A., Hannun, R. A., Wennberg, P. O., Ball, K., Lee, Y. R., Huey, L. G., Tanner, D. J., Arterburn, L., Blake, D. R., Blake, N. J., Barletta, B., Meinardi, S., Min, K.-E., Kang, H., Nam, W., Wisthaler, A., Piel, F., Wojnowski, W., Dibb, J., and Crawford, J.: Insights on Ozone Formation Sensitivity in Southeast and East Asian Megacities during ASIA-AQ, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-6434, 2026.
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Changmin Cho
Atmospheric Chemistry Observations & Modeling Laboratory, NSF National Center for Atmospheric Research, Boulder, CO, USA
Alessandro Franchin
CORRESPONDING AUTHOR
Atmospheric Chemistry Observations & Modeling Laboratory, NSF National Center for Atmospheric Research, Boulder, CO, USA
Frank Flocke
Atmospheric Chemistry Observations & Modeling Laboratory, NSF National Center for Atmospheric Research, Boulder, CO, USA
Kirk Lesko
Atmospheric Chemistry Observations & Modeling Laboratory, NSF National Center for Atmospheric Research, Boulder, CO, USA
Courtney Owen
Atmospheric Chemistry Observations & Modeling Laboratory, NSF National Center for Atmospheric Research, Boulder, CO, USA
Samuel R. Hall
Atmospheric Chemistry Observations & Modeling Laboratory, NSF National Center for Atmospheric Research, Boulder, CO, USA
Kirk Ullmann
Atmospheric Chemistry Observations & Modeling Laboratory, NSF National Center for Atmospheric Research, Boulder, CO, USA
Eric C. Apel
Atmospheric Chemistry Observations & Modeling Laboratory, NSF National Center for Atmospheric Research, Boulder, CO, USA
Alan J. Hills
Atmospheric Chemistry Observations & Modeling Laboratory, NSF National Center for Atmospheric Research, Boulder, CO, USA
Rebecca S. Hornbrook
Atmospheric Chemistry Observations & Modeling Laboratory, NSF National Center for Atmospheric Research, Boulder, CO, USA
Behrooz Roozitalab
Atmospheric Chemistry Observations & Modeling Laboratory, NSF National Center for Atmospheric Research, Boulder, CO, USA
Daun Jeong
Atmospheric Chemistry Observations & Modeling Laboratory, NSF National Center for Atmospheric Research, Boulder, CO, USA
now at: Chemical Sciences Laboratory, Boulder, National Oceanic and Atmospheric Administration, CO, USA
now at: Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA
Glenn S. Diskin
NASA Langley Research Center, Hampton, VA, USA
Yonghoon Choi
NASA Langley Research Center, Hampton, VA, USA
Analytical Mechanics Associates, Hampton, VA, USA
Joshua P. DiGangi
NASA Langley Research Center, Hampton, VA, USA
Jason Miech
NASA Langley Research Center, Hampton, VA, USA
Glenn M. Wolfe
NASA Goddard Space Flight Center, Greenbelt, MD, USA
Thomas F. Hanisco
NASA Goddard Space Flight Center, Greenbelt, MD, USA
Jason M. St. Clair
NASA Goddard Space Flight Center, Greenbelt, MD, USA
University of Maryland Baltimore County, Baltimore, MD, USA
Jin Liao
NASA Goddard Space Flight Center, Greenbelt, MD, USA
University of Maryland Baltimore County, Baltimore, MD, USA
Erin R. Delaria
NASA Goddard Space Flight Center, Greenbelt, MD, USA
University of Maryland, College Park, MD, USA
Abby Sebol
University of Maryland, College Park, MD, USA
Reem A. Hannun
NASA Ames Research Center, Moffett Field, CA, USA
Paul O. Wennberg
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA
Katherine Ball
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA
Young Ro Lee
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
now at: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
L. Gregory Huey
School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
David J. Tanner
School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
Linda Arterburn
School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
Donald R. Blake
Department of Chemistry, University of California Irvine, Irvine, CA, USA
Nicola J. Blake
Department of Chemistry, University of California Irvine, Irvine, CA, USA
Barbara Barletta
Department of Chemistry, University of California Irvine, Irvine, CA, USA
Simone Meinardi
Department of Chemistry, University of California Irvine, Irvine, CA, USA
Kyung-Eun Min
Department of Environmental and Energy Engineering, Gwangju Institute of Science and Technology, Gwangju, South Korea
Heejoo Kang
Department of Environmental and Energy Engineering, Gwangju Institute of Science and Technology, Gwangju, South Korea
now at: Department of Chemistry, University of Colorado Boulder, Boulder, CO, USA
Woohui Nam
Department of Environmental and Energy Engineering, Gwangju Institute of Science and Technology, Gwangju, South Korea
Armin Wisthaler
Department of Chemistry, University of Oslo, Oslo, Norway
Institute for Ion Physics and Applied Physics, University of Innsbruck, Innsbruck, Austria
Felix Piel
Department of Chemistry, University of Oslo, Oslo, Norway
Wojciech Wojnowski
Department of Chemistry, University of Oslo, Oslo, Norway
Department of Analytical Chemistry, Gdańsk University of Technology, Gdańsk, Poland
Jack Dibb
Earth Systems Research Center, University of New Hampshire, Durham, NH, USA
James Crawford
NASA Langley Research Center, Hampton, VA, USA
Short summary
Ozone pollution is a challenge in Asian cities due to its nonlinear response to precursor controls. Using aircraft measurements, we examined how ozone production depends on nitrogen oxides and volatile organic compounds using a new isopleth-based approach. Ozone production is limited by nitrogen oxides in Metro Manila and Tainan–Kaohsiung, mixed in Bangkok, and limited by volatile organic compounds in Seoul. These results inform city-specific control strategies to reduce ozone in urban areas.
Ozone pollution is a challenge in Asian cities due to its nonlinear response to precursor...
Cho et al. present a comprehensive aircraft-constrained box modeling framework to examine the sensitivity of POx formation to NOx and VOCs in several urban environments. The study is carefully executed and clearly presented. However, I would like to suggest a few additions and clarifications that could further strengthen the manuscript.
The choice of a 750 m altitude threshold to represent near-surface conditions is reasonable and supported by vertical profiles (Fig. 2). However, the authors should test the sensitivity of this altitude to selecting another altitude and how it would impact the 4 cities considered in this study, particularly given potential vertical gradients in NOx, VOCs, and photolysis rates. A brief sensitivity test or justification based on boundary-layer height variability would improve the methodology.
Averaging observations over 1–2 h flight tracks may obscure rapid chemical transitions and plume heterogeneity, especially in high-NOx urban environments. The authors should discuss the implications of this temporal smoothing for nonlinear ozone chemistry and whether shorter averaging windows were tested.
MCM is a widely accepted chemical mechanism. However, it can be biased due to the neglection of heterogeneous chemistry, for example, HO2 uptake on aerosols, and the impact of halogens on secondary chemistry and radical budget. The authors can include a brief discussion on the direction and impact of these uncertainties.
"A gridded parameter space was created by scaling the observed 155 mean mixing ratios of NOx and total VOCs from 1 to 300%." Please clarify this scaling approach to improve reproducibility.
Section 2.3 is conceptually clear but not sufficiently detailed for full reproducibility. For example, grid resolution, ridge identification algorithm, and transition-line fitting are not sufficiently specified to allow full reproducibility of the orthogonal distance metric.
Section 3.2 discusses the individual POx isopleths. However, the authors may consider adding a brief comparative discussion of regime differences across cities and times of day, as this could further strengthen the interpretation
The FSS model and its setup should be included in the methodology section.
The authors state that the box model is constrained using meteorological variables and photolysis rates. Please clarify the source of these parameters (e.g., in situ aircraft measurements, radiative transfer calculations, or model-derived values), and briefly describe how they were implemented in the box model and potentially the bias associated with this.
The manuscript would also benefit from a brief discussion of how uncertainties in observed NOx, VOCs, and photolysis rates propagate into POx estimates and regime classification, particularly near the transition line.