Insights on Ozone Formation Sensitivity in Southeast and East Asian Megacities during ASIA-AQ

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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.
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Funding Information
  • Earth Sciences Division (80NSSC21K1704)
  • Earth Sciences Division (80NSSC23K0818)
  • Earth Sciences Division (80NSSC23K0819)
  • Earth Sciences Division (80NSSC23K0822)
  • Earth Sciences Division (80NSSC23K0824)
  • Earth Sciences Division (80NSSC23K0825)
  • Earth Sciences Division (80NSSC23K0826)
  • Earth Sciences Division (80NSSC24K0005)
  • National Center for Atmospheric Research (1852977)
  • Bundesministerium für Klimaschutz, Umwelt, Energie, Mobilität, Innovation und Technologie (ASAP 2022, FO999900547)
  • Politechnika Gdańska (DEC-4/1/2024/IDUB/II.1b/Am)