Insights on ozone formation sensitivity in Southeast and East Asian megacities during ASIA-AQ

Controlling ozone (O 3 ) in rapidly urbanizing megacities in Southeast and East Asia remains a challenge. O 3 is a secondary pollutant formed through nonlinear photochemical reactions with its precursors: nitrogen oxides (NO x ) and volatile organic compounds (VOCs). Observation-based quantification of precursor sensitivity remains scarce, limiting actionable O 3 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 O x (PO x , O x = O 3 + NO 2 ). Precursor sensitivity regimes were characterized for each megacity by generating isopleths of PO x across varying levels of NO x and VOCs. The analysis revealed that Manila and Tainan–Kaohsiung exhibited predominantly NO x -sensitive conditions, favoring NO x reduction as an effective O 3 mitigation strategy, while Bangkok showed a more mixed sensitivity, suggesting combined NO x and VOC reductions. In contrast, Seoul exhibited a primarily VOC-sensitive regime associated with its higher NO x conditions relative to the other cities, 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 NO x -sensitive regimes in the afternoon. These findings provide critical insights for formulating effective, city-specific O 3 control policies in urban environments.