Abstract. Ozone (O₃) is an important secondary pollutant that impacts air quality and human health. Eastern Asia has high regional O₃ background due to the numerous sources and increasing and rapid industrial growth, which also impacts the Seoul Metropolitan Area (SMA). However, the SMA has also been experiencing increasing O₃ driven by decreasing NO_x emissions, highlighting the role of the local in situ O₃ production on the SMA. Here, comprehensive gas-phase measurements collected on the NASA DC-8 during the National Institute of Environmental Research (NIER)/NASA Korea–United States Air Quality (KORUS-AQ) study are used to constrain the instantaneous O₃ production rate over the SMA. The observed NO_x oxidized products support the importance of non-measured peroxy nitrates (PNs) in the O₃ chemistry in the SMA, as they accounted for ∼49 % of the total PNs. Using the total measured PNs (ΣPNs) and alkyl and multifunctional nitrates (ΣANs), unmeasured volatile organic compound (VOC) reactivity (R(VOC)) is constrained and found to range from 1.4–2.1 s⁻¹. Combining the observationally constrained R(VOC) with the other measurements on the DC-8, the instantaneous net O₃ production rate, which is as high as ∼10 ppbv h⁻¹, along with the important sinks of O₃ and radical chemistry, is constrained. This analysis shows that ΣPNs play an important role in both the sinks of O₃ and radical chemistry. Since ΣPNs are assumed to be in a steady state, the results here highlight the role that ΣPNs play in urban environments in altering the net O₃ production, but ΣPNs can potentially lead to increased net O₃ production downwind due to their short lifetime (∼1 h). The results provide guidance for future measurements to identify the missing R(VOCs) and ΣPN production.