Using observed urban NOx sinks to constrain VOC reactivity and the ozone and radical budget in the Seoul Metropolitan Area

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 NOx 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 NOx 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 (6PNs) and alkyl and multifunctional nitrates (6ANs), 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 6PNs play an important role in both the sinks of O3 and radical chemistry. Since 6PNs are assumed to be in a steady state, the results here highlight the role that 6PNs play in urban environments in altering the net O3 production, but 6PNs 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 6PN production.