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

Abstract

Ozone (O-3) is an important secondary pollutant that impacts air quality and human health. Eastern Asia has high regional O-3 background due to the numerous sources and increasing and rapid industrial growth, which impacts the Seoul Metropolitan Area (SMA). However, SMA has also been experiencing increasing O-3 driven by decreasing NOx emissions, highlighting the role of local, in-situ O-3 production on SMA. Here, comprehensive gas-phase measurements collected on the NASA DC-8 during the NIER/NASA Korea United States-Air Quality (KORUS-AQ) study are used to constrain the instantaneous O-3 production rate over the SMA. The observed NOx oxidized products support the importance of non-measured peroxy nitrates (PNs) in the O-3 chemistry in SMA, as they accounted for similar to 49 % of the total PNs. Using the total measured PNs (Sigma PNs) and alkyl and multifunctional nitrates (Sigma ANs), 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 O-3 production rate, which is as high as similar to 10 ppbv hr(-1), along with the important sinks of O-3 and radical chemistry, are constrained. This analysis shows that Sigma PNs play an important role in both the sinks of O-3 and radical chemistry. Since Sigma PNs are assumed to be in steady-state, the results here highlight the role Sigma PNs play in urban environments in reducing net O-3 production, but Sigma PNs can potentially lead to increased net O-3 production downwind due to their short lifetime (similar to 1 hr). The results provide guidance for future measurements to identify the missing R(VOCs) and Sigma PNs production.