Interpretation of decadal-scale ozone production efficiency in the Seoul Metropolitan Area: Implication for ozone abatement

Abstract

Understanding the drivers of urban ozone (O3) formation is challenging and requires uncovering the non-linear relationship between O3 and its precursors. We used a novel method to differentiate between background O3 levels and local photochemical O3 generation using the observation-based O3 production efficiency (OPE) from decade-long observations including nitrogen dioxide (NO2), nitrogen monoxide (NO), reactive nitrogen compounds (NOy), O3, and 56 volatile organic compounds (VOCs) covering eight years (2009–2016). In this study, we employed Ox (=O3+NO2), and defined OPEx as the number of Ox molecules produced from the oxidation of a single NOx molecule, and estimates the local contribution (in %) to total O3 from the slope of the Ox−NOz(=NOy–NOx) linear regression. We thus identified the local urban O3 formation regime using the relationship of several factors, including nitrogen oxide (NOx), VOC/NOx, NO2, NOz, and long-range transported Ox, and suggested how O3 can be reduced in the Seoul Metropolitan Area (SMA).The results show that the regional background dominates the annual level of Ox with a contribution of about 66 ± 11%, while photochemical production acts as a stronger factor influencing the annual variability of Ox than regional background. Regional background varies by transport evidenced by its good correlation with the west wind fraction, whereas photochemical production depends on the VOC/NOx ratio, which is high in summer and low in winter. The relationship between photochemical production and VOC/NOx indicates that the transition of the Ox formation regime occurs at a VOC/NOx ratio of approximately 4 in the SMA. Urban environments with NOx > 20 ppb typically lie in low VOC/NOx zone, where reduced NO2 emissions lead to decreased NOz but increased OPEx, with no discernible change in photochemical Ox production. Thus, NOx emission reductions in urban environments would have a negligible effect on Ox reduction. In contrast, suburban environments such as usually NOx < 10 ppb are located in the higher VOC/NOx zone, where decreased NO2 leads to decreased OPEx with no discernible change in NOz, consequently suppressing photochemical Ox production. Together, these results suggest that any urban O3 policy in the SMA should consider significant cuts in NOx emissions to allow the O3 regime to shift. The OPEx method can also be applied to other urban areas for the development of effective O3 reduction policies.