Observation-Based Modeling of O3–Precursor Relationships in Nanjing, China

Abstract

Surface ozone (O3) is formed through a series of photochemical reactions of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the atmosphere. In recent years, ozone concentrations in Nanjing have been increasing. To effectively mitigate ozone pollution, it is essential to understand the relationship between O3 and its precursors. In this study, the observation-based model (OBM) coupled with the Master Chemical Mechanism (MCM), was applied to investigate the O3-NOx-VOCs relationship in the Nanjing Metropolitan Area of China in summer 2015. The OBM model well reproduces the levels and diurnal variations of O3 with a high index of agreement value. Analysis of ozone formation potential (OFP) indicates that although observed alkanes are the most abundant VOCs (46.8%), their contributions to OFP are relatively small due to lower maximum incremental reactivity (MIR). Aromatics contribute the most to OFP (61.6%), followed by alkenes (18%). These two groups dominate in the top ten VOCs of OFP. In particular, m/p-Xylene shows a significant contribution to OFP with the highest OFP value over 30 µg/m3. The relative incremental reactivity (RIR) results demonstrate that the reduction of anthropogenic VOCs (AVOCs) is the most efficient way to mitigate local O3 pollution in Nanjing. Specifically, m/p-Xylene emissions should be reduced at it shows the highest RIR among all the AVOCs. Based on the results of OBM, a cutting ratio of AVOCs to NOx of more than 0.46 is proposed to implement efficient control measurements in Nanjing for the study period.