A Better Understanding of an Extremely High Ozone Episode with Ensemble Simulation

Severe ozone pollutions may occur in the Great Bay Area (GBA) when typhoons approach South China. However, numerical models often fail to capture the high ozone concentrations during the episodes, leading to uncertainties in understanding their formation mechanisms. This study conducted an ensemble simulation with 30 members (EMs) using the WRF-Chem model, coupled with a self-developed ozone source apportionment method, to analyze an extremely high ozone episode associated with Typhoon NIDA in the summer of 2016. The newly proposed index effectively distinguished between well-performing (good) and poorly performing (bad) EMs. Compared to the bad EMs, the good EMs accurately reproduced surface ozone variations, particularly capturing the extremely high concentrations observed in the afternoon of July 31. The formation of such high ozone levels was attributed to the retention of ozone in the residual layer at night and the enhanced photochemistry during daytime. As Typhoon NIDA approached, weak winds confined large amounts of ozone in the residual layer at night. The development of planetary boundary layer (PBL) facilitated the downward transport of ozone aloft, contributing to the rapid increase in surface ozone in the following morning. The enhanced photochemistry was primarily driven by increased ozone precursors resulting from favorable accumulation conditions and enhanced biogenic emissions. During the period of high ozone concentrations, contributions from local and surrounding regions increased. Additionally, ozone from southeastern Asia could transport to the GBA at high altitudes and then contribute to surface ozone when the PBL developed.当台风逼近华南地区时，大湾区（Great Bay Area, GBA）可能会出现严重的臭氧污染。然而，数值模式往往无法捕捉该类污染事件中的高臭氧浓度，导致对其形成机制的认知存在不确定性。本研究采用WRF-Chem模式（Weather Research and Forecasting with Chemistry, WRF-Chem），结合自主研发的臭氧源解析方法，开展了包含30个集合成员（Ensemble Members, EMs）的集合模拟试验，以分析2016年夏季与台风妮妲（Typhoon NIDA）相关的一次极端高臭氧污染事件。本文提出的新指标可有效区分模拟表现优异与表现不佳的集合成员。相较于表现不佳的集合成员，表现优异的集合成员能够准确再现地表臭氧的变化特征，尤其精准捕捉到7月31日午后观测到的极端高臭氧浓度。此类高臭氧浓度的形成，归因于夜间臭氧在残留层（residual layer）的滞留，以及日间光化学反应的增强。随着台风妮妲逼近，弱风场使得夜间大量臭氧被限制在残留层中。行星边界层（Planetary Boundary Layer, PBL）的发展促进了高空臭氧的向下输送，使得次日清晨地表臭氧浓度快速升高。增强的光化学反应主要由有利的累积条件下臭氧前体物浓度升高，以及生物源排放增强共同驱动。在高臭氧浓度时段，本地及周边区域的臭氧贡献占比有所提升。此外，当行星边界层发展时，源自东南亚的臭氧可通过高空输送抵达大湾区，并对地表臭氧浓度产生贡献。