Persistent O3 pollution over the North China Plain during the advancement of East Asian summer monsoon: Process analysis and interannual difference

Abstract

The North China Plain (NCP) experiences the most severe persistent (≥3 days) ozone pollution (POP) across China. The study investigates the POP events during the advancement of EASM using the regional climate-chemistry-ecology model RegCM-Chem-YIBs. Summers from May to August are divided into the Meiyu-flood (MYF) period, and the periods before and after. Over the past ten years, most POP events occurred in the MYF period and its preceding period, with the longest duration and highest intensity during the MYF period. A notable peak was observed in 2017, with a significant increase in POP frequency across all three monsoon periods. Process analysis and sensitivity experiments for the 2017 events were conducted to elucidate the underlying mechanisms. In the PreF period, POP events are associated with the Baikal ridge and a high-pressure system over the NCP region. Chemical reactions, vertical advection, and turbulent transport facilitate the accumulation of surface O₃, while horizontal advection has a negative effect. The most severe POP events occur in the MYF period, driven by a stable and strong Baikal ridge and a relatively weak Western Pacific Subtropical High (WPSH). Local chemical reactions and advections contribute significantly to these events. In the PostF period, POP events are rare due to a significant decrease in solar radiation. Compared to 2016, 2017 featured a stronger and earlier-established Baikal ridge and a weaker, later-jumped WPSH, leading to an increase of 2.0–2.3 ppb hr⁻¹ in the O₃ chemical production rate during early summer. Our findings indicate that the timing of the Baikal ridge establishment and the WPSH's strength and movement significantly influence POP events over the NCP. Interannual differences in the EASM process can modulate local atmospheric circulation and determine POP frequency.