Synthetically impacts of the topography and typhoon periphery on the atmospheric boundary layer structure and special regional pollution pattern of O3 in North China Plain

Westerly warm advection from the Loess Plateau to the over layer of North China Plain (NCP) suppresses the mixed layer during the daytime in summer, having already be confirmed as an important formation mechanism of regional O3 pollution all over NCP. However, when NCP was simultaneously affected by the westerly warm advection from the Loess Plateau and the typhoon periphery airflow from the Western Pacific tropical cyclone system, a rare pollution pattern of surface O3 was detected, characterized by a very low ozone pool in Beijing (LOP-BJ) contrast to much higher O3 in surrounding areas. To explore the causes and deep formation mechanism to this special pollution phenomenon, regional pollution characteristics of O3, characteristics of the atmospheric boundary layer (ABL) structure and vertical airflows affected by different weather systems were analyzed based on the analysis of observations and three dimensional simulations. It was found that the synthetic impacts of the topography and the typhoon peripheral flow promoted the stabilization of the ABL and introduced a nearly isolated air mass in terms of both thermodynamic and dynamic structures within ABL in Beijing region. In the stable and isolated ABL air masses, the significant lower ozone near the ground was primarily attributed to the strong loss by NO titration consumption from local excessive emissions and extremely weak transport contributions from surrounding areas. The conclusion was also confirmed by the integrated process rate (IPR) analysis results of contributions from chemistry tendency (CHEM) and total transport tendency (TRAN) to surface ozone. Our work insight the understanding of the formation mechanism of regional ozone pollution and improve the forecasting capabilities under comprehensive weather conditions over NCP.