Impacts of synoptic circulation types on nocturnal ozone increase in the North China Plain: meteorological drivers and physical mechanisms

Abstract. Nocturnal ozone increase (NOI) has become common worldwide, raising the risk of ozone pollution. Weather types serve as one of the critical drivers of NOI, but how they influence NOI remains poorly understood. Using ground observation and reanalysis datasets, this study identified the relationship between weather types and NOI days in a representative city on the North China Plain, and investigated the meteorological characteristics and formation mechanisms of NOI days under different weather types. During 2021–2023, 392–470 NOI days were observed, with the occurrence predominantly concentrated between 00:00 and 06:00 local time (71.35 %). Based on the Lamb-Jenkinson method, NOI days were classified into four primary weather types: A-type (57.36 %), C-type (11.17 %), S-type (20.81 %) and WNE-type (10.66 %). A-type was dominated by high-pressure anticyclones with mild weather condition, whereas C-type was controlled by low-pressure systems and influenced by moist airflows. S-type was located in the transition zone between high and low pressure, influenced by southern geostrophic winds, and WNE-type was characterized by high-pressure systems with W-N-E geostrophic flows. Notably, under all four weather types, the formation of NOI days was attributed to the synergy of regional and vertical transport. Specifically, under S-type and WNE-type, regional transport was influenced by geostrophic winds, while vertical transport was associated with high-pressure subsidence (A-type and WNE-type), cold pool subsidence (C-type), and shallow convection (S-type). This study elucidates the diverse dynamical pathways of weather-driven NOI, and provides mechanistic insights for improving ozone forecasting and mitigation strategies in urban environments.