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

Nocturnal ozone (O ₃ ) increase (NOI) has become common worldwide, raising the risk of O ₃ pollution. While weather types serve as critical drivers of NOI, their specific dynamic influences remain poorly understood. Using ground observations and reanalysis datasets, this study identified the relationship between weather types and NOI days in a representative city in the North China Plain, and revealed the meteorological characteristics and physical mechanisms driving these events. During 2021–2023, 370–444 NOI days were observed, with the occurrence predominantly concentrated between 00:00 and 06:00 local time (69.53 %). After screening, a total of 171 representative NOI days were finally identified to characterize the overall occurrence of local NOI events. Based on the Lamb-Jenkinson method, the representative NOI days were classified into four primary weather types: A-type (57.89 %), S-type (19.88 %), WNE-type (14.04 %), and C-type (8.19 %). A-type was dominated by high-pressure anticyclones with calm conditions, 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. Distinct mechanical triggers for NOI were further resolved for different types: A-type is dominated by high-pressure subsidence; C-type is driven by cold-pool downdrafts and convective motions; whereas both S-type and WNE-type are primarily controlled by synoptically-driven intense wind shear. This study elucidates the diverse dynamical pathways of weather-driven NOI, providing mechanistic insights for improving O ₃ forecasting and mitigation strategies in urban environments.