Quantitative impacts of vertical transport on the long-term trend of nocturnal ozone increase over the Pearl River Delta region during 2006–2019

Abstract. The Pearl River Delta (PRD) region in southern China has been subject to severe ozone (O3) pollution during daytime and anomalous nocturnal O3 increase (NOI) nighttime. In this study, the spatiotemporal variation of NOI events PRD from 2006 2019 is comprehensively analysed, role vertical transport occurrence quantified based on observed surface fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) dataset. results show that average annual frequency whole 14 year period estimated be 53 ± 16 d yr−1, with an 58 11 µg m−3 peak (NOP) concentration. Low-level jets (LLJs) are main meteorological processes triggering events, explaining 61 % events. Annual exhibit upward trend before 2011 (4.70 yr−1) a downward thereafter (−0.72 yr−1), which consistent LLJs (r=0.88, p<0.01). Although contribution convective storms (Conv) relatively small value %, Conv-induced steadily increased at rate 0.26 yr−1 due impact urbanisation. Seasonally, higher spring autumn, seasonal pattern maximum daily 8 h (MDA8) O3. Spatially, frequent eastern PRD, agrees well spatial distribution partially overlaps MDA8 concentration, suggesting plays more important than Research Forecasting (WRF) model coupled Community Multiscale Air Quality (CMAQ) profiles further applied illustrate mechanisms formation caused by Conv. confirm both Conv trigger inducing downdrafts difference being induce wind shear, while compensating downdrafts. Through observational modelling analysis, study presents long-term (2006–2019) trends quantifies first time, emphasising importance transport, as concentration