Quantitative impacts of vertical transport on 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 facing severe ozone (O₃) pollution during the day, as well as anomalous nocturnal O₃ increase (NOI) during the night. In this study, relying on observed surface and vertical O₃ and the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) dataset, the spatiotemporal variation of NOI events is comprehensively analysed and the role of vertical transport in the occurrence of NOI events is quantified in the PRD region from 2006 to 2019. Results show that the annual average frequency of NOI events is estimated to be 53 ± 16 d yr^-1 in the PRD region during the 14-year period, with 58 ± 11 μg m^-3 for the nocturnal O₃ peak (NOP) concentration on average. Low-level jets (LLJs) and convective storms (Conv) are the main meteorological processes that induce NOI events, explaining 61 % and 11 % of NOI events on average, respectively. Annually, NOI events exhibit an upward trend (4.70 d yr^-1) before 2011 and a downward trend (-0.72 d yr^-1) afterward, which is consistent with the annual variation of LLJs (r=0.88, p<0.01). Although the contribution of Conv to NOI events is relatively small, Conv-induced NOI events continuously increase at a rate of 0.26 d yr^-1 during this 14-year period owing to the effect of expanding urbanization. Seasonally, relatively higher frequency of NOI events is observed in spring and autumn, which is consistent with the seasonal pattern of LLJs and maximum daily average 8-h (MDA8) O₃. Spatially, NOI events are frequent in the eastern PRD, which matches well with the spatial distribution of LLJ frequency while partially overlapping with the distribution of MDA8 O₃ concentration, suggesting a more important role for vertical transport than for daytime O₃ concentration in NOI events, which is also supported by the difference in their annual trends between urban and rural areas. The WRF/CMAQ model and the observed vertical O₃ profiles are further applied to illustrate the mechanisms of NOI formation caused by LLJs and Conv. Results confirm that both LLJs and Conv trigger NOI events by inducing downdrafts, the difference being that LLJs induce them by shear and Conv by compensating downdrafts. Through an observational and modeling analysis, this study reveals the long-term (2006–2019) trends of NOI events in the PRD region and quantifies the contribution of meteorological processes for the first time, emphasizing the importance of vertical transport as well as daytime O₃ concentration for the occurrence of NOI events.