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

[摘要] The Pearl River Delta (PRD) region in southern China has been subject to severe ozone ( O 3 ) pollution during daytime and anomalous nocturnal O 3 increase (NOI) during nighttime. In this study, the spatiotemporal variation of NOI events in the PRD region from 2006 to2019 is comprehensively analysed, and the role of vertical transport in theoccurrence of NOI events is quantified based on observed surface andvertical O 3 and the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) dataset. The results show that the average annual frequency of NOI events in the whole PRD region during the 14 year period is estimated to be 53  ±  16  d yr −1 , with an average of 58  ±  11  µg m −3 for the nocturnal O 3 peak(NOP) concentration. Low-level jets (LLJs) are the main meteorologicalprocesses triggering NOI events, explaining on average 61 % of NOI events. Annual NOI events exhibit an upward trend before 2011 (4.70  d yr −1 ) and a downward trend thereafter ( − 0.72  d yr −1 ), which is consistent with the annual variation of LLJs ( r =0.88 , p <0.01 ). Although the contribution of convective storms (Conv) to NOI events is relatively small with an average value of 11 %, Conv-induced NOI events steadily increased at a rate of 0.26  d yr −1 during this 14 year period due to the impact of urbanisation. Seasonally, a relatively higher frequency of NOI events isobserved in spring and autumn, which is consistent with the seasonal pattern of LLJs and maximum daily 8 h average (MDA8) O 3 . Spatially, NOI events are frequent in the eastern PRD, which agrees well with the spatial distribution of the frequency of LLJs and partially overlaps with the distribution of MDA8 O 3 concentration, suggesting that vertical transport plays a more important role in NOI events than daytime O 3 concentration. The Weather Research and Forecasting (WRF) model coupled with the Community Multiscale Air Quality (CMAQ) model and the observed vertical O 3 profiles are further applied to illustrate the mechanisms of NOI formation caused by LLJs and Conv. The results confirm that both LLJs and Conv trigger NOI events by inducing downdrafts with the difference being that LLJs induce downdrafts by wind shear, while Conv by compensating downdrafts. Through observational and modelling analysis, this study presents the long-term (2006–2019) trends of NOI events in the PRD region and quantifies the contribution of meteorological processes for the first time, emphasising the importance of vertical transport, as well as daytime O 3 concentration for the occurrence of NOI events.

[学科分类] 医学（综合）