Preprints
https://doi.org/10.5194/acp-2022-310
https://doi.org/10.5194/acp-2022-310
 
11 May 2022
11 May 2022
Status: a revised version of this preprint is currently under review for the journal ACP.

Unexpected high frequency of nocturnal surface ozone enhancement events over China: Characteristics and mechanisms

Cheng He1,2, Xiao Lu1,2, Haolin Wang1,2, Haichao Wang1,2, Yan Li3, Guowen He1,2, Yuanping He1,2, Yurun Wang4, Youlang Zhang1,2, Yiming Liu1,2, Qi Fan1,2, and Shaojia Fan1,2 Cheng He et al.
  • 1School of Atmospheric Sciences, Sun Yat-sen University, and Key Laboratory of Tropical Atmosphere-Ocean System, Ministry of Education, Zhuhai, China
  • 2Guangdong Provincial Observation and Research Station for Climate Environment and Air Quality Change in the Pearl River Estuary, Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China
  • 3School of Environmental Science and Engineering, China West Normal University, Nanchong, China
  • 4Department of Land, Air, and Water Resources, University of California Davis, Davis, USA

Abstract. Surface ozone concentrations typically peak in daytime driven by active photochemical production and decrease gradually after sunset by chemical destruction and dry deposition. Here, we report that nocturnal ozone enhancement (NOE, defined as ozone increase by at least 5 ppbv hour-1 in one of any two adjacent hours in 20:00–06:00 local time) events are observed at multiple monitoring sites in China at a high frequency that has not been recognized in previous studies. We present an overview of the general characteristics of NOE events in China and explore the possible mechanisms based on six-year observations from the national monitoring network. We find that the annual mean frequency of NOE events is 41±10 % averaged over all 814 Chinese sites in 2014–2019, which is 46 % larger than those over Europe and US. The NOE event frequency is higher in industrialized city clusters (>50 %) than regions with lighter ozone pollution, and is higher in the warm (46 %) than cold season (36 %), consistent with the spatiotemporal evolution of ozone levels. The mean ozone peak during NOE events reaches 37±6 ppbv in the warm season. The ozone enhancements are within 5–15 ppbv hour-1 in 85 % of the NOE events, but in about 10 % of the cases the ozone increases can exceed 20 ppbv hour-1. We propose that the high photochemistry-induced ozone in the daytime provides rich ozone source in the nighttime residual layer, determining the overall high frequency of NOE events in China, and then the enhanced atmospheric mixing triggers NOE events by allowing the ozone-rich air in the residual layer to mix into the nighttime boundary layer. This is supported by our analyses that 70 % (65 %) of the NOE events are associated with increases in friction velocity (planetary boundary layer height), indicative of enhanced atmospheric mixing, and also supported by the observed sharp decreases in surface NO2 and CO concentrations with ozone increases in NOE events, a typical signal of mixing with air in the residual layer. Three case studies in Beijing and Guangzhou shows that synoptic processes such as convective storms and low-level jets can lead to the NOE event by aggravating vertical mixing. Horizontal transport of ozone-rich plumes may also be a supplementary driver of NOE event. Our results summarize for the first time the characteristics and mechanism of the NOE events in China based on nationwide and long-term observations, and call for more direct measurements and modeling studies on the nighttime ozone evolution from surface to the residual layer.

Cheng He et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-310', Anonymous Referee #1, 20 Jul 2022
  • RC2: 'Comment on acp-2022-310', Anonymous Referee #2, 25 Jul 2022

Cheng He et al.

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Short summary
We report that nocturnal ozone enhancement (NOE) events are observed at a high annual frequency of 41 % over 800 sites over China in 2014–2019, about 50 % larger than that over Europe and US. High daytime ozone provides rich ozone source in the nighttime residual layer, determining the overall high frequency of NOE events in China, and then the enhanced atmospheric mixing triggers NOE events by allowing the ozone-rich air in the residual layer to be mixed into the nighttime boundary layer.
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