Comment on acp-2022-310

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.