The Role of Low-Level Jet and Cold Front in Enhancing Nocturnal Surface Ozone

Abstract

The nocturnal surface ozone (O₃) enhancement (NSOE) is receiving increasing attention. To investigate the cause of NSOE, an observation campaign was conducted in the Bohai Bay region using ground, tower-based, aircraft detection, and ground-based remote sensing observations. A typical case on the night of November 9, 2019, was captured, characterized by two surface O₃ enhancements occurring in one night. The Low-Level Jet (LLJ) and cold front before precipitation caused the two NSOEs in succession. The LLJ-induced NSOE event recorded a peak O₃ of 57.0 µg/m³, a rise of 31.0 µg/m³, and a rate of 5.2 µg/(m³·h), whereas the cold front-induced event had a peak of 51.0 µg/m³, a rise of 21.0 µg/m³, and a rate of 7.2 µg/(m³·h). The occurrence of the warm, humid southwesterly LLJ resulted in a downward transmission of sensible heat flux in the first half of the night. This maintained high near-surface temperatures and weakened the stability of the nocturnal atmospheric stratification. Subsidence motion above LLJ and enhanced turbulence near the surface further promote the vertical mixing of O₃. The effect of the cold front in the second half of the night included both vertical and horizontal transport of O₃. The significant sinking motion behind the cold front contributed to the downward transport of upper-level O₃, thus forming an “O₃ front”. Subsequently, the “O₃ front” was transported horizontally by the cold front to enhance the surface O₃ in the region it passed through. The effects of two types of weather systems are covered in this study. However further comprehensive studies are necessary to fully understand the underlying causes of this phenomenon.