Large-scale land-sea interactions extend ozone pollution duration in coastal cities along northern China

Land-sea atmosphere interaction (LSAI) is one of the important processes affecting ozone (O₃) pollution in coastal areas. The effects of small-scale LSAIs like sea-land breezes have been widely studied. However, it is not fully clear how and to what extent the large-scale LSAIs affect O₃ pollution. Here we explored an O₃ episode to illuminate the role of large-scale LSAIs in O₃ pollution over the Bohai–Yellow Seas and adjacent areas through observations and model simulations. The results show that the northern Bohai Sea's coastal region, influenced by the Mongolian High, initially experienced a typical unimodal diurnal O₃ variation for three days, when O₃ precursors from Beijing–Tianjin–Hebei, Shandong, and Northeast China were transported to the Bohai–Yellow Seas. Photochemical reactions generated O₃ within marine air masses, causing higher O₃ levels over the seas than coastal regions. As the Mongolian High shifted eastward and expanded, southerly winds on its western edge transported O₃-rich marine air masses toward the coast, prolonging pollution for an additional three days and weakening diurnal variations. Subsequently, emissions from the Korean Peninsula and marine shipping significantly affected O₃ levels in the northern Bohai Sea (10.7% and 13.7%, respectively). Notably, Shandong's emissions played a substantial role in both phases (27.5% and 26.1%, respectively). These findings underscore the substantial impact of large-scale LSAIs driven by the Mongolian High on O₃ formation and pollution duration in coastal cities. This insight helps understand and manage O₃ pollution in northern Bohai Sea cities and broadly applies to temperate coastal cities worldwide.