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 (O) 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 O pollution. Here we explored an O episode to illuminate the role of large-scale LSAIs in O 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 O variation for three days, when O precursors from Beijing-Tianjin-Hebei, Shandong, and Northeast China were transported to the Bohai-Yellow Seas. Photochemical reactions generated O within marine air masses, causing higher O levels over the seas than coastal regions. As the Mongolian High shifted eastward and expanded, southerly winds on its western edge transported O-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 O 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 O formation and pollution duration in coastal cities. This insight helps understand and manage O pollution in northern Bohai Sea cities and broadly applies to temperate coastal cities worldwide.