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(3)) 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(3) pollution. Here we explored an O(3) episode to illuminate the role of large-scale LSAIs in O(3) 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(3) variation for three days, when O(3) precursors from Beijing–Tianjin–Hebei, Shandong, and Northeast China were transported to the Bohai–Yellow Seas. Photochemical reactions generated O(3) within marine air masses, causing higher O(3) levels over the seas than coastal regions. As the Mongolian High shifted eastward and expanded, southerly winds on its western edge transported O(3)-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(3) 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(3) formation and pollution duration in coastal cities. This insight helps understand and manage O(3) pollution in northern Bohai Sea cities and broadly applies to temperate coastal cities worldwide.