The effects of coastal local circulations and their interactions on ozone pollution in the Hangzhou metropolitan area

Abstract

Impacts of coastal local circulations and their interactions on ozone (O3) in Hangzhou on 5 June 2018 with relatively low temperatures were investigated using surface observations and three-dimensional air quality simulations. Local circulations and their interactions were primary causes of the episode instead of high temperatures. Northwesterly land breeze on the previous day transported O3 to sea, and sea breeze (SB) on the episode day blew pollution back to land. Inland penetration (east to west) of the combined SB front (SBF) was accelerated by urban heat island (UHI) induced easterly flow over coast and slowed by its westerly flow and surface roughness over inland urban areas. O3 accumulation at surface and in the elevated air over city was caused by mountain barrier effect and venting process. Subsidence heating associated with valley circulation accelerated UHI circulation (UHIC) before 1400 LT, indirectly strengthening the SB circulation (SBC). The UHIC and valley circulation decoupled with SBC at noon and in the late afternoon, respectively, leading to a stronger SBC. The stronger SBC brought additional O3 (>20 ppb) to further inland area, to elevated air with updrafts in SB head, and to nocturnal stable boundary layer and residual layer with its upper-level offshore flow.