Investigating the factors contributing to the maintenance of high levels of warm season nocturnal ozone in a basin city

Abstract

In recent years, ozone (O₃) pollution in many Chinese cities has worsened. Several cities have also experienced incidents where nocturnal O₃ concentrations did not decrease as expected, and instead remained at high levels (above 50 ppb). However, there have been few detailed studies on the causes of these events. The present study used air quality and meteorological monitoring data, along with the Hybrid Single-Particle Lagrangian Integrated Trajectory model, to investigate the causes contributing to nocturnal O₃ remaining at high values (NORHV) in Linfen City, a typical basin city in the Fenhe River Basin, characterized by severe O₃ pollution. These events are influenced by the individual or combined impacts of mountain-valley breezes, along with vertical and horizontal transport of O₃-rich air masses. In addition, the influencing mechanisms of three NORHV events were identified as: 1) the alternating effect of the descent of upper-level air masses enriched with O₃, CO, and SO₂ and downslope mountain breezes carrying the O₃-rich air mass; 2) the interaction between the transport of polluted air masses across mountains and downslope mountain breezes on both sides of the two mountains in urban areas; and 3) the O₃-rich air mass from high-altitude regions descended into the urban area following two LLJ events. These conclusions were supported by vertical distributions of O₃ concentrations in the three cases simulated using the WRF-CMAQ model. In addition, the NORHV events resulted in higher initial O₃ concentrations on the following day compared to normal days, exacerbating O₃ pollution. The results of this study demonstrate the significant contributions of complex synergistic effects, such as mountain-valley breezes and vertical transport, to the occurrence of NORHV events in basin cities. These findings may be applicable to other basin cities around the world.