An observational study on the impact of sea-land breeze and low-level jet on air pollutant transport in the Bohai Bay

Abstract

Nocturnal pollutant concentrations in Bohai Bay often exhibit significant uneven spatial distribution, even under weak atmospheric circulation and stable boundary layer conditions, posing a serious challenge for air pollution forecasting. This study analyzed the causes of inverse changes in nocturnal pollutant concentrations between inland and coastal areas based on a month-long observation campaign on the meteorology and air pollutants in Bohai Bay. The results indicated that the primary cause was the combined influence of sea-land breeze (SLB) and low-level jet (LLJ) under a weak synoptic pattern. Sea breezes (SB) can transport pollutants from the coast to inland areas. The occurrence of land breezes (LB) and southwest-west LLJ at night can lead to the transport of pollutants inland to the coast. The impacts of LLJ on pollutant transport can be observed in three ways. First, the LLJ enhanced the horizontal ventilation in the boundary layer. The horizontal wind speed in the stable boundary layer increased by 2–6 m/s. Second, the LLJ weakened the stability of the stable boundary layer. A thin layer of instability existed between the surface-based and elevated inversion layers. The height of the inversion layer top increased by approximately 200 m, and the strength of the inversion weakened by at least 1 °C. Finally, the LLJ enhanced atmospheric vertical diffusion. The increased turbulent kinetic energy and upward momentum flux in the lower layer of the boundary layer, as well as the pronounced vertical upward motion between the top of the boundary layer and the altitude of the LLJ, contributed to the vertical dispersion of ground pollutants. Pollutants diffusing to the altitude of the LLJ can be transported horizontally to the coastal areas along the southwest-west LLJ.