Sea-land breeze (SLB) circulation is a mesoscale process induced by the thermal difference between land and sea. After sunrise, the land surface is heated faster than the sea surface, which leads to a pressure gradient force and the air flow from sea to land to form a sea breeze. At night, there is a contrary thermal difference, and the air flows from land to sea to form a land breeze. SLB circulation plays an important role in the generation and transportation of air pollutants, which impacts the weather, climate, and air quality of coastal areas. Lying on the south of Liaodong Bay, Huludao is easily influenced by SLB. In recent years, regional pollution characterized by ozone (O₃) and particles have become increasingly serious under the impact of chemical industry production and automobile exhaust emissions in Huludao. SLB circulation will change the temperature and humidity structure in the coastal boundary layer which determines the photochemical reaction conditions. Meanwhile, it impacts the transport of pollutants in coastal areas. Influenced by local circulation, solar radiation, precursor concentration, and other factors, the O₃ concentration on SLB days is more complicated, which has important research significance. Coherent Doppler wind lidar (CDWL) has a high spatiotemporal resolution and continuous observational ability. It can obtain the complete SLB and detailed structure of the atmospheric boundary layer, which is of great significance for understanding the horizontal and vertical transport characteristics of pollutants during SLB circulation.

From March 1^st to April 30^th in 2021, wind profile observation was carried out with CDWL in Juehua Island, Huludao, Liaoning (120.78° E, 40.48° N). The obtained meteorological parameters include wind speed/direction and temperature in the Huludao area and O₃ concentrations measured by ground-based instruments during observation. Three main factors should be considered in SLB identification: 1. large-scale background wind field; 2. temperature difference between sea and land; 3. near-surface wind direction change. We identified the SLB days during observation depending on these three conditions and the coastline direction in the Huludao area. We gathered the temporal and spatial distributions of SLB circulation in Huludao, including arrival time, prevailing speed, main direction, and the height of the sea breeze. The impact of SLB on O₃ concentration was analyzed, with the ground air quality monitoring data taken into account. Weather Research and Forecasting (WRF) modeling was performed to investigate SLB and its impact on O₃ concentration.

A total of 11 SLB days were identified with the data from CDWL and automatic meteorological stations in Huludao, accounting for 18% of the observation days. The results show that the sea breeze started at 08:30 averagely. During 14:00—17:00, it developed stronger, and the average speed exceeded 7.0 m·s^-1. The height of the sea breeze was 0.3-0.5 km during 10:00—16:00 and reached above 0.9 km after 18:00. As the main direction was east, the sea breeze showed a tendency to deflect in a clockwise direction over time (Figs. 2, 3, and 4). The WRF model presents the sea breeze circulation in the vertical section on April 4th. Sea breeze moved to the Huludao area at 10:00, and a strong wind convergence zone formed along the coastal line at 12:00 (Fig. 5). Pollutants accumulated at the intersection of sea and land breezes and transported to the ground surface by cold air sinking at the sea breeze head simultaneously. The data from the environmental monitoring station shows that O₃ concentration rose faster and had a higher peak on SLB days (Fig. 6). The surface wind speed on SLB days was lower than on non-SLB days, and the difference was more than 2 m·s^-1 at the same time point (Fig. 7). Land breeze carried O₃ from inland to sea at night, and the sea breeze during daytime blew pollutants back to the land, causing the cyclic accumulation of pollutants. With April 4th as an example, the O₃ concentration rose faster after the sea breeze arrived at Huludao and peaked at 106 μg·m^-3 (Fig. 9). The local recirculation index of horizontal wind in Huludao was only 0.049 on April 4th (Fig. 10), indicating that the transmission capacity of wind field was weak, and thus pollutants were not easy to spread.

According to the criteria at home and abroad, we identified the SLB days during spring, 2021 in Huludao with the wind data from CDWL and ground stations. In addition, we analyzed the temporal and spatial distributions of SLB circulation in Huludao, including the arrival time, prevailing speed, main direction, and the height of the sea breeze. The result shows that sea breeze forms later at a high altitude than on the surface, and the wind direction changes clockwise. The mesoscale WRF model was used to analyze the development of the sea breeze circulation on April 4^th, which proved the results observed by CDWL. The O₃ concentration rises faster and has a higher peak on SLB days. The study case shows that the local recirculation of horizontal wind under SLB is low, indicating that it is not conducive to the spread of pollutants. Pollutants will recirculate to the inland area after moving away from the coast during the shift of sea breeze and land breeze, which causes the cyclic accumulation of pollutants.