Sea-Land Breeze And Its Impact On Ozone Pollution In Shanghai

The sea-land breeze is a mesoscale weather phenomenon with a diurnal cycle caused by the sea-land thermal contrast.The circulation of sea-land breeze plays a crucial role in the generation and transportation of air pollutants,and have an important impact on the weather,climate and air quality of coastal areas.Lies on the middle coast of China,Shanghai is easily affected by the circulation of sea-land breeze.In recent years,ozone（O₃）pollution has become more and more serious with the impact of chemical industry production and automobile exhaust emissions in this city.The sea-land breeze has an important impact on the generation and transport of O₃.Therefore,it is of great practical significance to carry out research on the effects of sea-land breeze and its impact on O₃ pollution in Shanghai.Firstly,based on both conventional meteorological data of Shanghai and the reanalysis data,the standards of the sea-land breeze were established and the characteristics of it were studied.Meanwhile,combined with the ground air quality monitoring data,the impact of the sea-land breeze on O₃ concentration was analyzed.The results showed that:（1）In Shanghai,the sea-land breeze occurrs more frequently from March to August and the frequency is around 20%to 30%,which significantly correlated with the temperature difference between land and sea.（2）Generally,the onset and cessation time are around 09:00 and 17:00 of the local standard time（LST）respectively,and the average maximum speed is 3.9m/s,which usually shows after 15:00 LST;The sea-land breeze usually occurs together with an increase of ozone concentration,and a delay of peak value.（3）The impact of sea-land breeze can reach Dian Shan Lake（DSL）,and the further the sea-land breeze goes,the smaller the influence on ozone concentration is.Secondly,theMozartschemeintheWeatherResearchand Forecasting-Chemistry（WRF-Chem）modelling system was coupled with the Urban Canopy Model（UCM）model to investigate the process of the sea-land breeze and its impact on O₃ concentrations in Shanghai.Two typical high O₃ days with sea-land breeze were selected to conduct case study.Based on both conventional meteorological data and the ground air quality monitoring data of Shanghai,the model performance was evaluated.The comparison between predictions and experimental data indicates that:（1）The numerical system successfully reproduces the process of the sea-land breeze in Shanghai.For the wind speed,the average mean bias（MB）is close to0.81m/s,while the root mean squared error（RMSE）to 1.6m/s.The index of agreement（IOA）is 0.62,while the correlation coefficient（COR）at 0.6.For the wind direction,the MB is 15°and the RMSE at 47°.For the temperature,the MB is-1.12℃,while the RMSE to 1.88℃,the IOA and the COR is 0.91.For the relative humidity,the MB is-0.71%,the RMSE is 9.04%,and the IOA and COR are 0.89 and 0.87,respectively.（2）With the sea breeze onset,convergence areas are identified in Shanghai and then they can reach DSL,the further the convergence areas go,the smaller the strength of the convergence is.Meanwhile,the sea-land breeze coupled with the urbanization-induced circulations play a vital role in the development of regional thermal circulation.Finally,based on the ground air quality monitoring data of Shanghai,the model performance on O₃ was evaluated.The comparison between predictions and experimental data indicates that:（1）The model simulates the ozone concentration well,with the MB of-15.36μg/m3 and the IOA of 0.83,while the COR is 0.75.The DSL station has the strongest correlation between simulation and measurements.（2）The overlapping breezes lead to O₃ accumulation in the convergence areas.The maximum predicted and observed O₃ concentrations are recorded close to the convergence line.The O₃ high value center has a good correspondence with the strong convergence zone.