Characteristics Of Sea-Land Breeze Circulation And Its Effect On PM_2.5 Pollution In China Coastal Area

The sea-land breeze is a local mesoscale circulation in coastal cities driven by thermodynamic differences between land and sea.It affects local weather by changing the temperature,humidity,wind fields and other meteorological variables through the boundary layer.The current researches on sea-land breeze are mostly localized due to the topographical and meteorological influences,and comparative studies on sea-land breeze in many coastal areas of China are still lacking.This paper uses meteorological observation data and reanalysis data from 2015 to 2018 to analyze the characteristics of sea-land breeze in major coastal cities of China and compare the differences between regions to study the impact of different sea-land breeze selection criteria such as pure sea-land breeze(P-type)and true sea-land breeze(T-type)on weather characteristics.The results showed that the P-type sea-land breeze occurred more frequently is less affected by the background wind field and the corresponding PM_2.5 concentration is lower.The frequency of T-type sea-land breeze is less than half that of P-type,and the corresponding PM_2.5 concentration is higher under the influence of background wind field.We use WRF model to simulate sea-land breeze circulation structure and combined with pollutant monitoring data and model HYSPLIT-4 to simulate and cluster the backward trajectory of the pollutants under the sea-land breeze for exploring the sources and paths of the pollutants.The impact of sea-land breeze on air quality provides a theoretical basis for air pollution control in coastal cities.The main conclusions of this paper are as follows:1)Meteorological characteristics and excitation conditions of sea-land breeze.Sea-land breeze occurs more in summer and autumn and less in winter in China;the sea breeze usually starts at 11-15 and lasting for 5-7 hours;the land breeze usually starts from 01 to 05 and ends at 08-10.As an asymmetric mesoscale weather system,the intensity and duration of sea breeze are stronger than land breeze.Generally,sea breeze is stronger in spring and summer and land breeze is stronger in winter.The sea-land thermal difference is the key factor driving the circulation.The trend of the sea-land breeze increases by 0.1m/s/hr for every 5?increase in the thermal difference,which is more obvious at night than during the day.2)Latitude is the main factor leading to the difference of the characteristics of sea-land breeze in different areas.With the decrease of latitude,the frequency of sea-land breeze becomes higher,and the start time of sea breeze is gradually later.The start time of P type sea breeze is 0.3hours earlier and the beginning time of T type sea breeze is 0.7 hours earlier when the increase of latitude by 10°.The trend of sea-land breeze is weaker in southern China due to the lower latitude.3)The difference between P-type and T-type is mainly affected by the background wind field.The presence of the background wind field will obscure part of the sea-land breeze circulation signals.The proportion of sea-land breeze under the P-type criteria exceeds20%,which is about 10%under the T-type standard.The seasonal difference reflects the influence of localized thermal forcing for circulation excluding the synoptic wind,which is contributed by the prevailing wind for the T-type sea-land breeze.The background wind field generally increases sea breeze and weakens land breeze for the duration of the sea breeze in the T-type being longer than that of the P-type,which also makes the seasonal difference in sea-land breeze growing.The trend of sea-land breeze is weaker for T-type under the same thermal condition because the thermal drive of the sea-land circulation is influenced by localized thermal forcing and large scale wind field.4)The structure of circulation,distribution of emission sources,geographical location and coastline twists are the main factors causing the higher PM_2.5 concentration of sea-land breeze day than other weather types.Generally speaking,the PM_2.5 concentration of sea-land breeze days is always higher than or close to that of land breeze days,and the pollutant concentration of sea breeze days is the lowest.The accumulation of pollutants under sea-land breeze day changes at different sites.Geographical factor and emission sources lead to the high-polluting events in Tianjin,whose unique structure of the inland sea and adjacent to the Beijing-Tianjin-Hebei economic development zone makes polluted days under three weather types.The high concentration of PM_2.5 under the sea-land breeze is due to the transportation of the surrounding emission sources by the wind.Due to the unique landform of the peninsula,the concentration of pollutants on the sea-land breeze days in Qingdao is between the sea breeze days and the land breeze days.The unique recirculation structure is the main reason for the pollution on the sea-land breeze days.The pollutant concentration in Hangzhou Bay is higher in sea-land breeze days than that in land breeze days,which is the lowest on sea breeze days.The irregular coastline leads to the accumulation of particulate matter more easily and the pollution situation in the north shore is more serious than that in the south shore.The high concentration emission sources in Suzhou-Wuxi-Changzhou also increases the number of days in which pollutant concentration exceeds the standard.South China is more influenced by the structure of sea-land breezes.The concentration in Xiamen and Zhuhai stations is relatively low and the weather is basically clean.The high emission sources in the surrounding areas are significantly less than those in eastern and northern China.The gyratory backward trajectory with a higher proportion reflects the higher concentration under the sea-land breeze is mainly due to the combined effect of the circulation structure of and the local emissions.The long-distance transport has little impact on this region.