Atmospheric Precipitation In Shanghai Area Characteristics Of Hydrogen And Oxygen Isotopes And Mercury And Their Environmental Significance

Atmospheric water vapor,as a precursor of precipitation,participates in the water cycle process,and its hydrogen and oxygen isotopic composition changes with the change of environmental conditions and records the corresponding environmental characteristics.Therefore,it can be used to trace the source of water vapor and material transport.At the same time,atmospheric mercury has the characteristics of long-distance transport due to its strong volatility,so the study of atmospheric mercury and its deposition is of great significance for analyzing the current situation and sources of atmospheric mercury pollution in the region,and proving the source and transport of regional substances.In this paper,369 groups of atmospheric water vapor samples were collected from June 2020 to December 2020 in Shanghai area.The hydrogen and oxygen isotopes of the samples were measured.The temporal variation characteristics of water vapor isotopes were analyzed,and the precipitation lines of different weather processes were discussed.Deuterium surplus and ¹⁷O surplus were used to trace the water vapor sources of different weather processes in Shanghai.At the same time,the atmospheric mercury deposition data from April to November 2020 and December 2020 in Shanghai area were collected,and the temporal variation characteristics of atmospheric mercury and atmospheric wet deposition mercury in Shanghai area were discussed,and the atmospheric mercury pollution and important pollution events in Shanghai area were analyzed.Through the study,the following conclusions are drawn:1.The variation characteristics of?D,?¹⁸O and?¹⁷O isotopes of atmospheric water vapor in Shanghai area are basically the same,and there is no obvious diurnal and seasonal variation in different weather processes.Isotope minimum values appear in the process of typhoon passing through,autumn heavy rainfall and winter cold front rainfall.This is because the condensation process of water vapor condenses heavy isotopes into precipitation.The residual isotopes in water vapor are lighter,and the value of atmospheric water vapor isotopes is lower in the process of accompanying precipitation.The climatic scale data show that the precipitation isotopes in El Nino years are positively skewed compared with La Nina years.2.There is a good correlation between?¹⁸O and?D in atmospheric water vapor in Shanghai area.The slope of precipitation line of atmospheric water vapor has the characteristics of low in flood and drought,low before and after the cold front crosses,and high in plum rain,typhoon,autumn and the cold front crosses.The intercept is characterized by drought,negative before and after the cold front crossing,and positive at other sampling periods.This is related to the strong secondary evaporation of raindrops before and after the passing of cold front in Shanghai area during drought weather.3.The D values of water vapor isotopes showed significant changes with different weather processes:in autumn October(40.90‰)>After the winter cold front passed through(39.65‰)>Autumn September(30.25‰)>Summer drought(28.50‰)>Summer typhoon(26.04‰)>Summer Meiyu(21.99‰)>Winter cold front transit(-6.75‰)>Characteristics of winter cold front before transit(-10.83‰).The D value of water vapor is higher than that of precipitation in the same period,and the seasonal variation trend is consistent,but the seasonal variation range is larger.Except in typhoon period,before and during the transit period of cold front in winter,D values in other sampling periods showed a law of decreasing gradually with the increase of relative humidity,and this correlation was the highest in the transit period of cold front in winter.This indicates that the atmospheric water vapor during the transit period of the cold front in Shanghai mainly comes from local evaporation,which can reflect the relationship between D value and relative humidity.4.The ?¹⁷O and?¹⁸O in the atmospheric water vapor isotopes in Shanghai mainly follow the mass fractionation effect in summer,typhoon season and September and October in autumn.The?¹⁷O and?¹⁸O in the atmospheric water vapor isotopes in Shanghai basically follow the mass fractionation effect.The slope and intercept of the cold front are smaller than that of the mass fractionation line,while the non-cold front is larger in winter.The ¹⁷O surplus of atmospheric water vapor reached the highest value of 86.17 per Meg when the cold front passed in winter,and the lowest value of-40.93per Meg when the cold front passed in winter.The standard deviation of the winter sample is the largest,indicating that the water vapor source in the study area is complex in winter.5.During the monitoring period,the average concentration of gaseous total mercury in Shanghai atmosphere was 1.29 ng/m³±0.73 ng/m³,lower than the background value in the northern hemisphere,and the atmospheric mercury pollution was small.There was a diurnal variation of the gaseous total mercury in Shanghai area,and the peak value appeared between 08:00 and 10:00 am and 20:00 and 24:00 at night.The reason is that the temperature and wind speed in the night are lower than that in the day,which makes the atmospheric boundary layer maintain a lower height,the diffusion of atmospheric pollutants is inhibited,and the total gaseous mercury in the atmosphere continues to accumulate in the boundary layer.The atmospheric mercury pollution in Shanghai is lower in summer and higher in spring and autumn,which is mainly related to the local wind direction and speed.In summer,clean easterly winds from the Yellow Sea or the East China Sea prevail.In spring,the northwest wind from Inner Mongolia and Hebei prevails;Autumn prevailing from Jiangxi,Zhejiang and other places south wind,southwest wind.The main factors affecting the atmospheric mercury concentration in Shanghai include two aspects:source intensity and atmospheric process.At the same time,in some special weather conditions,such as temperature inversion,sunny weather,no wind or weak wind,the atmospheric mercury pollution in Shanghai area may come from the mercury emission from human activities in the local source area.The HYSPLIT backward trajectory model shows that compared with low Hg concentration events,the air mass of high Hg concentration events is mainly from inland areas,and human activities emit more Hg waste gas,which has a greater impact.