The Chemical Characteristics And Environmental Significance Of Atmospheric Precipitation In Shanghai Area

As an important part of the water cycle,atmospheric precipitation will change its material composition with the environment during the cycle,thus preserving a lot of environmental information.Therefore,the source and transmission of atmospheric precipitation has become the frontier of current environmental evolution and global change research.The isotope composition of atmospheric precipitation is sensitive to environmental changes and contains important information on the source and transport of water vapor,which can be used to track weather systems and atmospheric circulation processes.In addition,the process of atmospheric precipitation in the process of landing to the ground absorbs trace gases and aerosols in the atmosphere through the cloud-capture collection process.By analyzing the ion concentration in the atmospheric precipitation,the source of atmospheric components can be further analyzed,and the precipitation on the surface ecosystem can be evaluated,and so on.This paper analyzes the characteristics of hydrogen and oxygen isotopes(?D,?¹⁸O and?¹⁷O values,atmospheric precipitation lines,etc.)in 124 rainwater samples from September 2016 to August 2018.We use the data to discusses their environmental effects.The d value and ¹⁷O_-excess value are used for water vapor source tracing.The environmental significance is analyzed by combining the pH value of the rain sample,the conventional ion concentration characteristics,and the correlation between the isotope composition and the conventional ion concentration.Through the research of the paper,the following main conclusions are drawn:1.In different years,the hydrogen and oxygen isotope composition of atmospheric precipitation in Shanghai has fluctuated less in recent years,and the annual fluctuation range has been decreasing year by year.Under different months,the annual variation trend of?D,?¹⁸O and?¹⁷O values is consistent and the fluctuation range is large,the dry season is positive,and the rainy season is negative.In different seasons,the values of?D,?¹⁸O and?¹⁷O are greater than the half year in winter.Under the single precipitation scale,the?D,?¹⁸O and?¹⁷O values in the continuous rainfall samples in Shanghai are consistent,and they all undergo a process from the beginning of rainfall to the enrichment of heavy isotopes to the end of rainfall.2.The Local Meteoric Water Line?LMWL?of?D^?18O in Shanghai area is:?D=8.07?¹⁸O+11.48?r=0.96,?=0.001,n=124?.Compared with the Global Meteoric Water Line?GMWL?equation,the slope and intercept of the equation are similar.Compared with Chinese Meteoric Water Line?CMWL?,the slope of the equation is similar and the intercept is large.The Shanghai area is closer to the ocean and the climate is generally milder and humid,so the slope and intercept of the established LMWL are larger than the global average.The oxygen isotope L MWL of?¹⁷O^?18O in Shanghai:?'¹⁷O=0.526×?'¹⁸O+0.00052?n=124,R²=0.99?.The slope of the equation is between 0.529 and 0.518,indicating that the water vapor in the area is in the ocean to land migration and is evaporated in the middle.3.The annual average?¹⁸O value of atmospheric precipitation in Shanghai shows significant precipitation effects and anti-temperature effects.There is less precipitation in winter and spring in Shanghai area,so the relative humidity is smaller,and the evaporation is stronger,the heavy isotopes are more concentrated.The opposite is summer and autumn,so the precipitation effect is presented.The anti-temperature effect is mainly in the Shanghai area along the Yangtze River and the sea.The relative humidity of the atmosphere is relatively large throughout the year,and the concentration of?¹⁸O is no longer prominent.Coupled with the influence of summer and autumn strong precipitation,the rainfall has a certain degree of interference on the positive correlation between?¹⁸O value and temperature.4.The d value of atmospheric precipitation in Shanghai has a large change in the whole year,with an average value of 11.01‰,so the evaporation effect in water vapor transport is strong.The d value in the season is half a year in winter>half a year in summer.This is because the summer monsoon is mainly from the ocean to the land,and the water vapor is less effective than the warm and humid evaporation;the winter wind is opposite,and the water vapor is more effective than the dry and cold evaporation.The variation of ¹⁷O_-excess in atmospheric precipitation in Shanghai is small,with an average of 13.45 per meg.The water vapor experienced evaporation during transport.The seasonal change of ¹⁷O_-excess is spring>summer>winter>autumn.The ¹⁷O_-excess and relative humidity?RH?are not correlated at the annual scale,but the season is negatively correlated with autumn and positively correlated with winter.This indicates that the water vapor in the Shanghai area is mainly marine air mass in the autumn;while the winter is mainly local evaporation,and other seasons are more complicated.5.The average annual pH value of the Shanghai area for two consecutive years was 5.54 and 5.50 respectively,and the interannual variation was relatively stable.In the different months of the year,the pH of the rain sample is at least 4.09,the highest is 6.43,and the fluctuation is large during the year.In different seasons,the pH value of rainwater samples is higher in summer and autumn,lower in winter and spring,and the seasons are significantly different.Among the 124 rainwater samples in Shanghai,the most common types of neutral and weak acid rainfall are 95%.Most of the precipitation in spring and autumn is weakly acidic,and the acidity of precipitation in winter is significantly enhanced.In summer,it is mostly neutral.Moreover,the results of this paper are compared with the previous ones.In recent years,the pH value of rainwater in Shanghai has increased significantly,and the degree of acid rain has been reduced.6.The order of the constant ion concentration in atmospheric precipitation in Shanghai is:SO₄^2->Cl^->Ca²⁺>NO₃^->NH₄⁺>Na⁺>Mg²⁺>K⁺.Among the total ions,the most important ion is SO₄^2-,accounting for 23.3%,the most important cations are Ca²⁺and NH₄⁺,accounting for 37%and 29%,the most important anions are SO₄^2-and Cl^-,accounting for 42%and 37%.Seasonal variation characteristics of anion concentration:Cl^-is winter>spring>autumn>summer.SO₄^2-and NO₃^-are winter>autumn>spring>summerin 16-18 years,spring>autumn>winter>summerin17-18 years.The seasonal variation characteristics of cation concentration are:K⁺is autumn>winter>spring>summer.Ca²⁺is winter>spring>autumn>summer.Both Na⁺and Mg²⁺are winter>autumn>spring>summer.NH₄⁺is spring>16>summer>autumn in 16-17 years,winter>spring>autumn>summer in 17-18 years.7.The correlation coefficient between hydrogen and oxygen isotope composition and constant ion concentration in atmospheric precipitation in Shanghai is characterized by:?D is significantly correlated with Na⁺and Cl^-at 0.01 level,and the correlation coefficients are 0.334 and 0.410.The ions are similar to the source of?D.The hydrogen and oxygen isotope is not significantly correlated with NH₄⁺and NO₃^-ions.The NH₄⁺and NO₃^-ion pairs are derived from human production and life.The relationship between the two shows that human production and life have no significant effect on hydrogen and oxygen isotope composition.