Distribution Characteristics And Cause Analysis Of Chemical Components In Precipitation Collected By A Sequential Sampler In Qianhu Area Of Nanchang

From May to September 2020,the APS-3A automatic precipitation sampler was used to collect precipitation in sections in the Qianhu area of Nanchang.During the study period,we observed 20 precipitation events,collected 88 segmented precipitation samples,and analyzed the concentration distribution characteristics and correlations of carbon components(WIOC,EC)and ion components(inorganic ions,organic acid radicals)in the segmented precipitation.Based on the distribution of chemical components in the segmented precipitation samples,quantitatively calculate the contribution of cloud water and under-cloud erosion to each component,analyze the type of regional rainfall,and analyze the source of each component in the precipitation.The main conclusions are as follows:1.Distribution characteristics of carbon component and ion composition in segmented precipitation.(1)The segmented precipitation collection method is more detailed than the whole precipitation collection method to grasp the distribution of various chemical components precipitation process,and more scientifically and objectively reflect the source of pollutants in precipitation.(2)The WIOC concentration in the segmented precipitation samples was0.2191?6.2235mg/L,with an average of 0.2341mg/L,and the EC concentration was0.0003?0.7702mg/L,with an average of 0.1612mg/L.Among the ionic components of the segmented precipitation,the concentration of Ca²⁺and NH₄⁺accounted for85%of the total concentration of cations;the concentration of NO₃^-and SO₄^2-accounted for 89%of the total concentration of anions;acetate,formate and oxalate are the main organic acid radicals,occupying 94%of the total concentration of organic acid radicals.(3)During the study period,the monthly variation trends of WIOC and EC concentrations in the regional precipitation samples were consistent,with the largest in May and the smallest in August and the fastest decrease from May to June and July to August,which was consistent with the regional wind direction.The scouring source under the cloud is related the monthly average concentration of inorganic ions in precipitation samples decreased significantly in May and June;the concentration of Ca²⁺and NO₃^-was the lowest in June,and the ion concentration in July and August showed an upward trend.The concentration of NH₄⁺was highest in July,the concentration of Ca²⁺and NO₃^-was the highest in August;the concentration of K⁺,Mg²⁺,Na⁺,Cl^-,NO₂^-did not change significantly between months;the acetate concentration in precipitation samples maintained high concentrations from June to August,then decline in September,the monthly changes of formate and oxalate are not significant.(4)Distribution characteristics of chemical components in segmented precipitation along with the precipitation process.The WIOC and EC in the segmented precipitation show a gradual decrease as the precipitation process continues,indicating that precipitation has the effect of removing pollutants in the air;however,WIOC and EC sometimes fluctuate in concentration during the precipitation process,reflecting rainfall intensity.The influence of meteorological factors,pollutant accumulation and cloud clusters on the carbon components in the precipitation samples.The change trend of inorganic ion and organic acid concentration is consistent with the change trend of WIOC and EC,and the overall change trend of anion NO₃^-and SO₄^2-is the closest.In a single long rainfall event,compared with the changes of ordinary anions and cations,WIOC and EC,organic acid radicals decrease more significantly with the change of rainfall intensity,and there is no obvious peak during the rainfall period,and the correlation with rainfall intensity is closer.The WIOC/EC ratio in the segmented precipitation is mainly concentrated at about 5,and the ratio often rises sharply in the late period of a single rainfall event.The WIOC and EC in the precipitation samples of the front and middle sections are mainly OC and EC in the scouring particles under the cloud,while the end section the concentration of WIOC and EC in precipitation samples is mainly determined by cloud water.The concentration of EC in cloud water is low,and the OC/EC ratio of cloud water is high;the WIOC/EC ratio of the precipitation samples in the front and middle sections is similar to the OC/EC ratio of regional atmospheric particulate matter,reflecting that the WIOC and EC in precipitation mainly come from the atmospheric particulate matter scoured under clouds.2.Contribution and cause analysis of cloud erosion and cloud water to carbon and ionic components.(1)In the long single precipitation event,the contribution of the under-cloud erosion to the chemical components in the first three precipitation samples gradually decreases.The under-cloud erosion in the early stage of precipitation is the main source of each component in the precipitation samples,and the chemical components in the precipitation samples in the late stage of precipitation the main contribution is Yunshui.The contribution rate of under-cloud erosion to NH₄⁺in precipitation samples is not absolutely declining in the first three sections,but the contribution rate of under-cloud erosion in the first section is slightly greater than that of the second section.The reason may be that NH₄⁺in precipitation is affected by particulate matter.And gas pollution.(2)The correlation between WIOC and EC in the precipitation samples of the first two sections is stronger than that in the precipitation samples of the latter two sections(the correlation coefficients are 0.558 and 0.245,respectively).The strong correlation between WIOC and EC in the first two paragraphs indicates that WIOC and EC have the same homology in the early stage of precipitation.In the precipitation samples,WIOC and EC in the early stage of precipitation mainly come from the erosion of atmospheric particles under the cloud.As the rainfall continues,the erosion under the cloud the contribution decreases,while the contribution of cloud water increases.OC in cloud water is the main cloud condensation nucleus,and the distribution of EC in cloud water is less,and the correlation between WIOC and EC in cloud water is weak.(3)The correlation between the inorganic ion components in the precipitation samples in the first two sections is slightly stronger than the correlation between the inorganic ions in the precipitation samples in the latter two sections.Among them,Mg²⁺and Ca²⁺are in the early and late periods of rainfall and in all rainfall samples.The significance of both is the highest,indicating that the precipitation in Nanchang is affected by the pollution of land-based dust sources.In the early,late and all rainfall samples,the correlation between organic acids and other inorganic ions is poor,and there are multiple sets of negative correlations.As the rainfall continued,WIOC and water-soluble ions decreased significantly,while the significant correlation between EC and Cl^-,NO₃^-increased as the rainfall continued.(4)According to the correlation in all rainfall samples,the weak correlation of Na⁺and Cl^-is in line with the situation in Nanchang that is far from the coast,and the sources of Na⁺and Cl^-are different.The high correlation between SO₄^2-and Ca²⁺,NO₃^-and Ca²⁺indicates that these ions in precipitation mostly exist in the state of Ca SO₄ and Ca(NO₃)₂.K⁺and NH₄⁺have a high correlation,and there may be sources of agricultural activities.Among organic acids,the correlation between formic acid and oxalic acid is higher than that between acetic acid and oxalic acid,and the correlation between formic acid and acetic acid is the lowest.There are multiple sets of negative correlations between organic acids and other ions,and the correlation between oxalate and inorganic ions is stronger than that of formic acid and acetic acid.At the same time,the correlation between organic acids and cations in precipitation is relatively poor.The correlation coefficient between WIOC,EC and other ion components is less than or equal to 0.5,and the correlation between WIOC and other ions is weaker than that of EC,indicating that the sources of carbon components and ion components are different.(5)The monthly average of the SO₄^2-/NO₃^-ratio in the precipitation samples is0.9548,indicating that the rainfall type is mixed;while the SO₄^2-/NO₃^-ratios in the segmented precipitation samples are mostly concentrated between 0.5 and 3,and are greater than 3.The situation is more than less than 0.5,indicating that the rainfall type in the region is mostly mixed type and sulfuric acid type.The monthly average value of the F/A ratio in the precipitation samples ranges from 0.2652 to 3.4454,and the monthly average value is 1.1205.The monthly difference of the F/A ratio varies greatly,indicating that the production of organic acids is affected by multiple factors.Most of the F/A ratios in the segmented precipitation samples are concentrated around1,and the cases below 1 are more than the ratios above 1,indicating that the organic acids in the precipitation mainly come from human activity emissions or organic matter combustion.Photochemical oxidation of unsaturated hydrocarbons.