Chemical Composition And Hygroscopicity Of Salt Components From Saline Lakes In Qaidam Basin

Salt/Alkali dust storm is a very disastrous natural weather phenomenon originating from arid and semi-arid areas.Salt/Alkali dust storm which contains mineral dust,fine-particle inorganic salt aerosols and heavy metal elements not only directly causes huge economic losses,but also affects the regional ecological environment,atmospheric chemistry processes and the global climate system due to the high hygroscopicity of salt aerosol particles and efficient ice nucleation ability of mineral dust.Saline lakes and playas are important sources of inorganic salt aerosols,so brine,lakebed salt and crust salt samples from four different saline lakes（Chaka Saline Lake,Keke Saline Lake,Qarhan Saline Lake and Mang′ai Saline Lake）in Qinghai province and brine samples from two different saline lakes（Xiao Chi Saline Lake and Yan Chi Saline Lake）in Shanxi province were collected,moreover,ionic data of brine,fresh and aged aerosol samples from other studies were summarized,respectively.By combining laboratory experiments and model analyses,the study analyzed the differences in chemical compositions of all salt samples,fresh and aged aerosols,and further explored the deliquescence relative humidity（DRH）and hygroscopic components of salt particles.This study provides a scientific theoretical basis for studying the influence of salt particles originating from saline lakes and playas on the atmospheric heterogeneous processes and aerosol-cloud-climate interactions.The main research results are as follows:（1）The Na⁺,Cl^-,Mg²⁺and SO₄^2-contents in salt samples（brine,crystallized salt,lakebed salt and crust salt）from the four saline lakes（Chaka Saline Lake,Keke Saline Lake,Qarhan Saline Lake and Mang′ai Saline Lake）in the Qaidam Basin are high,while the K⁺,Ca²⁺,NO₃^-contents are relatively low,and the Mg²⁺and SO₄^2-contents in the brine and crystallized salt samples are relatively higher than those in lakebed salt and crust salt samples.The correlation coefficients between the ionic components of the lakebed salt and crust salt samples of Chaka Saline Lake,Keke Saline Lake,Mang′ai Saline Lake are all 1,indicating that the geographical location and the brine components of the saline lakes have little influence on the chemical compositions of natural solid salts（lakebed salt and crust salt）,and their main component is NaCl.Although the chemical components of brine and crystallized salt samples from four saline lakes in the Qaidam Basin are similar,pH values of brine samples are lower than that of crystallized salt samples,absorption of ambient gases（CO₂,etc.）may be one of the reasons for the acidification of brine.The pH values of the salt sample solutions（brine,crystallized salt and lakebed salt）are negatively correlated with the Mg²⁺concentrations,the higher concentration of Mg²⁺can act as a weakly basic ion and cause acidification of these salt samples.（2）The ionic composition data of 96 groups of brines shows that the contents of Cl^-,Na⁺and Mg²⁺are relatively high,while the contents of K⁺,Ca²⁺and SO₄^2-are lower.There is a negative correlation between Na⁺and Mg²⁺,indicating that there may be a competitive relationship between Na⁺and Mg²⁺,and Ca²⁺is negatively correlated with SO₄^2-,which may be due to these two ions easily form Ca SO₄ precipitation in the aqueous phase.The concentrations of Na⁺,Cl^-,and Ca²⁺in fresh salt aerosols are higher,which is consistent with the components of brines and salt particles collected from saline lakes and playas,while the secondary inorganic aerosol components(NH₄⁺,NO₃^-,SO₄^2-)in aged salt aerosols are higher.（3）The hygroscopic experiments of salt particles in the Qaidam Basin show that the mass of particles increased gradually with the increase of relative humidity（RH）.Results of the AIOMFAC model show that initial mass growth of crystallized salts is strongly influenced by MgCl₂ composition,the initial hygroscopic growth of lakebed salt in Qarhan Saline Lake is related to NaCl and KCl compositions,while the initial mass growth of crust salts may be largely influenced by NaCl.（4）The model results of 96 groups of brines are consistent with the hygroscopic experimental results of crystallized salts in the Qaidam Basin,the DRHs of MgCl₂ are low,i.e.,the MgCl₂component is most likely to deliquescence,which affects the initial mass change of the brine samples,NaCl is the main component of most brines and affects the hygroscopic growth of the samples strongly,KCl has a great influence on the hygroscopicity of potassium-rich brines.Results of AIOMFAC model show that the DRHs of the MgCl₂ component of the fresh salt aerosols are lowest,while the DRHs of the NH₄NO₃ component of the aged salt mixtures are lowest,indicating that the initial hygroscopic property of the salt aerosols changed during transportation and aging process.The initial hygroscopic property of the fresh salt aerosols and aged salt aerosols are MgCl₂ and NH₄NO₃,respectively.