Hygroscopic Properties And Cloud Condensation Nucleation Activities Of Representative Calcium-containing,Magnesium-containing And Methanesulfonate Aerosols

Sea salt aerosol is generated by sea spray over oceans,and mineral dust aerosol is mainly emitted from arid and semiarid regions.They are the two types of tropospheric aerosols with largest emission fluxes,and their hygroscopicity and cloud condensation nucleation（CCN）activities have significant impacts on atmospheric chemistry,global radiation balance and climate change.Though calcium-and magnesium-containing salts and methanesulfonates are important component of sea salt and mineral dust particles,their hygroscopic properties and CCN activities are not well understood.In this work,hygroscopic properties and CCN activities of four calcium-containing salts,four magnesium-containing salts and three methanesulfonates were investigated using a hygroscopicity tandem differential mobility analyzer（HTDMA）,a vapor sorption analyzer（VSA）and a cloud condensation nuclei counter（CCNc）.The main results include:（1）HTDMA was employed to measure hygroscopic growth factors（GF）of eight types of sub-micron aerosols,including Ca（NO₃）₂,Mg（NO₃）₂,Ca Cl₂,Mg Cl₂,Ca（HCOO）₂,Mg（HCOO）₂,Ca（CH₃COO）₂and Mg（CH₃COO）₂ at different relative humidity（RH,5-90%）.All the eight types of aerosols examined exhibited continuous hygroscopic growth,indicating that these particles were probably amorphous under dry conditions.GF at 90%RH were determined to be 1.79±0.03 for Ca（NO₃）₂,1.67±0.03for Mg（NO₃）₂,1.71±0.03 for Ca Cl₂and Mg Cl₂,1.54±0.02 for Ca（HCOO）₂,1.69±0.03for Mg（HCOO）₂,1.26±0.04 for Ca（CH₃COO）₂ and 1.53±0.01 for Mg（CH₃COO）₂,respectively.All the eight Ca-and Mg-containing aerosols showed higher hygroscopicity than carbonates,suggesting that atmospheric heterogeneous reactions with acids（e.g.,HNO₃,HCl,HCOOH and CH₃COOH,etc.）would enhance the hygroscopicity of carbonate aerosols.In addition,compared with formates and acetates,nitrate and chloride showed higher hygroscopicity.（2）Hygroscopic properties of Na Cl and sea salt were studied using VSA and HTDMA.The ratio of particle mass at 90%RH to that under dry conditions,were measured to be 7.29±0.04 and 6.51±0.04 for Na Cl and sea salt,and hygroscopic GF were measured to be 2.37±0.01 and 2.22±0.02 for Na Cl and sea salt at 90%;the hygroscopicity of sea salt was slightly lower than that of Na Cl for>75%RH,due to the presence of Mg Cl₂ and Ca Cl₂（the hygroscopicity of which was lower than Na Cl）in the sea salt.Meanwhile,hygroscopic properties of CH₃SO₃Na,CH₃SO₃K and（CH₃SO₃）₂Ca at room temperature were also investigated using VSA and HTDMA.The ratios of particle mass at 90%RH to that under dry conditions,were measured to be4.002±0.053,3.479±0.031 and 3.663±0.019 for CH3SO3Na,CH3SO3K and（CH₃SO₃）₂Ca,and hygroscopic GF at 90%were determined to be 1.48±0.02 for CH₃SO₃Na,1.53±0.04 for CH₃SO₃K and 1.65±0.02 for（CH₃SO₃）₂Ca,suggesting that hygroscopicity of the three methanesulfonates were lower than Na Cl and sea salt.In addition,deliquescent relative humidity of CH₃SO₃Na were determined to be 70-71%,showing no change with temperature in the range of 15-35℃.（3）CCN activities of CH₃SO₃Na,CH₃SO₃K and（CH₃SO₃）₂Ca aerosols were explored using CCNc.The single hygroscopicity parameter（κ）were determined to be0.46±0.02,0.37±0.01 and 0.47±0.02 for CH₃SO₃Na,CH₃SO₃K and（CH₃SO₃）₂Ca,respectively,suggesting that the CCN activities of the three methanesulfonates were quite large.CCN activities of CH₃SO₃Na/Na Cl and CH₃SO₃Na/sea salt（with mass ratios of 1:1 and 1:10）were further examined.For internally mixed CH₃SO₃Na/Na Cl and CH₃SO₃Na/sea salt aerosol particles with a mass ration of 1:10,their CCN activities were found to be almost equivalent to those for Na Cl and sea salt particles.Since mass ratios of methanesulfonates to sea salt particles in the troposphere are usually much smaller than 1:10,it could be concluded that the effects of methanesulfonates on the CCN activities of sea salt aerosols should be very minor.To summarize,hygroscopic properties and cloud condensation nucleation activities of several major calcium-and magnesium-containing salts and methanesulfonates were investigated in this work.The high-quality experimental data provided in this work would help better understand the hygroscopicity and cloud condensation nucleation activity of sea salt and mineral dust aerosols and thus their environmental and climate impacts.