A New Method For Hygrosocpicity Measurement:Development And Applications

Hygroscopicity,which represents the ability of aerosol particles to absorb or lose water,affects their size,morphology,chemical composition,optical activity,and cloud condensation and ice nucleation activities,thus determining their environmental and climatic effects.However,it is still difficult to reliably determine hygroscopicity of aerosol particles which are nonspherical or of low hygroscopicity.In this work,we have developed a new method to measure hygroscopicity of atmospheric particles using a vapor sorption analyzer,and employed this method to investigate hygroscopicity of several types of particles,including pollen,mineral dust,and fly ash.The main results include:（1）A new method was developed to measure hygroscopicity of atmospheric particles using a vapor sorption analyzer which measured mass changes of particles,and no assumption on particles morphology was required.Its performance was experimentally verified:The measured DRH（deliquescence relative humidity）of six compounds at different temperatures（5-30 ^oC）,varying from～20%to～90%,agreed well with those reported in literature;the measured mass growth factors were in good agreement with those predicted by the extended aerosol inorganics model for（NH₄）₂SO₄ and Na Cl;in addition,a relative mass change of<0.025%could be reliably measured,demonstrating its outstanding sensitivity.（2）Three different techniques,including vapor sorption analysis,diffusion reflectance infrared Fourier transform spectroscopy and transmission Fourier transform infrared spectroscopy,were employed to investigate hygroscopic properties of six anemophilous pollen species as a function of temperature（5 or 15,25 and 37 ^oC）.At25 ^oC,the normalized mass of pollen species at 90%RH（relative to that at<1%RH）,ranged from 1.293±0.028 to 1.549±0.018,suggesting that these pollen species exhibited moderate hygroscopicity;moreover,pollen species which contained higher levels of OH groups usually had higher hygroscopicity.In addition,increase in temperature in general led to small but detectable decrease in pollen hygroscopicity.（3）The vapor sorption analyzer was used to study hygroscopic properties of eight Ca-and Mg-containing salts,including Ca（NO₃）₂?4H₂O,Mg（NO₃）₂?6H₂O,Mg Cl₂?6H₂O,Ca Cl₂?6H₂O,Ca（HCOO）₂,Mg（HCOO）₂?2H₂O,Ca（CH₃COO）₂?H₂O and Mg（CH₃COO）₂?4H₂O.At room temperature,DRH were lowest for Ca Cl₂?6H₂O（～28.5%）and highest for Ca（HCOO）₂and Mg（HCOO）₂?2H₂O（>95%）;in addition,the Clausius-Clapeyron equation could well describe the change of DRH with temperature（5-30 ^oC）.At 25 ^oC,the molar ratios of water to Ca²⁺or Mg²⁺in solutions formed due to deliquescence of Mg（NO₃）₂?6H₂O,Ca（NO₃）₂?4H₂O,Mg Cl₂?6H₂O,Ca Cl₂?6H₂O and Mg（CH₃COO）₂?4H₂O were determined to be 36.87±0.23,28.78±0.20,36.26±1.76,36.30±0.21 and 28.97±0.20 at 90%RH,while Ca（CH₃COO）₂?H₂O,Ca（HCOO）₂and Mg（HCOO）₂?2H₂O did not deliquesce at 90%RH.These results suggested that heterogeneous reaction of carbonate minerals with gaseous acid species will form more hygroscopic substances,thus significantly enhancing their hygroscopicity.（4）Hygroscopicity properties of 11 unconventional mineral dust particles were examined at room temperature,using a vapor sorption analyzer and diffusion reflectance infrared Fourier transform spectroscopy.Large variations in hygroscopicity were observed for these particles,and mass ratios of adsorbed water to dry samples,m_w/m₀,were found to vary from 0.0003 to 0.7340 at 90%RH.In addition,m_w/m₀ at 90%RH showed positive dependence on water-soluble ion contents and BET surface areas.Hygroscopicity of these particles could be described by the Freundlich adsorption isotherm model,and A_F and B_Fvalues were found to be in the range of 3.5×10^-4-1.29and 0.12-1.58,respectively.