Hygroscopic Growth and Phase Transitions of Na2CO3 and Mixed Na2CO3/Li2CO3 Particles: Influence of Li2CO3 on Phase Transitions of Na2CO3 and Formation of LiNaCO3

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The hygroscopic behaviors and phase changes of inorganic aerosols have been widely explored, but little is known on the hygroscopicity of soluble carbonates. The hydrated states of solid Na 2 CO 3 particles in an air environment remain largely unclear. In this work, the hygroscopic growth, hydrated form transformations, and influence of internal Li 2 CO 3 on phase transitions of Na 2 CO 3 particles are investigated in linear and pulsed relative humidity (RH) changing modes by the vacuum Fourier transform infrared (FTIR) technique. For pure Na 2 CO 3, aqueous droplets effloresced to a mixture of anhydrous Na 2 CO 3 and Na 2 CO 3 ·H 2 O with the initial efflorescence relative humidity (ERH) of 50.8%, probably concerning the formation of Na 2 CO 3 ·10H 2 O in the conversion from aqueous to anhydrous Na 2 CO 3 . A reverse process is presented during the three-stage deliquescence transition beginning at ∼60.1% RH; i.e., anhydrous Na 2 CO 3 transforms into aqueous Na 2 CO 3 and Na 2 CO 3 ·10H 2 O in stage I, Na 2 CO 3 ·10H 2 O dissolves to aqueous Na 2 CO 3 in stage II, and Na 2 CO 3 ·H 2 O dissolves into aqueous Na 2 CO 3 in stage III. For internally mixed Na 2 CO 3 /Li 2 CO 3 particles, a double salt, LiNaCO 3, is found in mixed crystalline phases for the first time, leading to the eutonic composition with Na 2 CO 3 . The experimental observations point to the excess of LiNaCO 3 and complete consumption of Na 2 CO 3 in eutonic composition formation, which results in the absence of Na 2 CO 3 hydrates during phase transitions. The results provide key data for model simulations of hygroscopic properties and phase transitions of Na 2 CO 3 as well as mixed soluble carbonates.