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. MBTCA (3-methyl-1,2,3-butanetricarboxylic acid), a low-volatile, highlyoxidized, secondary-generation product of monoterpenes, is one of the mostrelevant tracer compounds for biogenic secondary organic aerosols (SOAs). In this study,laboratory-generated, micrometer-sized, pure-MBTCA, mono-/di-/trisodiumMBTCA salts and MBTCA–NaCl mixture aerosol particles of four mixing ratios(molar ratios = 1 : 1, 1 : 2, 1 : 3, and 2 : 1) were examined systematically toobserve their hygroscopic behavior by varying the relative humidity (RH)using in situ Raman microspectrometry (RMS) assembled with either asee-through impactor, where the particles were deposited on a Si wafer, or alevitation system. The pure MBTCA droplets effloresced at RH = ∼ 30 %–57.8 % and did not deliquesce until RH &gt; 95 %. The mono- and disodium MBTCA salt aerosols did not show clearefflorescence RH (ERH) and deliquescence RH (DRH). In contrast, thetrisodium MBTCA salt exhibited ERH = ∼ 44.4 %–46.8 % andDRH = ∼ 53.1 % during the hygroscopic experiment cycle.The mixture aerosols generated from solutions of MBTCA : NaCl = 1 : 1 and 2 : 1showed no visible ERH and DRH in the see-through impactor because of thepartial and total consumption of NaCl, respectively, through chemicalreactions during the dehydration process. The mixture particles with a 1 : 1molar ratio in the levitation system exhibited a clear DRH at∼ 71 % and ERH at ∼ 50 %. This suggests lessreaction between the mixtures and a larger portion of NaCl remaining in thelevitation system. The other mixtures of MBTCA : NaCl = 1 : 2 and 1 : 3displayed single-stage efflorescence and deliquescence at ERH = ∼ 45 %–50 % and DRH = ∼ 74 %, respectively,because of the considerable amount of NaCl present in the mixture aerosolsin both systems. Observations and Raman analyses indicated that onlymonosodium MBTCA salt aerosols could be formed through a reaction betweenMBTCA and NaCl. The reaction occurred more rapidly with a more elevatedconcentration of either MBTCA or NaCl, and the controlling factor for thereactivity of the mixtures depended mostly on the availability of H+dissociated from the MBTCA tricarboxylic acid. The lower degree of reactionof the mixture particles in the levitation system might be caused by therelatively airtight circumstance inside, i.e., less release of HCl. Thestudy revealed that the interactions between the MBTCA and NaCl could modifythe properties of the organic acid in the atmosphere, leading to enhancedcapability of the probable heterogeneous chemistry in the aqueous aerosols.

Hygroscopic behavior of aerosols generated from solutions of 3-methyl-1,2,3-butanetricarboxylic acid, its sodium salts, and its mixtures with NaCl