11 Secondary organic aerosols (SOAs), which are formed and transformed through complex 12 physicochemical processes in the atmosphere, have attracted considerable attention over the past decades 13 because of their impacts on both climate change and human health. Recently, 3-methyl-1,2,3-14 butanetricarboxylic acid (MBTCA), a low volatile, highly oxidized, secondary generation product of 15 monoterpenes, is one of the most relevant tracer compounds for biogenic SOAs. Therefore, MBTCA was 16 selected to understand its hygroscopic properties better. In addition, interactions between the organic acid 17 and inorganic components have been reported, which may alter their hygroscopic properties mutually. In 18 this study, laboratory-generated, micrometer-sized, pure MBTCA, mono-/di-/tri-sodium MBTCA salts, 19 and MBTCA-NaCl mixture aerosol particles of four mixing ratios (molar ratios = 1:1, 1:2, 1:3, and 2:1) 20 were examined systematically to observe their hygroscopic behavior by varying the relative humidity 21 (RH) from RH = ~95% to ~1% through a dehydration process, followed by a humidification process from 22 RH = ~1% to ~95%, using in-situ Raman microspectrometry (RMS) assembled with a see-through 23 impactor where the particles were deposited on a Si wafer. The hygroscopic behavior of pure MBTCA 24 and MBTCA-NaCl mixture aerosol particles of three mixing ratios (molar ratios = 1:1, 1:2, and 1:3) were 25 also examined using a levitation system mounted on in-situ RMS through a humidification process from 26 2 RH = ~10% to ~80% after a quenching process from droplets, followed by dehydration from RH = ~80% 27 to ~10%. The pure MBTCA droplets effloresced at RH = ~30-57.8% and did not dissolve until RH > 28 95%. The mono-and di-sodium MBTCA salt aerosols did not show clear efflorescence RH (ERH) and 29 deliquescence RH (DRH). In contrast, the tri-sodium MBTCA salt exhibited ERH = ~44.4-46.8% and 30 DRH = ~53.1%, during the hygroscopic experiment cycle. The mixture aerosols generated from solutions 31 of MBTCA:NaCl = 1:1 and 2:1 showed no visible ERH and DRH in the see-through impactor because of 32 the partial and total consumption of NaCl, respectively, through chemical reactions during the 33 dehydration process. The mixture particles with a 1:1 molar ratio in the levitation system exhibited a clear 34 DRH at ~71% and ERH at ~50%. This suggests less reaction between the mixtures and a larger portion 35 of NaCl remaining in the levitation system. The other mixtures of MBTCA:NaCl = 1:2 and 1:3 displayed 36 single-stage efflorescence and deliquescence at ERH = ~45-50% and DRH = ~74%, respectively, because 37 of the considerable amount of NaCl present in the mixture aerosols in both systems. Observations and 38 Raman analyses indicated that only monosodium MBTCA salt aerosols could be formed through a 39 reaction between MBTCA and NaCl. The reaction occurred more rapidly with a more elevated 40 concentration of either MBTCA or NaCl, and the controlling factor for the reactivity of the mixtures 41 depended mostly on the availability of H + dissociated from the MBTCA tricarboxylic acid. The lower 42 degree of reaction of the mixture particles in the levitation system might be caused by the relatively 43 airtight circumstance inside, i.e., the less release of HCl. In addition, the quenching process, i.e., the 44 starting point of the hygroscopicity experiments, induced the solidification of MBTCA, and further, a 45 slow reaction between MBTCA and NaCl. The study revealed that the interactions between the MBTCA 46 and NaCl could modify the properties of the organic acid in the atmosphere, leading to enhanced 47 capability of the probable heterogeneous chemistry in the aqueous aerosols. 48 49