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

MBTCA (3-methyl-1,2,3-butanetricarboxylic acid), a low-volatile, highly oxidized, secondary-generation product of monoterpenes, is one of the most relevant tracer compounds for biogenic secondary organic aerosols (SOAs). In this study, laboratory-generated, micrometer-sized, pure-MBTCA, mono-/di-/trisodium MBTCA 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 to observe their hygroscopic behavior by varying the relative humidity (RH) using in situ Raman microspectrometry (RMS) assembled with either a see-through impactor, where the particles were deposited on a Si wafer, or a levitation system. The pure MBTCA droplets effloresced at RH = ∼ 30 %–57.8 % and did not deliquesce until RH > 95 %. The mono- and disodium MBTCA salt aerosols did not show clear efflorescence RH (ERH) and deliquescence RH (DRH). In contrast, the trisodium MBTCA salt exhibited ERH = ∼ 44.4 %–46.8 % and DRH = ∼ 53.1 % during the hygroscopic experiment cycle. The mixture aerosols generated from solutions of MBTCA : NaCl = 1 : 1 and 2 : 1 showed no visible ERH and DRH in the see-through impactor because of the partial and total consumption of NaCl, respectively, through chemical reactions during the dehydration process. The mixture particles with a 1 : 1 molar ratio in the levitation system exhibited a clear DRH at ∼ 71 % and ERH at ∼ 50 %. This suggests less reaction between the mixtures and a larger portion of NaCl remaining in the levitation system. The other mixtures of MBTCA : NaCl = 1 : 2 and 1 : 3 displayed single-stage efflorescence and deliquescence at ERH = ∼ 45 %–50 % and DRH = ∼ 74 %, respectively, because of the considerable amount of NaCl present in the mixture aerosols in both systems. Observations and Raman analyses indicated that only monosodium MBTCA salt aerosols could be formed through a reaction between MBTCA and NaCl. The reaction occurred more rapidly with a more elevated concentration of either MBTCA or NaCl, and the controlling factor for the reactivity of the mixtures depended mostly on the availability of H+ dissociated from the MBTCA tricarboxylic acid. The lower degree of reaction of the mixture particles in the levitation system might be caused by the relatively airtight circumstance inside, i.e., less release of HCl. The study revealed that the interactions between the MBTCA and NaCl could modify the properties of the organic acid in the atmosphere, leading to