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Although water uptake of aerosol particles plays an important role in the\udatmospheric environment, the effects of interactions between components on\udchemical composition and hygroscopicity of particles are still not well\udconstrained. The hygroscopic properties and phase transformation of oxalic\udacid (OA) and mixed particles composed of ammonium sulfate (AS) and OA with\uddifferent organic to inorganic molar ratios (OIRs) have been investigated by\udusing confocal Raman spectroscopy. It is found that OA droplets first\udcrystallize to form OA dihydrate at 71 % relative humidity (RH),\udand further lose crystalline water to convert into anhydrous OA\udaround 5 % RH during the dehydration process. The deliquescence and\udefflorescence point for AS is determined to be 80.1 ± 1.5 % RH and\ud44.3 ± 2.5 % RH, respectively. The observed efflorescence relative\udhumidity (ERH) for mixed OA ∕ AS droplets with OIRs of 1 : 3, 1 : 1 and 3 : 1 is\ud34.4 ± 2.0, 44.3 ± 2.5 and 64.4 ± 3.0 % RH,\udrespectively, indicating the elevated OA content appears to favor the\udcrystallization of mixed systems at higher RH. However, the deliquescence\udrelative humidity (DRH) of AS in mixed OA ∕ AS particles with OIRs of 1 : 3 and\ud1 : 1 is observed to occur at 81.1 ± 1.5 and 77 ± 1.0 % RH, respectively. The Raman spectra of mixed OA ∕ AS droplets indicate the\udformation of ammonium hydrogen oxalate (NH4HC2O4) and\udammonium hydrogen sulfate (NH4HSO4) from interactions between OA\udand AS in aerosols during the dehydration process on the time scale of hours,\udwhich considerably influence the subsequent deliquescence behavior of\udinternally mixed particles with different OIRs. The mixed OA ∕ AS particles\udwith an OIR of 3 : 1 exhibit no deliquescence transition over the RH range\udstudied due to the considerable transformation of (NH4)2SO4\udinto NH4HC2O4 with a high DRH. Although the hygroscopic\udgrowth of mixed OA ∕ AS droplets is comparable to that of AS or OA at high RH\udduring the dehydration process, Raman growth factors of mixed particles after\uddeliquescence are substantially lower than those of mixed OA ∕ AS droplets\udduring the efflorescence process and further decrease with elevated OA\udcontent. The discrepancies for Raman growth factors of mixed OA ∕ AS particles\udbetween the dehydration and hydration process at high RH can be attributed to\udthe significant formation of NH4HC2O4 and residual OA, which\udremain solid at high RH and thus result in less water uptake of mixed\udparticles. These findings improve the understanding of the role of reactions\udbetween dicarboxylic acid and inorganic salt in the chemical and physical\udproperties of aerosol particles, and might have important implications for\udatmospheric chemistry

Hygroscopic behavior and chemical composition evolution of internally mixed aerosols composed of oxalic acid and ammonium sulfate