Abstract
Abstract. Although water uptake of aerosol particles plays an important role in the atmospheric environment, the effects of interactions between components on chemical composition and hygroscopicity of particles are still not well constrained. The hygroscopic properties and phase transformation of oxalic acid (OA) and mixed particles composed of ammonium sulfate (AS) and OA with different organic to inorganic molar ratios (OIRs) have been investigated by using confocal Raman spectroscopy. It is found that OA droplets first crystallize to form OA dihydrate at 71 % relative humidity (RH), and further lose crystalline water to convert into anhydrous OA around 5 % RH during the dehydration process. The deliquescence and efflorescence point for AS is determined to be 80.1 ± 1.5 % RH and 44.3 ± 2.5 % RH, respectively. The observed efflorescence relative humidity (ERH) for mixed OA ∕ AS droplets with OIRs of 1 : 3, 1 : 1 and 3 : 1 is 34.4 ± 2.0, 44.3 ± 2.5 and 64.4 ± 3.0 % RH, respectively, indicating the elevated OA content appears to favor the crystallization of mixed systems at higher RH. However, the deliquescence relative humidity (DRH) of AS in mixed OA ∕ AS particles with OIRs of 1 : 3 and 1 : 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 formation of ammonium hydrogen oxalate (NH4HC2O4) and ammonium hydrogen sulfate (NH4HSO4) from interactions between OA and AS in aerosols during the dehydration process on the time scale of hours, which considerably influence the subsequent deliquescence behavior of internally mixed particles with different OIRs. The mixed OA ∕ AS particles with an OIR of 3 : 1 exhibit no deliquescence transition over the RH range studied due to the considerable transformation of (NH4)2SO4 into NH4HC2O4 with a high DRH. Although the hygroscopic growth of mixed OA ∕ AS droplets is comparable to that of AS or OA at high RH during the dehydration process, Raman growth factors of mixed particles after deliquescence are substantially lower than those of mixed OA ∕ AS droplets during the efflorescence process and further decrease with elevated OA content. The discrepancies for Raman growth factors of mixed OA ∕ AS particles between the dehydration and hydration process at high RH can be attributed to the significant formation of NH4HC2O4 and residual OA, which remain solid at high RH and thus result in less water uptake of mixed particles. These findings improve the understanding of the role of reactions between dicarboxylic acid and inorganic salt in the chemical and physical properties of aerosol particles, and might have important implications for atmospheric chemistry.
Highlights
Atmospheric aerosols have vital impacts on the Earth’s climate directly by scattering, reflecting and absorbing solar radiation, and indirectly by influencing formation of clouds and precipitation (Tang and Munkelwitz, 1994b; Jacobson et al, 2000; Penner et al, 2001; Pöschl, 2005; Von Schneidemesser et al, 2015)
Ammonium sulfate (AS) droplets effloresce at 44.3 ± 2.5 % relative humidity (RH), as indicated by the disappearance of the water peak centered at 3437 cm−1 and a red-shift in the νs(SO24−) peak position from 979 to 974 cm−1 during the dehydration process
The deliquescence of AS particles is observed to occur at 80.1 ± 1.5 % RH, resulting in an abrupt increase in the absorbance of the water peak centered at 3437 cm−1 and a blue-shift in the νs(SO24−) peak position from 974 to 979 cm−1
Summary
Atmospheric aerosols have vital impacts on the Earth’s climate directly by scattering, reflecting and absorbing solar radiation, and indirectly by influencing formation of clouds and precipitation (Tang and Munkelwitz, 1994b; Jacobson et al, 2000; Penner et al, 2001; Pöschl, 2005; Von Schneidemesser et al, 2015). Previous studies have found that chloride depletion could occur in the NaCl / dicarboxylic acid mixed aerosols during the dehydration or efflorescence process, which led to the formation of organic salts and in turn affected subsequent deliquescence behaviors of aerosols (Laskin et al, 2012; Ghorai et al, 2014). The incorporation of organic salts formed from interactions between inorganic salts and organic acids is crucial to the modeling of hygroscopic properties of mixed organic / inorganic particles It merits further investigation on the interactions between OA and AS and the related influence on the water uptake behaviors of aerosols during the dehydration and hydration processes. We explored the effects of reactions between OA and AS on the chemical compositions and hygroscopic properties of mixed OA / AS droplets
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