Effect of inorganic salts on the volatility of organic acids

Abstract

Particulate phase reactions between organic and inorganic compounds may significantly alter aerosol chemical properties, for example, by suppressing particle volatility. Here, chemical processing upon drying of aerosols comprised of organic (acetic, oxalic, succinic, or citric) acid/monovalent inorganic salt mixtures was assessed by measuring the evaporation of the organic acid molecules from the mixture using a novel approach combining a chemical ionization mass spectrometer coupled with a heated flow tube inlet (TPD-CIMS) with kinetic model calculations. For reference, the volatility, i.e. saturation vapor pressure and vaporization enthalpy, of the pure succinic and oxalic acids was also determined and found to be in agreement with previous literature. Comparison between the kinetic model and experimental data suggests significant particle phase processing forming low-volatility material such as organic salts. The results were similar for both ammonium sulfate and sodium chloride mixtures, and relatively more processing was observed with low initial aerosol organic molar fractions. The magnitude of low-volatility organic material formation at an atmospherically relevant pH range indicates that the observed phenomenon is not only significant in laboratory conditions but is also of direct atmospheric relevance.

Figure 1

Method for determining F_org,model and ΔH_vap of an organic acid in an inorganic salt mixture—example case (OA/NaCl, F_org = 0.2 in the atomized solution). (a) Optimal F_org,model and ΔH_vap pair found by minimizing the difference between the ratios (mixture vs pure) of (b) observed CIMS signals and (c) modeled evaporated masses. Red refers to TPD-CIMS measurements and black to the model. Solid lines illustrate the evaporation of oxalic acid from pure aerosol and dashed lines its evaporation from oxalic acid/NaCl mixture (in b and c). The pure oxalic acid data set used in the illustration has been used in previous studies.^,

Figure 2

Comparing existing literature (open markers) with (a) p_sat and (b) ΔH_vap of pure oxalic acid and succinic acid obtained by minimizing the difference between measured and modeled evaporation of these organics (red circles and blue triangles).

Figure 3

(a) Comparing the model derived initial molar fraction of organic acid in the dry mixed aerosol (F_org,model) to the initial organic molar fraction of the solute in the atomized solution (F_org). (b) Based on the difference in the F_org,model and F_org (presented in a), the fraction of effectively nonvolatile organic matter of the initial organic molar fraction (NVF) was determined and presented as a function of F_org. Markers of blue color scheme refer to succinic acid and markers of red color scheme to oxalic acid data. Circles and triangles illustrate sodium chloride and ammonium sulfate mixtures, respectively. Black dashed lines illustrate the limit where F_org,model is comparable to F_org and, thus, indicate no formation of low-volatility organic material.

Figure 4

Fraction of the effectively nonvolatile organic matter (NVF) as a function of (a) aqueous aerosol pH and (b) pure component saturation vapor pressure of the organic acid. Bluish markers refer to succinic acid, reddish markers to oxalic acid, yellow markers to acetic acid, and black markers to citric acid data. Circles and triangles illustrate the data of organic acid mixed with sodium chloride and ammonium sulfate, respectively. Black dashed lines illustrate the limit where no formation of low-volatility organic material is observed. Note that the values for acetic acid and citric acid pure component vapor pressures and NVFs are qualitative estimates based on our data and literature data.