Deliquescence, efflorescence, and phase miscibility of mixed particles of ammonium sulfate and isoprene-derived secondary organic material

Abstract. The hygroscopic phase transitions of ammonium sulfate mixed with isoprene-derived secondary organic material were investigated in aerosol experiments. The organic material was produced by isoprene photo-oxidation at 40% and 60% relative humidity. The low volatility fraction of the photo-oxidation products condensed onto ammonium sulfate particles. The particle-phase organic material had oxygen-to-carbon ratios of 0.67 to 0.74 (±0.2) for mass concentrations of 20 to 30 μg m⁻³. The deliquescence, efflorescence, and phase miscibility of the mixed particles were investigated using a dual arm tandem differential mobility analyzer. The isoprene photo-oxidation products induced deviations in behavior relative to pure ammonium sulfate. Compared to an efflorescence relative humidity (ERH) of 30 to 35% for pure ammonium sulfate, efflorescence was eliminated for aqueous particles having organic volume fractions ϵ of 0.6 and greater. Compared to a deliquescence relative humidity (DRH) of 80% for pure ammonium sulfate, the DRH steadily decreased with increasing ϵ, approaching a DRH of 40% for ϵ of 0.9. Parameterizations of the DRH(ϵ) and ERH(ϵ) curves were as follows: DRH(ϵ)= ∑_i c_i,d ϵⁱ valid for 0 ≤ ϵ ≤0.86 and ERH(ϵ)= ∑ _i c_i,e ϵⁱ valid for 0 ≤ ϵ ≤ 0.55 for the coefficients c_0,d= 80.67, c_0,e = 28.35, c_1,d = −11.45, c_1,e = −13.66, c_2,d = 0, c_2,e = 0, c_3,d = 57.99, c_3,e = -83.80, c_4,d = −106.80, and c_4,e = 0. The molecular description that is thermodynamically implied by these strongly sloped DRH(ϵ) and ERH(ϵ) curves is that the organic isoprene photo-oxidation products, the inorganic ammonium sulfate, and water form a miscible liquid phase even at low relative humidity. This phase miscibility is in contrast to the liquid-liquid separation that occurs for some other types of secondary organic material. These differences in liquid-liquid separation are consistent with a prediction recently presented in the literature that the bifurcation between liquid-liquid phase separation versus mixing depends on the oxygen-to-carbon ratio of the organic material. The conclusions are that the influence of secondary organic material on the hygroscopic properties of ammonium sulfate varies with organic composition and that the degree of oxygenation of the organic material, which is a measurable characteristic of complex organic materials, is an important variable influencing the hygroscopic properties of mixed organic-inorganic particles.