Phase transitions and phase miscibility of mixed particles of ammonium sulfate, toluene-derived secondary organic material, and water.

The phase states of atmospheric particles influence their roles in physicochemical processes related to air quality and climate. The phases of particles containing secondary organic materials (SOMs) are still uncertain, especially for SOMs produced from aromatic precursor gases. In this work, efflorescence and deliquescence phase transitions, as well as phase separation, in particles composed of toluene-derived SOM, ammonium sulfate, and water were studied by hygroscopic tandem differential mobility analysis (HTDMA) and optical microscopy. The SOM was produced in the Harvard Environmental Chamber by photo-oxidation of toluene at chamber relative humidities of <5 and 40%. The efflorescence and deliquescence relative humidities (ERH and DRH, respectively, studied by HTDMA) of ammonium sulfate decreased as the SOM organic fraction ε in the particle increased, dropping from DRH = 80% and ERH = 31% for ε = 0.0 to DRH = 58% and ERH = 0% for ε = 0.8. For ε < 0.2, the DRH and ERH to first approximation did not change with the organic volume fraction. This observation is consistent with independent behaviors for ε < 0.2 of water-infused toluene-derived SOM and aqueous ammonium sulfate, suggesting phase immiscibility between the two. Optical microscopy of particles prepared for ε = 0.12 confirmed phase separation for RH < 85%. For ε from 0.2 to 0.8, the DRH and ERH values steadily decreased, as studied by HTDMA. This result is consistent with one-phase mixing of ammonium sulfate, SOM, and water. Optical microscopy for particles of ε = 0.8 confirmed this result. Within error, increased exposure times of the aerosol in the HTDMA from 0.5 to 30 s affected neither the ERH(ε) nor DRH(ε) curves, implying an absence of kinetic effects on the observations over the studied time scales. For ε > 0.5, the DRH values of ammonium sulfate in mixtures with SOM produced at <5% RH were offset by -3 to -5% RH compared to the results for SOM produced at 40% RH, suggesting differences in SOM chemistry. The observed miscibility gap (i.e., phase separation) between toluene-derived SOM and aqueous ammonium sulfate across a limited range of organic volume fractions differentiates this SOM from previous reports for isoprene-derived SOM of full miscibility and for α-pinene-derived SOM of nearly full immiscibility with aqueous ammonium sulfate.