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.