A large fraction
of atmospheric aerosols can be characterized as
primary organic aerosol (POA) and secondary organic aerosol (SOA).
Knowledge of the phase behavior, that is, the number and type of phases
within internal POA + SOA mixtures, is crucial to predict their effect
on climate and air quality. For example, if POA and SOA form a single
phase, POA will enhance the formation of SOA by providing organic
mass to absorb SOA precursors. Using microscopy, we studied the phase
behavior of mixtures of SOA proxies and hydrocarbon-like POA proxies
at relative humidity (RH) values of 90%, 45%, and below 5%. Internal
mixtures of POA and SOA almost always formed two phases if the elemental
oxygen-to-carbon ratio (O/C) of the POA was less than 0.11, which
encompasses a large fraction of atmospheric hydrocarbon-like POA from
fossil fuel combustion. SOA proxies mixed with POA proxies having
0.11 ≤ O/C ≤ 0.29 mostly resulted in particles with
one liquid phase. However, two liquid phases were also observed, depending
on the type of SOA and POA surrogates, and an increase in phase-separated
particles was observed when increasing the RH in this O/C range. The
results have implications for predicting atmospheric SOA formation
and policy strategies to reduce SOA in urban environments.
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