Size-Resolved Sea Spray Aerosol Particles Studied
by Vibrational Sum Frequency Generation
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Abstract
We present vibrational sum frequency
generation (SFG) spectra of
the external surfaces and the internal interfaces of size-selected
sea spray aerosol (SSA) particles generated at the wave flume of the
Scripps Hydraulics Laboratory. Our findings support SSA particle models
that invoke the presence of surfactants in the topmost particle layer
and indicate that the alkyl chains of surfactant-rich SSA particles
are likely to be disordered. Specifically, the SFG spectra suggest
that across the range of sizes studied, surfactant-rich SSA particles
contain CH oscillators that are subject to molecular orientation distributions
that are broader than the narrow molecular distribution functions
associated with well-ordered and well-aligned alkyl chains. This result
is consistent with the interpretation that the permeability of organic
layers at SSA particle surfaces to small reactive and nonreactive
molecules may be substantial, allowing for much more exchange between
reactive and nonreactive species in the gas or the condensed phase
than previously thought. The SFG data also suggest that a one-component
model is likely to be insufficient for describing the SFG responses
of the SSA particles. Finally, the similarity of the SFG spectra obtained
from the wave flume microlayer and 150 nm-sized SSA particles suggests
that the SFG active CH oscillators in the topmost layer of the wave
flume and the particle accumulation mode may be in similar chemical
environments. Needs for additional research activities are discussed
in the context of the results presented