Abstract. Viscosities and diffusion rates of organics within secondary
organic aerosol (SOA) remain uncertain. Using the bead-mobility technique, we
measured viscosities as a function of water activity (aw) of
SOA generated by the ozonolysis of limonene followed by browning by exposure
to NH3 (referred to as brown limonene SOA or brown LSOA). These
measurements together with viscosity measurements reported in the literature
show that the viscosity of brown LSOA increases by 3–5 orders of magnitude
as the aw decreases from 0.9 to approximately 0.05. In addition,
we measured diffusion coefficients of intrinsic fluorescent organic molecules
within brown LSOA matrices using rectangular area fluorescence recovery after
photobleaching. Based on the diffusion measurements, as the aw
decreases from 0.9 to 0.33, the average diffusion coefficient of the
intrinsic fluorescent organic molecules decreases from 5.5×10-9 to 7.1×10-13 cm2 s−1 and
the mixing times of intrinsic fluorescent organic molecules within 200 nm
brown LSOA particles increases from 0.002 to 14 s. These results suggest
that the mixing times of large organics in the brown LSOA studied here are
short (<1 h) for aw and temperatures often found in
the planetary boundary layer (PBL). Since the diffusion coefficients and mixing
times reported here correspond to SOA generated using a high mass loading
(∼1000 µg m−3), biogenic SOA particles found in the
atmosphere with mass loadings ≤10 µg m−3 are likely to
have higher viscosities and longer mixing times (possibly 3 orders of
magnitude longer). These new measurements of viscosity and diffusion were
used to test the accuracy of the Stokes–Einstein relation for predicting
diffusion rates of organics within brown LSOA matrices. The results show that
the Stokes–Einstein equation gives accurate predictions of diffusion
coefficients of large organics within brown LSOA matrices when the viscosity
of the matrix is as high as 102 to 104 Pa s. These results have important implications for predicting diffusion
and mixing within SOA particles in the atmosphere.
Water diffusion in atmospherically relevant α-pinene secondary organic material
