2 research outputs found
Fundamental Time Scales Governing Organic Aerosol Multiphase Partitioning and Oxidative Aging
Traditional descriptions of gas–particle
partitioning of
organic aerosols (OA) rely solely on thermodynamic properties (e.g.,
volatility). Under realistic conditions where phase partitioning is
dynamic rather than static, the transformation of OA involves the
interplay of multiphase partitioning with oxidative aging. A key challenge
remains in quantifying the fundamental time scales for evaporation
and oxidation of semivolatile OA. In this paper, we use isomer-resolved
product measurements of a series of normal-alkanes (C<sub>18</sub>, C<sub>20</sub>, C<sub>22</sub>, and C<sub>24</sub>) to distinguish
between gas-phase and heterogeneous oxidation products formed by reaction
with hydroxyl radicals (OH). The product isomer distributions when
combined with kinetics measurements of evaporation and oxidation enable
a quantitative description of the multiphase time scales to be simulated
using a single-particle kinetic model. Multiphase partitioning and
oxidative transformation of semivolatile normal-alkanes under laboratory
conditions is largely controlled by the particle phase state, since
the time scales of heterogeneous oxidation and evaporation are found
to occur on competing time scales (on the order of 10<sup>–1</sup> h). This is in contrast to atmospheric conditions where heterogeneous
oxidation time scales are expected to be much longer (on the order
of 10<sup>2</sup> h), with gas-phase oxidation being the dominant
process regardless of the evaporation kinetics. Our results demonstrate
the dynamic nature of OA multiphase partitioning and oxidative aging
and reveal that the fundamental time scales of these processes are
crucial for reliably extending laboratory measurements of OA phase
partitioning and aging to the atmosphere
Ambient Gas-Particle Partitioning of Tracers for Biogenic Oxidation
Exchange
of atmospheric organic compounds between gas and particle
phases is important in the production and chemistry of particle-phase
mass but is poorly understood due to a lack of simultaneous measurements
in both phases of individual compounds. Measurements of particle-
and gas-phase organic compounds are reported here for the southeastern
United States and central Amazonia. Polyols formed from isoprene oxidation
contribute 8% and 15% on average to particle-phase organic mass at
these sites but are also observed to have substantial gas-phase concentrations
contrary to many models that treat these compounds as nonvolatile.
The results of the present study show that the gas-particle partitioning
of approximately 100 known and newly observed oxidation products is
not well explained by environmental factors (e.g., temperature). Compounds
having high vapor pressures have higher particle fractions than expected
from absorptive equilibrium partitioning models. These observations
support the conclusion that many commonly measured biogenic oxidation
products may be bound in low-volatility mass (e.g., accretion products,
inorganic–organic adducts) that decomposes to individual compounds
on analysis. However, the nature and extent of any such bonding remains
uncertain. Similar conclusions are reach for both study locations,
and average particle fractions for a given compound are consistent
within ∼25% across measurement sites