4 research outputs found
Connecting Bulk Viscosity Measurements to Kinetic Limitations on Attaining Equilibrium for a Model Aerosol Composition
The
growth, composition, and evolution of secondary organic aerosol
(SOA) are governed by properties of individual compounds and ensemble
mixtures that affect partitioning between the vapor and condensed
phase. There has been considerable recent interest in the idea that
SOA can form highly viscous particles where the diffusion of either
water or semivolatile organics within the particle is sufficiently
hindered to affect evaporation and growth. Despite numerous indirect
inferences of viscous behavior from SOA evaporation or “bounce”
within aerosol instruments, there have been no bulk measurements of
the viscosity of well-constrained model aerosol systems of atmospheric
significance. Here the viscous behavior of a well-defined model system
of 9 dicarboxylic acids is investigated directly with complementary
measurements and model predictions used to infer phase state. Results
not only allow us to discuss the atmospheric implications for SOA
formation through this representative mixture, but also the potential
impact of current methodologies used for probing this affect in both
the laboratory and from a modeling perspective. We show, quantitatively,
that the physical state transformation from liquid-like to amorphous
semisolid can substantially increase the importance of mass transfer
limitations within particles by 7 orders of magnitude for 100 nm diameter
particles. Recommendations for future research directions are given
Measured Saturation Vapor Pressures of Phenolic and Nitro-aromatic Compounds
Phenolic and nitro-aromatic compounds
are extremely toxic components
of atmospheric aerosol that are currently not well understood. In
this Article, solid and subcooled-liquid-state saturation vapor pressures
of phenolic and nitro-aromatic compounds are measured using Knudsen
Effusion Mass Spectrometry (KEMS) over a range of temperatures (298–318
K). Vapor pressure estimation methods, assessed in this study, do
not replicate the observed dependency on the relative positions of
functional groups. With a few exceptions, the estimates are biased
toward predicting saturation vapor pressures that are too high, by
5–6 orders of magnitude in some cases. Basic partitioning theory
comparisons indicate that overestimation of vapor pressures in such
cases would cause us to expect these compounds to be present in the
gas state, whereas measurements in this study suggest these phenolic
and nitro-aromatic will partition into the condensed state for a wide
range of ambient conditions if absorptive partitioning plays a dominant
role. While these techniques might have both structural and parametric
uncertainties, the new data presented here should support studies
trying to ascertain the role of nitrogen containing organics on aerosol
growth and human health impacts
Reaction between CH<sub>3</sub>O<sub>2</sub> and BrO Radicals: A New Source of Upper Troposphere Lower Stratosphere Hydroxyl Radicals
Over the last two decades it has
emerged that measured hydroxyl
radical levels in the upper troposphere are often underestimated by
models, leading to the assertion that there are missing sources. Here
we report laboratory studies of the kinetics and products of the reaction
between CH<sub>3</sub>O<sub>2</sub> and BrO radicals that shows that
this could be an important new source of hydroxyl radicals:BrO + CH<sub>3</sub>O<sub>2</sub> → products (1). The temperature
dependent value in Arrhenius form of <i>k</i>(<i>T</i>) is <i>k</i><sub>1</sub> = (2.42<sub>–0.72</sub><sup>+1.02</sup>) × 10<sup>–14</sup> exp[(1617
± 94)/<i>T</i>] cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>. In addition, CH<sub>2</sub>OO and HOBr are
believed to be the major products. Global model results suggest that
the decomposition of H<sub>2</sub>COO to form OH could lead to an
enhancement in OH of up to 20% in mid-latitudes in the upper troposphere
and in the lower stratosphere enhancements in OH of 2–9% are
inferred from model integrations. In addition, reaction 1 aids conversion
of BrO to HOBr and slows polar ozone loss in the lower stratosphere
Direct Measurements of Unimolecular and Bimolecular Reaction Kinetics of the Criegee Intermediate (CH<sub>3</sub>)<sub>2</sub>COO
The
Criegee intermediate acetone oxide, (CH<sub>3</sub>)<sub>2</sub>COO,
is formed by laser photolysis of 2,2-diiodopropane in the presence
of O<sub>2</sub> and characterized by synchrotron photoionization
mass spectrometry and by cavity ring-down ultraviolet absorption spectroscopy.
The rate coefficient of the reaction of the Criegee intermediate with
SO<sub>2</sub> was measured using photoionization mass spectrometry
and pseudo-first-order methods to be (7.3 ± 0.5) × 10<sup>–11</sup> cm<sup>3</sup> s<sup>–1</sup> at 298 K and
4 Torr and (1.5 ± 0.5) × 10<sup>–10</sup> cm<sup>3</sup> s<sup>–1</sup> at 298 K and 10 Torr (He buffer). These
values are similar to directly measured rate coefficients of <i>anti</i>-CH<sub>3</sub>CHOO with SO<sub>2</sub>, and in good
agreement with recent UV absorption measurements. The measurement
of this reaction at 293 K and slightly higher pressures (between 10
and 100 Torr) in N<sub>2</sub> from cavity ring-down decay of the
ultraviolet absorption of (CH<sub>3</sub>)<sub>2</sub>COO yielded
even larger rate coefficients, in the range (1.84 ± 0.12) ×
10<sup>–10</sup> to (2.29 ± 0.08) × 10<sup>–10</sup> cm<sup>3</sup> s<sup>–1</sup>. Photoionization mass spectrometry
measurements with deuterated acetone oxide at 4 Torr show an inverse
deuterium kinetic isotope effect, <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = (0.53 ± 0.06), for reactions with SO<sub>2</sub>, which may be consistent with recent suggestions that the
formation of an association complex affects the rate coefficient.
The reaction of (CD<sub>3</sub>)<sub>2</sub>COO with NO<sub>2</sub> has a rate coefficient at 298 K and 4 Torr of (2.1 ± 0.5) ×
10<sup>–12</sup> cm<sup>3</sup> s<sup>–1</sup> (measured
with photoionization mass spectrometry), again similar to rate for
the reaction of <i>anti</i>-CH<sub>3</sub>CHOO with NO<sub>2</sub>. Cavity ring-down measurements of the acetone oxide removal
without added reagents display a combination of first- and second-order
decay kinetics, which can be deconvolved to derive values for both
the self-reaction of (CH<sub>3</sub>)<sub>2</sub>COO and its unimolecular
thermal decay. The inferred unimolecular decay rate coefficient at
293 K, (305 ± 70) s<sup>–1</sup>, is similar to determinations
from ozonolysis. The present measurements confirm the large rate coefficient
for reaction of (CH<sub>3</sub>)<sub>2</sub>COO with SO<sub>2</sub> and the small rate coefficient for its reaction with water. Product
measurements of the reactions of (CH<sub>3</sub>)<sub>2</sub>COO with
NO<sub>2</sub> and with SO<sub>2</sub> suggest that these reactions
may facilitate isomerization to 2-hydroperoxypropene, possibly by
subsequent reactions of association products