11 research outputs found
Sensitivity of Aerosol Refractive Index Retrievals Using Optical Spectroscopy
<div><p>Accurate refractive index values are required to determine the effects of aerosol particles on direct radiative forcing. Theoretical retrievals using extinction data alone or extinction plus absorption data have been simulated to determine the sensitivity of each retrieval. A range of aerosol types with a range of different refractive indices were considered. The simulations showed that the extinction-only retrieval was not able to accurately or precisely retrieve refractive index values, even for purely scattering compounds, but the addition of a simulated absorption measurement greatly improved the retrieval.</p><p>Copyright 2014 American Association for Aerosol Research</p></div
Vapor–Wall Deposition in Chambers: Theoretical Considerations
In
order to constrain the effects of vapor–wall deposition
on measured secondary organic aerosol (SOA) yields in laboratory chambers,
researchers recently varied the seed aerosol surface area in toluene
oxidation and observed a clear increase in the SOA yield with increasing
seed surface area (Zhang, X.; et al. <i>Proc. Natl. Acad. Sci.
U.S.A.</i> <b>2014</b>, <i>111</i>, 5802). Using
a coupled vapor–particle dynamics model, we examine the extent
to which this increase is the result of vapor–wall deposition
versus kinetic limitations arising from imperfect accommodation of
organic species into the particle phase. We show that a seed surface
area dependence of the SOA yield is present only when condensation
of vapors onto particles is kinetically limited. The existence of
kinetic limitation can be predicted by comparing the characteristic
time scales of gas-phase reaction, vapor–wall deposition, and
gas–particle equilibration. The gas–particle equilibration
time scale depends on the gas–particle accommodation coefficient
α<sub>p</sub>. Regardless of the extent of kinetic limitation,
vapor–wall deposition depresses the SOA yield from that in
its absence since vapor molecules that might otherwise condense on
particles deposit on the walls. To accurately extrapolate chamber-derived
yields to atmospheric conditions, both vapor–wall deposition
and kinetic limitations must be taken into account
Heating-Induced Evaporation of Nine Different Secondary Organic Aerosol Types
The volatility of the compounds comprising
organic aerosol (OA)
determines their distribution between the gas and particle phases.
However, there is a disconnect between volatility distributions as
typically derived from secondary OA (SOA) growth experiments and the
effective particle volatility as probed in evaporation experiments.
Specifically, the evaporation experiments indicate an overall much
less volatile SOA. This raises questions regarding the use of traditional
volatility distributions in the simulation and prediction of atmospheric
SOA concentrations. Here, we present results from measurements of
thermally induced evaporation of SOA for nine different SOA types
(i.e., distinct volatile organic compound and oxidant pairs) encompassing
both anthropogenic and biogenic compounds and O<sub>3</sub> and OH
to examine the extent to which the low effective volatility of SOA
is a general phenomenon or specific to a subset of SOA types. The
observed extents of evaporation with temperature were similar for
all the SOA types and indicative of a low effective volatility. Furthermore,
minimal variations in the composition of all the SOA types upon heating-induced
evaporation were observed. These results suggest that oligomer decomposition
likely plays a major role in controlling SOA evaporation, and since
the SOA formation time scale in these measurements was less than a
minute, the oligomer-forming reactions must be similarly rapid. Overall,
these results emphasize the importance of accounting for the role
of condensed phase reactions in altering the composition of SOA when
assessing particle volatility
OH-Initiated Heterogeneous Oxidation of Internally-Mixed Squalane and Secondary Organic Aerosol
Recent work has established that
secondary organic aerosol (SOA)
can exist as an amorphous solid, leading to various suggestions that
the addition of SOA coatings to existing particles will decrease the
reactivity of those particles toward common atmospheric oxidants.
Experimental evidence suggests that O<sub>3</sub> is unable to physically
diffuse through an exterior semisolid or solid layer thus inhibiting
reaction with the core. The extent to which this suppression in reactivity
occurs for OH has not been established, nor has this been demonstrated
specifically for SOA. Here, measurements of the influence of adding
a coating of α-pinene+O<sub>3</sub> SOA onto squalane particles
on the OH-initiated heterogeneous oxidation rate are reported. The
chemical composition of the oxidized internally mixed particles was
monitored online using a vacuum ultraviolet-aerosol mass spectrometer.
Variations in the squalane oxidation rate with particle composition
were quantified by measurement of the effective uptake coefficient,
γ<sub>eff</sub>, which is the loss rate of a species relative
to the oxidant-particle collision rate. Instead of decreasing, the
measured γ<sub>eff</sub> increased continuously as the SOA coating
thickness increased, by a factor of ∼2 for a SOA coating thickness
of 42 nm (corresponding to ca. two-thirds of the particle mass). These
results indicate that heterogeneous oxidation of ambient aerosol by
OH radicals is not inhibited by SOA coatings, and further that condensed
phase chemical pathways and rates in organic particles depend importantly
on composition
Real-Time Black Carbon Emission Factor Measurements from Light Duty Vehicles
Eight
light-duty gasoline low emission vehicles (LEV I) were tested
on a Chassis dynamometer using the California Unified Cycle (UC) at
the Haagen-Smit vehicle test facility at the California Air Resources
Board in El Monte, CA during September 2011. The UC includes a cold
start phase followed by a hot stabilized running phase. In addition,
a light-duty gasoline LEV vehicle and ultralow emission vehicle (ULEV),
and a light-duty diesel passenger vehicle and gasoline direct injection
(GDI) vehicle were tested on a constant velocity driving cycle. A
variety of instruments with response times ≥0.1 Hz were used
to characterize how the emissions of the major particulate matter
components varied for the LEVs during a typical driving cycle. This
study focuses primarily on emissions of black carbon (BC). These measurements
allowed for the determination of BC emission factors throughout the
driving cycle, providing insights into the temporal variability of
BC emission factors during different phases of a typical driving cycle
Volatility of Primary Organic Aerosol Emitted from Light Duty Gasoline Vehicles
Primary
organic aerosol (POA) emitted from light duty gasoline
vehicles (LDGVs) exhibits a semivolatile behavior in which heating
the aerosol and/or diluting the aerosol leads to partial evaporation
of the POA. A single volatility distribution can explain the median
evaporation behavior of POA emitted from LDGVs but this approach is
unable to capture the full range of measured POA volatility during
thermodenuder (TD) experiments conducted at atmospherically relevant
concentrations (2–5 μg m<sup>–3</sup>). Reanalysis
of published TD data combined with analysis of new measurements suggest
that POA emitted from gasoline vehicles is composed of two types of
POA that have distinctly different volatility distributions: one low-volatility
distribution and one medium-volatility distribution. These correspond
to fuel combustion-derived POA and motor oil POA, respectively. Models
that simultaneously incorporate both of these distributions are able
to reproduce experimental results much better (<i>R</i><sup>2</sup> = 0.94) than models that use a single average or median distribution
(<i>R</i><sup>2</sup> = 0.52). These results indicate that
some fraction of POA emitted from LDGVs is essentially nonvolatile
under typical atmospheric dilution levels. Roughly 50% of the vehicles
tested in the current study had POA emissions dominated by fuel combustion
products (essentially nonvolatile). Further testing is required to
determine appropriate fleet-average emissions rates of the two POA
types from LDGVs
The Influence of Molecular Structure and Aerosol Phase on the Heterogeneous Oxidation of Normal and Branched Alkanes by OH
Insights
into the influence of molecular structure and thermodynamic
phase on the chemical mechanisms of hydroxyl radical-initiated heterogeneous
oxidation are obtained by identifying reaction products of submicrometer
particles composed of either <i>n</i>-octacosane (C<sub>28</sub>H<sub>58</sub>, a linear alkane) or squalane (C<sub>30</sub>H<sub>62</sub>, a highly branched alkane) and OH. A common pattern
is observed in the positional isomers of octacosanone and octacosanol,
with functionalization enhanced toward the end of the molecule. This
suggests that relatively large linear alkanes are structured in submicrometer
particles such that their ends are oriented toward the surface. For
squalane, positional isomers of first-generation ketones and alcohols
also form in distinct patterns. Ketones are favored on carbons adjacent
to tertiary carbons, while hydroxyl groups are primarily found on
tertiary carbons but also tend to form toward the end of the molecule.
Some first-generation products, viz., hydroxycarbonyls and diols,
contain two oxygen atoms. These results suggest that alkoxy radicals
are important intermediates and undergo both intramolecular (isomerization)
and intermolecular (chain propagation) hydrogen abstraction reactions.
Oxidation products with carbon number less than the parent alkane’s
are observed to a much greater extent for squalane than for <i>n</i>-octacosane oxidation and can be explained by the preferential
cleavage of bonds involving tertiary carbons
Real-Time Emission Factor Measurements of Isocyanic Acid from Light Duty Gasoline Vehicles
Exposure to gas-phase isocyanic acid
(HNCO) has been previously
shown to be associated with the development of atherosclerosis, cataracts
and rheumatoid arthritis. As such, accurate emission inventories for
HNCO are critical for modeling the spatial and temporal distribution
of HNCO on a regional and global scale. To date, HNCO emission rates
from light duty gasoline vehicles, operated under driving conditions,
have not been determined. Here, we present the first measurements
of real-time emission factors of isocyanic acid from a fleet of eight
light duty gasoline-powered vehicles (LDGVs) tested on a chassis dynamometer
using the Unified Driving Cycle (UC) at the California Air Resources
Board (CARB) Haagen-Smit test facility, all of which were equipped
with three-way catalytic converters. HNCO emissions were observed
from all vehicles, in contrast to the idealized laboratory measurements.
We report the tested fleet averaged HNCO emission factors, which depend
strongly on the phase of the drive cycle; ranging from 0.46 ±
0.13 mg kg<sub>fuel</sub><sup>–1</sup> during engine start
to 1.70 ± 1.77 mg kg<sub>fuel</sub><sup>–1</sup> during
hard acceleration after the engine and catalytic converter were warm.
The tested eight-car fleet average fuel based HNCO emission factor
was 0.91 ± 0.58 mg kg<sub>fuel</sub><sup>–1</sup>, within
the range previously estimated for light duty diesel-powered vehicles
(0.21–3.96 mg kg<sub>fuel</sub><sup>–1</sup>). Our results
suggest that HNCO emissions from LDGVs represent a significant emission
source in urban areas that should be accounted for in global and regional
models
Parameterized Yields of Semivolatile Products from Isoprene Oxidation under Different NO<sub><i>x</i></sub> Levels: Impacts of Chemical Aging and Wall-Loss of Reactive Gases
We
developed a parametrizable box model to empirically derive the
yields of semivolatile products from VOC oxidation using chamber measurements,
while explicitly accounting for the multigenerational chemical aging
processes (such as the gas-phase fragmentation and functionalization
and aerosol-phase oligomerization and photolysis) under different
NO<sub><i>x</i></sub> levels and the loss of particles and
gases to chamber walls. Using the oxidation of isoprene as an example,
we showed that the assumptions regarding the NO<sub><i>x</i></sub>-sensitive, multigenerational aging processes of VOC oxidation
products have large impacts on the parametrized product yields and
SOA formation. We derived sets of semivolatile product yields from
isoprene oxidation under different NO<sub><i>x</i></sub> levels. However, we stress that these product yields must be used
in conjunction with the corresponding multigenerational aging schemes
in chemical transport models. As more mechanistic insights regarding
SOA formation from VOC oxidation emerge, our box model can be expanded
to include more explicit chemical aging processes and help ultimately
bridge the gap between the process-based understanding of SOA formation
from VOC oxidation and the bulk-yield parametrizations used in chemical
transport models
Analysis of Organic Anionic Surfactants in Fine and Coarse Fractions of Freshly Emitted Sea Spray Aerosol
The inclusion of organic compounds
in freshly emitted sea spray
aerosol (SSA) has been shown to be size-dependent, with an increasing
organic fraction in smaller particles. Here we have used electrospray
ionization-high resolution mass spectrometry in negative ion mode
to identify organic compounds in nascent sea spray collected throughout
a 25 day mesocosm experiment. Over 280 organic compounds from ten
major homologous series were tentatively identified, including saturated
(C<sub>8</sub>–C<sub>24</sub>) and unsaturated (C<sub>12</sub>–C<sub>22</sub>) fatty acids, fatty acid derivatives (including
saturated oxo-fatty acids (C<sub>5</sub>–C<sub>18</sub>) and
saturated hydroxy-fatty acids (C<sub>5</sub>–C<sub>18</sub>), organosulfates (C<sub>2</sub>–C<sub>7</sub>, C<sub>12</sub>–C<sub>17</sub>) and sulfonates (C<sub>16</sub>–C<sub>22</sub>). During the mesocosm, the distributions of molecules within
some homologous series responded to variations among the levels of
phytoplankton and bacteria in the seawater. The average molecular
weight and carbon preference index of saturated fatty acids significantly
decreased within fine SSA during the progression of the mesocosm,
which was not observed in coarse SSA, sea-surface microlayer or in
fresh seawater. This study helps to define the molecular composition
of nascent SSA and biological processes in the ocean relate to SSA
composition