4 research outputs found
Closing the ultrafine particle number concentration budget at road-to-ambient scale: Implications for particle dynamics
<p>Freshly emitted vehicle exhaust particles are diluted quickly as they mix into ambient air, but the contribution of evaporation, coagulation, and/or nucleation of new particles to the number concentration has been the subject of some debate. We analyzed one-second time resolution size distribution data from an early morning field campaign, data collected at a time at which dilution has a smaller (but still dominant; ∼70−80%) impact on particle concentrations. Because the plume is diluted over an hour, and a distance of 1500 m, we can constrain the processes with higher accuracy. We find that concentrations in the smaller size bins (5.6–23.7 nm) peak further downwind than the reference particles (42.1–562 nm), and decay significantly faster than larger particles particularly in the area 100−400 m downwind. Comparisons of the cumulative contributions of van der Waals enhanced coagulation, dry deposition, and dilution and the observed decay curves, imply that for up to the first 50–100 m there is nucleation and/or growth of particles smaller than 5.6 nm. In contrast, in the ∼100–400 m region, some of the smaller particles evaporate. In the further downwind areas (>400 m) the particles all appear to decay at rates consistent with the sum of dilution, coagulation, and deposition. We also find that a dry deposition parameterization at the low end of those available in the literature is most consistent with the observational data.</p> <p>© 2016 American Association for Aerosol Research</p
Real Refractive Indices and Formation Yields of Secondary Organic Aerosol Generated from Photooxidation of Limonene and α-Pinene: The Effect of the HC/NO<sub><i>x</i></sub> Ratio
The refractive index is an important property affecting aerosol
optical properties, which in turn help determine the aerosol direct
effect and satellite retrieval results. Here, we investigate the real
refractive indices (<i>m</i><sub>r</sub>) of secondary organic
aerosols (SOA) generated from the photooxidation of limonene and α-pinene
with different HC/NO<sub><i>x</i></sub> ratios. Refractive
indices were obtained from polar nephelometer data using parallel
and perpendicular polarized 532 nm light combined with measured size
distributions, and retrievals were performed using a genetic algorithm
and Mie–Lorenz scattering theory. The absolute error associated
with the <i>m</i><sub>r</sub> retrieval is ±0.03, and
reliable retrievals are possible for mass concentrations above 5–20
μg/m<sup>3</sup> depending on particle size. The limonene SOA
data suggest the most important factor controlling the refractive
index is the HC/NO<sub><i>x</i></sub> ratio; the refractive
index is much less sensitive to the aerosol age or mass concentration.
The refractive index ranges from about 1.34 to 1.56 for limonene and
from 1.36 to 1.52 for α-pinene, and generally decreases as the
HC/NO<sub><i>x</i></sub> ratio increases. Especially for
limonene, the particle diameter is also inversely related to the HC/NO<sub><i>x</i></sub> ratio; the final size mode increases from
220 to 330 nm as the HC/NO<sub><i>x</i></sub> ratio decreases
from 33 to 6. In an effort to explore the ability of models from the
literature to explain the observed refractive indices, a recent limonene
oxidation mechanism was combined with SOA partitioning and a structure–property
relationship for estimating refractive indices of condensing species.
The resulting refractive indices fell in a much narrower range (1.475
± 0.02) of <i>m</i><sub>r</sub> than observed experimentally.
We hypothesize the experimentally observed high <i>m</i><sub>r</sub> values are due to oligomerization and the low values
to water uptake, small soluble molecules such as glyoxal and other
factors, each of which is not included in the oxidation mechanism.
Aerosol formation yields were measured over the mass concentration
range from 6 to ∼150 μg/m<sup>3</sup>, over which they
increased steadily, and were higher for high HC/NO<sub><i>x</i></sub> ratio experiments
Dependence of Real Refractive Indices on O:C, H:C and Mass Fragments of Secondary Organic Aerosol Generated from Ozonolysis and Photooxidation of Limonene and α-Pinene
<div><p>The refractive index is a fundamental property controlling aerosol optical properties. Secondary organic aerosols have variable refractive indices, presumably reflecting variations in their chemical composition. Here, we investigate the real refractive indices (m<sub>r</sub>) and chemical composition of secondary organic aerosols (SOA) generated from the oxidation of α-pinene and limonene with ozone and NO<sub>x</sub>/sunlight at different HC/NO<sub>x</sub> ratios. Refractive indices were retrieved from polar nephelometer measurements using parallel and perpendicular polarized 532-nm light. Particle chemical composition was monitored with a high-resolution time-of-flight aerosol mass spectrometer (HR-Tof-AMS). For photochemically generated SOA, the values of refractive indices are consistent with prior results, and ranged from about 1.34 to 1.55 for limonene and from 1.44 to 1.47 for α-pinene, generally increasing as the particles grew. While AMS fragments are strongly correlated to the refractive index for each type of SOA, the relationships are in most cases quite different for different SOA types. Consistent with its wide range of refractive index, limonene SOA shows larger variations compared to α-pinene SOA for most parameters measured with the AMS, including H:C, O:C, f<sub>43</sub> (<i>m/z</i> 43/organic), f<sub>C4H7</sub><sup>+</sup>, and others. Refractive indices for α-pinene ozonolysis SOA also fell in narrow ranges; 1.43–1.45 and 1.46–1.53 for particles generated at 19–22 and 23–29°C, respectively, with corresponding small changes of f<sub>43</sub> and H:C ratio and other parameters. Overall, H:C ratio, m/z 43 and 55 (C<sub>2</sub>H<sub>3</sub>O<sup>+</sup>, C<sub>4</sub>H<sub>7</sub><sup>+</sup>) were the best correlated with refractive index for all aerosol types investigated. The relationships between m<sub>r</sub> and most fragments support the notion that increasing condensation of less oxygenated semivolatile species (with a possible role for a concomitant decrease in low refractive index water) is responsible for the increasing m<sub>r</sub>s observed as the experiments progress. However, the possibility that oligomerization reactions play a role cannot be ruled out.</p>
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HULIS Enhancement of Hydroxyl Radical Formation from Fe(II): Kinetics of Fulvic Acid–Fe(II) Complexes in the Presence of Lung Antioxidants
Oxidative
stress mediated by reactive oxygen species (ROS) is a
hypothesized mechanism for particulate-matter related health effects.
FeÂ(II) is a key player in ROS formation in surrogate lung fluid (SLF)
containing antioxidants. Humic-like substances (HULIS) in particulate
matter such as biomass burning aerosol chelate FeÂ(II), but the effect
on ROS formation in the presence of lung antioxidants is not known.
We use Suwanee River Fulvic Acid (SRFA) as a surrogate for HULIS and
investigate its effect on OH formation from FeÂ(II). For the first
time, a chemical kinetics model was developed to explain behavior
of FeÂ(II) and SRFA in SLF. Model and experimental results are used
to find best-fit rate coefficients for key reactions. Modeling results
indicate SRFA enhances Fe-mediated reduction of O<sub>2</sub> to O<sub>2</sub><sup>–</sup> and destruction of H<sub>2</sub>O<sub>2</sub> to OH to 5.1 ± 1.5 and (4.3 ± 1.4) × 10<sup>3</sup> M<sup>–1</sup> s<sup>–1</sup> respectively.
Best-fit rates for Citrate–FeÂ(II) mediated O<sub>2</sub> to
O<sub>2</sub><sup>–</sup> and H<sub>2</sub>O<sub>2</sub> to
OH were 3.0 ± 0.7 and (4.2 ± 1.7) × 10<sup>3</sup> M<sup>–1</sup> s<sup>–1</sup> respectively. The kinetics
model agrees with both the experimental results and thermodynamic
model calculations of chemical speciation for 0 and 5 μg/mL
SRFA, but both models are less successful at predicting further enhancements
to OH formation at higher SRFA Concentrations