26 research outputs found
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Physical, chemical, and toxicological characteristics of particulate emissions from current technology gasoline direct injection vehicles
Worldwide data sets constrain the water vapor uptake coefficient in cloud formation
Cloud droplet formation depends on the condensation of water vapor on ambient aerosols, the rate of which is strongly affected by the kinetics of water uptake as expressed by the condensation (or mass accommodation) coefficient, α_c. Estimates of α_c for droplet growth from activation of ambient particles vary considerably and represent a critical source of uncertainty in estimates of global cloud droplet distributions and the aerosol indirect forcing of climate. We present an analysis of 10 globally relevant data sets of cloud condensation nuclei to constrain the value of αc for ambient aerosol. We find that rapid activation kinetics (α_c > 0.1) is uniformly prevalent. This finding resolves a long-standing issue in cloud physics, as the uncertainty in water vapor accommodation on droplets is considerably less than previously thought
The Molecular Identification of Organic Compounds in the Atmosphere: State of the Art and Challenges
The influence of molecular structure and aerosol phase on the heterogeneous oxidation of normal and branched alkanes by OH
Surface Organic Monolayers Control the Hygroscopic Growth of Submicrometer Particles at High Relative Humidity
Although
many organic molecules commonly found in the atmosphere
are known to be surface-active in macroscopic aqueous solutions, the
impact of surface partitioning of organic molecules to a microscopic
aqueous droplet interface remains unclear. Here we measure the droplet
size formed, at a relative humidity (∼99.9%) just below saturation,
on submicrometer particles containing an ammonium sulfate core and
an organic layer of a model compound of varying thickness. The 12
model organic compounds are a series of dicarboxylic acids (C<sub>3</sub> to C<sub>10</sub>), <i>cis</i>-pinonic, oleic,
lauric, and myristic acids, which represent a broad range in solubility
from miscible (malonic acid) to insoluble. The variation in droplet
size with increasing organic aerosol fraction cannot be explained
by assuming the organic material is dissolved in the bulk droplet.
Instead, the wet droplet diameters exhibit a complex and nonlinear
dependence on organic aerosol volume fraction, leading to hygroscopic
growth that is in some cases smaller and in others larger than that
predicted by bulk solubility alone. For palmitic and stearic acid,
small droplets at or below the detection limit of the instrument are
observed, indicating significant kinetic limitations for water uptake,
which are consistent with mass accommodation coefficients on the order
of 10<sup>–4</sup>. A model based on the two-dimensional van
der Waals equation of state is used to explain the complex droplet
growth with organic aerosol fraction and dry diameter. The model suggests
that mono- and dicarboxylic acids with limited water solubility partition
to the droplet surface and reduce surface tension only after a two-dimensional
condensed monolayer is formed. Two relatively soluble compounds, malonic
and glutaric acid, also appear to form surface phases, which increase
hygroscopicity. There is a clear alternation in the threshold for
droplet growth observed for odd and even carbon number diacids, which
is explained in the model by differences in the excluded molecular
areas of even (∼40 Å<sup>2</sup>/molecule) and odd (∼20
Å<sup>2</sup>/molecule) diacids. These differences are consistent
with the odd diacids arranged at the droplet interface in “end-to-end”
configurations with only one acid group in contact with the aqueous
phase, which is in contrast to even carbon numbered diacids forming
“folded” films with both acid groups in contact with
the bulk phase. Organic matter produced by the ozonolysis of α-pinene
forms surface films that exhibit similar behavior and become thinner
with oxidation, allowing for greater water uptake. These results reveal
a new and complex relationship between the composition of an organic
aerosol and its hygroscopicity, suggesting that organic surface films
might strongly influence cloud droplet formation as well as the multiphase
chemistry of organic aerosols
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An interfacial mechanism for cloud droplet formation on organic aerosols.
Accurate predictions of aerosol/cloud interactions require simple, physically accurate parameterizations of the cloud condensation nuclei (CCN) activity of aerosols. Current models assume that organic aerosol species contribute to CCN activity by lowering water activity. We measured droplet diameters at the point of CCN activation for particles composed of dicarboxylic acids or secondary organic aerosol and ammonium sulfate. Droplet activation diameters were 40 to 60% larger than predicted if the organic was assumed to be dissolved within the bulk droplet, suggesting that a new mechanism is needed to explain cloud droplet formation. A compressed film model explains how surface tension depression by interfacial organic molecules can alter the relationship between water vapor supersaturation and droplet size (i.e., the Köhler curve), leading to the larger diameters observed at activation
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Isomeric product detection in the heterogeneous reaction of hydroxyl radicals with aerosol composed of branched and linear unsaturated organic molecules.
The influence of molecular structure (branched vs linear) on product formation in the heterogeneous oxidation of unsaturated organic aerosol is investigated. Particle phase product isomers formed from the reaction of squalene (C30H50, a branched alkene with six C═C double bonds) and linolenic acid (C18H30O2, a linear carboxylic acid with three C═C double bonds) with OH radicals are identified and quantified using two-dimensional gas chromatography-mass spectrometry. The reactions are measured at low and high [O2] (∼1% vs 10% [O2]) to understand the roles of hydroxyalkyl and hydroxyperoxy radical intermediates in product formation. A key reaction step is OH addition to a C═C double bond to form a hydroxyalkyl radical. In addition, allylic alkyl radicals, formed from H atom abstraction reactions by hydroxyalkyl or OH radicals play important roles in the chemistry of product formation. Functionalization products dominate the squalene reaction at ∼1% [O2], with the total abundance of observed functionalization products being approximately equal to the fragmentation products at 10% [O2]. The large abundance of squalene fragmentation products at 10% [O2] is attributed to the formation and dissociation of tertiary hydroxyalkoxy radical intermediates. For linolenic acid aerosol, the formation of functionalization products dominates the reaction at both ∼1% and 10% [O2], suggesting that the formation and dissociation of secondary hydroxyalkoxy radicals are minor reaction channels for linear molecules. The distribution of linolenic acid functionalization products depends upon [O2], indicating that O2 controls the reaction pathways of the secondary hydroxyalkyl radical. For both reactions, alcohols are formed in favor of carbonyl functional groups, suggesting that there are some key differences between heterogeneous reactions involving allylic radical intermediates and those reactions of OH radicals with simple saturated hydrocarbons
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Physical, chemical, and toxicological characteristics of particulate emissions from current technology gasoline direct injection vehicles
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