3 research outputs found
Products and Mechanism of the Reaction of 1‑Pentadecene with NO<sub>3</sub> Radicals and the Effect of a −ONO<sub>2</sub> Group on Alkoxy Radical Decomposition
The linear C<sub>15</sub> alkene,
1-pentadecene, was reacted with NO<sub>3</sub> radicals in a Teflon
environmental chamber and yields of secondary organic aerosol (SOA)
and particulate β-hydroxynitrates, β-carbonylnitrates,
and organic peroxides (β-nitrooxyhydroperoxides + dinitrooxyperoxides)
were quantified using a variety of methods. Reaction occurs almost
solely by addition of NO<sub>3</sub> to the Cî—»C double bond
and measured yields of β-hydroxynitrate isomers indicate that
92% of addition occurs at the terminal carbon. Molar yields of reaction
products determined from measurements, a proposed reaction mechanism,
and mass-balance considerations were 0.065 for β-hydroxynitrates
(0.060 and 0.005 for 1-nitrooxy-2-hydroxyÂpentadecane and 1-hydroxy-2-nitrooxyÂpentadecane
isomers), 0.102 for β-carbonylnitrates, 0.017 for organic peroxides,
0.232 for β-nitrooxyalkoxy radical isomerization products, and
0.584 for tetradecanal and formaldehyde, the volatile C<sub>14</sub> and C<sub>1</sub> products of β-nitrooxyalkoxy radical decomposition.
Branching ratios for decomposition and isomerization of β-nitrooxyalkoxy
radicals were 0.716 and 0.284 and should be similar for other linear
1-alkenes ≥ C<sub>6</sub> whose alkyl chains are long enough
to allow for isomerization to occur. These branching ratios have not
been measured previously, and they differ significantly from those
estimated using structure–activity relationships, which predict
>99% isomerization. It appears that the presence of a −ONO<sub>2</sub> group adjacent to an alkoxy radical site greatly enhances
the rate of decomposition relative to isomerization, which is otherwise
negligible, and that the effect is similar to that of a −OH
group. The results provide insight into the effects of molecular structure
on mechanisms of oxidation of volatile organic compounds and should
be useful for improving structure–activity relationships that
are widely used to predict the fate of these compounds in the atmosphere
and for modeling SOA formation and aging
Quantification of Byproduct Formation from Portable Air Cleaners Using a Proposed Standard Test Method
In response to the COVID-19 pandemic, air cleaning technologies
were promoted as useful tools for disinfecting public spaces and combating
airborne pathogen transmission. However, no standard method exists
to assess the potentially harmful byproduct formation from air cleaners.
Through a consensus standard development process, a draft standard
test method to assess portable air cleaner performance was developed,
and a suite of air cleaners employing seven different technologies
was tested. The test method quantifies not only the removal efficiency
of a challenge chemical suite and ultrafine particulate matter but
also byproduct formation. Clean air delivery rates (CADRs) are used
to quantify the chemical and particle removal efficiencies, and an
emission rate framework is used to quantify the formation of formaldehyde,
ozone, and other volatile organic compounds. We find that the tested
photocatalytic oxidation and germicidal ultraviolet light (GUV) technologies
produced the highest levels of aldehyde byproducts having emission
rates of 202 and 243 μg h–1, respectively.
Additionally, GUV using two different wavelengths, 222 and 254 nm,
both produced ultrafine particulate matter
Hygroscopicity of Organic Compounds as a Function of Carbon Chain Length and Carboxyl, Hydroperoxy, and Carbonyl Functional Groups
The albedo and microphysical properties of clouds are controlled
in part by the hygroscopicity of particles serving as cloud condensation
nuclei (CCN). Hygroscopicity of complex organic mixtures in the atmosphere
varies widely and remains challenging to predict. Here we present
new measurements characterizing the CCN activity of pure compounds
in which carbon chain length and the numbers of hydroperoxy, carboxyl,
and carbonyl functional groups were systematically varied to establish
the contributions of these groups to organic aerosol apparent hygroscopicity.
Apparent hygroscopicity decreased with carbon chain length and increased
with polar functional groups in the order carboxyl > hydroperoxy
>
carbonyl. Activation diameters at different supersaturations deviated
from the −3/2 slope in log–log space predicted by Köhler
theory, suggesting that water solubility limits CCN activity of particles
composed of weakly functionalized organic compounds. Results are compared
to a functional group contribution model that predicts CCN activity
of organic compounds. The model performed well for most compounds
but underpredicted the CCN activity of hydroperoxy groups. New best-fit
hydroperoxy group/water interaction parameters were derived from the
available CCN data. These results may help improve estimates of the
CCN activity of ambient organic aerosols from composition data