9 research outputs found
Measurements of cluster ions using a nano radial DMA and a particle size magnifier in CLOUD
We built a new instrumental setup for measuring ion distributions in the size range [1.3-6] nm. The implementation of an high transmission inlet increased the total transmission efficiency to more than 6% at 1.47 nm mobility equivalent diameter, allowing the detection of ions at atmospheric concentrations. The size resolution of our measurements is as high as 6. We characterized the instrument in the laboratory and carried out measurements during the CLOUD7 campaign. We compared the results obtained with the Neutral cluster and Air Ion spectrometer finding very good agreement
Evolution of α-Pinene Oxidation Products in the Presence of Varying Oxidizers: Negative APi-TOF Point of View
Laboratory experiments conducted in the frame of the CLOUD project at CERN investigated the oxidation of α-pinene oxidation products in a carefully controlled environment and with different oxidation conditions: 1) pure ozonolysis (with the use of an hydroxyl radical (⋅OH) scavenger), 2) ozonolysis without use of a scavenger, and 3) pure ⋅OH oxidation using nitrous acid (HONO) to produce ⋅OH. The anions and negatively charged clusters present in the chamber were analyzed and their chemical composition compared for the different oxidation pathways
Measuring Composition & Growth of Ion Clusters of Sulfuric Acid, Ammonia, Amines & Oxidized Organics as First Steps of Nucleation in the CLOUD Experiment
The mechanisms behind the nucleation of vapors forming new particles in the atmosphere had been proven difficult to establish. One main aim of the CLOUD experiment was to explore in detail these first steps of atmospheric new particle formation by performing extremely well controlled laboratory experiments. We examined nucleation and growth in the presence of different mixtures of vapors, including sulfuric acid, ammonia, dimethylamine, and oxidation products of pinanediol or α-pinene. Among the employed state-of-the-art instrumentation, a high-resolution mass spectrometer that directly sampled negatively charged ions and clusters proved particularly useful. We were able to resolve most of the chemical compositions found for charged sub-2nm clusters and to observe their growth in time. These compositions reflected the mixture of condensable vapors in the chamber and the role of each individual vapor in forming sub-2nm clusters could be explored. By inter-comparing between individual experiments and ambient observations, we try to establish which vapors participate in nucleation in the actual atmosphere, and how
Gas-Phase Ozonolysis of Selected Olefins: The Yield of Stabilized Criegee Intermediate and the Reactivity toward SO<sub>2</sub>
The gas-phase reaction of ozone with olefins represents
an important
path for the conversion of unsaturated hydrocarbons in the atmosphere.
The current interest is focused on the formation of stabilized Criegee
intermediates (sCI) and possible further reactions of sCI. We report
results from the ozonolysis of 2,3-dimethyl-2-butene (TME), trans-2-butene
and 1-methyl-cyclohexene (MCH) carried out in an atmospheric pressure
flow tube at 293 ± 0.5 K and RH = 50% using chemical ionization
atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometry
to detect H<sub>2</sub>SO<sub>4</sub> produced from SO<sub>2</sub> oxidation by sCI. The yields of sCI were found to be in good agreement
with recently observed data: 0.62 ± 0.28 (TME), 0.53 ± 0.24
(trans-2-butene) and 0.16 ± 0.07 (MCH). The rate coefficients
for sCI + SO<sub>2</sub> from our experiment, (0.9–7.7) ×
10<sup>–13</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>, are within the range of recommendations from
indirect determinations as given so far in the literature. Our study
helps to assess the importance of sCI in atmospheric chemistry, especially
for the oxidation of SO<sub>2</sub> to H<sub>2</sub>SO<sub>4</sub>
Effects of Chemical Complexity on the Autoxidation Mechanisms of Endocyclic Alkene Ozonolysis Products: From Methylcyclohexenes toward Understanding α‑Pinene
Formation of highly oxidized, multifunctional
products in the ozonolysis of three endocyclic alkenes, 1- methylcyclohexene,
4-methylcyclohexene, and α-pinene, was investigated using a
chemical ionization atmospheric pressure interface time-of-flight
(CI-APi-TOF) mass spectrometer with a nitrate ion (NO<sub>3</sub><sup>–</sup>) based ionization scheme. The experiments were performed
in borosilicate glass flow tube reactors at room temperature (<i>T</i> = 293 ± 3 K) and at ambient pressure. An ensemble
of oxidized monomer and dimer products was detected, with elemental
compositions obtained from the high-resolution mass spectra. The monomer
product distributions have O/C ratios from 0.8 to 1.6 and can be explained
with an autocatalytic oxidation mechanism (=autoxidation) where the
oxygen-centered peroxy radical (RO<sub>2</sub>) intermediates internally
rearrange by intramolecular hydrogen shift reactions, enabling more
oxygen molecules to attach to the carbon backbone. Dimer distributions
are proposed to form by homogeneous peroxy radical recombination and
cross combination reactions. These conclusions were supported by experiments
where H atoms were exchanged to D atoms by addition of D<sub>2</sub>O to the carrier gas flow. Methylcyclohexenes were observed to autoxidize
in accordance with our previous work on cyclohexene, whereas in α-pinene
ozonolysis different mechanistic steps are needed to explain the products
observed
The Formation of Highly Oxidized Multifunctional Products in the Ozonolysis of Cyclohexene
The prompt formation of highly oxidized
organic compounds in the
ozonolysis of cyclohexene (C<sub>6</sub>H<sub>10</sub>) was investigated
by means of laboratory experiments together with quantum chemical
calculations. The experiments were performed in borosilicate glass
flow tube reactors coupled to a chemical ionization atmospheric pressure
interface time-of-flight mass spectrometer with a nitrate ion (NO<sub>3</sub><sup>–</sup>)-based ionization scheme. Quantum chemical
calculations were performed at the CCSD(T)-F12a/VDZ-F12//ωB97XD/aug-cc-pVTZ
level, with kinetic modeling using multiconformer transition state
theory, including Eckart tunneling corrections. The complementary
investigation methods gave a consistent picture of a formation mechanism
advancing by peroxy radical (RO<sub>2</sub>) isomerization through
intramolecular hydrogen shift reactions, followed by sequential O<sub>2</sub> addition steps, that is, RO<sub>2</sub> autoxidation, on
a time scale of seconds. Dimerization of the peroxy radicals by recombination
and cross-combination reactions is in competition with the formation
of highly oxidized monomer species and is observed to lead to peroxides,
potentially diacyl peroxides. The molar yield of these highly oxidized
products (having O/C > 1 in monomers and O/C > 0.55 in dimers)
from
cyclohexene ozonolysis was determined as (4.5 ± 3.8)%. Fully
deuterated cyclohexene and <i>cis</i>-6-nonenal ozonolysis,
as well as the influence of water addition to the system (either H<sub>2</sub>O or D<sub>2</sub>O), were also investigated in order to strengthen
the arguments on the proposed mechanism. Deuterated cyclohexene ozonolysis
resulted in a less oxidized product distribution with a lower yield
of highly oxygenated products and <i>cis</i>-6-nonenal ozonolysis
generated the same monomer product distribution, consistent with the
proposed mechanism and in agreement with quantum chemical modeling
Nitrate Radicals Suppress Biogenic New Particle Formation from Monoterpene Oxidation
Highly
oxygenated organic molecules (HOMs) are a major
source of
new particles that affect the Earth’s climate. HOM production
from the oxidation of volatile organic compounds (VOCs) occurs during
both the day and night and can lead to new particle formation (NPF).
However, NPF involving organic vapors has been reported much more
often during the daytime than during nighttime. Here, we show that
the nitrate radicals (NO3), which arise predominantly at
night, inhibit NPF during the oxidation of monoterpenes based on three
lines of observational evidence: NPF experiments in the CLOUD (Cosmics
Leaving OUtdoor Droplets) chamber at CERN (European Organization for
Nuclear Research), radical chemistry experiments using an oxidation
flow reactor, and field observations in a wetland that occasionally
exhibits nocturnal NPF. Nitrooxy-peroxy radicals formed from NO3 chemistry suppress the production of ultralow-volatility
organic compounds (ULVOCs) responsible for biogenic NPF, which are
covalently bound peroxy radical (RO2) dimer association
products. The ULVOC yield of α-pinene in the presence of NO3 is one-fifth of that resulting from ozone chemistry alone.
Even trace amounts of NO3 radicals, at sub-parts per trillion
level, suppress the NPF rate by a factor of 4. Ambient observations
further confirm that when NO3 chemistry is involved, monoterpene
NPF is completely turned off. Our results explain the frequent absence
of nocturnal biogenic NPF in monoterpene (α-pinene)-rich environments
Molecular Understanding of the Enhancement in Organic Aerosol Mass at High Relative Humidity
The mechanistic pathway
by which high relative humidity (RH) affects
gas–particle partitioning remains poorly understood, although
many studies report increased secondary organic aerosol (SOA) yields
at high RH. Here, we use real-time, molecular measurements of both
the gas and particle phase to provide a mechanistic understanding
of the effect of RH on the partitioning of biogenic oxidized organic
molecules (from α-pinene and isoprene) at low temperatures (243
and 263 K) at the CLOUD chamber at CERN. We observe increases in SOA
mass of 45 and 85% with increasing RH from 10–20 to 60–80%
at 243 and 263 K, respectively, and attribute it to the increased
partitioning of semi-volatile compounds. At 263 K, we measure an increase
of a factor 2–4 in the concentration of C10H16O2–3, while the particle-phase concentrations
of low-volatility species, such as C10H16O6–8, remain almost constant. This results in a substantial
shift in the chemical composition and volatility distribution toward
less oxygenated and more volatile species at higher RH (e.g., at 263
K, O/C ratio = 0.55 and 0.40, at RH = 10 and 80%, respectively). By
modeling particle growth using an aerosol growth model, which accounts
for kinetic limitations, we can explain the enhancement in the semi-volatile
fraction through the complementary effect of decreased compound activity
and increased bulk-phase diffusivity. Our results highlight the importance
of particle water content as a diluting agent and a plasticizer for
organic aerosol growth
Insight into Acid–Base Nucleation Experiments by Comparison of the Chemical Composition of Positive, Negative, and Neutral Clusters
We investigated the nucleation of
sulfuric acid together with two bases (ammonia and dimethylamine),
at the CLOUD chamber at CERN. The chemical composition of positive,
negative, and neutral clusters was studied using three Atmospheric
Pressure interface-Time Of Flight (APi-TOF) mass spectrometers: two
were operated in positive and negative mode to detect the chamber
ions, while the third was equipped with a nitrate ion chemical ionization
source allowing detection of neutral clusters. Taking into account
the possible fragmentation that can happen during the charging of the ions or within the first stage of the mass spectrometer, the cluster formation proceeded via essentially one-to-one acid–base addition for all of the clusters, independent of the type of the base. For the positive clusters, the charge is carried by one excess protonated base, while for the negative clusters it is carried by a deprotonated acid; the same is true for the neutral clusters after these have been ionized. During the experiments involving sulfuric acid and dimethylamine, it was possible to study the appearance time for all the clusters (positive, negative, and neutral). It appeared that, after the formation of the clusters containing three molecules of sulfuric acid, the clusters grow at a similar speed, independent of their charge. The growth rate is then probably limited by the arrival rate of sulfuric acid or cluster–cluster collision