9 research outputs found
Probing the pH dependence of brown carbon formation: Insights from laboratory studies on aerosol particles and bulk phase solutions
Light-absorbing organic aerosol (brown carbon, BrC) can have a significant impact on the radiative balance of the Earth’s atmosphere. However there are still substantial uncertainties regarding the formation, composition, and radiative properties of BrC. In this study, we conducted laboratory experiments to investigate the pH dependence of BrC formation in both aerosol particles and bulk phase solutions. Using glyoxal, ammonia, and ammonium salts, we generated precursor solutions under varying bulk pH conditions ranging from 0.69 to 8.43. Drying the solutions either in the bulk or aerosol phase resulted in BrC formation. The resulting organic material was analyzed to determine its chemical composition and optical properties. Under the set of conditions investigated here, neutral to basic conditions of relevance to cloud water favored BrC formation for both aerosols and bulk solutions. In contrast, BrC products were formed under acidic conditions only in the aerosol phase. Due to rapid equilibration with the gas phase and evaporative losses of water, the aerosols probed here likely had extremely low pH values, well below the bulk pH of 0.69. By achieving such acidic conditions in the aerosol phase, new acid-catalyzed pathways are possible to form BrC. These findings indicate brown carbon formation is favored at both high and very low pH, and further point to the importance of using aerosol samples in studies of pH dependent chemistry of relevance to the atmosphere. Copyright © 2023 American Association for Aerosol Research</p
Depositional Ice Nucleation on Monocarboxylic Acids: Effect of the O:C Ratio
The heterogeneous ice nucleation efficiency of a series
of thin
C3–C6 monocarboxylic acid films between 180 and 200 K has been
investigated using a Knudsen cell flow reactor. At each temperature,
the critical ice saturation ratio for depositional nucleation as well
as the effective contact angle was found to be strongly dependent
on the chemical nature of the film. For the organic acids used in
this study, increasing the O:C ratio lowered the supersaturation required
for the onset of heterogeneous ice nucleation and decreased the effective
angle of contact. This could be the result of an increase in surface
hydrophilicity, which allows the film to better adsorb a metastable,
icelike layer of water that serves as a template for the new phase
of ice. These ice nucleation results are in excellent agreement with
ice nucleation on laboratory generated α-pinene secondary organic
aerosol as well as on predominantly organic particles collected in
Mexico City
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
Immersion and Contact Efflorescence Induced by Mineral Dust Particles
The phase state of
inorganic salt aerosols impacts their properties,
including the ability to undergo hygroscopic growth, catalyze heterogeneous
reactions, and act as cloud condensation nuclei. Here, we report the
first observation of contact efflorescence by mineral dust aerosol.
The efflorescence of aqueous ammonium sulfate ((NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>) and sodium chloride (NaCl) droplets by contact
with three types of mineral dust particles (illite, montmorillonite,
and NX illite), were examined using an optical levitation chamber.
Immersion mode efflorescence was also studied for comparison. We find
that in the presence of mineral dust particles, crystallization occurred
at a higher relative humidity (RH) when compared to the homogeneous
phase transition. Additionally, crystallization by contact mode efflorescence
occurred at a higher RH than the corresponding immersion mode. Crystallization
efficiencies in the contact mode exhibited an ion-specific trend consistent
with the Hoffmeister series. Estimates for lifetimes of a salt droplet
to collide with dust particles suggests that collisions between the
two aerosol types are likely to occur before the salt aerosol is removed
by other atmospheric processes. Such collisions could then lead to
the crystallization of salt droplets that would otherwise have remained
liquid, changing the overall impact that salt aerosols have on atmospheric
chemistry and climate
Heterogeneous Ice Nucleation on Simulated Secondary Organic Aerosol
In this study, we have explored the
phase behavior and the ice
nucleation properties of secondary organic aerosol made from aqueous
processing (aqSOA). AqSOA was made from the dark reactions of methylglyoxal
with methylamine in simulated evaporated cloud droplets. The resulting
particles were probed from 215 to 250 K using Raman spectroscopy coupled
to an environmental cell. We find these particles are in a semisolid
or glassy state based upon their behavior when exposed to mechanical
pressure as well as their flow behavior. Further, we find that these
aqSOA particles are poor depositional ice nuclei, in contrast to previous
studies on simple mixtures of glassy organics. Additionally, we have
studied the effect of ammonium sulfate on the phase, morphology, and
ice nucleation behavior of the aqSOA. We find that the plasticizing
effect of ammonium sulfate lowers the viscosity of the aqSOA, allowing
the ammonium sulfate to effloresce within the aqSOA matrix. Upon humidification,
the aqSOA matrix liquefies before it can depositionally nucleate ice,
and the effloresced ammonium sulfate can act as an immersion mode
ice nucleus. This change in the mode of nucleation is accompanied
by an increase in the overall ice nucleation efficiency of the aqSOA
particles
Elemental Analysis of Complex Organic Aerosol Using Isotopic Labeling and Unit-Resolution Mass Spectrometry
Elemental analysis of unit-mass resolution
(UMR) mass spectra is
limited by the amount of information available to definitively elucidate
the molecular formula of a molecule ionized by electron impact. The
problem is compounded when a mixture of organic molecules (such as
those found in organic aerosols) is analyzed without the benefit of
prior separation. For this reason, quadrupole mass spectrometry is
not usually suited to the elemental analysis of organic mixtures.
Here, we present a mathematical method for the elemental analysis
of UMR mass spectra of a complex organic aerosol through the use of
isotopic labeling. Quadrupole aerosol mass spectrometry was used to
obtain UMR data of <sup>13</sup>C-labeled and unlabeled aerosol generated
by far ultraviolet (FUV) photochemistry of gas mixtures containing
0.1% of either CH<sub>4</sub> or <sup>13</sup>CH<sub>4</sub> in N<sub>2</sub>. In this method, the differences in the positions of ion
groups in the resulting spectra are used to estimate the mass fraction
of carbon in the aerosol, and estimation of the remaining elements
follows. Analysis of the UMR data yields an elemental composition
of 63 ± 7% C, 8 ± 1% H, and 29 ± 7% N by mass. Unlabeled
aerosols formed under the same conditions are found by high-resolution
time-of-flight aerosol mass spectrometry to have an elemental composition
of 63 ± 3% C, 8 ± 1% H, 20 ± 4% N, and 9 ± 3%
O by mass, in good agreement with the UMR method. This favorable comparison
verifies the method, which expands the UMR mass spectrometry toolkit
Long Working-Distance Optical Trap for in Situ Analysis of Contact-Induced Phase Transformations
A novel optical trapping technique
is described that combines an
upward propagating Gaussian beam and a downward propagating Bessel
beam. Using this optical arrangement and an on-demand droplet generator
makes it possible to rapidly and reliably trap particles with a wide
range of particle diameters (∼1.5–25 μm), in addition
to crystalline particles, without the need to adjust the optical configuration.
Additionally, a new image analysis technique is described to detect
particle phase transitions using a template-based autocorrelation
of imaged far-field elastically scattered laser light. The image analysis
allows subtle changes in particle characteristics to be quantified.
The instrumental capabilities are validated with observations of deliquescence
and homogeneous efflorescence of well-studied inorganic salts. Then,
a novel collision-based approach to seeded crystal growth is described
in which seed crystals are delivered to levitated aqueous droplets
via a nitrogen gas flow. To our knowledge, this is the first account
of contact-induced phase changes being studied in an optical trap.
This instrument offers a novel and simple analytical technique for
in situ measurements and observations of phase changes and crystal
growth processes relevant to atmospheric science, industrial crystallization,
pharmaceuticals, and many other fields
Impact of Organic Coating on Optical Growth of Ammonium Sulfate Particles
Light
extinction by particles in Earth’s atmosphere is strongly
dependent on particle size, chemical composition, hygroscopic growth
properties, and particle mixing state. Here, the influence of an organic
coating on particle optical growth was studied. The particle optical
growth factor, <i>f</i>RH<sub>ext</sub>, was measured using
cavity ring-down aerosol extinction spectroscopy at 532 nm. The particles
were composed of ammonium sulfate (AS), 1,2,6-hexanetriol, and mixed
particles containing a wet or dry ammonium sulfate core and a 1,2,6-hexanetriol
coating. Dry, coated particles were generated by atomization followed
by drying. Wet, coated particles were formed via liquid–liquid
phase separation (LLPS). LLPS was achieved by deliquescing and then
drying the particles to a relative humidity (RH) between the phase
separation RH and the efflorescence RH. For the LLPS particles, the <i>f</i>RH<sub>ext</sub> at each RH was between the <i>f</i>RH<sub>ext</sub> of ammonium sulfate and that of 1,2,6-hexanetriol.
In contrast, for the mixed dry, coated particles, the <i>f</i>RH<sub>ext</sub> was the same as 1,2,6-hexanetriol particles. At
room temperature, the water uptake properties of AS coated with 1,2,6-hexanetriol
are largely dictated by the phase of the AS. Thus, the total water
uptake depends on the RH history of the particle and the resulting
phase of AS
Chemical and Physical Transformations of Aluminosilicate Clay Minerals Due to Acid Treatment and Consequences for Heterogeneous Ice Nucleation
Mineral dust aerosol is one of the
largest contributors to global
ice nuclei, but physical and chemical processing of dust during atmospheric
transport can alter its ice nucleation activity. In particular, several
recent studies have noted that sulfuric and nitric acids inhibit heterogeneous
ice nucleation in the regime below liquid water saturation in aluminosilicate
clay minerals. We have exposed kaolinite, KGa-1b and KGa-2, and montmorillonite,
STx-1b and SWy-2, to aqueous sulfuric and nitric acid to determine
the physical and chemical changes that are responsible for the observed
deactivation. To characterize the changes to the samples upon acid
treatment, we use X-ray diffraction, transmission electron microscopy,
and inductively coupled plasma–atomic emission spectroscopy.
We find that the reaction of kaolinite and montmorillonite with aqueous
sulfuric acid results in the formation of hydrated aluminum sulfate.
In addition, sulfuric and nitric acids induce large structural changes
in montmorillonite. We additionally report the supersaturation with
respect to ice required for the onset of ice nucleation for these
acid-treated species. On the basis of lattice spacing arguments, we
explain how the chemical and physical changes observed upon acid treatment
could lead to the observed reduction in ice nucleation activity