32 research outputs found
Technical Note: New methodology for measuring viscosities in small volumes characteristic of environmental chamber particle samples
Herein, a method for the determination of viscosities of small sample volumes is introduced, with important implications for the viscosity determination of particle samples from environmental chambers (used to simulate atmospheric conditions). The amount of sample needed is < 1 μl, and the technique is capable of determining viscosities (η) ranging between 10<sup>−3</sup> and 10<sup>3</sup> Pascal seconds (Pa s) in samples that cover a range of chemical properties and with real-time relative humidity and temperature control; hence, the technique should be well-suited for determining the viscosities, under atmospherically relevant conditions, of particles collected from environmental chambers. In this technique, supermicron particles are first deposited on an inert hydrophobic substrate. Then, insoluble beads (~1 μm in diameter) are embedded in the particles. Next, a flow of gas is introduced over the particles, which generates a shear stress on the particle surfaces. The sample responds to this shear stress by generating internal circulations, which are quantified with an optical microscope by monitoring the movement of the beads. The rate of internal circulation is shown to be a function of particle viscosity but independent of the particle material for a wide range of organic and organic-water samples. A calibration curve is constructed from the experimental data that relates the rate of internal circulation to particle viscosity, and this calibration curve is successfully used to predict viscosities in multicomponent organic mixtures
Adsorptive uptake of water by semisolid secondary organic aerosols
Aerosol climate effects are intimately tied to interactions with water. Here we combine hygroscopicity measurements with direct observations about the phase of secondary organic aerosol (SOA) particles to show that water uptake by slightly oxygenated SOA is an adsorption-dominated process under subsaturated conditions, where low solubility inhibits water uptake until the humidity is high enough for dissolution to occur. This reconciles reported discrepancies in previous hygroscopicity closure studies. We demonstrate that the difference in SOA hygroscopic behavior in subsaturated and supersaturated conditions can lead to an effect up to about 30% in the direct aerosol forcinghighlighting the need to implement correct descriptions of these processes in atmospheric models. Obtaining closure across the water saturation point is therefore a critical issue for accurate climate modeling.Peer reviewe
Liquid–liquid phase separation in particles containing organics mixed with ammonium sulfate, ammonium bisulfate, ammonium nitrate or sodium chloride
As the relative humidity varies from high to low values in the atmosphere,
particles containing organic species and inorganic salts may undergo
liquid–liquid phase separation. The majority of the laboratory work on this
subject has used ammonium sulfate as the inorganic salt. In the following we
studied liquid–liquid phase separation in particles containing organics
mixed with the following salts: ammonium sulfate, ammonium bisulfate,
ammonium nitrate and sodium chloride. In each experiment one organic was
mixed with one inorganic salt and the liquid–liquid phase separation
relative humidity (SRH) was determined. Since we studied 23 different
organics mixed with four different salts, a total of 92 different particle
types were investigated. Out of the 92 types, 49 underwent liquid–liquid
phase separation. For all the inorganic salts, liquid–liquid phase
separation was never observed when the oxygen-to-carbon elemental ratio
(O : C) ≥ 0.8 and was always observed for O : C < 0.5. For 0.5
≤ O : C < 0.8, the results depended on the salt type. Out of the 23
organic species investigated, the SRH of 20 organics followed the trend:
(NH4)2SO4 ≥ NH4HSO4 ≥ NaCl ≥
NH4NO3. This trend is consistent with previous salting out studies
and the Hofmeister series. Based on the range of O : C values found in the
atmosphere and the current results, liquid–liquid phase separation is likely
a frequent occurrence in both marine and non-marine environments
Observations and implications of liquid–liquid phase separation at high relative humidities in secondary organic material produced by <i>α</i>-pinene ozonolysis without inorganic salts
Particles consisting of secondary organic material (SOM) are abundant in the
atmosphere. To predict the role of these particles in climate, visibility and
atmospheric chemistry, information on particle phase state (i.e., single
liquid, two liquids and solid) is needed. This paper focuses on the phase
state of SOM particles free of inorganic salts produced by the ozonolysis of
<i>α</i>-pinene. Phase transitions were investigated in the laboratory using
optical microscopy and theoretically using a thermodynamic model at 290 K
and for relative humidities ranging from  <  0.5 to 100 %. In the
laboratory studies, a single phase was observed from 0 to 95 % relative
humidity (RH) while two liquid phases were observed above 95 % RH. For
increasing RH, the mechanism of liquid–liquid phase separation (LLPS) was
spinodal decomposition. The RH range over which two liquid phases were
observed did not depend on the direction of RH change. In the modeling
studies, the SOM took up very little water and was a single organic-rich
phase at low RH values. At high RH, the SOM underwent LLPS to form an
organic-rich phase and a water-rich phase, consistent with the laboratory
studies. The presence of LLPS at high RH values can have consequences for the
cloud condensation nuclei (CCN) activity of SOM particles. In the simulated
Köhler curves for SOM particles, two local maxima were observed.
Depending on the composition of the SOM, the first or second maximum can
determine the critical supersaturation for activation. Recently researchers
have observed inconsistencies between measured CCN properties of SOM
particles and hygroscopic growth measured below water saturation (i.e.,
hygroscopic parameters measured below water saturation were inconsistent with
hygroscopic parameters measured above water saturation). The work presented
here illustrates that such inconsistencies are expected for systems with LLPS
when the water uptake at subsaturated conditions represents the
hygroscopicity of an organic-rich phase while the barrier for CCN activation
can be determined by the second maximum in the Köhler curve when the
particles are water rich
Effect of varying experimental conditions on the viscosity of <i>α</i>-pinene derived secondary organic material
Knowledge of the viscosity of particles containing secondary organic material
(SOM) is useful for predicting reaction rates and diffusion in SOM particles.
In this study we investigate the viscosity of SOM particles as a function of
relative humidity and SOM particle mass concentration, during SOM synthesis.
The SOM was generated via the ozonolysis of <i>α</i>-pinene at < 5 % relative humidity (RH). Experiments were carried out using the
poke-and-flow technique, which measures the experimental flow time
(<i>Ï„</i><sub>exp, flow</sub>) of SOM after poking the material with a needle. In the
first set of experiments, we show that <i>Ï„</i><sub>exp, flow</sub> increased by a
factor of 3600 as the RH increased from < 0.5 RH to 50 % RH,
for SOM with a production mass concentration of 121 µg m<sup>−3</sup>. Based on
simulations, the viscosities of the particles were between 6  ×  10<sup>5</sup> and 5  ×  10<sup>7</sup> Pa s at
< 0.5 % RH and between 3  ×  10<sup>2</sup> and 9  ×  10<sup>3</sup> Pa s
at 50 % RH. In the second set of experiments we show that under dry
conditions <i>Ï„</i><sub>exp, flow</sub> decreased by a factor of 45 as the production
mass concentration increased from 121 to 14 000 µg m<sup>−3</sup>. From simulations
of the poke-and-flow experiments, the viscosity of
SOM with a production mass concentration of 14 000 µg m<sup>−3</sup> was
determined to be between 4  ×  10<sup>4</sup> and 1.5  ×  10<sup>6</sup> Pa s compared to between
6  ×  10<sup>5</sup> and 5  ×  10<sup>7</sup> Pa s for SOM with a production mass concentration
of 121 µg m<sup>−3</sup>. The results can be rationalized by a dependence of
the chemical composition of SOM on production conditions. These results
emphasize the shifting characteristics of SOM, not just with RH and precursor
type, but also with the production conditions, and suggest that production
mass concentration and the RH at which the viscosity was determined should be
considered both when comparing laboratory results and when extrapolating
these results to the atmosphere
Measurement and modeling of the multiwavelength optical properties of uncoated flame-generated soot
Optical properties of flame-generated black carbon (BC) containing soot particles were quantified at multiple wavelengths for particles produced using two different flames: a methane diffusion flame and an ethylene premixed flame. Measurements were made for (i) nascent soot particles, (ii) thermally denuded nascent particles, and (iii) particles that were coated and then thermally denuded, leading to the collapse of the initially lacy, fractal-like morphology. The measured mass absorption coefficients (MACs) depended on soot maturity and generation but were similar between flames for similar conditions. For mature soot, here corresponding to particles with volume-equivalent diameters > ∼ 160nm, the MAC and absorption Ångström exponent (AAE) values were independent of particle collapse while the single-scatter albedo increased. The MAC values for these larger particles were also size-independent. The mean MAC value at 532nm for larger particles was 9.1±1.1m2g-1, about 17% higher than that recommended by Bond and Bergstrom (2006), and the AAE was close to unity. Effective, theory-specific complex refractive index (RI) values are derived from the observations with two widely used methods: Lorenz-Mie theory and the Rayleigh-Debye-Gans (RDG) approximation. Mie theory systematically underpredicts the observed absorption cross sections at all wavelengths for larger particles (with x > 0.9) independent of the complex RI used, while RDG provides good agreement. (The dimensionless size parameter x = π dp/λ, where dp is particle diameter and λ is wavelength.) Importantly, this implies that the use of Mie theory within air quality and climate models, as is common, likely leads to underpredictions in the absorption by BC, with the extent of underprediction depending on the assumed BC size distribution and complex RI used. We suggest that it is more appropriate to assume a constant, size-independent (but wavelength-specific) MAC to represent absorption by uncoated BC particles within models
Measurement and modeling of the multiwavelength optical properties of uncoated flame-generated soot
© Author(s) 2018. Optical properties of flame-generated black carbon (BC) containing soot particles were quantified at multiple wavelengths for particles produced using two different flames: a methane diffusion flame and an ethylene premixed flame. Measurements were made for (i) nascent soot particles, (ii) thermally denuded nascent particles, and (iii) particles that were coated and then thermally denuded, leading to the collapse of the initially lacy, fractal-like morphology. The measured mass absorption coefficients (MACs) depended on soot maturity and generation but were similar between flames for similar conditions. For mature soot, here corresponding to particles with volume-equivalent diameters > ∼ 160nm, the MAC and absorption Ångström exponent (AAE) values were independent of particle collapse while the single-scatter albedo increased. The MAC values for these larger particles were also size-independent. The mean MAC value at 532nm for larger particles was 9.1±1.1m2g-1, about 17% higher than that recommended by Bond and Bergstrom (2006), and the AAE was close to unity. Effective, theory-specific complex refractive index (RI) values are derived from the observations with two widely used methods: Lorenz-Mie theory and the Rayleigh-Debye-Gans (RDG) approximation. Mie theory systematically underpredicts the observed absorption cross sections at all wavelengths for larger particles (with x > 0.9) independent of the complex RI used, while RDG provides good agreement. (The dimensionless size parameter x = π dp/λ, where dp is particle diameter and λ is wavelength.) Importantly, this implies that the use of Mie theory within air quality and climate models, as is common, likely leads to underpredictions in the absorption by BC, with the extent of underprediction depending on the assumed BC size distribution and complex RI used. We suggest that it is more appropriate to assume a constant, size-independent (but wavelength-specific) MAC to represent absorption by uncoated BC particles within models
Changing shapes and implied viscosities of suspended submicron particles
The change in shape of atmospherically relevant organic particles is used to estimate the viscosity of the particle material without the need for removal from aerosol suspension. The dynamic shape factors χ of particles produced by α-pinene ozonolysis in a flow tube reactor, under conditions of particle coagulation, were measured while altering the relative humidity (RH) downstream of the flow tube. As relative humidity was increased, the results showed that χ could change from 1.27 to 1.02, corresponding to a transition from aspherical to nearly spherical shapes. The shape change could occur at elevated RH because the organic material had decreased viscosity and was therefore able to flow to form spherical shapes, as favored by the minimization of surface area. Numerical modeling was used to estimate the particle viscosity associated with this flow. Based on particle diameter and RH exposure time, the viscosity dropped from 10(8.7±2.0) to 10(7.0±2.0) Pa s (two sigma) for an increase in RH from < 5 to 58 % at 293 K. These results imply that the equilibration of the chemical composition of the particle phase with the gas phase can shift from hours at mid-range RH to days at low RH.Published versio