8 research outputs found
Particle Classification by the Tandem Differential Mobility AnalyzerāParticle Mass Analyzer System
<div><p>Particle mass analyzers, such as the aerosol particle mass analyzer (APM) and the Couette centrifugal particle mass analyzer (CPMA), are frequently combined with a differential mobility analyzer (DMA) to measure particle mass <i>m</i><sub><i>p</i></sub> and effective density Ļ<sub>eff</sub> distributions of particles with a specific electrical mobility diameter <i>d</i><sub><i>m</i></sub>. Combinations of these instruments, which are referred to as the DMAāAPM or DMAāCPMA system, are also used to quantify the mass-mobility exponent <i>D</i><sub><i>m</i></sub> of non-spherical particles as well as to eliminate multiple charged particles. This study investigates the transfer functions of these setups, focusing especially on the DMAāAPM system. The transfer function of the DMAāAPM system was derived by multiplying the transfer functions of DMA and APM. The APM transfer function can be calculated using either the uniform or parabolic flow models. The uniform flow model provides an analytical function, while the parabolic flow model is more accurate. The resulting DMAāAPM transfer functions were plotted on log(<i>m</i><sub><i>p</i></sub>)-log(<i>d</i><sub><i>p</i></sub>) space. A theoretical analysis of the DMAāAPM transfer function demonstrated that the resolution of the setup is maintained when the rotation speed Ļ of APM is scanned to measure distribution. In addition, an equation was derived to numerically calculate the minimum values of the APM resolution parameter Ī»<sub><i>c</i></sub> for eliminating multiple charged particles.</p><p>Copyright 2015 American Association for Aerosol Research</p></div
1-octanol-water partitioning as a classifier of water soluble organic matters: Implication for solubility distribution
<p>Water-soluble organic matters (WSOMs) play an important role in determining magnitudes of climatic and environmental impacts of organic aerosol particles because of their contributions to hygroscopic growth and cloud formation. These processes are dependent on water solubility as well as distribution of this property in a particle, yet no method has been available to quantify such characteristics. In this study, we developed a theoretical framework to classify WSOM by 1-octanol-water partitioning that has a strong correlation with water solubility. 1-octanol-water partitioning coefficient also has a strong correlation with a traditional solid phase extraction method, facilitating interpretation of data from the technique. The theoretical analysis demonstrated that the distributions of WSOM classified by 1-octanol-water partitioning depend on (1) the volume ratio of 1-octanol and aqueous phases, and (2) extraction steps. The method was tested by using organic aerosol particles generated by smoldering of a mosquito coil, which serves as a surrogate for biomass burning particles. The WSOM extracted from the mosquito coil burning particles was classified by 1-octanol-water partitioning at different volume ratios. These solutions, including both the 1-octanol and aqueous phases, were nebulized to generate particles for measurements using an online aerosol mass spectrometer. The mass spectra indicated that highly oxygenated species tend to be highly soluble, while high molecular weight compounds are less soluble. Linear combinations of these mass spectra allowed the estimation of the mass fractions of WSOM partitioned to 1-octanol and aqueous phases, thereby facilitating the evaluation of the mass fractions of cloud condensation nuclei (CCN) active materials.</p> <p>Ā© 2017 American Association for Aerosol Research</p
An Analytic Equation for the Volume Fraction of Condensationally Grown Mixed Particles and Applications to Secondary Organic Material Produced in Continuously Mixed Flow Reactors
<div><p>Secondary condensation of organic material onto primary seed particles is one pathway of particle growth in the atmosphere, and many properties of the resulting mixed particles depend on organic volume fraction. Environmental chambers can be used to simulate the production of these types of particles, and the optical, hygroscopic, and other properties of the mixed particles can be studied. In the interpretation of the measured properties, the probability density function <i>p</i>(<i>Ļµ</i>;<i>d</i>) of volume fraction <i>Ļµ</i> of the condensing material for particle diameter <i>d</i> in the outflow of the chamber is typically needed. In this article, analytic equations are derived <i>p</i>(<i>Ļµ</i>;<i>d</i>) for condensational growth in a continuously mixed flow reactor. The equation predictions are compared to measurements for the condensation of secondary organic material on quasi-monodisperse sulfate seed particles. Equations are presented herein for discrete, Gaussian, and triangular distribution functions for the seed particle numberādiameter distributions, including generalization to any linearly segmented distributions. The analytic equations are useful both for the interpretation of laboratory data from environmental chambers, such as the construction of probability density functions for use in interpretation of hygroscopic growth data, cloudācondensationānuclei data, or other laboratory data sets dependent on organic volume fraction, as well as for understanding atmospheric processes at times that condensational growth processes prevail.</p><p>Copyright 2014 American Association for Aerosol Research</p></div
Using Elemental Ratios to Predict the Density of Organic Material Composed of Carbon, Hydrogen, and Oxygen
A governing equation was developed to predict the density
Ļ<sub>org</sub> of organic material composed of carbon, oxygen,
and hydrogen
using the elemental ratios O:C and H:C as input parameters: Ļ<sub>org</sub> = 1000 [(12 + 1Ā(H:C) + 16Ā(O:C)]/[7.0 + 5.0Ā(H:C) + 4.15Ā(O:C)]
valid for 750 < Ļ<sub>org</sub> < 1900 kg m<sup>ā3</sup>. Comparison of the actual to predicted Ļ<sub>org</sub> values
shows that the developed equation has an accuracy of 12% for more
than 90% of the 31 atmospherically relevant compounds used in the
training set. The equation was further validated for secondary organic
material (SOM) produced by isoprene photo-oxidation and by Ī±-pinene
ozonolysis. Depending on the conditions of SOM production, Ļ<sub>org/SOM</sub> ranged from 1230 to 1460 kg m<sup>ā3</sup>,
O:C ranged from 0.38 to 0.72, and H:C ranged from 1.40 to 1.86. Atmospheric
chemistry models that simulate particle production and growth can
employ the developed equation to simulate particle physical properties.
The equation can also extend atmospheric measurements presented as
van Krevelen diagrams to include estimates of the material density
of particles and their components. Use of the equation, however, is
restricted to particle components having negligible quantities of
additional elements, most notably nitrogen
Using Elemental Ratios to Predict the Density of Organic Material Composed of Carbon, Hydrogen, and Oxygen
A governing equation was developed to predict the density
Ļ<sub>org</sub> of organic material composed of carbon, oxygen,
and hydrogen
using the elemental ratios O:C and H:C as input parameters: Ļ<sub>org</sub> = 1000 [(12 + 1Ā(H:C) + 16Ā(O:C)]/[7.0 + 5.0Ā(H:C) + 4.15Ā(O:C)]
valid for 750 < Ļ<sub>org</sub> < 1900 kg m<sup>ā3</sup>. Comparison of the actual to predicted Ļ<sub>org</sub> values
shows that the developed equation has an accuracy of 12% for more
than 90% of the 31 atmospherically relevant compounds used in the
training set. The equation was further validated for secondary organic
material (SOM) produced by isoprene photo-oxidation and by Ī±-pinene
ozonolysis. Depending on the conditions of SOM production, Ļ<sub>org/SOM</sub> ranged from 1230 to 1460 kg m<sup>ā3</sup>,
O:C ranged from 0.38 to 0.72, and H:C ranged from 1.40 to 1.86. Atmospheric
chemistry models that simulate particle production and growth can
employ the developed equation to simulate particle physical properties.
The equation can also extend atmospheric measurements presented as
van Krevelen diagrams to include estimates of the material density
of particles and their components. Use of the equation, however, is
restricted to particle components having negligible quantities of
additional elements, most notably nitrogen
Phase Transitions and Phase Miscibility of Mixed Particles of Ammonium Sulfate, Toluene-Derived Secondary Organic Material, and Water
The phase states of atmospheric particles
influence their roles
in physicochemical processes related to air quality and climate. The
phases of particles containing secondary organic materials (SOMs)
are still uncertain, especially for SOMs produced from aromatic precursor
gases. In this work, efflorescence and deliquescence phase transitions,
as well as phase separation, in particles composed of toluene-derived
SOM, ammonium sulfate, and water were studied by hygroscopic tandem
differential mobility analysis (HTDMA) and optical microscopy. The
SOM was produced in the Harvard Environmental Chamber by photo-oxidation
of toluene at chamber relative humidities of <5 and 40%. The efflorescence
and deliquescence relative humidities (ERH and DRH, respectively,
studied by HTDMA) of ammonium sulfate decreased as the SOM organic
fraction Īµ in the particle increased, dropping from DRH = 80%
and ERH = 31% for Īµ = 0.0 to DRH = 58% and ERH = 0% for Īµ
= 0.8. For Īµ < 0.2, the DRH and ERH to first approximation
did not change with the organic volume fraction. This observation
is consistent with independent behaviors for Īµ < 0.2 of water-infused
toluene-derived SOM and aqueous ammonium sulfate, suggesting phase
immiscibility between the two. Optical microscopy of particles prepared
for Īµ = 0.12 confirmed phase separation for RH < 85%. For
Īµ from 0.2 to 0.8, the DRH and ERH values steadily decreased,
as studied by HTDMA. This result is consistent with one-phase mixing
of ammonium sulfate, SOM, and water. Optical microscopy for particles
of Īµ = 0.8 confirmed this result. Within error, increased exposure
times of the aerosol in the HTDMA from 0.5 to 30 s affected neither
the ERHĀ(Īµ) nor DRHĀ(Īµ) curves, implying an absence of kinetic
effects on the observations over the studied time scales. For Īµ
> 0.5, the DRH values of ammonium sulfate in mixtures with SOM
produced
at <5% RH were offset by ā3 to ā5% RH compared to
the results for SOM produced at 40% RH, suggesting differences in
SOM chemistry. The observed miscibility gap (i.e., phase separation)
between toluene-derived SOM and aqueous ammonium sulfate across a
limited range of organic volume fractions differentiates this SOM
from previous reports for isoprene-derived SOM of full miscibility
and for Ī±-pinene-derived SOM of nearly full immiscibility with
aqueous ammonium sulfate
Water Solubility Distribution of Organic Matter Accounts for the Discrepancy in Hygroscopicity among Sub- and Supersaturated Humidity Regimes
Water uptake properties of organic matter (OM) are critical
for
aerosol direct and indirect effects. OM contains various chemical
species that have a wide range of water solubility. However, the role
of water solubility on water uptake by OM has poorly been investigated.
We experimentally retrieved water solubility distributions of water-soluble
OM (WSOM) from combustion of mosquito coil and tropical peat using
the 1-octanolāwater partitioning method. In addition, hygroscopic
growth and cloud condensation nuclei (CCN) activity of solubility-segregated
WSOM were measured. The dominant fraction of WSOM from mosquito coil
smoldering was highly soluble (water solubility (S) > 10ā2 g cmā3), while that
from peat combustion contained ā¼40% of less-soluble species
(S ā3 g cmā3). The difference in water solubility distributions induced changes
in the roles of less water-soluble fractions (S <
10ā3 g cmā3) on CCN activity.
Namely, the less water-soluble fraction from mosquito coil combustion
fully dissolved at the point of critical supersaturation, while that
for tropical peat smoldering was limited by water solubility. The
present result suggests that water solubility distributions of OM,
rather than its bulk chemical property, need to be quantified for
understanding the water uptake process
Uptake of Epoxydiol Isomers Accounts for Half of the Particle-Phase Material Produced from Isoprene Photooxidation via the HO<sub>2</sub> Pathway
The
oxidation of isoprene is a globally significant source of secondary
organic material (SOM) of atmospheric particles. The relative importance
of different parallel pathways, however, remains inadequately understood
and quantified. SOM production from isoprene photooxidation was studied
under hydroperoxyl-dominant conditions for <5% relative humidity
and at 20 Ā°C in the presence of highly acidic to completely neutralized
sulfate particles. Isoprene photooxidation was separated from SOM
production by using two continuously mixed flow reactors connected
in series and operated at steady state. Two online mass spectrometers
separately sampled the gas and particle phases in the reactor outflow.
The loss of specific gas-phase species as contributors to the production
of SOM was thereby quantified. The produced SOM mass concentration
was directly proportional to the loss of isoprene epoxydiol (IEPOX)
isomers from the gas phase. IEPOX isomers lost from the gas phase
accounted for (46 Ā± 11)% of the produced SOM mass concentration.
The IEPOX isomers comprised (59 Ā± 21)% (molecular count) of the
loss of monitored gas-phase species. The implication is that for the
investigated reaction conditions the SOM production pathways tied
to IEPOX isomers accounted for half of the SOM mass concentration