ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-13601-2016Unexpectedly acidic nanoparticles formed in dimethylamine–ammonia–sulfuric-acid nucleation experiments at CLOUDLawlerMichael J.mlawler@uci.eduWinklerPaul M.KimJaeseokAhlmLarsTröstlJasminhttps://orcid.org/0000-0002-2807-0348PraplanArnaud P.SchobesbergerSiegfriedhttps://orcid.org/0000-0002-5777-4897KürtenAndreasKirkbyJasperhttps://orcid.org/0000-0003-2341-9069BianchiFedericohttps://orcid.org/0000-0003-2996-3604DuplissyJonathanHanselArminhttps://orcid.org/0000-0002-1062-2394JokinenTuijaKeskinenHelmiLehtipaloKatrianneLeimingerMarkusPetäjäTuukkahttps://orcid.org/0000-0002-1881-9044RissanenMattihttps://orcid.org/0000-0003-0463-8098RondoLindaSimonMariohttps://orcid.org/0000-0002-4900-7460SipiläMikkoWilliamsonChristinaWimmerDanielahttps://orcid.org/0000-0002-5539-9958RiipinenIlonaVirtanenAnneleSmithJames N.https://orcid.org/0000-0003-4677-8224Department of Chemistry, University of California, Irvine, Irvine, CA,
92697, USAFaculty of Physics, University of Vienna, 1090 Vienna, AustriaDepartment of Applied Physics, University of Eastern Finland, Kuopio,
FinlandArctic Research Center, Korea Polar Research Institute, Yeonsu-gu,
Incheon 21990, Republic of KoreaDepartment of Environmental Science and
Analytical Chemistry, Stockholm University, Stockholm, SwedenPaul Scherrer Institute, Villigen, SwitzerlandDepartment of Physics, University of Helsinki, 00014 Helsinki,
FinlandFinnish Meteorological Institute, 00101 Helsinki, FinlandDepartment of Atmospheric Sciences, University of Washington,
Seattle, WA 98195, USAInstitute for Atmospheric and
Environmental Sciences, Goethe University of Frankfurt, 60438 Frankfurt am Main, GermanyEuropean Organization for Nuclear Research (CERN), Geneva,
SwitzerlandInstitute for Ion and Applied Physics, University of Innsbruck, 6020
Innsbruck, AustriaCooperative Institute for Research in Environmental Sciences,
University of Colorado Boulder, Boulder, CO, USAChemical Sciences
Division, NOAA Earth System Research Laboratory, Boulder, CO, USAvisitor at: National Center for Atmospheric Research, Atmospheric
Chemistry Observations and Modeling Lab, Boulder, CO, 80301, USAformerly at: University of Eastern Finland, Department of Applied
Physics, Kuopio, FinlandMichael J. Lawler (mlawler@uci.edu)3November20161621136011361827April201624June201616September201629September2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/13601/2016/acp-16-13601-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/13601/2016/acp-16-13601-2016.pdf
New particle formation driven by acid–base chemistry was initiated in the
CLOUD chamber at CERN by introducing atmospherically relevant levels of gas-phase sulfuric acid and dimethylamine (DMA). Ammonia was also present in the
chamber as a gas-phase contaminant from earlier experiments. The composition
of particles with volume median diameters (VMDs) as small as 10 nm was
measured by the Thermal Desorption Chemical Ionization Mass Spectrometer
(TDCIMS). Particulate ammonium-to-dimethylaminium ratios were higher than the
gas-phase ammonia-to-DMA ratios, suggesting preferential uptake of ammonia
over DMA for the collected 10–30 nm VMD particles. This behavior is not
consistent with present nanoparticle physicochemical models, which predict a
higher dimethylaminium fraction when NH3 and DMA are present at similar
gas-phase concentrations. Despite the presence in the gas phase of at least
100 times higher base concentrations than sulfuric acid, the recently formed
particles always had measured base : acid ratios lower than 1 : 1. The
lowest base fractions were found in particles below 15 nm VMD, with a strong
size-dependent composition gradient. The reasons for the very acidic
composition remain uncertain, but a plausible explanation is that the
particles did not reach thermodynamic equilibrium with respect to the bases
due to rapid heterogeneous conversion of SO2 to sulfate. These results
indicate that sulfuric acid does not require stabilization by ammonium or
dimethylaminium as acid–base pairs in particles as small as 10 nm.
Introduction
Atmospheric
new particle formation (NPF) refers to the formation and subsequent growth of
condensed-phase particles from gas-phase precursors. The mechanisms and
chemical species responsible for NPF have been the subject of many recent
studies, with the motivation of understanding the impacts of these processes
on air quality and climate. NPF is an important source of cloud condensation
nuclei (CCN) and therefore may significantly affect the global radiative
energy balance (Wang and Penner, 2009; Kazil et al., 2010), but the
mechanisms and chemical species driving NPF are still poorly understood
(e.g.,
Riipinen et al., 2012). Sulfuric acid is widely accepted as one of the most
important species for particle formation in the atmosphere (e.g., Kirkby et
al., 2011; Kuang et al., 2008; Kulmala et al., 2007; Weber et al., 1997).
However, particle growth rate and composition measurements have indicated
that species other than sulfuric acid dominate growth past the smallest
particle sizes in many environments (O'Dowd et al., 2002; Weber et al., 1997;
Smith et al., 2008; Kuang et al., 2010; Bzdek at al., 2012). Three
fundamental types of growth pathways for nanoparticles have been identified
in the literature: reversible condensational growth by low-volatility
species, reactive uptake, and acid–base interactions (Riipinen et al., 2012,
and references therein). Recent evidence has supported an important role for
highly oxidized, very low-volatility organic molecules in the growth of
atmospheric nanoparticles (Ehn et al., 2014; Zhao et al., 2013; Riccobono et
al., 2014). Reactive uptake involves the multiphase transformation of a gas-phase species into a new condensed-phase species that is less volatile than
its precursor (e.g., Wang et al., 2010). Growth due to acid–base chemistry
results from strong ionic interactions, which could be reversed depending on
the chemical conditions within the cluster or particle (Bzdek et al., 2010;
Barsanti et al., 2009).
In the sulfuric acid–dimethylamine–ammonia–water system, the most likely
growth pathways involve electrostatic interactions arising from proton
transfer from sulfuric acid to the base species, direct condensational growth
by sulfuric acid due to its very low saturation vapor pressure, and
coagulation of particles and/or clusters. Dimethylamine (DMA) and ammonia on
their own are quite volatile gases, and significant growth by water vapor
condensation alone is unfavorable due to the Kelvin effect until the
particles are larger than about 50 nm in diameter. Nonetheless, all of these
compounds may be incorporated together in a stable matrix as the particles
grow, and water vapor has been shown to be an important contributor to the
initial steps of new particle formation (e.g., Zollner et al., 2012, and
references therein). While ammonia is present at much higher concentrations
than DMA in the remote atmosphere, amines such as DMA can effectively compete
with ammonia in the growth of nanoparticles of ∼ 10 nm diameter (Smith
et al., 2010; Barsanti et al., 2009). Measurements made at the CLOUD chamber
in CERN of molecular clusters in the sulfuric acid–base system show a
nucleation process involving stepwise additions of H2SO4 and base,
typically in a 1 : 1 ratio (Kirkby et al., 2011; Schobesberger et al.,
2013, 2015; Almeida et al., 2013; Kürten et al., 2014; Bianchi et al.,
2014). At this early stage in the particle formation process, when there are
fewer than about 20 molecules in the clusters, ionic acid–base interactions
clearly drive the growth. For these experiments, sulfuric acid was present at
much lower concentrations than were the bases, so it was the chemical species
that limited the growth.
Here we present chemical composition measurements of recently formed
nanoparticles in the size range of 10–30 nm in diameter (volume median
diameter for collected particles), from experiments conducted in the CLOUD
chamber at CERN. These are the first direct compositional measurements of
newly formed particles in this chemical system. These observations place
constraints on the nature of particle growth after nucleation in the
DMA–NH3–H2SO4–water system, with implications for new particle
formation in the atmosphere.
Overview of experimental conditions for the TDCIMS measurements
described. The chamber temperature was 278 K and the relative humidity was
38 %. DMA and NH3 measurements are given (parts per trillion by
volume) when coincident with the TDCIMS observations, and otherwise averages
for the same experimental conditions close in time are given as estimates,
indicated with parentheses. O3 and SO2 are reported in parts per
billion by volume. Larger UV aperture fractions allow more ultraviolet light
into the chamber, resulting in greater rates of SO2 oxidation to
H2SO4.
Run no.DMA targetDMANH3O3SO2UV apertureH2SO4pptpptpptppbppb%× 107 cm-3103240512223631001.3–1.4103340(46)(28)23631001.3103540(46)(28)2363400.5–0.8103640(46)(28)23631001.1–1.3104010(23)(19)2363200.4104310(23)(19)2363400.9–1.0104710282886631002.4–2.710560< 5< 523721003.5Laboratory conditions
The experiments were undertaken at the seventh Cosmics Leaving Outdoor
Droplets (CLOUD7) campaign at CERN during October 2012. The CLOUD chamber is
a 26.1 m3 cylindrical chamber made from electropolished stainless
steel. Efforts were taken in its construction to make it the cleanest
possible chamber used for studying atmospheric particle nucleation (Kirkby et
al., 2011; Schnitzhofer et al., 2014; Duplissy et al., 2016). It can be
precisely temperature controlled, and UV light at 250–400 nm can be
provided to the chamber with minimal heating via an array of fiber-optic
bundles (Kupc et al., 2011). It is possible to eliminate ions from the
chamber using a 20 kV m-1 clearing field and conversely to increase
the number of chamber ions by directing a 3.5 GeV pion beam into the
chamber. The latter state results in ion concentrations that are up to an
order of magnitude greater than what is present naturally from cosmic ray
bombardment at the Earth's surface (Franchin et al., 2015).
Clean air and reagents were added to the chamber on a continuous basis,
yielding a roughly 4 h overturning time for the chamber. The chamber
conditions were set to simulate atmospheric conditions relevant to the lower
troposphere. The main constituent gases were N2 (79 %) and O2
(21 %) from cryogenic liquid sources. The chamber was maintained at
38 % relative humidity with ultrapure water generated by a Millipore
filter system with UV irradiation (Merck Millipore). The trace gases added
were ozone (O3), DMA, and sulfur dioxide (SO2).
O3 was usually kept at 23.5 parts per billion by volume (ppb), and DMA
was set to either 10 or 40 parts per trillion by volume (ppt) (see Table 1).
SO2 was set to 63 ppb to facilitate the formation of H2SO4 at
atmospherically relevant rates, leading to levels from 5 to
35 × 106 cm-3. SO2 was the only species added at
levels well above likely background atmospheric mixing ratios. The addition
of UV light resulted in OH radical formation, causing SO2 oxidation to
H2SO4 and formation of new particles. The ozone mixing ratio was
increased to 86 ppb for one experiment to increase H2SO4 levels
and the new particle formation rate. During CLOUD7 the chamber air was
maintained at 278 K. Aerosol cleaning cycles were performed regularly
between experiments. For these cleanings, the chamber was flushed with clean
humidified air and the particles were ionized with a pion beam generated from
the CERN Proton Synchrotron, with the high voltage clearing field being cycled
on and off to allow charging and then removal of charged particles and
clusters. An extended cleaning of the chamber was performed prior to the
campaign. This involved rinsing the chamber walls with ultrapure water for
24 h, followed by raising the temperature to 373 K for a further 24 h and
flushing with ultrapure synthetic air to drive contaminants off the chamber
walls. A further overnight heating cycle at 373 K was performed prior to the
experiment with no added DMA (the final experiment described here).
Despite the efforts to minimize contamination, ammonia was present as an
impurity during the DMA experiments as a carryover from previous experiments
during the campaign. A few days prior to the DMA–sulfuric acid experiments
described in this study, nucleation experiments with intentional addition of
ammonia gas at levels of 20 and 40 ppt were performed. After that time, no
ammonia was intentionally added. However, ammonia was detected using ion
chromatography (IC), as described in Praplan et al. (2012). The IC was not
working during every experiment described here, but it provided measurements
of both NH3 and DMA under several important experimental conditions. DMA
was measured at an average mixing ratio of 46 ± 12 ppt
(1 SD) for nominal 40 ppt DMA experiments, for which a total of 18
particle collections are reported here, and 23 ± 7 ppt for nominal
10 ppt DMA experiments, for which a total of 21 particle collections are
reported here (Simon et al., 2016). Over the respective periods, the average
NH3 was 28 ± 9 and 19 ± 10 ppt. Therefore in both the high
and low DMA cases, the DMA and NH3 mixing ratios were comparable. It is
worth noting that the IC is likely to measure particulate ammonium and
dimethylaminium as well as gas-phase ammonia and dimethylamine. During the
high-temperature cleaning, ammonia reached extremely high levels (over
1000 ppt) due to evaporation from the chamber walls, whereas DMA only
reached about 30 ppt. After the cleaning, both species were below the 5 ppt
detection limit for the experiment with no added DMA.
Many instruments sampled the chamber air to assess the characteristics of
the gases, molecular clusters, and particles present in the chamber. These
included a scanning mobility particle sizer (SMPS), a hygroscopicity tandem
differential mobility analyzer (HTDMA; Kim et al., 2016; Keskinen et al.,
2013), and several mass spectrometers. Atmospheric pressure interface
time-of-flight mass spectrometers (APi-TOFs, Tofwerk AG) measured gas-phase
ions (Kirkby et al., 2011; Schobesberger et al., 2013; Almeida et al., 2013;
Bianchi et al., 2014), and two chemical ionization APi-TOFs measured neutral
gas-phase species (Jokinen et al., 2012; Kürten et al., 2014).
TDCIMS instrument
The thermal desorption chemical ionization mass spectrometer (TDCIMS) has
been described in detail elsewhere (Smith et al., 2004; Lawler et al., 2014).
Significant improvements in ammonium sensitivity were made since the
measurements described in Lawler et al. (2014) by modifying the differential
pumping and ion transfer optics. For these experiments, particles were
sampled continuously from the CLOUD chamber at 3.3 L min-1 (Fig. 1).
Sampled particles were charged with a pair of unipolar chargers (UPCs;
McMurry et al., 2009; Chen and Pui, 1999). The charged particles were then
collected by electrostatic deposition onto a Pt filament. After a collection
period of typically 30 min, the filament was translated into the ion source
of the mass spectrometer and the temperature was ramped to
∼ 600 ∘C to desorb the collected mass. Desorbed molecules and
decomposition products were ionized and ultimately detected using a high-resolution time-of-flight mass spectrometer (HTOF, Tofwerk AG). Instrument
backgrounds were assessed for each collection by performing the identical
procedure but without applying a collection high voltage. All measurements
reported here are corrected for this background. The TDCIMS is capable of
detecting both positive and negative ions, but only one polarity can be
monitored for each sample collected. Chemical ionization reagent ions are
generated by an 241Am source kept in a clean N2 flow. Impurity
H2O and O2 in the N2 gas result in (H2O)nH+
reagent ions for positive mode and (H2O)nO2- reagent ions
in negative mode. Particle-phase dimethylaminium (C2H8N+, or
DMAH+) and ammonium (NH4+) are detected as molecular ions and as
ions clustered with water in positive mode. Sulfate is detected in negative
ion mode primarily as the SO5- ion, which is formed from the reaction
of the sulfate salt decomposition product SO3 with O2-. The
TDCIMS observations are presented as sums of background-corrected detected
ions, integrated over the desorption period, for each collected sample, or as
ratios of these ion sums.
Schematic of the TDCIMS instrument as deployed at CLOUD7. Dashed
boxes show components that were not always connected. Particles from sampled
chamber air were negatively charged in the UPCs (unipolar chargers),
optionally size-selected by radial differential mobility analyzers (RDMAs),
and electrostatically precipitated onto a Pt collection filament (shown here
in collection position) held at +4 kV over a period of 15 or 30 min. The
filament was then translated vertically into the ion source of a
time-of-flight (TOF) mass spectrometer and the sampled material was thermally
desorbed and/or decomposed by a temperature ramp of the wire at atmospheric
pressure in a dry N2 environment. The resulting compounds were ionized
and passed into the TOF via electrostatic and octopole ion guides. CPC:
condensation particle counter. MFC: mass flow controller. Vacuum pumps and
vacuum lines have been omitted from the figure.
During most of the CLOUD7 experiments, the sampled particles were not
mobility-selected, and the particle sizes that made up most of the collected
mass were dependent on the chamber particle size distribution as it evolved
over the course of the particle formation events. For one experiment, a
mobility size of 20 nm was selected using a pair of radial differential
mobility analyzers (RDMAs; Zhang et al., 1995) placed between the unipolar
chargers and the electrostatic precipitator. This mostly served to exclude
small (< 15 nm) particles from analysis. A nano-SMPS sampled particles
after charging in the UPCs. The nano-SMPS consisted of a differential
mobility analyzer (TSI Inc., model 3085; Nano DMA) and an ultrafine
condensation particle counter (CPC; TSI Inc., model 3027) and was capable of
measuring particles of 4–42 nm mobility diameter. The nano-SMPS system was
temporarily moved downstream of the electrostatic precipitator to assess the
size-resolved efficiency with which the precipitator captured sampled
particles. Additionally, a post-campaign test was performed using a
long-column differential mobility analyzer (TSI Inc., model 3081; long DMA)
to assess the capture efficiency of 12–140 nm particles under the same
instrumental operating conditions. The region of overlap between these two
collection efficiency measurements compared well. The two efficiency
measurements were combined for correcting the experimental data, and the
absolute cutoff for collection was determined to be about 60 nm (Fig. 2).
Charged particles smaller than about 15 nm appeared to be captured with
∼ 100 % efficiency once inside the precipitator region.
Solid line: estimated TDCIMS particle sampling efficiency as a
function of mobility diameter for bulk (non-size-resolved) sampling. Dashed
line: the same number efficiency scaled by the volume of an individual
particle at each size and normalized to fit on the same plot (arbitrary
units). These estimates include the effects of sampling losses, (multiple)
charging efficiency, and efficiency of capture by the electrostatic
precipitator. Small (< 15 nm) particles are efficiently captured due to
their high mobility. The efficiency of collection at sizes below 4 nm could
not be assessed using the available experimental apparatus, but particles at
smaller sizes are not likely to have contributed detectable amounts of
particle mass during these experiments.
To assess the net size-resolved collection efficiency of the instrument,
sampling losses and (multiple) charging efficiency had to be taken into
account. This was achieved by a statistical optimization procedure that
relied on the CLOUD chamber SMPS system and a CPC downstream of the TDCIMS
precipitator, using a procedure similar to that described in Lawler et
al. (2014). Briefly, a size-resolved sampling and charging efficiency filter
was optimized to obtain agreement between the chamber particle distribution
and the TDCIMS CPC when there was no precipitator voltage applied and after
correcting for UPC-generated particles (see next paragraph). This information
was combined with the precipitator capture efficiency to determine the sizes
and total volume of particles collected. The volume median diameter (VMD) was
assessed as a measure of the center of the collected particle distributions.
When sampling chamber air with the TDCIMS, there was a persistent mode of
small (∼ 6 nm) particles generated in the UPCs. This was a result of
ionization and oxidation processes driven by the UPC radioactive sources. The
particles were present even in the absence of H2SO4 in the chamber,
indicating that oxidation of SO2 to form H2SO4 was probably
involved. The alpha-emitting radioactive sources are known to produce OH
radicals from H2O present in the sampled air (e.g., He and Hopke, 1995).
When these charger-produced particles were the only source of particles to
the TDCIMS, the resultant background-corrected particle-phase ion signals
were usually not detectable, indicating that these fine particles did not
contribute significantly to the TDCIMS chamber measurements. However, an
attempt was made to try to reduce any possible impact of this oxidation
process, as it also seemed possible that charger-generated H2SO4
could influence the composition of sampled particles. During many of the
experiments, a flow of 5 % H2 in Ar (industrial-grade welding gas)
was added to the TDCIMS sampling inlet to achieve a ∼ 1 % H2
mixing ratio. This was done to scavenge OH and other oxidants generated in
the UPCs and resulted in a reduction in the spurious small particle mode by a
factor of 3–4. Measured particle sulfate fractions did not appear to be
substantially affected by this change (see Sect. 4). However, this test demonstrated that n-methyl
formamide (MFA) detected in the particles was the result of oxidation processes
within the charger (see Sect. 4). Adding the H2 gas resulted in higher
background signals and variability for many of the signals due to
contaminants in the H2/ Ar gas cylinder.
Instrument calibrationsCalibrations during the CLOUD7 measurements
The TDCIMS was calibrated for ammonium sulfate (AS) particles once during the
campaign by using an atomizer to generate aerosol from a dilute AS solution. The wet particles were passed through a diffusion dryer
before entering the UPCs. This calibration aerosol was mobility-selected to
either 15 or 20 nm using a pair of RDMAs, one downstream of each UPC, to
control the amount of mass available for collection on the filament. By
selecting for 15 nm particles and collecting for 5 min, about
0.3 × 10-9 cm3 of particle volume was collected. This
calibration showed SO5- and (H2O)NH4+ signals in the
low-to-mid range of those observed in the nucleation experiments (Figs. 3 and
4). By selecting 20 nm particles, an estimated particle volume of
1.3 × 10-9 cm3 was collected, resulting in sulfate
signals that were similar to mid-to-large chamber aerosol samples and ammonia
signals over twice as large as any observed for chamber aerosol (Figs. 3 and
4). These AS aerosol calibrations indicate that the TDCIMS was roughly
equally sensitive on a molar basis to ammonium and sulfate during CLOUD7,
with a 2.3 : 1 ammonium-to-sulfate-signal ratio for AS.
(a) Sum of oxidized sulfur (“sulfate”) signal vs.
collected particle volume. (b) Sum of N base (DMAH+ and
NH4+) signal vs. collected particle volume. Red crosses are from
chamber particle collections of DMA + H2SO4 nucleation
experiments, and black circles are from calibration ammonium sulfate aerosol.
Ion counting errors are smaller than the symbols and are excluded. The
negative ion data from the chamber particles are broadly consistent with
particles dominated in volume by sulfate. However, the chamber particles have
a much lower base fraction than the ammonium sulfate calibration aerosol.
Calibration ammonium sulfate aerosol mass spectrum for (a)
negative and (b) positive ions. SO5- and SO4- are
the main sulfate ions, and NO2- indicates a nitrate contaminant in
the standard, most likely introduced by the water or air used for the
atomization. The instrument is more sensitive to ammonium nitrate (detected
as NO2-) than to ammonium sulfate by a factor of ∼ 100 (Bzdek
et al., 2014), so the nitrate contamination is likely a small fraction of the
sampled aerosol mass. NH4+ and its clusters with water and NH3
are the main ammonium ions. All species plotted have one elemental charge.
Results of a post-CLOUD7 calibration of the TDCIMS with salts
generated by atomizing liquid solutions of ammonium sulfate,
(NH4)2SO4, and dimethylaminium bisulfate (DMABS),
(C2H8N)HSO4. Particles of roughly 25 nm volume median
diameter were used for this calibration, and larger mass collections were
achieved by collecting for longer periods. For these calibrations, particle
volume was corrected to mass using the densities of DMAS (1.35 g cm-3)
and AS (1.78 g cm-3). (a) Signal in negative ion mode plotted
against particle mass, with a second-order polynomial fit. The sulfate
sensitivity for the two salts appears to be indistinguishable.
(b) Signal in positive ion mode for the two salts. There was
contamination of the DMABS by ammonium at some point between solution
generation and analysis. Based on laboratory tests, this most likely occurs
as a result of trace ammonia present in the carrier N2 and transport
lines. For this reason, the DMAH+ signal, NH4+ signal, and their
sum are all given. The sum signal for DMABS is very similar to the AS signal,
suggesting that both aerosols may actually have similar base : acid ratios
after atomization and drying.
Post-CLOUD7 calibrations
Aerosol calibrations were performed after the CLOUD7 experiments to assess
the relative sensitivity of the TDCIMS to particle ammonium and
dimethylaminium and to assess the linearity of the response. Aqueous
solutions of 2 mM AS and dimethylaminium bisulfate (DMABS) were
atomized in a stainless steel atomizer, dried by passing through a thermal
denuder at 373 K, and sampled by TDCIMS. All transport, sheath, and atomizer
gas flows used N2 gas delivered from a liquid N2 dewar. Tests in
our lab have shown that even brief exposure to lab air or even “clean” zero
air can cause significant contamination of the particles with ammonium. The
AS solution was generated using AS salt
(> 99 %, Sigma-Aldrich) and deionized water (Millipore). The
dimethylamine bisulfate solution was made using 40 % (by mass)
dimethylamine in H2O (Sigma-Aldrich), H2SO4 (95–98 %,
Sigma-Aldrich), and deionized water (Millipore) to form a 1 : 1 molar ratio
of dimethylaminium to sulfate. The TDCIMS RDMAs were used to size-select the
sampled particles at 25 nm mobility diameter, and a nano-SMPS was used to
characterize the resulting size distribution. During particle collection, the
nano-differential mobility analyzer was bypassed so that the number
concentration difference between sample and background could be used to
assess the number of particles collected for brief collection periods. Sample
time was varied to achieve different collection masses, which were estimated
using the known size distribution, the number concentrations downstream of
the wire during collections, and the wire collection efficiency, which was
determined using a comparison of back-to-back stable collection and
background particle concentrations. For these calibrations, the dry particle
densities were assumed to correspond to dimethylamine sulfate
(1.35 g cm-3) and AS (1.78 g cm-3), respectively (Qiu and
Zhang, 2012), and collected masses are reported. It should be noted that the
actual base : acid ratio of the calibration aerosol is not known exactly
because of loss to the gas phase during evaporation. For example, Wong et
al. (2015) found that their wet AS aerosol had an ammonium : sulfate ratio
of 1.72 due to this process. In addition, a given salt may only have certain
stable configurations when dried. A dimethylamine sulfate droplet was shown
to go from a 2 : 1 solution to a 1.5 : 1 DMAH+ : SO42-
ratio particle when dried (Chan and Chan, 2013). Ouyang et al. (2015) showed
a tendency for dried DMA : H2SO4 nanoclusters to reach a
consistent density of 1.567 g cm-3, independent of the bulk
composition of the initial electrospray solution. Despite using solution
DMA : H2SO4 molar ratios ranging from 1 : 10 to 2 : 1, the
particles settled on the same final composition. Similarly, during “droplet
tests” in our lab in which dilute salt solutions are directly applied to the
TDCIMS collection wire, allowed to evaporate, then analyzed normally, we
found only a few percent increase in DMAH+ signal when using a 2 : 1
DMAH+ : SO42- solution compared with a 1 : 1 solution,
indicating that the additional DMA simply evaporated rather than being
incorporated into the salt (the molar ratio of which remains uncertain).
The sensitivity of the TDCIMS to sulfate in the negative ion mode was
basically indistinguishable for the two salt types (Fig. 5). The signal
dependence was nonlinear with respect to sampled mass and the relationship is
well-fit by a second-order polynomial. Similar to the negative ion signal,
the base signals showed nonlinear sensitivity with respect to mass when a
large range of collected mass was considered, and the relationship could be
described using a second-order polynomial. NH4+ was detected as a
contaminant in the DMABS aerosols. Contamination of the
particles by ammonia most likely occurred sometime prior to collection on the
wire (i.e., either during generation or transport of the particles), based on
tests of temporal stability of the particle composition after precipitation
onto the collection wire. The solutions themselves showed very little
ammonium contamination, based on a test in which a dilute solution was
directly applied to the precipitator wire. This test also indicated that
there was no significant ammonia released upon thermal decomposition of the
dimethylaminium salt. These observations underscore the facility with which
ammonia is taken up into DMA–H2SO4 aerosol. The DMAH+ signals
for the DMABS calibration particles were slightly less than but comparable to
the NH4+ signals for the AS calibration particles. If the DMABS
calibration aerosol was exactly 1 : 1 DMAH+ : SO42- and the
AS aerosol was exactly 2 : 1 NH4+ : SO42-, this would
indicate higher sensitivity to DMAH+ than to NH4+ such that the
reported CLOUD7 experimental DMAH+ : NH4+ ratios are
overestimates. In this worst case, the DMAH+ sensitivity is about
50 % higher than the NH4+ sensitivity. However, based on the
discussion presented above, the actual composition ratios were probably
closer for the two calibration aerosols, and any inferred sensitivity
difference would be smaller. If the actual composition ratios for the AS and
DMABS aerosols were 1.75 : 1 NH4+ : SO42- and 1.25 : 1
DMAH+ : SO42-, respectively, the inferred sensitivity to both
bases would be the same. The base : acid signal ratios were roughly linear
over a wide range of collected masses (Fig. 6), allowing a straightforward
analysis of this quantity for the chamber aerosol. Overall, these
calibrations indicate no significant sensitivity bias for the salts studied,
but there is uncertainty in the relative sensitivity of DMAH+ and
NH4+ due to uncertainty about the calibration aerosol composition.
Base (DMAH++ NH4+) ion signals plotted against
sulfate ion signals for ammonium sulfate and dimethylaminium bisulfate
calibration aerosol overestimated sample loadings of 0 to 1.8 ng, conducted
in the laboratory at NCAR after CLOUD7 (the same tests as presented in
Fig. 5). Signals were
scaled by collected mass to match positive and negative analyses. Despite the
nonlinear character in the individual ions' dependence on the sample mass
(Fig. 5), the base : acid ratio remains constant
over a range of collected masses for the same calibration aerosol, in
agreement with the ammonium sulfate aerosol calibration performed at CLOUD7
(Fig. 5). Note that relative sensitivities to base and acid were different
during this test than during the calibrations at CLOUD7 (Fig. 5), which
should be considered the better comparison for the experimental particle data
since they were conducted under CLOUD7 conditions.
An additional test was performed after the CLOUD7 experiments to determine
the stability of DMA–H2SO4 nanoparticles on the TDCIMS filament.
Nanoparticles were formed using high concentrations of DMA and
H2SO4 in a glass flow reactor modeled after that described in
Glasoe et al. (2015). Particles of ∼ 5–50 nm were generated and
sampled with the TDCIMS. Particles were analyzed both in the usual way, with
desorption proceeding a few seconds after the conclusion of sampling, and
with an additional “rest” period three times as long as the sampling period
before desorption to allow any volatile compounds to desorb. There was no
significant difference in the results from the two methods, indicating that,
for these particles at least, continuous evaporation of base was not an
issue. This test does not exclude the possibility that some DMA is lost
rapidly during sampling, i.e., as the result of the evaporation of an aqueous
phase. However, the calibrations performed with atomized solutions indicate
that (DMA)H2SO4 and (NH3)2H2SO4 aerosol behave
similarly in this regard.
Overview of calibrations
To summarize the calibration results, the TDCIMS shows identical sensitivity
to the sulfate fraction of the two calibration aerosols tested, DMABS and AS.
However, there is some uncertainty in the relative sensitivity of
the basic components NH4+ and DMAH+. If there is indeed any
difference, the instrument is more sensitive to DMAH+ and therefore may
overestimate both DMAH+ : NH4+ and base : acid ratios.
However, the measured experimental ratios were in fact lower than expected
(see Sect. 4). The AS calibration performed at CLOUD7 should be
considered the best assessment of the relative sensitivities between positive
(base ions) and negative (sulfate ions) ion modes for these measurements
because it was performed under identical operating conditions. This
calibration showed the TDCIMS to be slightly more sensitive towards
particulate ammonium than towards particulate sulfate, but given the very low
base : acid ratios measured for chamber aerosol (see Sect. 4 below), no
attempt was made to correct for this, and the base : acid ratios presented
are strictly the ratios of the uncorrected ion signals.
Results
During the CLOUD7 experiments, negative ion TDCIMS spectra were dominated by
SO5- (Fig. 7a). This was also the strongest signal for AS in the calibrations (Fig. 4a) and results from the reaction of
SO3+ O2- in the ion source following the decomposition of
sulfate salts. There were other sulfur masses that tracked SO5-,
including SO3-, SO4-, and HSO4-. The oxidized sulfur
signal per estimated collected mass is consistent with the AS
calibration aerosol, indicating that sulfate dominated the collected mass in
the particles measured from the chamber, as expected (Fig. 3). Overall the
spectra were very clean, but some apparent contaminant organic molecules were
also detected in the particles, including C3H3O3- (an
oxidized carboxylic acid or fragment), CHNO2- (likely fragment ion),
and C8H4O3- (phthalic anhydride). Phthalic anhydride is
most likely derived from phthalate contamination by plastics.
Negative (a) and positive (b) background-corrected
30 min collection particle mass spectra for a particle formation experiment
with nominal 40 ppt DMA, 23.5 ppb O3, and
2.8 × 106 cm-3 cm-3 H2SO4, run 1033 on
16–17 October 2012. All species plotted have one elemental charge. The
volume median diameter of collected particles was 10.9 nm for positive ions
and 13.1 nm for the negative ions. Particulate ammonium and dimethylaminium
levels were similar, despite not adding gas-phase ammonia intentionally to
the chamber. N-methyl formamide was a contaminant generated in the particle
chargers. The negative spectrum is dominated by the sulfate-derived peak
SO5-. The instrument is more sensitive to ammonium nitrate (detected
as NO2-) than to ammonium sulfate by a factor of ∼ 100 (Bzdek
et al., 2014), so the nitrate contamination is likely a small fraction of the
sampled aerosol mass.
Dimethylaminium and ammonium were major components of the positive ion
spectra (Fig. 7b). The main ammonium peak was its first water cluster,
NH4+(H2O), which is also the main ammonium peak in the AS standards (Fig. 4b). There was also a large C2H6NO+
peak, which appears to be the DMA oxidation product MFA,
a contaminant generated in the UPCs (see below and Fig. 8). In all
experiments, more ammonium was detected in the particles than dimethylaminium
and no clear difference between nominal 40 and 10 ppt DMA runs was observed
(Fig. 9). The DMAH+ : NH4+ ratio was more likely to be lower
for smaller amounts of collected mass (Fig. 9), though a clear dependence on
particle size was not observed. The collected mass dependence could be due to
a limitation of the available contaminant ammonia or the aging time necessary
to change the composition. Small mass collections were often the result of
sampling at the end of runs, when the remaining particles were few, large,
and aged, with no more sub-10 nm particles present and collected VMDs closer
to 30 nm. In the run conducted with no DMA added, the DMAH+ signal was
about 1 % of the NH4+ signal, indicating that DMA did not remain
as a significant contaminant in the chamber, unlike NH3.
(a) Particulate ion signals for sulfate (SO5-),
dimethylamine, n-methyl formamide, and ammonia during a nucleation experiment
with 10 ppt DMA and 2.6 × 107 cm-3 H2SO4 (run
1047). For most of this run, 1 % H2 was added to the TDCIMS inlet
as a gas-phase radical scavenger, except for the period between the dashed
vertical lines. During this period, the n-methyl formamide increased about
30-fold, while the other species underwent more modest changes.
(b) Size-resolved particle mass collected during this period. White
gaps indicate background periods when no particles were collected. Open
circles show the volume mean diameter for collected particles.
The ratio of summed DMAH+ to summed NH4+ ion signals
plotted against the collected particle volume for DMA + H2SO4
particle formation experiments (circles) and for the H2SO4 only (no
added DMA) nucleation experiment (triangle). Collections of less than
0.1 × 10-9 cm-3 of sample were excluded due to low
signal-to-noise ratio. The filled symbols are periods with nominally 10 ppt DMA in
the chamber and the open symbols are with 40 ppt DMA. Blue symbols indicate
periods with 1 % H2 added to the TDCIMS inlet to scavenge radicals,
and red symbols are periods without H2. The vertical error bars indicate
the standard error of the ion signals. Higher ammonium fractions were more
likely for low collected masses (resulting from low chamber aerosol mass),
possibly because less contaminant ammonia was required to alter the
composition of the particles (see Sect. 4). DMAH+ was never observed to
be more abundant than NH4+ in the particles during any of
experiments.
To investigate the apparent spurious MFA (C2H6NO+) formation in the UPCs, a test was performed in which
the oxidant-scavenging H2 flow to the TDCIMS inlet was turned off midway
through a particle nucleation experiment. This experimental condition was
maintained for a period of over 3 h during the nucleation experiment,
such that two measurements under “high charger oxidant” conditions were
made for both positive and negative ion modes. Most of the main species
observed were only minimally affected by this test, but
C2H6NO+ in the particles increased by over an order of
magnitude (Fig. 8). The DMAH+ signal was not strongly affected,
indicating that DMA present in the sampled particles was not appreciably
oxidized in the absence of the H2 scavenger. This suggests that the DMA
oxidation to MFA occurred in the gas phase and the MFA partitioned to the
particles afterward. It is also possible that the instrument sensitivity for
MFA is much greater than for DMAH+ and only a small fraction of particle-phase DMAH+ was oxidized. In either case, the large MFA signal is
clearly spurious and we have no evidence suggesting that the particle-phase
DMA signal was significantly affected by the MFA production. MFA signal was
therefore excluded from calculations of base : acid and
DMAH+ : NH4+ ratios. Another concern was that other species,
especially H2SO4, could be generated in the chargers and partition
to the particles. The addition of H2 reduced the fraction of
charger-generated OH that could react with SO2 by almost 2 orders of
magnitude. However, no consistent bias in base : acid ratios is evident
when H2 was added (see next paragraph, and Fig. 10). Overall, therefore,
there is nothing to indicate that the measured base : acid ratios were
strongly biased by sampling artifacts.
Base : acid TDCIMS signal ratios for DMA–H2SO4
experiments (circles) and the H2SO4 alone experiment (triangle)
plotted against the volume median particle diameter for collected particles;
signal-weighted APi-TOF positive ion cluster composition (black X markers)
(Bianchi et al., 2014); HTDMA-based estimates (black squares) assuming
1 : 1 ammonium : dimethylaminium ratio, with standard deviations (n=3–6) from Ahlm et al. (2016) and Kim et al. (2016). The TDCIMS
base : acid ratio is the summed NH4+ and DMAH+ ions and their
clusters divided by summed sulfate peaks. Collections of less than
0.1 × 10-9 cm3 of material were excluded. Only
experiments for which the collected mass for the two polarities were within a
factor of 2 are included in the plot, and the ratios are scaled to correct
for differences in sampled mass. The filled symbols are experiments with
nominally 10 ppt DMA and the open symbols are with 40 ppt DMA. Blue symbols
indicate periods with 1 % H2 added to the TDCIMS inlet and red
symbols are without H2. The vertical error bars represent the standard
error of the particle ion signals, and the horizontal error bars indicate the
range of particle diameters that contributed to the collected mass (minimum
2 % contribution). The TDCIMS results show consistently lower
base : acid ratios than inferred by the HTDMA, but they show a similar size
dependence and high variability for the smallest detectable sizes.
Ratios of the sum of base (DMA and ammonia) to acid (sulfate) integrated
peaks were calculated to assess the acidity of the particles in each
experiment (Fig. 10). Because acid and base compounds are each detected in a
different polarity (negative and positive, respectively), there is always a
time gap between the observations, during which the particle size and
potentially composition have changed. However, reasonably consistent results
were achieved when comparisons were made for collections close in time for
similar volume median diameters of sampled particles, with similar amounts of
collected mass. There was not a strong relationship between gas-phase
sulfuric acid concentration and particle composition. Kim et al. (2016)
similarly found a non-dependence of 10 nm particle hygroscopicity on
sulfuric acid but did observe a decrease in base : acid ratio for 15 nm
particles as gas-phase sulfuric acid increased. For essentially all
collections, the TDCIMS base : acid ratio was below 0.5, indicating that
the particles did not reach a fully neutralized state. For some collections
of small (∼ 10 nm VMD) particles, this ratio was near 0.1, though
there was high variability for this size range. The HTDMA data show a similar
size dependence but indicate higher base : acid ratios (Fig. 10). A
notable exception to the very acidic particles was a case for which particles
were mobility-selected at 20 nm before collection and the base : acid
ratio was 1.23. This is most likely due to contamination of the particles by
ammonia in the recirculating sheath flow of the mobility analyzers and so the
results of this experiment are therefore excluded from Figs. 9 and 10. The
DMAH+ : NH4+ ratio was low (∼ 0.15) for this experiment,
indicating a substantial change in particle composition relative to other
experiments. The mass of small particles excluded during this test was too
small to account for the large change in the base : acid ratio, even if the
excluded particles contained no DMA or ammonia. It should be noted that the
same mobility analyzers were used to size-select the calibration AS aerosol (see Sect. 3.1).
However, it is unlikely that the calibration particles could become
contaminated by ammonia to the extent that they achieve a base : acid ratio
higher than that of the stable AS salt (i.e., 2 : 1), which is
roughly the expected composition of the calibration aerosol. So, while the
size-selection process significantly impacted the composition of the acidic
chamber particles, this process was unlikely to influence the composition of
the calibration aerosol greatly.
During these experiments, contaminant ammonia was present in the chamber at
significant levels as a result of earlier experiments with intentional
ammonia additions. TDCIMS measurements indicate that this ammonia was
incorporated into growing sulfuric acid particles of ∼ 10–30 nm with
an efficiency comparable to or greater than DMA. Observations of nucleated
clusters in the CLOUD chamber show that DMA is an important constituent of
recently nucleated particles when it is present in the chamber at
concentrations as small as 5 ppt (Almeida et al., 2013; Kürten et al.,
2014), whereas ammonia was only detected in clusters with more than 7
H2SO4 molecules (Bianchi et al., 2014). However, it is possible
that ammonia molecules were present in smaller clusters in the chamber and
evaporated during analysis in the APi-TOF of CI-APi-TOF instruments. The
abundance of ammonia in the TDCIMS-measured particles indicates either that
the molecular clusters that lead to > 10 nm particles contain more
ammonia than is evident from cluster observations or that ammonia is
incorporated into particles by a process distinct from cluster formation, or
both.
A particle composition model based on chemical equilibria for aqueous
well-mixed particles predicts roughly equal fractions of DMAH+ and
NH4+ ions in the particles when 15 ppt ammonia and 10 ppt DMA are
present in the gas phase (Ahlm et al., 2016). At smaller DMA concentrations
ammonium is expected to dominate. For gas-phase concentrations of 40 ppt DMA
and 15 ppt ammonia, however, the particulate-phase base should be primarily
DMAH+ with a DMAH+ : NH4+ ratio of about 5 due to the
higher basicity and lower saturation vapor pressure of DMA (Ahlm et al.,
2016). The relative DMAH+ : NH4+ ratio is thus expected to be
highly dependent on the gas-phase concentration of DMA, a feature which is
not observed in the TDCIMS data. This may be explained in part by the fact
that the DMA : NH3 ratio in the chamber, as measured by IC, was actually
comparable in both the 40 and 10 ppt DMA experiments. That is, the ammonia
contamination was somewhat lower for the 10 ppt DMA experiments. However,
for DMA : NH3 gas-phase ratios above 1, there should be more
DMAH+ than NH4+ in the particles. The particle-phase
DMAH+ : NH4+ ratios are close to 1 in many cases, such that a
relatively small difference in TDCIMS sensitivity to the two bases, or the
relatively large uncertainty in the gas-phase DMA : NH3 ratio, would
be enough to remove disagreement from expectations. However, there are several
cases for which the DMAH+ signal is constrained by error bars to be less
than two-thirds of the NH4+ signal and, based on calibrations, the
instrument sensitivity to DMAH+ appears to be the same or higher than to
NH4+. This unexpected result has at least two potential explanations:
(1) a deficiency in the thermodynamic calculations of the particle-phase
composition, such as a kinetic limitation for surface uptake of the bases or
water; or (2) a measurement error or artifact in the particle- or gas-phase
composition. In the following we discuss each of these possibilities.
Models that are based on chemical equilibria for well-mixed aqueous particles
may not adequately describe nanoparticles. Recently formed sulfuric acid
nanoparticles may have structures and corresponding intermolecular
interactions that are different from those in a well-mixed solution. For
example, steric effects that favor the presence of ammonia over DMA may be
important at these small sizes. Thermodynamic calculations and previous
experimental studies on larger particles suggest aqueous mixtures of aminium
and ammonium sulfates with base : acid ratios from 2 : 1 to 1 : 1 are
the stable composition at the RH of the CLOUD chamber experiments (Chan and
Chan, 2013; Sauerwein et al., 2015). However, there were unfortunately no
direct observations on the phase and water content of the nanoparticles in
these experiments.
Kinetic limitations to surface uptake of the bases are probably only possible
for non-aqueous particles, since the diffusion time into an aqueous 10 nm
particle is very short compared to the lifetime of a particle in the chamber.
However, recent work on nanoparticle phase suggests that these small
particles should have a liquid phase (Cheng et al., 2015). Supersaturated AS
nanoparticles, such as those likely to be produced in new particle formation,
are expected to be liquid or mixed phase (depending on particle size) at the
temperature and sizes considered in these experiments (see Sect. 5 for more
details). The growth rates of 2 nm particles in the CLOUD7 experiments are
around 10 times faster than would be predicted if the growth were due to
perfectly efficient condensation of the measured gas-phase sulfuric acid
monomers (Lehtipalo et al., 2016). This suggests that cluster–particle
collisions were the main driver of the growth of the newly formed sub-2 nm
particles. The calculations presented by Ahlm et al. (2016) show that
coagulation was also an important factor driving the growth of the larger
particles. The composition of the smaller clusters and particles could
therefore be reflected in the composition of the growing particles, if the
timescale of the growth was significantly shorter than the equilibration
timescale of the particles with the gas-phase bases (such as would occur for
high-viscosity, non-aqueous particles). However, there is no evidence
indicating an enhancement of NH4+ relative to DMAH+ in smaller
particles and clusters, and, according to present understanding, most
particles should have reached thermodynamic equilibrium in the chamber before
sampling.
Experimental errors that would affect these interpretations are limited to
differential measurement sensitivity between DMAH+ and NH4+ in
the particle phase and between DMA and NH3 in the total (mostly gas-phase) observations. In the case of the TDCIMS particle-phase measurements,
this could arise from a sampling artifact involving either preferential loss
of DMA relative to ammonia or contamination of the sample with ammonia. DMA
is expected to form a more stable salt with sulfuric acid than ammonia,
making the former possibility unlikely. Calibrations with known standards
also show no relative deficiency of the DMA signal (see Sect. 3.1).
Significant contamination of the particles by ammonia within the TDCIMS is
also unlikely given the insensitivity of the measured base : acid ratios
and DMAH+ : NH4+ ratios to the ammonium background measured by
the TDCIMS. Ammonium backgrounds were over an order of magnitude higher when
H2 gas was added, but the particle-phase signals were not measurably
influenced (Figs. 9 and 10). Furthermore, when H2 gas was not added to
the TDCIMS inlet, most of the background ammonium signal came from the
chamber. After the high-temperature chamber cleaning, which did not affect
the TDCIMS (it was disconnected), the ammonium background went down by a
factor of 5. This indicates that most of the ammonia that was available to be
picked up by particles was provided by desorption from the chamber walls, not
by contamination within the instrument. The gas-phase DMA observations could
have been overestimated with respect to ammonia if the DMA was preferentially
in the particle phase and ammonia was not, and if the IC
system was sensitive to both particle-phase and gas-phase DMA. The TDCIMS
measurements are not sensitive to gas-phase compounds, however, and they
indicate that there was not preferential particle uptake of DMA compared with
ammonia. Nonetheless, for the larger mass collections, a significant fraction
of both the DMA and NH3 present in the chamber was present in the
particle phase.
To summarize, the DMAH+ : NH4+ ratios in the particles appear
to be lower than expected based on thermodynamic equilibrium calculations.
However, there is significant uncertainty in the actual gas-phase mixing
ratios of DMA and ammonia in the chamber, and it is also possible that the
TDCIMS showed differential sensitivity with respect to DMAH+ and
NH4+ in the salts that constituted the particles in the chamber.
Given these uncertainties, we cannot exclude the possibility of no deviation
in the DMAH+ : NH4+ ratios from thermodynamic expectations.
However, the observed base : acid ratios in the particles were far from
their expected values, as discussed below.
Particle-phase base : acid ratios
The TDCIMS results indicate a base : acid ratio below 0.5 for particles
10 nm and above, despite a significant excess of gas-phase DMA and ammonia
with respect to sulfuric acid. This is counter to the expectation that the
particles should reach thermodynamic equilibrium and be closer to pH-neutral,
with more ammonium and dimethylaminium ions than sulfate ions (Ahlm et al.,
2016). This discrepancy may be explained by uncertainties in the
thermodynamics of small particles, but oxidative chemistry may also play a
role in this case. Possible experimental biases are discussed below as well.
As mentioned previously, CLOUD measurements and numerical calculations of
cluster and nanoparticle growth show that cluster–cluster, cluster–particle,
and particle–particle collisions are important drivers of growth in the
DMA–sulfuric acid system (Lehtipalo et al., 2016; Ahlm et al., 2016). The
composition of clusters could therefore be important for determining the
composition of > 10 nm particles for timescales shorter than the
equilibration timescales of the particles. Positive ion APi-TOF mass
spectrometer measurements indicate that up to the maximum
observable sizes for this technique (∼ 2 nm), the composition of
ammonia–DMA–sulfuric acid clusters may be acidic for the smallest clusters
but approaches a 1.2 : 1 base : acid composition (Bianchi et al., 2014;
Fig. 10). It seems possible that the least-neutralized (most acidic) clusters
could have the highest accommodation coefficients and therefore be the most
important clusters for nanoparticle growth, but we could find no evidence to
support this in the literature. This hypothesis could explain why the
observed > 5 nm particles are more acidic than most of the observed
< 2 nm clusters. The most effective clusters may have shorter lifetimes
to accommodation and may therefore be less prominent in the APi-TOF spectra.
It should be noted that equilibrium cluster distribution measured by the
APi-TOF gives essentially no information about the relative importance of the
different clusters for nanoparticle growth due to a lack of knowledge about
the cluster formation rates and lifetimes. In any case, it appears clear that
the growth of particles above 5 nm is not due to a stepwise addition of
H2SO4 and DMA molecules as appears to be the case for the smallest
clusters.
In view of the apparent production of H2SO4 in the UPCs, it is
necessary to consider the possibility that the low base : acid ratios
resulted from contamination of the particles by sulfuric acid within the
instrument. There are two distinct periods when such contamination could have
occurred: (1) while the particles were within the unipolar chargers and
(2) while the particles rested on the precipitator wire after being
collected. Significant growth in the first, brief period can be excluded by
comparing the appearance times of particles of a given size in the chamber
with those for the particles exiting the UPCs, as measured with the
differential mobility analyzer and CPC downstream of the UPCs. There was no
discernible growth during this period. Addition of compounds to the
precipitator wire should be inefficient due to the flow of N2 sheath gas
around it. Such contamination does nonetheless occur, but its effects are in
general subtracted out by the background correction. A scenario in which this
background correction would not be sufficient would be if the gas-phase
H2SO4 were more efficiently incorporated into sampled particles on
the wire than onto a particle-free wire. Figure 3 suggests that this was not
a significant issue, since there was no evidence for increased sulfate signal
in chamber samples with respect to calibration standards of similar sampled
mass. The estimation of collected particle mass cannot be influenced by
processes occurring after collection onto the wire.
The TDCIMS data overall indicate a large increase of base fraction as the
particles grew from 10 to 20 nm (Fig. 10), though variability among
individual experiments was high for ∼ 10 nm particle composition. The
direction of this composition change over this range of diameter is
corroborated by aerosol hygroscopicity results from the same experiments.
These measurements showed that 15 nm particles had significantly lower
hygroscopicity than 10 nm particles. Kim et al. (2016) estimated a
base : acid ratio of roughly 1.0 for 15 nm particles and 0.3 for 10 nm
particles, based on the measured hygroscopicities. Our results indicate even
lower ratios. Given the uncertainty of TDCIMS sensitivities to very acidic
particles, the TDCIMS ion ratio results are estimates below a base : acid
ratio of ∼ 1.0. The differences in the absolute base : acid ratio
between the TDCIMS and HTDMA-based measurements could be related to the
handling of the particles during sampling. In the case of the TDCIMS
observations, the sampled particles were exposed to clean N2 gas for up
to 30 min. This is unlikely to introduce much contamination but may result
in the loss of molecules that do not form a stable salt while the particles
dry in the N2 flow. The final composition and phase of the drying
particles is uncertain and probably depends on size. However, the many
calibrations performed during and after CLOUD7 indicate that the TDCIMS
analysis does not show a strong tendency to lose bases prior to analysis for
ammonium or dimethylaminium sulfate aerosol (see Sect. 3.1). That the
base : acid ratio is not strongly biased within the instrument is shown by
two observations of very different particle-phase base : acid ratios from
most of the DMA–H2SO4 experiments described here: (a) we measured
calibration aerosol with much higher base : acid ratios than the aerosol
formed in the experiments described here (see Sect. 3.1); (b) when we
exposed the particles to some air of unknown composition by size-selecting
them in one experiment, we contaminated the particles with ammonium and found
the base : acid ratio to increase (see Sect. 4). The operation of the HTDMA
instrument depends on the addition of humidified and dried HEPA-filtered air.
It seems likely that these observations of base : acid ratio could be
biased higher than the TDCIMS observations due to contamination by ammonia.
In any case, the independent observations of particle chemistry and physical
properties provided by the TDCIMS and HTDMA agree that particles at
10–20 nm sizes had a substantially larger base ion fraction than particles
near 10 nm.
The large change in particle composition around 10 nm indicates a sharp
change in particle compositional growth. This could result from a phase
change from a purely liquid state to a mixed-phase state with a solid core.
Recent results have shown that the phase state of nanoparticles is
size dependent, such that particles are expected to be completely liquid
below some size threshold, even for supersaturated droplets (Cheng et al.,
2015). The location of this transition depends on particle size, composition,
water content, and temperature, and it was demonstrated to occur for AS nanoparticles at around 10 nm at 298 K for a particulate AS mass fraction of 0.63 (Cheng et al., 2015). At the slightly lower
temperature in the present study (273 K), this transition occurs at a
slightly smaller particle diameter, closer to 9 nm. For the
dimethylammonium–ammonium–sulfate nanoparticles studied here, a similar
size-dependent transition seems plausible. However, less is known about the
thermodynamics of these mixed ammonium–aminium sulfate salts. Even for large
droplets, where size-dependent effects are not important for phase state,
pure dimethylaminium sulfate does not form a solid phase, even at
< 3 % RH. However, the exposure of such an amorphous liquid droplet
to ammonia results in the formation of a solid phase which contains ammonium
and aminium and is resistant to further exchange (Chan and Chan, 2013). If a
phase transition does occur for the mixed ammonium–dimethylaminium–sulfate
nanoparticles in the present study, the molar concentrations of the species
present in the smaller, purely liquid particles would be higher than those in
the liquid phase of the slightly larger mixed-phase particles. This could
explain the strong difference between 10 nm and larger particles, through a
few possible mechanisms. There may be an enhancement of interfacial and/or
aqueous-phase reaction rates for the smaller particles relative to the
larger, phase-separated particles. In addition, base molecules present in the
aqueous phase are more likely to be lost as the particles evaporate during
sampling than base molecules held in salts in a solid phase, which must
conform to a regular repeating chemical structure. Therefore, larger
phase-separated particles may be better able to maintain their base molecules
when sampled by the TDCIMS. However, the HTDMA measurements also show a lower
base : acid ratio in the 10 nm particles, and sampled particles in this
instrument do not experience a long period in dry, clean air during which
base molecules can slowly evaporate. Overall, the evidence suggests that the
apparent high acidity of the small particles is a feature of the chamber
particles and not an artifact of the experimental methods.
We can also consider whether the high chamber SO2 mixing ratios could
have led to low base : acid ratios in the particles by rapid sulfate
production which outstripped the ability of the gas-phase bases to neutralize
the nanoparticles. Ahlm et al. (2016) found that for sub-10 nm particles,
observed growth rates were a factor of 2 to 5 times as large as predicted
when considering growth by H2SO4 condensation, cluster collisions,
and coagulation with other nanoparticles. This is consistent with the
possibility that additional chemistry played a role in nanoparticle growth in
this system. SO2, or S(IV), can be oxidized to sulfate in aqueous
droplets by H2O2 and O3 (e.g., Caffrey et al., 2001; Hoyle et
al., 2016). We are aware of only one study examining the potential for known
aqueous-phase S(IV) oxidation reactions to cause significant particle growth
in nanoparticles (Kerminen et al., 2000). This numerical study showed that
known pathways cannot result in enough growth for newly formed nanoparticles
to reach CCN size, given typical tropospheric particle lifetimes and oxidant
and SO2 mixing ratios of 100 ppt. However, the SO2 mixing ratio in
the present experiments was 63 ppb, typical of pollution plumes, and there
was no larger aerosol mode to act as a coagulation sink for the small
particles and thereby limit their growth. Furthermore, H2O2 levels
were also probably higher than typical ambient levels, as this species is the
main HO2 radical sink under the CLOUD chamber conditions of extremely
low NOx. For particles of pH < 4 and for low liquid water content, as
in the case of the recently formed particles in this study, H2O2 is
the dominant oxidant of S(IV) in the atmosphere. To estimate the potential of
aqueous-phase S(IV) oxidation to contribute to nanoparticle growth, we
considered the rate of H2O2+ S(IV) reaction in a 5.4 nm
diameter ammonium bisulfate particle as a proxy for the chamber particles. We
used the Extended Aerosol Thermodynamics Model (E-AIM) to estimate the pH and
assumed that [SO2] and [H2O2] were in Henry's law equilibrium
with the gas phase. For a gas-phase H2O2 level of 1 ppb, S(IV)
oxidation was estimated at about 0.002 molecules s-1 for the small
particle. For comparison, H2SO4 collisions with the particle would
occur at a rate of about 0.2 molecules s-1 for an H2SO4
concentration of 1 × 107 cm-3, with still larger
contributions from cluster collisions and coagulation. This aqueous chemistry
was therefore an insignificant contributor to the particle growth and
composition.
However, in addition to the well-known aqueous-phase S(IV) oxidation, a
recent study has demonstrated an extremely rapid surface reaction of SO2
on acidic micro-droplets to produce sulfate and other oxidized sulfur species
in the condensed phase, in the absence of any added oxidants other than
O2 (Hung and Hoffman, 2015). The observed process was fastest for
droplets with a pH ∼ 3, for which the reaction rate was about 4 orders
of magnitude faster than S(IV) oxidation for typical ambient H2O2
levels. Several recent studies indicate a likely role for such heterogeneous
SO2 conversion to sulfate in major haze–fog events in polluted Chinese
megacities (e.g., Xue et al., 2016, and references therein). Since the
SO2 mixing ratio was over 3 orders of magnitude larger than the DMA
mixing ratio, the rates of nanoparticle collisions with SO2 were more
frequent than collisions with DMA of NH3 by a similar or even much
greater factor, when considering that a significant fraction of the nitrogen
bases present in the chamber were likely bound in the particle phase. Sulfur
dioxide, by contrast, should be > 99.99 % in the gas phase given the
high mixing ratio added to the chamber, sulfur dioxide's relatively low
solubility, and the low aerosol loading in these experiments. If 0.05 %
of SO2 collisions with a 5 nm particle irreversibly formed particle-phase sulfate, even the largest discrepancies between predicted and observed
growth rates could be reconciled. This hypothetical rate would be about 3
times as fast as nitrogen base collisions with the same particle if there
were 10 ppt of the nitrogen base (ammonia or DMA) in the gas phase, which is
within the range of possibility. Under these conditions, the particle-phase
sulfate production rate could exceed the rate at which nitrogen bases would
be able to neutralize the particles. Therefore we consider it likely
that the unexpectedly low base : acid ratios are explained by a rapid
surface reaction of SO2.
Implications for ambient nanoparticles
Newly formed nanoparticles in the planetary boundary layer are unlikely to
contain only sulfuric acid and nitrogen bases once they reach the sizes
considered in this study. However, by observing this chemically simple
system, it was possible to probe what appears to be a rapid heterogeneous
reaction which may play a role in some atmospheric regions. If the inferred
reaction probability for SO2 is applicable in other circumstances, then
this reaction is competitive with H2SO4 uptake when the SO2
concentration is about a factor of 2000 times larger than the H2SO4
concentration (e.g., about 1 ppb SO2 and
1 × 107 cm-3 at sea level). This is most likely to occur
at night in regions with strong SO2 emissions, when short-lived,
photochemically produced H2SO4 is at low concentrations but
SO2 levels remain high. This is consistent with the findings of Xue et
al. (2016), who show that a diurnally unvarying SO2–surface reaction is
likely an important contributor to secondary aerosol formation in haze–fog
events in Chinese megacities and that it represents a significant fraction
(about a third) of the sulfate production at night. SO2 levels for these
conditions were similar to the present study, in the range of tens of ppb.
For comparison, if we assume a 50 ppb SO2 mixing ratio and a particle
distribution for which the available particle-phase surface area is
represented by 100 nm diameter condensation nuclei with a concentration of
5000 cm-3, a 0.05 % SO2–surface reaction probability leads to
a 5 µg m-3 sulfate production rate, which is of the right
order of magnitude to explain the polluted megacity observations. However,
this surface mechanism is much less likely to be important for new particle
formation in remote, unpolluted areas. For example, at the well-studied
Hyytiälä site in the Finnish boreal forest, SO2 concentrations
around 0.1 ppb are typical. At this low level, the SO2–surface reaction
could only contribute about 0.1 nm h-1 to growth, which is a small
fraction of typical growth rates observed at this site (Yli-Juuti et al.,
2011).
Conclusions
Nanoparticles were formed by reactions involving sulfuric acid, DMA, water vapor, and contaminant ammonia. DMA is the more effective
stabilizing base for sulfuric acid during the initial nucleation and growth
to at least 2 nm, but at sizes around 10 nm and greater, ammonia was taken
up into the particles with comparable or even greater efficiency. Given the
greater abundance of ammonia than amines in most atmospheric regions, ammonia
may be more important than DMA for forming particulate sulfate salts in
growing nanoparticles. The sulfuric acid particles were not observed to be
fully neutralized by bases, despite the presence in the gas phase of at least
an order of magnitude more DMA and NH3 than H2SO4. For many
particle collections at the smallest measurable particle sizes (around
10 nm), the base : acid ratio was a few times lower than for
∼ 20 nm particles. This could be the result of a phase transition, in
which particles moved from a supersaturated liquid to a phase-separated
particle with a solid core as they grow. The base ions (ammonium and
dimethylaminium) may have been maintained more stably in the solid phase of
the larger particles upon sampling, and we also consider it possible that the
chemistry of particle growth was different for the particles of different
phase states due to the different ion concentrations in their mobile phases.
However, further work is needed to characterize the phase states of mixed
ammonium–aminium–sulfate nanoparticles to assess these possibilities and to
understand the growth pathways of such particles. The very low particulate
base : acid ratios observed do not have an unequivocal explanation, but we
propose that the sampled particles were not at thermodynamic equilibrium with
the gas-phase vapors due to a rapid SO2–surface reaction. These
observations suggest that effective stabilization of sulfuric acid in
nanoparticles requires fewer base molecules than would be in a 2 : 1 or
even a 1 : 1 salt. The very low base : acid ratios observed in these
laboratory-generated particles may not be expected during new particle
formation under more typical ambient conditions of lower SO2 and higher
NOx.
Data availability
Particle composition and size distribution data are available through contact
with the authors (mlawler@uci.edu).
Acknowledgements
We would like to thank CERN for supporting CLOUD with important technical and
financial resources, and for providing a particle beam from the CERN Proton
Synchrotron. This research has received funding from the EC Seventh Framework
Programme (Marie Curie Initial Training Network “CLOUD-ITN” grant
no. 215072, the ERC-Advanced grant “ATMNUCLE” (no. 227463), the German
Federal Ministry of Education and Research (project no. 01LK0902A), the Swiss
National Science Foundation (project nos. 206621 125025 and 206620 130527),
the Academy of Finland Centre of Excellence program (project no. 1118615),
Academy of Finland (project no. 138951), the Austrian Science Fund (FWF;
project no. J3198-N21), the Portuguese Foundation for Science and Technology
(project no. CERN/FP/116387/2010), the US National Science Foundation, and
the Russian Foundation for Basic Research (grant N08-02-91006-CERN).
James N. Smith acknowledges funding from the Finnish Academy (Grant
No. 251007) and US Department of Energy (grant no. DE-SC0014469). The
National Center for Atmospheric Research is sponsored by the National Science
Foundation. Edited by: J.
Liggio Reviewed by: two anonymous referees
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