ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-12513-2016Chemical ionization of clusters formed from sulfuric acid and dimethylamine
or diaminesJenCoty N.jenco@berkeley.eduhttps://orcid.org/0000-0002-3633-4614ZhaoJunhttps://orcid.org/0000-0001-9954-9471McMurryPeter H.HansonDavid R.Department of Mechanical Engineering, University of Minnesota, Twin
Cities, 111 Church St. SE, Minneapolis, MN 55455, USADepartment of Chemistry, Augsburg College, 2211 Riverside Ave.,
Minneapolis, MN 55454, USAnow at: Department of Environmental Science, Policy, and Management,
University of California, Berkeley, Hilgard Hall, Berkeley, CA 94720, USAnow at: Institute of Earth Climate and Environment System, Sun Yat-sen
University, 135 West Xingang Road, 510275 Guangzhou, ChinaCoty N. Jen (jenco@berkeley.edu)7October2016161912513125299June20161July201613September201621September2016This 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/16/12513/2016/acp-16-12513-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/12513/2016/acp-16-12513-2016.pdf
Chemical ionization (CI) mass spectrometers are used to
study atmospheric nucleation by detecting clusters produced by reactions of
sulfuric acid and various basic gases. These instruments typically use
nitrate to deprotonate and thus chemically ionize the clusters. In this
study, we compare cluster concentrations measured using either nitrate or
acetate. Clusters were formed in a flow reactor from vapors of sulfuric acid
and dimethylamine, ethylene diamine, tetramethylethylene diamine, or
butanediamine (also known as putrescine). These comparisons show that
nitrate is unable to chemically ionize clusters with high base content. In
addition, we vary the ion–molecule reaction time to probe ion processes
which include proton-transfer, ion–molecule clustering, and decomposition of
ions. Ion decomposition upon deprotonation by acetate/nitrate was observed.
More studies are needed to quantify to what extent ion decomposition affects
observed cluster content and concentrations, especially those chemically
ionized with acetate since it deprotonates more types of clusters than
nitrate.
Model calculations of the neutral and ion cluster formation pathways are
also presented to better identify the cluster types that are not efficiently
deprotonated by nitrate. Comparison of model and measured clusters indicate
that sulfuric acid dimers with two diamines and sulfuric acid trimers with two
or more base molecules are not efficiently chemical ionized by nitrate. We
conclude that acetate CI provides better information on cluster abundancies
and their base content than nitrate CI.
Introduction
Atmospheric nucleation is an important source of global atmospheric
particles
(IPCC,
2014). In the atmospheric boundary layer, sulfuric acid often participates
in nucleation (Weber et al., 1996; Kuang et al., 2008; Kulmala et al.,
2004; Riipinen et al., 2007) by reacting with other trace compounds to
produce stable, electrically neutral molecular clusters; these compounds
include ammonia (Kirkby et al., 2011; Coffman and Hegg, 1995; Ball et
al., 1999), amines (Almeida et al., 2013; Zhao et al., 2011; Glasoe et
al., 2015), water (Leopold, 2011), and oxidized organics
(Schobesberger et al., 2013). The primary instruments used for detecting
freshly nucleated, sulfuric-acid-containing clusters are atmospheric-pressure chemical ionization mass spectrometers (CIMS) such as the Cluster
CIMS (Zhao et al., 2010; Chen et al., 2012) and the chemical ionization (CI) atmospheric-pressure interface–time-of-flight mass spectrometer (CI-APi-ToF; Jokinen et al., 2012). Both mass spectrometers use
nitrate to chemically ionize neutral sulfuric acid clusters. Depending upon
conditions, NO3- core ions generally have one or more HNO3
and possibly several H2O ligands The signal ratio of the ion cluster to
the reagent ion translates to the neutral cluster concentration
(Berresheim et al., 2000; Hanson and Eisele, 2002; Eisele and Hanson,
2000).
The amounts and types of ions detected by the mass spectrometer are affected
by four key processes: the abundance of neutral clusters, their ability to
be chemically ionized, product ion decomposition, and clustering reactions
of the product ions (ion-induced clustering, IIC). The first process,
neutral cluster formation, follows a sequence of acid–base reactions
(Chen et al., 2012; Jen et al., 2014; Almeida et al., 2013; McGrath et
al., 2012) whereby sulfuric acid vapor and its subsequent clusters react
with basic molecules to produce clusters that are more stable than aqueous
sulfuric acid clusters. The concentration of a specific cluster type depends
on its stability (i.e., evaporation rates of the neutral cluster) and the
concentrations of precursor vapors (i.e., the formation rate).
Neutral clusters then need to be ionized to be detected with a mass
spectrometer. In most prior work, this has been accomplished by chemical
ionization with the nitrate ion whereby the neutral clusters are exposed to
nitrate for a set amount of time known as the chemical ionization reaction
time (or ion–molecule reaction time). CI can be
conceptualized as another acid–base reaction where an acid (sulfuric acid)
donates a proton to the basic reagent ion (nitrate, the conjugate base of
nitric acid). To illustrate, the CI reaction of an aminated sulfuric acid
dimer, (H2SO4)2⚫ DMA, is shown in
Reaction (R1).
H2SO42⚫DMA⚫H2Ox+HNO3⚫NO3-→k2HSO4-⚫H2SO4⚫DMA+2HNO3+xH2O
This dimer of sulfuric acid contains a dimethylamine (DMA) molecule and
x water molecules. At room temperature, water molecules evaporate
upon ionization or entering the vacuum region and are assumed to not
significantly affect chemical ionization rates. The forward rate constant,
k2, is assumed to be the collisional rate coefficient of 1.9 × 10-9 cm3 s-1 (Su and Bowers, 1973), while the reverse rate
constant is 0.
Reaction (R1) can be extended to CI reactions for larger neutral clusters of
sulfuric acid, with the assumption that every collision between nitrate and
a sulfuric acid cluster results in an ionized cluster. However, Hanson and
Eisele (2002) presented evidence that some clusters of
sulfuric acid and ammonia were not amenable to ionization by
(HNO3)1-2⚫ NO3-. Acetate CI has been used
previously to detect organic acids less acidic than sulfuric acid in the
atmosphere, providing evidence that its higher proton affinity could
chemically ionize more basic clusters (Veres et al., 2008).
Subsequently, Jen et al. (2015) showed that CI with
(HNO3)1-2⚫ NO3- leads to significantly lower
neutral concentrations of clusters with three or more sulfuric acid molecules
and varying numbers of DMA molecules compared to results using acetate
reagent ions. Furthermore, neutral cluster concentrations detected using
acetate CI are in overall better agreement with values measured using a
diethylene glycol mobility particle sizer (DEG MPS). As no other
experimental conditions changed except the CI reagent ion, we hypothesized
that nitrate's proton affinity, which is lower than that of acetate, renders it less
able to chemically ionize clusters that contain nearly equal amounts of
sulfuric acid and base. Poor CI efficiency reduces the amount and types of
ions detected by the mass spectrometer.
After neutral clusters are ionized, the resulting ion may decompose.
Experimental studies have shown ion decomposition in the ammonia–sulfuric
acid system at 275 K (Hanson and Eisele, 2002), and
computational chemistry studies present evaporation rates of ion clusters of
sulfuric acid with various bases on the order of the CI reaction time used
here (Kurtén et al., 2011; Lovejoy and Curtius, 2001; Ortega et al.,
2014). For example, these studies predict an evaporation rate, Ed (Reaction R2), of DMA from a sulfuric acid dimer ion with one DMA molecule of
∼ 100 s-1 at 298 K (Ortega et al.,
2014).
HSO4-⚫H2SO4⚫DMA⟶EdHSO4-⚫H2SO4+DMA
Experimental observations at room temperature have never seen the aminated
sulfuric acid dimer ion, even at CI reaction times as short as a few milliseconds.
Thus, the decomposition rate is likely even faster than the computed value
of ∼ 100 s-1 at 298 K (Ortega et
al., 2014).
Ion clusters can also be produced by ion-induced clustering (IIC) whereby
the bisulfate ion (HSO4-), formed by CI of sulfuric acid
monomer, further reacts with H2SO4 (with
ligands) and larger clusters. Charged clusters can also cluster with
neutrals to form larger ion clusters. The signal due to these IIC products
must be subtracted from the observed signals to determine neutral cluster
concentrations. Specifically, the sulfuric dimer ion can be formed via the
IIC pathway given in Reaction (R3), with ligands not
shown.
HSO4-+H2SO4⟶k21HSO4-⚫H2SO4
The forward rate constant, k21, is the collisional rate constant of
2 × 10-9 cm3 s-1 because this reaction involves switching
ligands between the two clusters. Both reactants also contain water,
nitrate, and/or base ligands that detach during measurement. The IIC-produced
dimer signal interferes with the CI-detected neutral dimer but can be
calculated from measured sulfuric acid vapor concentrations and CI reaction
times (Chen et al., 2012; Hanson and Eisele, 2002).
IIC can also produce larger clusters, but in general its contribution is
less than for the dimer, even if all rates are assumed to be collisional.
Furthermore, bisulfate may not efficiently cluster with chemically
neutralized sulfate salt clusters formed by reactions of sulfuric acid and
basic compounds. If so, assuming the collisional rate constant for all
IIC-type reactions would lead to an overcorrection of the neutral cluster
concentrations.
Measured CIMS signals reflect the combined influences of all these
processes, with each occurring on timescales that depend on the chemistry,
experimental parameters, and techniques. Assuming a process is either
dominant or negligible can lead to large errors in reported neutral cluster
compositions and concentrations. Here, neutral cluster formation, chemical
ionization, IIC, and ion decomposition are examined experimentally and
theoretically to determine the influence of each process on the abundance of
ion clusters composed of sulfuric acid and various bases. These bases
include DMA, ethylene diamine (EDA), trimethylethylene diamine (TMEDA), and
butanediamine (also known as putrescine, Put). The diamines, recently
implicated in atmospheric nucleation, react with sulfuric acid vapors to
very effectively produce particles compared to monoamines (Jen et
al., 2016). We present observations that (1) show a clear difference between
acetate and nitrate CI for all clusters larger than the sulfuric acid dimer
with any of the bases, (2) provide evidence of ion decomposition, and (3) identify specific bases that influence the detectability of the dimer
neutral clusters. Also presented are modeling results that help elucidate
specific processes that influence measurement: neutral cluster formation
pathways, cluster types that do not undergo nitrate CI, and clusters that
are formed by IIC.
Method
Sulfuric acid clusters containing either DMA, EDA, TMEDA, or Put were
produced in a flow reactor that allows for highly repeatable observations
(see Jen et al., 2014; Glasoe et al., 2015). Glasoe et al. (2015) showed that the system has a
high cleanliness level: 1 ppqv level or below for amines. Each amine was
injected into the flow reactor at a point to yield ∼ 3 s
reaction time between the amine and sulfuric acid (see Jen et al., 2014, for a schematic). The initial sulfuric acid concentration
([A1]o) before reaction with basic gas was controlled at specified
concentrations. The base concentration, [B], was measured by the Cluster
CIMS in positive ion mode (see the Supplement of Jen et al., 2014, for further
details) and confirmed with calculated concentrations (Zollner et al.,
2012; Freshour et al., 2014). The dilute amines were produced by passing
clean nitrogen gas over either a permeation tube (for DMA and EDA) or a
liquid reservoir (TMEDA and Put) and further diluted in a process described
in Zollner et al. (2012). The temperature of the flow
reactor was held constant throughout an experiment but varied day-to-day
from 296 to 303 K to match room temperature. This was done to minimize thermal
convection, which induces swirling near the Cluster CIMS sampling region. The
relative humidity was maintained at ∼ 30 %, and measurements
were done at ambient pressure (∼ 0.97 atm). The total reactor
N2 flow rate was 4.0 L min-1 at standard conditions of 273 K and 1 atm.
(a, c) Comparison of specific cluster concentrations
([Am⚫ Bj]) using acetate (red squares) and nitrate
(black triangles) reagent ions at two different [DMA] and a constant initial
sulfuric acid concentration, [A1]o∼ 4 × 109 cm-3. Each cluster species is shown at its ion mass. The brackets
represent the number of DMA molecules in a cluster with a given number of
sulfuric acid. The half-filled symbols show the tetramers, and the outlined
symbols are the pentamers. Bar graphs (b) and (d) compare total cluster
concentration of a given size ([Nm]) between acetate (red) and nitrate
(black) for the same [DMA] and [A1]o as (a) and (b), respectively.
Two types of experiments were conducted: one set where specific base, base
concentration ([B]), and [A1]o were varied at constant CI reaction
time (similar to those in Jen et al., 2014) and the second
set where CI reaction time was varied for a subset of reactant conditions
(see Hanson and Eisele, 2002; Zhao et al., 2010). The resulting concentrations were measured with the
Cluster CIMS using either nitrate or acetate as the CI reagent ion. Nitrate
and acetate were produced by passing either nitric acid or acetic anhydride
vapor over Po-210 sources. Separate Po-210 sources and gas lines were used
for the acetate and nitrate to avoid cross-contamination. The measured
reagent ions for nitrate CI was (HNO3)1-2⚫ NO3-, and the reagent ions for acetate CI were
H2O ⚫ CH3CO2-, CH3CO2H ⚫ CH3CO2-,
and CH3CO2- (in order of abundance). The nitrate dimer and
trimer are assumed to chemically ionize at equal rate constants, and the
three acetate ions are assumed to chemically ionize in identical manners.
The inferred neutral cluster concentrations were calculated from the CI
reaction time, measured and extrapolated mass-dependent sensitivity (see
the Supplement), and the assumed collisional rate constant between
CI ion and sulfuric acid clusters (see Jen et al., 2014,
2015, for a discussion on the data inversion process). The CI
reaction time, tCI, was determined from the inlet dimensions
and electric field strength inside the sampling region; for this set of
experiments, tCI was fixed at 18 ms for nitrate and 15 ms for
acetate.
Varying tCI at fixed [B] and [A1]o was achieved by
changing the electric field used to draw ions across the sample flow into
the inlet. Similar experiments have been performed with other atmospheric-pressure, CI mass spectrometer inlets (Hanson and Eisele, 2002; Zhao et
al., 2010; Chen et al., 2012) with the detailed mathematical relationship
between tCI and ion signal ratios developed more in depth in
the following sections and the Supplement.
Acetate vs. nitrate comparison
Figure 1a and c compare inferred cluster concentrations derived from
measured signals (assuming the collisional rate constant, kc, and no
ion breakup) using acetate (red squares) and nitrate (black triangles)
reagent ions at a constant [A1]o∼ 4 × 109 cm-3 for two different [DMA]. The grouped points represent
clusters that contain an equivalent number of sulfuric acid molecules (N1
is the monomer, N2 is the dimer, etc.) but with a different number of DMA
molecules (e.g., A4-⚫ DMA0-3, where A is sulfuric
acid). The number of base molecules in each cluster is given by the grouping
bracket. Since the tetramers and pentamers have similar mass ranges, N4
clusters are given as half-filled symbols and N5 clusters as outlined
symbols. Note that N1 is detected at different masses between the two
reagent ions, with nitrate at 160 amu = HSO4-⚫ HNO3
and acetate at 97 amu = HSO4-. The total cluster concentrations,
[Nm], compared between the two CI ions are shown in Fig. 1b and d.
The notation used here differs slightly from Jen et al. (2014)
such that [Nm] denotes the total concentration for clusters that
contain m sulfuric acids molecules (i.e.,
[Nm] = [Am]+[Am⚫ B1]+[Am⚫ B2]…) and Am⚫ Bj represents a specific
cluster type with m sulfuric acid molecules and j basic
molecules (B). The measured [N1] and [N2] obtained using nitrate
and acetate are in good agreement for DMA. In the set of bases studied in
Jen et al. (2014) (ammonia, methylamine, DMA, and trimethylamine),
DMA is the strongest clustering agent, and these results reaffirm the
accuracy of previously reported values of [N1] and [N2] in Jen et al. (2014) at high [A1]o.
(a, c) Comparison of specific cluster concentrations
([Am⚫ Bj]) using acetate (red squares) and nitrate
(black triangles) reagent ions at two different [EDA] and a constant initial
sulfuric acid concentration, [A1]o∼ 4 × 109 cm-3. Each cluster species is shown at its ion mass. The brackets
represent the number of EDA molecules in a cluster with a given number of
sulfuric acid. The half-filled symbols show the tetramers, outlined symbols
as the pentamers, and crossed symbols as hexamer. Bar graphs (b) and (d) compare
total cluster concentration of a given size ([Nm]) between acetate (red)
and nitrate (black) for the same [EDA] and [A1]o as (a) and
(b), respectively.
Figures 2, 3, and 4 show the acetate and nitrate comparison for EDA, TMEDA,
and Put, respectively. Although nitrate appears to consistently detect less
[N1] than acetate, the estimated systematic uncertainty of acetate-detected [N1] is higher than with nitrate due to higher background
signals detected by acetate, sensitivity to the low masses (see the Supplement), and the possible influence of diamines on the ion throughput in the mass
spectrometer. Other factors that may influence the detected [N1] are
discussed in the Supplement. The true acetate [N1] could be up to a factor of 5
lower. Therefore, for monomer clusters formed from diamines, it is difficult
to conclude that acetate and nitrate lead to significant differences in
measured [N1].
Unlike the other bases, Put was observed in the monomer using either nitrate
or acetate CI (Fig. 4). The presence of A1-⚫ Put
indicates that its binding energy must be higher than monomers containing the
other bases. However, this ion still decomposes in roughly the
tCI= 15 ms as it is ∼ 0.1 % of [N1]. Elm
et al. (2016) have shown that the binding energy of
A1⚫ EDA is -11.1 kcal mol-1 and A1⚫ Put is -15.4 kcal mol-1,
with A1⚫ DMA closely matching A1⚫ EDA at -11.38 kcal mol-1 (Nadykto et al., 2014; Bork et al., 2014). The
higher neutral binding energies of A1⚫ Put may translate to
stronger ion binding energies than the other aminated monomers, though more
studies are needed to confirm this. Both acetate and nitrate primarily
detect the bare dimer, with [N2] up to a factor of 5 higher with
acetate CI than nitrate. The systematic uncertainties of the acetate
measurement have similar reasons as those for [N1] and could lead
to a factor of 2–3 times lower [N2] than reported here. These
comparisons suggest that for clusters formed from diamines, nitrate does not
detect as many types of N2 as does acetate; however, the large
uncertainty in acetate [N2] prevents a definitive conclusion as to
whether or not nitrate chemically ionizes all types of dimers. More
information is gained from experiments that vary tCI as they
are more sensitive to the various formation pathways. These results are
presented in the subsequent sections.
(a, c) Comparison of specific cluster concentrations
([Am⚫ Bj]) using acetate (red squares) and nitrate
(black triangles) reagent ions at two different [TMEDA] and a constant initial
sulfuric acid concentration, [A1]o∼ 4 × 109 cm-3. Each cluster species is shown at its ion mass. The brackets
represent the number of TMEDA molecules in a cluster with a given number of
sulfuric acid. The half-filled symbols show the tetramers and outlined
symbols as the pentamers. Bar graphs (b) and (d) compare total cluster
concentration of a given size ([Nm]) between acetate (red) and nitrate
(black) for the same [TMEDA] and [A1]o as (a) and (b), respectively.
Figures 1 through 4b and d clearly show that more of the larger clusters
(N3 and higher) were detected by acetate CI than nitrate. For all
bases, the [N3] measured by acetate is 2 to 100 times higher than
concentrations measured by nitrate CI. Nitrate detected small amounts of
N4 and no N5, likely due to the ionizable fraction of [N4]
and [N5] falling below detection limits (< 105 cm-3).
In addition as [B] increases, the differences between acetate and nitrate
cluster concentrations become more pronounced. This likely occurs because
sulfuric acid clusters become more chemically neutral as [B] increases,
thereby decreasing their tendencies to donate protons to nitrate ions. The
differences between acetate- and nitrate-measured cluster concentrations
cannot be explained only by the larger uncertainties in the acetate
measurements. The systematic uncertainties in acetate-detected larger
clusters is at most a factor of 2 below reported concentrations. Thus,
acetate is more efficient than nitrate at chemically ionizing the larger
cluster population.
The large differences between nitrate- and acetate-measured [N3] and
[N4] provide information to better understand recent atmospheric and
chamber measurements. Chen et al. (2012) and Jiang et al. (2011) published [N3] and [N4] measured in the
atmosphere using a larger version of the Cluster CIMS (Zhao
et al., 2010). For both studies, the measurements were conducted using
nitrate CI and only at the clusters' bare masses (A3 and A4).
Trimers and tetramers may have been under-detected, though this is uncertain
because the atmosphere contains numerous compounds that may behave
differently than DMA and diamines. If the actual concentrations of trimers
and tetramers were higher than those reported by Jiang et al. (2011), then the fitted evaporation rate of
E3=0.4 ± 0.3 s-1 from Chen et al. (2012)
is too high and the true value would be closer to 0 s-1
(collision-controlled or kinetic limit) that was reported by Kürten et
al. (2014) at 278 K. In addition, Kürten et al. (2014) measured [N3]
and [N4] about a factor of 10 lower than the collision-controlled
limit. They attribute this discrepancy to decreased sensitivity to the
larger ions, but it could also be due to inefficient CI by nitrate.
(a, c) Comparison of specific cluster concentrations
([Am⚫ Bj]) using acetate (red squares) and nitrate
(black triangles) reagent ions at two different [Put] and a constant initial
sulfuric acid concentration, [A1]o∼ 4 × 109 cm-3. Each cluster species is shown at its ion mass. The brackets
represent the number of Put molecules in a cluster with a given number of
sulfuric acid. The half-filled symbols show the tetramers and outlined
symbols as the pentamers. Bar graphs (b) and (d) compare total cluster
concentration of a given size ([Nm]) between acetate (red) and nitrate
(black) for the same [Put] and [A1]o as (a) and (b), respectively.
Comparing our results to the CLOUD (Cosmics Leaving OUtdoor Droplets) experiments, the amount of clusters
detected via nitrate CI using the Cluster CIMS differ from those detected by
nitrate using the CI-APi-ToF (Kürten et al., 2014). They observed
more ion clusters that contained nearly equal number of sulfuric acid and
DMA molecules (e.g., A3⚫ DMA2). Our experiments suggest
that such highly neutralized clusters are not efficiently ionized by our
nitrate core ions. We do not fully understand this difference but longer
acid–base reaction times, the amount of ligands on the nitrate core ions,
various inlet designs (e.g., corona discharge vs. our Po-210 or high vs. our
low flow rates), temperature (278 K compared to our 300 K), and ion breakup
upon sampling may all play a role.
Chemical ionization efficiency clearly plays a role in both the types and
amounts of clusters that can be detected. However, the concentrations in
Figs. 1 through 4 were calculated by assuming negligible contributions of IIC
and ion decomposition. The validity of these assumptions was tested by
examining the ion behavior with CI reaction time (tCI) for a
variety of bases. Presented in the following sections are ion signal
variations with tCI and a discussion of possible scenarios
that explain these observations. To help understand these measurements, we
developed a model to describe these complex series of reactions that govern
neutral cluster formation, chemical ionization, IIC, and ion decomposition.
The model combines two box models: one for neutral cluster formation and one
for the ion processes. When compared to observations, the model was useful
in identifying the controlling process for the monomers and dimers but, due to
the numerous reactions, only provided general scenarios to explain
observations for the larger clusters.
Monomer, N1
Over the 3 s neutral reaction time in this flow reactor (i.e., the reaction
time between neutral sulfuric acid vapor and the basic gas), initial monomer
concentration ([N1]) is depleted as it forms larger clusters/particles
and is lost to walls; N1 may reenter the gas phase by evaporation of
larger clusters. Two types of N1 may have significant abundances in the
sulfuric acid and DMA system: A1 and A1⚫ DMA. One
computational chemistry study predicts that the latter has an evaporation rate of
10-2 s-1 (all computed rates at 298 K unless otherwise
stated; Ortega et al., 2012), with others suggesting
an evaporation rate closer to 10 s-1 (Nadykto et al., 2014; Bork et
al., 2014).
Following the neutral clustering reactions, the remaining monomer is readily
chemically ionized and the product ion can decompose and undergo IIC with
the monomer or clusters. For example, the decomposition rate of
A1-⚫ DMA is predicted to be 109 s-1 (Ortega et al., 2014). Therefore, whether or not
A1⚫ DMA is a significant fraction of the total monomer
concentration, A1- is the only ion with significant abundance.
This agrees with our experimental observations.
Neutral [N1] can be estimated from mass spectrometry signals because
there is negligible ion breakup in the Cluster CIMS that leads to
A1-. As discussed above, a number of experiments and the current
results have shown this to be the case (Hanson and Eisele, 2002; Eisele
and Hanson, 2000; Lovejoy and Bianco, 2000). The signal ratio of the
sulfuric acid monomer at 160 amu for nitrate (S160) to the nitrate ion
at 125 amu (S125) can be converted to neutral [N1] following Eq. (1) (Eisele and Hanson, 2000), where tCI is the CI
reaction time.
S160S125=k1N1tCI
For N1+HNO3⚫ NO3-, k1= 1.9 × 10-9 cm3 s-1 (Viggiano et al., 1997), which is assumed to not
depend on whether water or bases are attached to the monomer. Equation (1)
was derived for short tCI where reagent ion and neutral
N1 are not depleted. These assumptions are tenuous at long
tCI; however, the rigorous analytical solution to the
population balance equations (derived in the Supplement and given in Eq. S6) shows
that Eq. (1) is a good approximation: at tCI=15 or 18 ms,
the differences between Eqs. (1) and (S6) are ∼ 1 %.
Measured (a, b) and modeled (c, d) sulfuric acid monomer to nitrate
signal ratio (S160/S125) as a function of CI reaction time for DMA
(a, c) and EDA (b, d). The measurements were conducted with nitrate as the
reagent ion and at [A1]o∼ 4 × 109 cm-3. Each
color represents a different [B] with the linear regressions for the
measurements given in colored text.
Figure 5a and b show the signal ratios as a
function of tCI for DMA and EDA as detected by nitrate CI at
equivalent [A1]o= 4 × 109 cm-3. TMEDA and Put graphs look
very similar to EDA (see the Supplement). The green points shown in this figure and
subsequent figures provide measurements at a base concentration of 0 pptv from
eight different days and offer a useful guide for the measurement
uncertainty. For all base concentrations as tCI increases, more [N1] is chemically ionized, leading to higher
S160/S125. As [B] increases, the signal ratios and therefore the
slopes of the lines decrease. This indicates that [N1] is depleted
during the 3 s neutral reaction time via uptake into large clusters that
increase with [B].
Summary of possible pathways for neutral monomer formation and
chemical ionization.
Neutral formationNitrate CI and ion decompositionDMA and diaminesDMAA1+B↔A1.BA1+NO3-→kcHNO3⚫A1-A1⚫B+NO3-→kcHNO3⚫A1-⚫BHNO3⚫A1-⚫B→fastHNO3⚫A1-+BDiaminesA1+NO3-→kcHNO3⚫A1-A1⚫B+NO3-→?HNO3⚫A1-⚫BHNO3⚫A1-⚫B→fastHNO3⚫A1-+B
The model, as mentioned above, was used to interpret the results presented
in Fig. 5 and subsequent graphs. The neutral cluster concentrations after
[A1]o and [B] react over the 3 s neutral reaction time are modeled
first. This portion of the model also takes into account base dilution from
its injection point in the flow reactor (see Jen et al., 2014),
wall loss, and particle coagulation. However, the model does not take into
account the possible dilution of N1 by the base addition flow, which may
affect measured [N1] as explained in the Supplement. The neutral model is then
coupled to the ion model, which simulates chemical ionization and IIC. Ion
decomposition is implicitly included by assuming that certain cluster types
instantly decompose into the observed ion.
For the monomer, the model has identical neutral cluster formation pathways
for all sulfuric acid and base systems. The acetate vs. nitrate comparison
suggests that monomers containing various bases are chemically ionized
similarly, with a slight possibility that nitrate may not chemically ionize
sulfuric acid monomers that contain a diamine. The modeled reactions
pertaining to the monomer are given in Table 1, where kc is 2 × 10-9 cm3 s-1. The full list of
modeled reactions, including loss of monomer to form larger clusters, is
given in the Supplement.
Figure 5c and d display the modeled results for DMA and EDA at the same
[B] and [A1]o as the measurements presented in panels a and b. The
model predicts the linear dependence of S160/S125 on
tCI as seen in Eq. (1). In addition, the predicted values of
S160/S125 and their dependence on [B] are in good qualitative
agreement with observations. Including or excluding nitrate CI of
A1⚫ diamine has little effect on S160/S125 because
[B] is typically less than [A1]o in these experiments. As a
result, the majority of monomers will remain as A1 even if the
evaporation rate of the A1⚫ B (E1) is very small.
Further experiments that quantify the fraction of A1⚫ diamine
in N1 are needed to definitively conclude on the efficacy of nitrate in
chemically ionizing all N1.
Dimer, N2
Neutral dimers (N2) largely form by collision of the two types of
monomers (A1 and A1⚫ B) and, to a much lesser extent,
decomposition of larger clusters. For sulfuric acid + DMA, the N2
likely exists as A2⚫ DMA and A2⚫ DMA2,
with both clusters predicted to have low evaporation rates of
∼ 10-5 s-1 (Ortega et al.,
2012) with another study suggesting a higher evaporation rate of
A2⚫ DMA2 which is ∼ 104 times higher
(Leverentz et al., 2013). Chemically ionizing these dimers
results in ions that undergo IIC and ion decomposition. Computational
chemistry predicts that A2-⚫ DMA2 and
A2-⚫ DMA have DMA evaporation rates of 108
and 102 s-1, respectively (Ortega et al.,
2014). However, the computed evaporation rate of A2-⚫ DMA may be too low because during the 18 ms CI reaction time used here, all
N2 are detected as A2- (195 amu). Similarly, the diamine
molecule is lost from A2-⚫ diamine as all dimers were
detected as A2-.
Measured sulfuric acid dimer to monomer signal ratio
(S195/S160) as a function of tCI for DMA (a), EDA (b), and TMEDA (c)
measured by nitrate CI at [A1]o∼ 4 × 109 cm-3. The tables in (a)–(c) provide the measured
[A1] at that [B] after the 3 s acid–base reaction time. Observations
were fitted according to Eq. (2) with the y intercept shown by the dashed
line. Panels (d)–(f) present modeled results for each base.
A2- can also be created from IIC between A1- and N1
(see Reaction R2), which proceeds with a rate coefficient of k21.
Including both processes in the cluster balance equations leads to the ratio
of sulfuric acid dimer (195 amu) to monomer (160 amu) signal intensities
shown in Eq. (2). This relationship includes a time-independent term (the
tCI= 0 s intercept) that is proportional to the neutral dimer-to-monomer ratio in the sampled gas and a term due to IIC that increases
linearly with tCI (Chen et al., 2012; Hanson and Eisele,
2002).
S195S160=k2k1N2N1+12k21N1tCI
The rate constant, k21, is the collisional rate constant of 2 × 10-9 cm3 s-1. Equation (2) was also derived from the assumption of
short tIC. The relation for S195/S160 vs.
tCI for long tCI is also derived in the Supplement.
Equation (2) is a good approximation of the more rigorous solution even at
long tIC.
Figure 6a, b, and c show measured S195/S160
as a function of tCI for DMA, EDA, and TMEDA, respectively, as
detected by nitrate CI at [A1]o= 4 × 109 cm-3. Put is
similar to EDA and is presented in Fig. 7 (left). For all bases, increasing
the CI reaction time leads to more IIC dimers. The observed linear increase
in the S195/S160 ratio for all bases provides evidence for the
influence of IIC on dimer measurements (Eq. 2). However, the y intercepts
for DMA exhibit a pattern that is distinctly different from those observed
for the diamines, indicating different trends for the neutral-monomer-to-dimer concentration ratios. For DMA, the y intercept increases with
increasing [B]. This is due to higher concentrations of base depleting the
monomer and enhancing dimer concentrations. A different trend was observed
for the diamines, with the intercepts showing no clear dependence on diamine
concentration.
Measured dimer-to-monomer signal ratios (S195/S160 for
nitrate or S97 for acetate) as a function of CI reaction time using
nitrate (a) and acetate CI (c). In both cases, [A1]o was held
constant at 4 × 109 cm-3. Panel (b) shows the modeled results for
Put. The tables inside (a) and (c) provide the measured [A1] after
the 3 s acid–base reaction time.
There are a number of scenarios that could partly explain the diamine
trends. First, the neutral trimer evaporation rate(s) could be very low such
that the formation of trimer and larger clusters will deplete both [N2]
and [N1]. The A1 evaporation rate from A3⚫ DMA is
predicted to be ∼ 1 s-1 (Ortega et
al., 2012) and likely lower for clusters with diamines (Jen et
al., 2016). The second possibility is that A2- could be the
decomposition product of larger ions such as A3-⚫ diamine forming A2-+A1⚫ diamine. A third
possibility is that A2⚫ diamine2 cannot be readily
ionized by nitrate as compared to A2⚫ DMA2, possibly due
to differences in cluster configurations and dipole moments. As [diamine]
increases, the fraction of dimers containing two diamines increases,
resulting in a growing fraction of N2 that may not be ionizable by
nitrate. For example, the model predicts that [A2⚫ EDA] is 10 %
of [A2⚫ EDA2] when [EDA] = 90 pptv.
Summary of possible pathways for neutral and ion dimer formation.
Neutral formationNitrate CI and ion decomposition reactionsIIC reactions (only A1-)DMA, Put, EDADMAAll basesA1⚫B+A1→kA2⚫BA2⚫B+NO3-→kcA2-⚫B+HNO3A1-+A1→kcA2-A1⚫B+A1⚫B→kA2⚫B2A2-⚫B→fastA2-+BA1-+A1⚫B→kcA2-⚫BA2⚫B+B→kA2⚫B2A2⚫B2+NO3-→kcA2⚫B2-+HNO3A2⚫B2→E2BA2⚫B+BA2-⚫B2→fastA2-⚫BTMEDADiaminesA1⚫B+A1→kA2⚫BA2⚫B+NO3-→kcA2-⚫BA1⚫B+A1⚫BA2⚫B2A2-⚫B→fastA2-+BA2⚫B+BA2⚫B2A2⚫B2+NO3-A2-⚫B2
Summary of possible pathways for neutral and ion trimers formed from
sulfuric acid and DMA, excluding decomposition of tetramer and larger ions.
Neutral formationNitrate CI and ion decomposition reactionsIIC reactions (only A1-)A2⚫B+A1→kA3⚫BA3⚫B+NO3-→kcA3-⚫B+HNO3A2-+A1→kcA3-A3⚫B+B→kA3⚫B2A3-⚫B→EdA2-+A1⚫BA1-+A2⚫B→kcA3-⚫BA3⚫B2+B→kA3⚫B3A3⚫B3+NO3-A3-⚫B3+HNO3A2⚫B2+A1→kA3⚫B2A3⚫B2+NO3-A3-⚫B2+HNO3A2⚫B+A1⚫B→kA3⚫B2A2⚫B2+A1⚫B→kA3⚫B3
The dimer (S195)-to-monomer signal (S97) ratio for sulfuric
acid + Put dimers measured using acetate CI as a function of
tCI was examined to better understand which of these
explanations is the most relevant. As mentioned previously, acetate detects
the sulfuric acid monomer as 97 amu, but the detected dimer is at 195 amu
for both nitrate and acetate. Figure 7 shows the
ratio of these signals for Put between nitrate (a) and acetate (c). At
[Put] = 40 pptv, acetate shows a S195/S97y intercept 25 times
higher than the intercepts shown in the nitrate graph. The higher
y intercepts are most likely due to improved CI efficiency. A decreased
detection efficiency of 97 amu and an increased contribution due to
A3-⚫ diamine decomposition due to better CI of N3
by acetate may also contribute (although high [A3-⚫ diamine] in Fig. 4 suggests that these ions are stable enough during the acetate
tCI= 15 ms). More acetate results similar to Fig. 7c are
needed to draw a more definitive conclusion, but these comparisons do
suggest that dimers containing 1–2 diamines are not efficiently chemically
ionized by nitrate in these experiments.
The model adds more clarity on why N2-containing diamines behave
differently than DMA using nitrate CI. For DMA, the best fit to the
observations was achieved by assuming all clusters can undergo nitrate
ionization and can be formed by IIC. In addition, base evaporation rates
from A2⚫ B2 and sulfuric acid evaporation rates from the
trimer were set to 0 s-1; increasing these evaporation rates (up to 10
and 5 s-1) had little effect on the ratio trends. The
model also assumed that A3-⚫ B does not decompose into
A2-. Figure 6d shows modeling results for DMA. To reproduce
S195/S160 trends of EDA and Put, the model followed that of DMA
except that A2⚫ B2 cannot be ionized by nitrate. For TMEDA,
the model also assumed that A2⚫ TMEDA2 does not form. Modeled
results are shown in Fig. 6e and f for EDA and TMEDA, respectively, and
Fig. 7b for Put. The modeled pathways for N2 are listed in
Table 2. For all three diamines, we were unable to
reproduce the observations with other combinations of reactions and
evaporation rates. The model only matched the observed trends by turning off
the CI or formation of A2⚫ diamine2.
However, several of the modeled reactions are simplified versions of
multistep reactions. For example, preventing the formation of
A2⚫ TMEDA2 could also mean that A2⚫ TMEDA2 forms at the collision rate but instantly decomposes into
A2⚫ TMEDA. Furthermore, differences between DMA and diamine
observations could instead be explained by semi-efficient nitrate CI of
A2⚫ diamine because the existence of high [A2⚫ diamine2] is unlikely due to its high basicity. Preventing
A2⚫ diamine2 from forming and semi-efficient CI of
A2⚫ diamine could lead to identical results as shown
in the model for EDA and TMEDA. Additional thermochemical data (e.g., from
more targeted experiments and computational chemistry) are needed to better
inform the model. Regardless, our observations and modeling show that a dimer's neutral formation pathways and/or the nitrate CI differs between the
DMA and diamine systems.
The model also provides an estimate of the fraction of [A2-]
formed by IIC at tCI= 18 ms (used for the nitrate CI
experiments). For base concentration of 0 pptv, the model is very similar to
what was measured in Fig. 6, indicating that A2- is almost
completely formed by A1-+A1 (i.e., is an IIC artifact) and
not by the CI of A2. The abundance of A2 is low at 300 K
(Hanson and Lovejoy, 2006), below the detection limit of the Cluster
CIM. For DMA, IIC dimers typically account for 1 % (less at high [DMA]) of
the total dimer signal which agrees with the conclusions drawn in Jen et al. (2015). In contrast, the IIC fraction of A2- using
nitrate for EDA and Put is ∼ 50 %, due to the potentially
large fraction of N2 not undergoing chemical ionization. The nitrate ion's
inability to chemically ionize some of the dimers is further highlighted
since IIC is suppressed in the diamine system: less N1 is available
(due to the formation of larger clusters); thus, both [A1] and
[A1-] are depressed. IIC-produced A2- accounts for
∼ 20 % of the total dimer signal for TMEDA. However, these
numbers are uncertain due to the assumptions in the model and uncertainties
in the measurement. For instance, the model is not sensitive to whether
A1- can cluster with A1⚫ B, which would
significantly influence the amount of IIC dimer without significantly
affecting S195/S160. IIC contributes much less A2- when
acetate is used as the reagent ion because acetate detects up to 5 times
more total neutral dimer concentration ([N2]) than nitrate when base is
present. Acetate measurements show that IIC produced ∼ 3 %
of the [A2-] when [Put]=2 pptv and near 0 when [Put] = 40 pptv (Fig. 7c).
Trimer, N3
Neutral trimers (N3) are primarily formed by combining one of the two
types of monomers with one of the two types of dimers; evaporation of large
clusters also contributes. In the sulfuric acid+DMA system, computational
chemistry predicts that A3⚫ DMA2 and A3⚫ DMA3 are relatively stable, with A3⚫ DMA3
exhibiting the lowest evaporation rate (Ortega et
al., 2012). Also, A3⚫ DMA may be present in significant
amounts due to a high production rate via A2⚫ DMA+A1.
CI of N3 leads to ions such as (i) A3-⚫ DMA3, which evaporate at a rate of 104 s-1 into A3-⚫
DMA2, and (ii) A3-⚫ DMA2 and
A3-⚫ DMA, which have predicted DMA evaporation
rates of ∼ 10-1 and 10-2 s-1 (Ortega et al., 2014), respectively, resulting in
lifetimes comparable to tCI used here. From Fig. 1, nitrate CI
resulted in A3-⚫ DMA2 (only at [DMA] = 110 pptv),
A3-⚫ DMA, and A3-. The DMA-containing clusters
were detected to a much lesser extent than with acetate CI.
Measured bare sulfuric acid trimer to monomer signal ratio
(S293/S160) as a function of tCI for DMA (a),
EDA (b), and TMEDA (c) detected by nitrate CI at
[A1]o= 4 × 109 cm-3.
Acetate CI results help shed light on these processes with much higher
[A3-⚫ DMA1,2] than with nitrate CI (Fig. 1), which
could be due to the decomposition of larger ion clusters. The acetate CI results
depicted in Fig. 1 show that A3-⚫ DMA2 is the most
abundant type of trimer ion, suggesting that the dominant neutral clusters
are A3⚫ DMA2-3, with any A3-⚫ DMA3 quickly decomposing into A3-⚫ DMA2.
Neutral A3⚫ DMA3 is predicted by our model to be
dominant at high [DMA]. This picture is consistent with our postulate that
nitrate cannot ionize A3⚫ DMA3 (and also, possibly,
A3⚫ DMA2) and thus little A3-⚫ DMA1,2 is observed using nitrate CI.
The trimer ions observed using acetate CI may have contributions from the decomposition of large clusters. For example, A3-⚫ DMA2 could be
formed by the decomposition of A4-⚫ DMA2 or A4-⚫ DMA3 via the loss of A1 or
A1⚫ DMA, respectively. If these types of processes are
significant, they might explain some of the differences in the trimer ion
observations between nitrate and acetate CI. Highly aminated tetramer
neutrals would be more readily ionized by acetate and result in larger
contributions to the trimer ion signals than compared nitrate CI. Thus, this
may be one drawback to acetate CI: a possible shift downwards in sulfuric
acid content in the distribution of ions vs. the neutrals.
Nitrate-measured signal ratio between A3⚫ B and
sulfuric acid monomer (SA3⚫B/S160) as a function of
tCI for DMA (a), EDA (b), and TMEDA (c) at
[A1]o=4 × 109 cm-3.
The sulfuric acid + diamine system shows the nitrate CI detection of
A3-⚫ diamine0-2 but at much lower abundances than
acetate CI, particularly for EDA. Interestingly, the most abundant trimer
ions after acetate CI contain on average one diamine molecule compared to two in
the DMA system. This is consistent with particle measurements that show one
diamine molecule is able to stabilize several sulfuric acid molecules and
thus form a stable particle, while at least two DMA molecules are required for
the same effect (Jen et al., 2016). The two amino groups on the
diamine molecule can both effectively stabilize trimers, and this size is
stable for the relevant timescales in this flow reactor (Glasoe et al.,
2015; Jen et al., 2016). Therefore, larger clusters can be produced with
higher acid-to-base ratios.
To better understand the trimer ion behaviors, we monitored the bare trimer
signal (A3-, S293) and monomer signal (S160) as a
function of CI reaction time, tCI.
Figure 8 shows S293/S160 for nitrate CI
for DMA, EDA, and TMEDA at [A1]o=4 × 109 cm-3.
Note that equivalent measurements for Put are similar to those of EDA. Low values of
S293/S160 for all conditions indicate minimal creation of
A3- from the CI of N3. Thus, IIC-produced A3- can
be a significant fraction of observed A3-. Without base present,
IIC is the only way to produce detectable amounts of A3- (green
circles in Fig. 8).
A3- can also be formed by the decomposition of larger ions such as
A3-⚫ B. Evidence of this decomposition can be seen in
Fig. 9, where SA3⚫B/S160 measured
using nitrate CI is shown as a function of tCI. For diamines
at high concentrations and short tCI, SA3⚫B/S160 decrease with tCI and can be attributed to the decomposition of this ion. Shorter tCI allows the instrument
to capture short-lived ions. A3-⚫ diamine decomposes at
longer times and could form A3-, thereby decreasing SA3⚫B/S160 and increasing S293/S160. However,
S293/S160 for the diamines does not increase with
tCI, indicating that A3-⚫ diamine likely
decomposes into products other than A3-. The DMA system also
exhibits a very small decrease in SA3⚫B/S160 at short
tCI, but ratio values are within measurement uncertainties.
Thus, no conclusion can be drawn from this decrease of SA3⚫DMA/S160 at short tCI.
Nitrate-measured signal ratio between A4⚫ B and
sulfuric acid monomer (SA4.diamine/S160) as a function of CI
reaction time for EDA (a), Put (b), and TMEDA (c).
Another, more likely scenario to explain these time dependent behaviors for
the trimer ion signals is if A3-⚫ B decays into
A2- and a neutral A1⚫ B at short tCI.
Assuming we have captured most of the initial A3-⚫ B
signal at the shortest tCI= 15 ms in Fig. 9a–c, the
increase in A2- due to this mechanism would be small compared to
the observed A2- signal. Acetate data for Put (Fig. 7c) provide
some evidence supporting this because the slope of the [Put]=2 pptv is 3.7
and is higher than the 2.6 slope of [B] = 0 pptv case. Since A2-
when [B] = 0 pptv is primarily produced by IIC, a higher slope when
[Put] = 2 pptv indicates larger ion decomposition contributing to the
A2- signal.
Scenarios deduced from these trimer ion observations and previous
computational chemistry studies for the sulfuric acid and DMA system are
summarized in Table 3. These reactions have little effect on the modeled dimer results
since they introduce minor sources of dimer ions. In contrast, each trimer
pathway adds large uncertainty to the modeled trimer behavior. For example,
including ion decomposition reactions of larger ions (tetramer and larger),
postulated from the acetate CI results, may greatly influence concentration
of smaller trimer ions which already exhibit very low signals using nitrate
CI. In addition, nitrate inefficient ionization of neutral trimers leads to
large uncertainties in modeling the unobserved trimer types. More detailed
observations of the chemically neutral trimers and computational chemistry
studies on evaporation rates for sulfuric acid+diamine systems will
improve future efforts to model these processes.
Tetramer, N4
Nitrate CI leads to very low amounts of tetramer ions and primarily as
A4-⚫ DMA1-3 and A4-⚫ diamine1,2. Computational chemistry suggests that the sulfuric
acid+DMA tetramer likely exists as A4⚫ DMA2-4, with
A4⚫ DMA4 dominating the population
(Ortega et al., 2012). The acetate data appear to
confirm this, with A4-⚫ DMA3 as the most abundant
tetramer ion, which likely originated predominately from the decomposition of
A4⚫ DMA4 upon ionization (Ortega
et al., 2014). Nitrate may efficiently chemically ionize A4⚫ DMA1-2; however, their concentrations after the 3 s neutral reaction
time are likely below the detection limit of the Cluster CIMS (< 105 cm-3). Furthermore, the A4-⚫ DMA1,2
ions may be subject to the elimination of A1⚫ DMA. Nitrate CI
results show ∼ 100 times higher [A4-⚫ diamine] than [A4-⚫ DMA] at about equivalent initial
reactant concentrations. This suggests that the most stable neutral
tetramers contain fewer diamine molecules than DMA. In addition, the acetate
CI results for the diamines show that the majority of N4 contain one diamine,
further supporting the conclusions drawn in Jen et al. (2016) that
only one diamine molecule is needed to form a stable particle.
Due to the very low observed concentration of A4-⚫ DMA,
we focus on the ions of the diamine systems. The stability and behavior of
A4-⚫ diamine can be examined by looking at
nitrate-detected signal ratios of A4⚫diamine- and
the monomer (SA4⚫diamine/S160) as a function of CI
reaction time, given in Fig. 10. Similar to A3-⚫ EDA,
SA4⚫EDA/S160 and SA4⚫Put/S160 decreases
with time at short tCI, indicating that they decompose with a
lifetime shorter than a few tens of milliseconds. SA4⚫TMEDA/S160 also shows a decrease at short tCI, but it is less
evident. It could have a fast decay rate leading to a lifetime of a few milliseconds, and
our measurements would have mostly missed them. Nonetheless, decomposition
of A4-⚫ diamine likely entails the evaporation of N1 or
N2 instead of a lone diamine from the cluster as [A4-] was
below the detection limit of the Cluster CIMS using nitrate. At long CI reaction
time, SA4⚫EDA/S160 remained constant, indicating a negligible
contribution of IIC to the A4-⚫ EDA signal. In contrast,
SA4⚫Put/S160 and SA4⚫TMEDA/S160 increase
at long tCI. This could be due to IIC or larger ion
decomposition.
Pentamer, N5
Nitrate CI did not detect any pentamer (N5), but pentamer was detected
using acetate CI. In the diamine system, acetate detected N5 with fewer
diamine molecules (1–2) than DMA (4). However, A5-⚫ EDA>3, A5-⚫ TMEDA>1, and
A5-⚫ Put>2 fall outside the Cluster CIMS mass range
of 710 amu. Thus, we may not have measured the complete pentamer population.
The most abundant N5 is A5-⚫ DMA4, and it
increases in both concentration and in fraction of N5 population with
increasing [DMA]. This ion could be the result of the loss of a DMA molecule
after CI of A5⚫ DMA5. This would follow similar trends
predicted by computation chemistry for smaller clusters. However, since
[DMA] ≪ [A1]o (i.e., [B] / [A1]o is high)
and stable particles need ∼ 2 DMA to form
(Glasoe et al., 2015), [A5⚫ DMA5] as
high as 107 cm-3 would not be expected. The presence of
A5-⚫ DMA4 could also then be the result of large
ion decomposition via the evaporation of A1 or A1⚫ DMA.
Measurements of ions larger than 700 amu are needed to better understand how
they evaporate upon acetate CI and what fraction of the pentamers are not
ionizable by nitrate.
Conclusions
This study presents measurements of the behavior of neutral and ionized
sulfuric acid clusters containing various bases. The results show the
complexities of the coupled neutral cluster formation pathways with the ion
processes (e.g., chemical ionization, ion-induced clustering, and ion
decomposition). We provide various scenarios to describe the observed
trends. Our most definitive conclusions are as follows.
Nitrate very likely does not chemically ionize all types of sulfuric acid
dimers containing diamines. The model indicates that A2⚫ diamine2 cannot be chemically ionized by nitrate. However, the model
did not consider semi-efficient nitrate CI of A2⚫ diamine, which could also explain our observations.
Nitrate only chemically ionizes a small fraction of trimer and larger
clusters in both the DMA and diamine with sulfuric acid systems.
Measurements suggest that the more chemically neutral clusters are not
chemically ionized by nitrate but are by acetate.
Acetate and nitrate CI measurements of sulfuric acid+DMA clusters
generally agree with the qualitative trends of neutral and ion cluster
predicted from computational chemistry (Ortega et al., 2012, 2014). However, these measurements suggest that
A3-⚫ B decomposes into A2- and
A1⚫ B.
Nitrate measurements of A3-⚫ B and
A4-⚫ B show that these ions decompose at roughly the
same timescales as the CI reaction time at room temperature. In principle,
ionization of neutral clusters leads to potentially large artifacts even
before they are sampled into a vacuum system. These decomposition reactions
will likely affect the calculated concentrations of the neutral clusters.
In an acid-rich environment where [B] / [A1]< 1, A2-
and A3- are primarily produced via IIC pathways and contribute
negligible amounts to overall dimer and trimer signals when any of these
bases are present and at our 18 ms CI reaction time. If some fraction of the
dimer is not chemically ionized by nitrate, then IIC-produced A2-
is a significant fraction of the dimer signal.
Additional computed neutral and ion evaporation rates and a more complex
model combined with multivariable parameter fitting would provide more
clarity to these results. In addition, more acetate CI measurements of ion
signal ratios as a function of CI reaction time are needed to provide more
details on specific ion behaviors. However, measurements using the acetate
ion (which includes acetate, acetate ⚫ water, and acetic
acid ⚫ acetate) exhibit high backgrounds in the low masses, leading
to up to a factor of 5 uncertainty in measured monomer concentration
([N1]) and a factor of 2–3 for dimer concentration ([N2]). A
higher-resolution mass spectrometer is needed to resolve the background
signals and reduce the uncertainties.
Data availability
All data presented in this study are available upon request from the
corresponding author.
The Supplement related to this article is available online at doi:10.5194/acp-16-12513-2016-supplement.
Coty N. Jen designed and performed the experiments, analyzed the results,
developed the model, and prepared the paper. Jun Zhao provided useful
comments on the paper. David R. Hanson helped with performing the
experiments, interpreting the data, and providing comments on the
paper. Peter H. McMurry assisted with data interpretation and paper preparation.
The authors declare that they have no conflict of interest.
Acknowledgements
Support from NSF Awards AGS1068201, AGS1338706, and AGS0943721 is gratefully
acknowledged. Coty N. Jen acknowledges support from NSF GRFP award 00006595, UMN
DDF, and NSF AGS Postdoctoral Fellowship award 1524211. Jun Zhao acknowledges
support from SYSU 100 Talents Program.
Edited by: A. Laskin
Reviewed by: three anonymous referees
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