Introduction
Atmospheric aerosol particles influence the Earth's radiation balance via
aerosol–radiation and aerosol–cloud interactions, the latter effect being
one of the largest sources of uncertainty in predicting the current and
future climate change (IPCC, 2013). An important source of
atmospheric aerosol particles is the formation of molecular clusters from
gas-phase precursors (vapors) and their subsequent growth to larger sizes by
vapor condensation and other processes. Such new-particle formation gives a
potentially large contribution to regional and even global cloud
condensation nuclei (CCN) populations (Merikanto et al., 2009; Kerminen
et al., 2012; Lee et al., 2013), thereby affecting aerosol–cloud
interactions and ultimately climate (Kazil et al., 2010; Makkonen et al.,
2012; Ghan et al., 2013). However, the very first steps of the atmospheric
new-particle formation process are still poorly understood and a subject of
ongoing research (Kulmala et al., 2014). An important task
in this respect is to find out the factors and underlying mechanisms that
determine the initial formation from vapors of molecular clusters and
particles smaller than 2 nm diameter, and how this process varies throughout
the atmosphere.
It is still largely unknown which vapors participate in atmospheric
new-particle formation. The only compound that certainly plays a major role
is sulfuric acid (H2SO4) (Weber et al., 1996; Kulmala et al.,
2004b; Kulmala et al., 2006; Riipinen et al., 2007). Together with
ubiquitous water vapor (H2O), H2SO4 is believed to be the
main source of new particles in the middle and upper troposphere
(Lovejoy et al., 2004). However, most measurements of
new-particle formation have been made close to the ground, and these
particle formation events have been observed to be confined into the lower
tropospheric boundary layer (Kulmala et al., 2004b; Kulmala and Kerminen,
2008; O'Dowd et al., 2009; Schobesberger et al., 2013b). Within this
relatively warm boundary layer, H2SO4 alone cannot explain either
the particle formation rate or the subsequent growth rate; H2SO4
concentrations are too low, typically below one part per trillion by volume
(< 1 pptv, corresponding to 2.5 × 107 molecules cm-3) (Kirkby et al., 2011). Other compounds are thus believed to
participate in the process of new-particle formation by stabilizing
H2SO4 molecules during the formation of initial clusters (e.g.,
Petäjä et al., 2011; Sipilä et al., 2010). Candidate compounds
for facilitating such stabilization are ions (Lovejoy et al., 2004;
Kirkby et al., 2011), bases such as ammonia (NH3) (Coffman and Hegg,
1995; Hanson and Eisele, 2002; Ortega et al., 2008; Kirkby et al., 2011) and
amines (Kurtén et al., 2008; Paasonen et al., 2012; Almeida et al.,
2013), and a possibly wide range of oxygenated organic molecules (Kulmala
et al., 1998; Zhang et al., 2004; Metzger et al., 2010; Schobesberger et
al., 2013a; Ehn et al., 2014; Riccobono et al., 2014).
Ammonia (NH3) and its stabilizing effect on the
H2SO4–H2O system is probably the most thoroughly researched
among all those alternatives. The saturation vapor pressure of
H2SO4 is several orders of magnitude lower above bulk
H2SO4–H2O–NH3 solutions compared to
H2SO4–H2O solutions (Marti et al., 1997).
The addition of NH3 vapor to a system of H2O and H2SO4
vapors leads to a large enhancement of the rates of aerosol particle
formation (Ball et al., 1999; Kirkby et al., 2011). On the molecular
scale, investigations of negatively charged H2SO4 and
NH3–H2SO4 clusters obtained by ionizing neutral clusters
showed that the NH3-containing clusters can form more readily
(specifically at warmer temperatures) than pure H2SO4 clusters
(Eisele and Hanson, 2000; Hanson and Eisele, 2002). Theoretical ab initio
studies show that NH3 forms strong bonds with H2SO4, greatly
enhancing the stability of H2SO4-containing clusters, for both
electrically neutral and charged clusters (e.g., Kurtén et al.,
2007b; Ortega et al., 2008; DePalma et al., 2012; Ortega et al., 2012).
Generally, these studies predict a maximum base : acid ratio of 1 : 1;
however, the maximum cluster size is usually computationally limited, e.g., to up to
about 8 molecules in Ortega et al. (2014) or to about
20 molecules in DePalma et al. (2012). Experimentally,
small ion clusters of the types (NH3)m ⋅ (H2SO4)n ⋅ HSO4- and
(NH3)m ⋅ (H2SO4)n ⋅ NH4+, containing up to about 15 molecules, have been produced in
various laboratory setups, allowing studies of their formation and stability
(Hanson and Eisele, 2002; Bzdek et al., 2011; Froyd and Lovejoy, 2012).
Ratios m/n≤1 were observed, in agreement with the theoretical
expectations.
Experiments at the Cosmics Leaving OUtdoor Droplets (CLOUD) facility at CERN
addressed new-particle formation from NH3, H2SO4 and H2O
in an aerosol chamber setup. The results from these experiments connected
the same (NH3)m ⋅ (H2SO4)n ⋅ ion± clusters directly to new-particle formation at atmospherically
relevant rates (Kirkby et al., 2011). Formation rates comparable to those
in the ambient atmosphere were only obtained when either H2SO4
concentrations were at least 1 order of magnitude higher than typical
ambient concentrations, or when the temperature was very low (-25 ∘C), ruling out NH3, H2SO4 and H2O as the
sole participants in new-particle formation in most regions of the
atmospheric boundary layer.
Clusters of NH3, H2SO4 and H2O may nevertheless play an
important role in the very first steps of new-particle formation in the
atmosphere. It was recently shown that a critical step may be the
stabilization of small H2SO4-containing clusters by NH3,
amines or organic compounds (Kulmala et al., 2013). In that study, these
stabilized clusters grew relatively slowly up to an activation size
(1.5–1.9 nm mobility diameter), and were only then able to grow faster by
the enhanced uptake of additional compounds (likely organics). Indeed, the
only clusters that have so far been unambiguously identified in the
atmosphere and directly linked to new-particle formation are clusters of
H2SO4 plus NH3 or amines or both (Ehn et al., 2010; Zhao
et al., 2011; Kulmala et al., 2013).
Gaseous NH3 concentrations vary widely in the atmosphere, both with
location and time, from < 10 pptv to several parts per billion by volume (Ziereis and
Arnold, 1986; Janson et al., 2001; Riipinen et al., 2007; Gong et al., 2011;
Osada et al., 2011). The lower limit is uncertain; low concentrations of
NH3 or other bases, such as amines, remain challenging to measure
accurately in the atmosphere (e.g., Chang et al., 2003; Huang et al.,
2009; von Bobrutzki et al., 2010; VandenBoer et al., 2011). However, recent
laboratory experiments have shown a great enhancement of the formation of
particles from H2SO4 by the addition of only tens of parts per trillion by volume of
NH3 (Kirkby et al., 2011) or just a few parts per trillion by volume of dimethylamine
(Almeida et al., 2013). Therefore, amines are likely to be important for
atmospheric particle formation in regions near to amine sources. It remains
to be determined which base is the dominant stabilizer of
H2SO4-containing clusters in the atmospheric boundary layer. Some
theoretical studies suggest that the stabilizing effect of NH3
dominates for typical atmospheric conditions due to relatively low gas-phase
amine concentrations (Nadykto et al., 2011). Indeed, a dominant role
for NH3 is consistent with the observation that clusters during
new-particle formation in the boreal forest contain more NH3 than
dimethylamine (Schobesberger et al., 2013a). Another experimental study
reported on the important role of small bases in new-particle formation in
Mexico City and Atlanta (Chen et al., 2012). The
stabilizing effect due to NH3 could not be differentiated from the
effect due to amines, but NH3 concentrations were found to clearly
exceed amine concentrations.
This paper presents a comprehensive set of observations of clusters
containing mainly H2SO4 and NH3 during new-particle formation
experiments at the CLOUD facility at CERN. These are growing ion clusters,
negatively or positively charged, that directly lead to the formation of
aerosol particles in the CLOUD aerosol chamber (Kirkby et al., 2011;
Keskinen et al., 2013). The chamber features precise control of experimental
parameters and exceptional cleanliness. It provides environments with very
low levels of contaminants (Schnitzhofer et
al., 2014) and allows for the exploration of a wide range of conditions
including very low concentrations of critical trace vapors such as NH3
and amines.
The main goal of this work is to provide a comprehensive picture of the role
of NH3 in the initial cluster formation, and subsequent new-particle
formation, in the NH3–H2SO4–H2O system. The specific
scientific questions we aim to answer here include the following: (1) what is the detailed
molecular structure of the observed clusters under different atmospherically
relevant conditions? (2) What are the roles of NH3 and H2SO4
concentrations and temperature in determining the cluster composition, and
thereby the plausible cluster formation mechanism, especially at the limits
of low and high NH3 to H2SO4 gas concentration ratios? And (3)
how are the clusters affected by trace amounts of other bases, such as
amines, that are usually present as contaminants in experimental systems? We
will also discuss the role of different charge carriers involved in these
kinds of cluster measurements, and compare our observations with field
observations and theoretical expectations. We approached the problem by
investigating both negatively and positively charged ions and ion clusters
up to 3300 Th, corresponding to up to about 2.1 nm in mobility-equivalent
diameter, by using a high-resolution ion mass spectrometer. Our experimental
conditions ranged from -25 to +20 ∘C for temperature, 21 %
to 90 % for relative humidity (RH), < 5 pptv to
> 1 ppbv for NH3 mixing ratio, and 3.3 × 106 to 1.4 × 109 cm-3 (corresponding to 0.1 to 56 pptv) for
H2SO4 concentration.
Methods
The results presented here are based on the CLOUD 2 (June and July 2010)
and CLOUD 3 (October and November 2010) campaigns at the CLOUD chamber at
CERN.
The CLOUD chamber
A description of the general experimental setup is given in more detail in
Kirkby et al. (2011). The CLOUD chamber is a cylindrical stainless-steel
container with an inner volume of 26.1 m3. It is filled principally
with air (79 % nitrogen, 21 % oxygen) that is obtained from the
evaporation of cryogenic liquids, with selected additional trace gases.
Ozone (O3) concentrations in the chamber typically range from 200 to
1000 ppbv. RH typically varies between 21 % and 90 %, but is mostly held
fixed between 37 % and 41 %. The trace gases sulfur dioxide (SO2)
and NH3 are added on demand via individual independent lines.
Fresh,
humidified air and trace gases are fed into the chamber continuously at a
total rate of 85 L min-1, while air is extracted by the measuring
instruments. The desired concentration of each gas is achieved by continuous
constant injection at the appropriate flow rate. The chamber is usually
operated at an overpressure of 5 mbar to avoid contamination from outside
the chamber. A pair of fans facilitate the mixing of the chamber contents
(Voigtländer et al., 2012). The inside of the chamber
is irradiated on demand by UV light from the top of the chamber
(Kupc et al., 2011). This UV light induces
photolytic reactions, in particular the oxidation of SO2 (at
concentrations of 15 to 34 ppbv) to form H2SO4. The temperature
inside the chamber is actively controlled and stable within 0.01 ∘C for the typical length of an experiment.
Some ionization always occurs inside the chamber via natural galactic cosmic
rays. In addition, the chamber can be exposed to 3.5 GeV/c pions (π+) that are provided by the CERN Proton Synchrotron in one to three
spills per minute. The intensity of the spills can be regulated, and the
mean total ion pair production rate in the chamber is therefore adjustable
between 2 cm-3 s-1 (π+ beam off) and 42 cm-3 s-1 (at the usual maximum available π+ beam intensity). An
electrical clearing field of 20 kV m-1 can be applied by means of a
pair of field cage electrodes, mounted at the top and the bottom of the
chamber. This field will sweep out all ions in the chamber in about 1 s, providing an environment practically free of ions, when needed.
During the CLOUD 2 and CLOUD 3 campaigns, a wide array of instruments was
arranged around the chamber, continuously analyzing its contents via 16
sampling probes. These sampling probes were mounted radially around the
chamber and projected 0.5 m into the chamber. The instrumentation included
an atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometer
to measure the chemical composition of ions (up to about 2 nm in size).
Results from the APi-TOF are the main subject of this study, and the
instrument is described below. The rest of the instrumentation included an
Airborne Neutral cluster and Air Ion Spectrometer (NAIS)
(Mirme et al., 2010), used to measure ions from 0.8 to 40 nm in mobility-equivalent diameter. A comprehensive suite of particle
counting and sizing instruments facilitated aerosol number size distribution
measurements, covering the range from 1.3 to 100 nm (Kirkby et al.,
2011). A chemical ionization mass spectrometer (CIMS)
(Kürten et al., 2011, 2012) measured
H2SO4 concentrations down to about 105 cm-3 (4 × 10-3 pptv) at an accuracy of +100 %/-50 %. During
CLOUD 3 only, a proton transfer reaction mass spectrometer (PTR-MS)
(Norman et al., 2007) and a LOng Path Absorption
Photometer (LOPAP) (Bianchi et al., 2012) were used to
measure NH3 concentrations down to 35 pptv.
Setup of the APi-TOF at the CLOUD chamber during the CLOUD
2 and CLOUD 3 campaigns. The APi-TOF shared one sampling probe (22.1 mm ID)
with the NAIS. The flow was split via a Y-splitter. Before reaching the
critical orifice inlet of the APi-TOF, the inner tube diameter reduced from
the Y-splitter's 21.2 mm to 7 mm.
Setup of the APi-TOF at the CLOUD chamber
The APi-TOF sampled air from the CLOUD chamber using one of the radially
mounted sampling probes. The sampling probe's inner diameter (ID) was 22.1 mm and its total length was 1.2 m, of which 0.5 m projected into the
chamber. The APi-TOF shared the same sampling probe with the NAIS. The total
sample flow from the chamber of 27.8 L min-1 (at -24 ∘C) to
34.5 L min-1 (at 19 ∘C) was split at 45∘ using a
Y-splitter (Fig. 1). The flow from the Y-splitter to the APi-TOF (9.8 to
11.5 L min-1) was directed at the APi-TOF's orifice inlet, where 0.8 L min-1 was drawn into the instrument and the rest discarded.
Figure 1 shows that much of the sampling line was exposed to room temperature
(> 5 ∘C). We thermally insulated the lines using
Armaflex pipe insulation with aluminum tape on the outside, to minimize
unwanted heating of the air taken from the chamber. The 0.8 L min-1
sample drawn into the APi-TOF was taken from the center of a ∼ 10 L min-1 flow, further mitigating heating of the sample. Simulations
of the heat flux from warm ambient air into cool air flowing in a tube,
insulated by a jacket of air, indicate that the APi-TOF sample may be heated
up to several degrees before reaching the APi-TOF (e.g., from -25 to
-16.5 ∘C or from 5 to 8.5 ∘C, ± 2 ∘C, at a
room temperature of 20 ∘C). However, such heating would not
qualitatively influence the conclusions regarding temperature effects in
this study. In fact, it would only reduce the magnitude of the observed
temperature effects.
The APi-TOF
The APi-TOF is a time-of-flight mass spectrometer built by Tofwerk AG and
Aerodyne Research, Inc. A detailed description of the instrument and its
capabilities is found in Junninen et al. (2010). The APi-TOF is designed
to measure the mass-to-charge ratio of ambient ions of either positive or
negative polarity. No ionization of the sample is performed, so only ions
that are formed in the CLOUD chamber are detected. Air is sampled directly
from atmospheric pressure via a critical orifice. In the interface (APi),
ions are focused and guided through differentially pumped chambers to the
time-of-flight mass spectrometer (TOF), where the pressure is reduced to
10-6 mbar.
During CLOUD 2 and CLOUD 3, the mass accuracy was better than 10 ppm. The
resolving power (determined from the peak width at half maximum) was up to
4900 Th/Th (CLOUD 2) or up to 5300 Th/Th (CLOUD 3) for negative ions, and up
to 4300 Th/Th for positive ions. The instrument was set to obtain
mass-to-charge ratios up to either about 2115 Th (in positive mode and some
experiments in negative mode) or 3300 Th (most experiments in negative
mode). At all times during these measurements, the APi-TOF detected only
singly charged ions; therefore, the unit thomson (Th) can also be thought of
as atomic mass unit (u) or dalton (Da). To provide a comparison with
condensation particle counters and mobility spectrometers, a singly charged
ammonium bisulfate cluster ion at 3300 Th corresponds to about 2.1 nm in
mobility-equivalent diameter, using the conversion procedure described by
Ehn et al. (2011). The APi-TOF's ion transmission efficiency was set to
have its maximum at about 900 to 1400 Th for negative ions, and at about 100
to 300 Th for positive ions. In the CLOUD campaigns, we recorded spectra at
a time resolution of 5 s. The signal-to-noise ratio usually resulted in a
maximum practical time resolution of 30 s.
Sampled ions may be subject to fragmentation inside the APi-TOF. Such
fragmentation was mainly manifest by the usual near-absence of any H2O
clustered with, for instance, sulfuric acid. The evaporation rate of
H2O from these clusters is too rapid to survive detection in the
non-equilibrium environment of the APi-TOF. However, many more strongly
bound clusters can be detected, as will be shown here and has been shown
before (e.g., Ehn et al., 2010; Junninen et al., 2010). Also, comparisons
with ion mobility spectrometers demonstrated a good agreement with the
APi-TOF's results (Ehn et al., 2011; Schobesberger et al., 2013a).
Comparisons between the APi-TOF and the NAIS for our measurements produced a
fair agreement as well, so the ion mass spectra obtained by the APi-TOF are,
in general, representative of the actual population of small ions and ion
clusters. However, a few molecules other than H2O may also be lost from
some clusters during the sampling, as has also been suggested by comparisons
between APi-TOF results and cluster simulations (Olenius et al., 2013b;
Ortega et al., 2014).
The data obtained from the APi-TOF measurements were processed and analyzed
using tofTools, a software package based on MATLAB and under continuous
development, mainly at the University of Helsinki. Details on the analysis
of APi-TOF data are found elsewhere (Schobesberger et al., 2013a).
Gas-phase concentrations of NH3
The primary means of obtaining the gas-phase concentration of NH3
([NH3]) were the results from the LOPAP (Bianchi et
al., 2012). It was only available during CLOUD 3 and above 0 ∘C.
Below 0 ∘C, measurements of [NH3] are available from the
PTR-MS for some experiments in CLOUD 3. Ammonia concentrations could also be
estimated from the calibrated mass flow controller settings.
In practice, [NH3] was directly measured whenever NH3 had been
added during most of the CLOUD 3 campaign. Without the deliberate addition
of NH3, values of [NH3] were below the detection limit of 35 pptv.
More refined measurements during later campaigns showed that the contaminant
level of [NH3] was in fact likely < 2 pptv for experiments at
5 ∘C (Almeida et al., 2013). The most plausible source of this
contaminant NH3 was evaporation from the inside walls of the chamber.
Therefore, we assumed that contaminant levels of [NH3] were 2 pptv at
5 ∘C and directly proportional to the desorption rate of NH3,
assuming an activation energy of 33 kJ mol-1. Thus calculated
contaminant levels of [NH3] ranged from 0.4 pptv (at -24.7 ∘C) to 4.1 pptv (at 19.8 ∘C).
Ammonia concentrations also had to be calculated for a selection of
experiments below 0 ∘C when no direct measurement results of
[NH3] were available. In the beginning of these experiments, [NH3]
was above contaminant levels, but no NH3 was being added to the
chamber. Therefore, a decay of [NH3] as measured previously by the
LOPAP was used for our calculations, in addition to the proportionality to
the desorption rate.
During the few experiments when NH3 was added during CLOUD 2, estimates
for [NH3] were made using the settings of the mass flow controllers
that control the gas flows into and out of the CLOUD chamber.
Ambient measurements in the boreal forest
The same APi-TOF as in the CLOUD campaigns was deployed also at the Station
for Measuring Ecosystem-Atmosphere Relations (SMEAR II) (Hari and
Kulmala, 2005), where it measured negatively charged ions during spring
2011. The SMEAR II station is located in Hyytiälä, southern Finland,
within a boreal forest. Tampere (population 213 000) is the closest larger
town, 50–60 km southwest of the station. The station is the site of a host
of continuing atmospheric observations, which includes extensive aerosol
measurements that can be used to detect and analyze new-particle formation
events (Kulmala et al., 2004a). For the results
shown in this study, [NH3] was measured by a Monitoring instrument for
Aerosols and Gases (MARGA) (Makkonen et al., 2014),
and [H2SO4] was measured by a CIMS, similar to the one used at the
CLOUD experiments.
The APi-TOF was situated inside a container in the forest, directly sampling
ambient air in a setup similar to that used at the CLOUD chamber (details
in Schobesberger et al., 2013a). It should be noted that the APi-TOF was
tuned differently for those measurements, resulting in a reduced ion
transmission efficiency at high m/z compared to the experiments at CLOUD.
Summary of a typical new-particle formation experiment in
the CLOUD chamber during the CLOUD 2 campaign, with no added NH3, at 20 ∘C,
60 % relative humidity, 3.7 × 108 cm-3
[H2SO4] (15 pptv), estimated 4 pptv [NH3] (none added),
< 1 pptv [C2H7N], pion beam on. (a) Measurements
of sulfuric acid concentration ([H2SO4]) by CIMS, showing the
marked increase in [H2SO4] after the start of UV illumination.
(b) Consequent new-particle formation event as observed by the NAIS
negative ion channel, showing negatively charged ions that grow from
originally well below 2 nm to larger sizes. The black box marks the time
period of steady new-particle formation that was used for averaging APi-TOF
data, and the size range covered by the APi-TOF mass spectra. (c)
Mass defect diagram for the APi-TOF mass spectrum, averaged over the shown
particle formation event. The diagram reveals the composition of the growing
negatively charged ions between about 1 and 2 nm. These are ion clusters,
growing by the addition of H2SO4 and contaminant NH3
molecules (red and blue). Some clusters also contain contaminant amines
(green and light blue).
Results
Negatively charged ions during new-particle formation experiments from H2SO4 (no NH3 added)
During the CLOUD 2 and 3 campaigns, the conditions in the CLOUD chamber were
set to precisely maintained conditions, specifically to a temperature of
typically either 5 or 19–20 ∘C, an RH of typically 37 % to
41 %, an SO2 concentration between 15 and 34 ppbv, and an O3
concentration between 200 and 1000 ppbv. In the initial experiments, no
NH3 was fed into the chamber.
A typical new-particle formation experiment in the CLOUD chamber was
initiated by the UV lights being turned on, leading to a marked increase of
[H2SO4] (Fig. 2a), which in turn triggered new-particle formation.
The formation and subsequent growth of particles was measured by the
particle or ion counting and sizing instrumentation, including the NAIS
(Fig. 2b). For most of the investigated gas mixtures, the NAIS showed that
ion-induced nucleation proceeded only or predominantly in negative polarity.
Therefore the APi-TOF was mostly run in the negative mode (for detecting
negatively charged ions) during both campaigns. Naturally, the main focus of
this study also lies on negatively charged ions.
The APi-TOF measurements provide high-resolution mass spectra of ions and
ion clusters up to about 2.1 nm in mobility-equivalent diameter, capturing
exactly the critical first steps of the ion-induced pathway of new-particle
formation (illustrated in Fig. 2b). The elemental compositions of ions are
identified primarily by their exact mass. Therefore, it is advantageous to
present mass spectra as mass defect diagrams (Fig. 2c). In such a diagram,
the mass defect for each ion (i.e., the deviation from its nominal mass) is
plotted against its mass-to-charge ratio. Any given ion will occupy a unique
position in this diagram, and an addition of a specified atom or molecule
will move an ion by a characteristic vector (e.g., see Fig. 2c insert).
Note that the APi-TOF spectra shown and analyzed in this study are averages
over the duration of the steady-state conditions during a new-particle
formation experiment (illustrated in Fig. 2). The steady-state periods were
defined as the period during which no change in the APi-TOF ion spectrum
occurred. Their duration ranged from 200 s to over 6 h.
The new-particle formation experiments in the CLOUD chamber covered a range
in [H2SO4] levels from 3.3 × 106 to 1.4 × 109 molecules cm-3 (0.1 to 56 pptv). During a typical experiment,
the dominant negatively charged ions were small sulfuric acid anion
clusters, with the strongest signal, in most cases, from the trimer,
(H2SO4)2 ⋅ HSO4- (Fig. 2c). Heavier ion
clusters (> 350 Th; containing > 3 sulfur atoms) were
considerably less abundant for most experimental conditions. These heavier
clusters consisted mostly of H2SO4 molecules. However, they were
observed not only as “pure” sulfuric acid clusters but also as clusters
with base molecules, specifically molecules of NH3 or of various
organic bases, mainly amines.
In general, larger clusters contained more base molecules. The predominant
base in these clusters was NH3, yielding clusters of the form
(NH3)m ⋅ (H2SO4)n ⋅ HSO4- (Fig. 2c). Only certain numbers of NH3 molecules (m)
were seen for each number of H2SO4 molecules (n), depending on
experimental conditions. This dependency will be discussed below in more
detail. Note that neither NH3 nor amines had been deliberately fed into
the chamber for these experiments. Instead, they were unintended impurities.
Some of the negatively charged clusters that grew by the addition of
H2SO4 and base molecules had an additional oxygen atom (Fig. 2c).
This can be explained by the growth starting from HSO5- instead of
HSO4-. The role of HSO5-, as opposed to HSO4-,
in the composition and growth of ion clusters, as well as its origin, will
be described and discussed in Sects. 3.2 and 4.2.
Note that the APi-TOF did not detect any growing positively charged clusters
under the typical experimental conditions discussed in this section; that is, when no NH3
was fed into the chamber, temperature was either 5 or 19 ∘C and RH was 40 %.
This is consistent with simultaneous NAIS
measurements, which did not show any growth starting from small positively
charged ions (< 2.5 nm).
Charge carriers different from HSO4-
Practically all anion clusters that included the bisulfate ion
(HSO4-) were also observed in the form where HSO4- was
replaced either with HSO5- or, to a lesser extent, with
SO5-. The HSO5- and SO5- ions appear to be
less efficient than HSO4- in forming the initial clusters with
H2SO4 molecules, based on the observed ratios of anion
sulfuric-acid dimer signals to those of the bare anion, i.e.:
[H2SO4⋅HSO4-]/[HSO4-]>[H2SO4⋅HSO5-]/[HSO5-]>[H2SO4⋅SO5-]/[SO5-].
The concentrations of HSO5- and SO5- in CLOUD were
particularly high compared to the concentration of HSO4- when the
concentration of H2SO4, the dominant precursor of HSO4-,
was low. The relative amounts of charge carriers were also affected by the
π+ beam intensity (i.e., the total ion concentration) and by the
O3 concentration: higher beam intensity led to a higher fraction of
HSO5- ions, whereas practically no HSO5- or
SO5- was observed in experiments without O3 present in the
CLOUD chamber. In addition, the abundance of HSO5--based ion
clusters relative to HSO4--based clusters in CLOUD increased
chiefly together with an increasing role of NH3. In the most extreme
case – i.e., high [NH3], low [H2SO4] and high beam intensity
– about 60 % of the large clusters (those containing 5–19 S atoms) were
associated with HSO5-. A maximum of 7 % of the larger clusters
were associated with SO5-, and a maximum of < 3 % with
H2O11NS2-, probably in the form
H2S2O8 ⋅ NO3-.
The cluster compositions were very similar regardless of which ion carried
their charge. The most important difference between the different charge
carriers was that we observed NH3 ⋅ (H2SO4)2 ⋅ HSO5- clusters, whereas the
smallest ammonia-containing cluster associated with HSO4- was
NH3 ⋅ (H2SO4)3 ⋅ HSO4-.
The counts for NH3 ⋅ (H2SO4)2 ⋅ HSO5- clusters were usually more than an order of magnitude lower
than the counts for similar clusters with one more H2SO4 molecule,
NH3 ⋅ (H2SO4)3 ⋅ HSO5-,
whereas the cluster NH3 ⋅ (H2SO4)2 ⋅ HSO4- was totally absent.
Subsequently, the average ratio between the number of NH3 molecules
(m) and the number of H2SO4 molecules (n) was initially higher in
HSO5--based clusters than in the corresponding
HSO4--based clusters. However, this difference decreased with an
increasing cluster size, disappearing or staying approximately constant at
about n≥9. The implications of these observations will be discussed in
Sect. 4.2.
Contaminant amines in growing anion clusters
No amines were deliberately added into the chamber for the experiments
discussed here, i.e., throughout the CLOUD 2 and CLOUD 3 campaigns. Amine
contamination originated probably from the same source as NH3 (see
Sect. 2.4). We can give some estimate of the contaminant levels of the
dominant amine, C2H7N, based on measurements from later CLOUD
campaigns when dimethylamine was also added into the chamber in several
experiments (Praplan et al., 2012; Almeida et al., 2013). These estimates
are based on direct measurements of dimethylamine concentrations down to 0.2 pptv performed during the later experiments and on measurements of the
content of C2H7N in clusters seen by the APi-TOF. Based on those
results, we speculate that gas-phase contaminant concentrations of
C2H7N were between 0.1 and 1 pptv during the CLOUD 2 campaign, and
about 0.1 pptv or even less during the CLOUD 3 campaign.
Details on the composition of the growing negatively
charged clusters during the new-particle formation experiment presented in
Fig. 2, binned by the number x of H2SO4 molecules in the cluster.
Note that only ion clusters based on the HSO4- ion are shown, for
simplicity. However, these ions constitute the majority of all ions, and
practically all ions at x > 3 (i.e., beyond 350 Th, as can be seen
in Fig. 2C). Note that besides contaminant NH3, a wide range of
contaminant nitrogen-containing organic compounds are seen in these clusters
if x > 3, especially for x = 4. By far the most observed of
these compounds is C2H7N (dimethylamine or ethylamine). Most
likely all these organics are amines or amides (such as CH4N2O,
probably urea), their high proton affinities facilitating the formation of
clusters with H2SO4.
In the experiments discussed here, the highest abundance of the clusters
containing contaminant organic bases (amines or amides) was usually seen on
those clusters that contained a sulfuric acid tetramer anion. Tetramers were
observed either without any base, such as
(H2SO4)3 ⋅ HSO4-, clustered with
NH3, or clustered with a basic organic compound. The organic base with
the highest signal has the formula C2H7N (dimethylamine or
ethylamine). Other bases observed in these clusters were CH5N
(methylamine), CH4N2O (urea) and larger amines or amides (Fig. 3).
Note that in some cases, we are unable to resolve whether one oxygen atom
was part of the organic constituent or whether the ion was HSO5-
instead of HSO4-. C2H7N was also seen to be bound to the
sulfuric acid trimer anion, forming C2H7N ⋅ (H2SO4)2 ⋅ HSO4-, although with a
signal about 2 orders of magnitude smaller than that of the cluster
C2H7N ⋅ (H2SO4)3 ⋅ HSO4-. Notably, the corresponding cluster with NH3 instead
of the amine, NH3 ⋅ (H2SO4)2 ⋅ HSO4-, was not observed, indicating its weaker base nature.
The clusters containing amines (or other organic bases) also evidently grew
by the accretion of H2SO4 and NH3 molecules when amines were
present at contaminant levels ([C2H7N] < 1 pptv):
increasingly larger clusters of the type Y ⋅ (NH3)m ⋅ (H2SO4)n ⋅ HSO4- were formed, where Y was almost always one N-containing
organic compound, and at maximum two such compounds ((C2H7N)2
or CH5N ⋅ C2H7N). In addition, the fraction of
clusters that included N-containing organic compounds was smaller for larger
clusters (n≥4) (Figs. 2c and Fig. 3).
Composition of NH3–H2SO4 clusters under different experimental conditions
In later experiments, NH3 was deliberately fed into the chamber to
investigate new-particle formation over a range of [NH3] from
contaminant levels (< 5 pptv; cf. Sect. 2.4) up to 1090 pptv. The
range of investigated temperatures reached from +20 down to -25 ∘C. (Note the possibility of air sampled from the chamber at low
temperatures being heated somewhat before reaching the APi-TOF, cf. Sect. 2.2.)
The average number of NH3 molecules (m) in clusters
with a certain amount of H2SO4 molecules (n), for negatively
charged clusters (NH3)m ⋅ (H2SO4)n ⋅ HSO4- (CLOUD experiments,
Hyytiälä), positively charged clusters (NH3)m ⋅ (H2SO4)n ⋅ NH4+ (CLOUD experiments) and
neutral clusters (NH3)m ⋅ (H2SO4)n (ACDC
model). Experiments are grouped by temperature into panels (a) (19 to
20 ∘C), (b) (5 to 6 ∘C), (c) (-26 to
-25 ∘C); mean relative humidities were 21 % to 84 % (anions,
CLOUD), 20 % to 40 % (cations, CLOUD), 41 % and 58 % (anions,
Hyytiälä). The resulting curves are near linear from about n=4 (anions) or n=1 (cations, neutrals) onwards. The principal factor
determining the slopes is the ratio of gas-phase concentrations
[NH3] / [H2SO4] (color scale).
For all experimental conditions, negative ion clusters with more than 4 or
5 sulfur atoms grew by the accretion of NH3 and H2SO4
molecules, forming progressively larger (NH3)m ⋅ (H2SO4)n ⋅ HSO4- clusters. The number
of NH3 molecules added on average per added H2SO4 molecule
remained nearly constant from 4 or 5 sulfur atoms up to the upper detection
limit of about 27 sulfur atoms, within the measurement uncertainties. These
findings are illustrated in Fig. 4, which shows the average number of
NH3 molecules (m) in clusters containing a certain amount of
H2SO4 molecules (n), for each experiment and grouped by
temperature.
We define the average number of added NH3 molecules per added
H2SO4 molecule as Δm / Δn. This ratio corresponds to
the slope of linear fits in m-vs.-n plots as in Fig. 4. For anions, we
calculated values of Δm / Δn for n≥4, and found that
Δm / Δn is well suited to describe the whole anion spectra
during new-particle formation events in the NH3–H2SO4 system:
two spectra with the same Δm / Δn were practically identical
(unless Δm / Δn was close to zero, see Sect. 3.5), and, for a
given temperature and RH, Δm / Δn was only dependent on the
ratio between the NH3 and H2SO4 gas-phase concentrations,
i.e., on [NH3] / [H2SO4] (color scale in Fig. 4, horizontal axis
in Fig. 5). In our analysis for this study, values of Δm / Δn
were calculated over the range 4≤n≤18 in the case of anion
clusters, because Δm / Δn was approximately constant for n≥4 and we obtained a signal from clusters up to at least n=18 in most of
the experiments.
Number of gained NH3 molecules per gained
H2SO4 molecule, Δm / Δn, plotted against the ratio of
NH3 and H2SO4 gas-phase concentrations,
[NH3] / [H2SO4], for particle formation experiments at 19 to
20 ∘C, showing a positive correlation. Circles are for anion
clusters (NH3)m ⋅ (H2SO4)n ⋅ HSO4- and experiments at a relative humidity (RH) of 37 % to
40 %. Colored squares denote experiments at lower RH (30 %) or higher RH
(> 59 %). Red pluses show Δm / Δn for cation
clusters (NH3)m ⋅ (H2SO4)n ⋅ NH4+, which are only observed at sufficiently high
[NH3] / [H2SO4], as indicated by the red-shaded area, at an RH
of 39 % to 40 %. Values for Δm / Δn were determined for
anion clusters in the range 4≤n≤18∗, and for cation
clusters in the range 1≤n≤16∗. The lower limits of
these ranges correspond to the sizes, from which onward Δm / Δn were constant. The upper limits were chosen, because up to these sizes a
good signal was obtained for most experiments.
∗ Actual upper limits varied depending on obtained signals. The median
maximum n was 16 for both anion and cation clusters.
At a given temperature and RH, the resulting Δm / Δn generally
increased with an increasing value of [NH3] / [H2SO4], then
flattened off when approaching the value of 1, and eventually reached a
saturation value slightly larger than 1. At 19 ∘C, the maximum
value of Δm/Δn of 1.1 to 1.2 was reached at the concentration
ratio [NH3] / [H2SO4] ≈ 100 (Fig. 5). This
concentration ratio was roughly coincident with the threshold for observing
cation clusters (at 19 ∘C and 40 % RH) of the form
(NH3)m ⋅ (H2SO4)n ⋅ NH4+ with m≈n and Δm/Δn≥1 (Figs. 4,
5). In an analogous way, the NAIS observed a formation of positively charged
ions only when positively charged NH3–H2SO4 clusters were
observable by the APi-TOF. Note that for the cation clusters, Δm/Δn was nearly constant from the monomer (n=1) onward and it was
generally calculated over the range 1≤n≤17.
The relationship between Δm/Δn and [NH3] / [H2SO4]
was similar under all experimental conditions, but the exact functional form
of this relation was temperature dependent (Fig. 6a). For example, the value
of Δm/Δn=0.2 was reached at [NH3] / [H2SO4] ≈ 0.1 when the temperature was 19 ∘C, but at
[NH3] / [H2SO4] ≈ 0.7 when it was 5 ∘C, and
at [NH3] / [H2SO4] > 1 when it was -25 ∘C. Also, the maximum observed values of Δm/Δn seemed to be
reached at lower values of [NH3] / [H2SO4], and these maximum
values were slightly higher at lower temperatures (e.g., a maximum Δm/Δn of 1.1 to 1.2 at 19 ∘C, versus a maximum Δm/Δn of 1.3 at -25 ∘C).
Note that in practically all our experiments with
[NH3] / [H2SO4] < 1, only contaminant levels of
[NH3] were present. These contaminant levels were not directly
measured, but calculated under the assumptions described in Sect .2.4. In
particular the temperature dependence of these low values of [NH3] is
subject to those assumptions. Note also that most experiments at CLOUD were
run with the RH between 37 % and 41 %, so the potential RH effects could
not be thoroughly investigated. However, an increase of RH to > 68 %
increased the value of Δm / Δn (Fig. 5). No significant
effect on negatively charged clusters was observed due to RH changes in the
range 30 % < RH < 60 %.
The composition of anion clusters,
(NH3)m ⋅ (H2SO4)n ⋅ HSO4-, during new-particle formation experiments in the CLOUD
chamber, shown as circles. RH was in the range 37–41 %; temperature
is given by the color scale. (a) Number of gained NH3 molecules
per gained H2SO4 molecule, Δm / Δn, plotted against
the ratio of NH3 and H2SO4 gas-phase concentrations,
[NH3] / [H2SO4]. Δm / Δn for anion clusters was
calculated for the range 4≤n≤18∗ for the results from
the CLOUD 2 and 3 campaigns, and for the range 4≤n≤9 for the
single result from CLOUD 5. Also shown are Δm / Δn for cation
clusters (NH3)m ⋅ (H2SO4)n ⋅ NH4+ (for the range 1≤n≤16∗), which are
observed only at sufficiently high [NH3] / [H2SO4]. The green
box and diamond markers show the corresponding results from ambient
observations in the boreal forest. These are for
(NH3)m ⋅ (H2SO4)n ⋅ HSO4-, 4 ≤n≤8∗, observed during new-particle
formation in Hyytiälä in spring 2011. RH varied from 36 % to
61 %, temperature from -0.5 to +8.5 ∘C. Dashed lines are
ACDC model calculations of Δm / Δn for neutral clusters
(NH3)m ⋅ (H2SO4)n in the range 1≤n≤5. (b) Fraction of pure, NH3-free sulfuric acid anion
clusters, (H2SO4)n ⋅ HSO4-, calculated
for the same ranges of n as in (a), and again plotted against
[NH3] / [H2SO4]. The legends in (a) apply to (b) as
well.
∗ Actual upper limits varied depending on obtained signals. For anion
clusters in the CLOUD 2 and 3 campaigns, median (nmax) = 16.5; for
cation clusters, median (nmax) = 16; for anion clusters in
Hyytiälä, median (nmax) = 7.
Negatively charged (NH3–)H2SO4 clusters at very low [NH3]/[H2SO4]
The role of NH3 in the formation of clusters became negligible at very
low values of [NH3] / [H2SO4], i.e., when Δm/Δn
dropped below 0.1 and the formation of pure binary clusters dominated (Fig. 6). Binary cluster formation is observed by the APi-TOF as pure sulfuric
acid anion clusters, (H2SO4)n ⋅ HSO4-,
because H2O molecules are lost in the sampling process (Sect. 2.3).
The fraction of pure, NH3-free sulfuric acid clusters was calculated
over the same cluster range as was used for calculating Δm/Δn (4≤n≤18), and like Δm/Δn, it was a function of
[NH3] / [H2SO4]. The fraction of NH3-free sulfuric acid
clusters increased with a decreasing value of [NH3] / [H2SO4],
and such clusters were observed only below some temperature-dependent
threshold value of [NH3] / [H2SO4] (Fig. 6b). Taken together,
the presence of pure, NH3-free sulfuric acid clusters was favored by
lower [NH3] / [H2SO4] ratios and lower temperatures.
Note that the NH3 contamination in the CLOUD chamber sets a lower limit
to the minimum achievable level of [NH3]. Therefore, pure binary
sulfuric-acid–water cluster formation can only be obtained at sufficiently
high [H2SO4] level or at low temperatures. A low temperature both
decreases the contaminant level of [NH3] (Sect. 2.4) and increases
the threshold [NH3] / [H2SO4] below which binary cluster
formation occurs (Fig. 6b).
Discussion
The composition of negative ions during new-particle formation experiments (no NH3 added)
Without adding any NH3 into the CLOUD chamber, most anions during a
new-particle formation experiment in the CLOUD chamber were small sulfuric
acid anion clusters. Heavier anion clusters, containing > 3
sulfur atoms, were not only “pure” sulfuric acid clusters but also
clusters associated with base molecules, namely NH3 or various organic
bases, mainly amines (Figs. 2c and 3). These findings agree qualitatively
with observations made in the ambient atmosphere (Ehn et al., 2010), with
independent measurements of such clusters in the laboratory
(Bzdek et al., 2011), as well as with the results of
simulations that consider classical collisions and evaporation rates based
on quantum chemical methods (Olenius et al., 2013b). Both the
earlier experimental and theoretical studies, as well as our results here,
show that NH3 molecules can only cluster with anionic (= deprotonated) sulfuric acid clusters,
i.e., form (NH3)m ⋅ (H2SO4)n ⋅ HSO4-, if n> 2.
This lower size limit for the inclusion of NH3 molecules is explained
by HSO4- itself acting as an electron donor (= Lewis base), in
competition with regular bases such as NH3. Only when
n> 2
is the cluster acidic enough to accept NH3 molecules. If NH3 or
amines are available, their inclusion into larger clusters (n> 2)
substantially enhances cluster stability, leading to a higher abundance as
well as detectability in our measurements, compared to the corresponding
pure sulfuric acid cluster (Ortega et al., 2014).
The observation of both ammonia and amines in these clusters here is
remarkable because neither NH3 nor amines were deliberately fed into
the CLOUD chamber for these experiments. All NH3 and amines were
unintended impurities ([NH3] < 5 pptv, [C2H7N]
< 1 pptv), yet they were found to play a crucial role in the
chemistry of growing ion clusters.
The role of charge carriers different from HSO4-
In order of importance, the charge carriers in the observed base–sulfuric-acid clusters were HSO4-, HSO5-, SO5-, and
H2O11NS2-. The HSO4- ion strongly correlated
with [H2SO4] and was certainly formed by the de-protonation of
H2SO4. A formation mechanism for SO5- has been
investigated by Möhler et al. (1992). The initial step of
this mechanism is the formation of SO3- from SO2 by the
transfer of O- from O3-. Subsequently, SO5- forms
from SO3- and O2 with a mediating N2 or H2O. We
observed that O3 in the CLOUD chamber was required for the formation of
both SO5- and HSO5-, supporting these or similar
ion–molecule reactions as a source. Possible origins of the HSO5-
ions were ion–molecule reactions similar to those probably leading to the
formation of SO5-, a proton-transfer reaction with
peroxymonosulfuric acid H2SO5, or the electronic charging of
HSO5, while HSO5 itself had possibly been formed as described in
Kurtén et al. (2009). H2O11NS2-
was probably H2S2O8 ⋅ NO3-.
H2S2O8 might have been formed from hydrated SO5 or from
HSO5 (Kurtén et al., 2009). Nitric acid
(HNO3) and its conjugate base NO3- were present as
contaminants in the chamber.
By far the most abundant ions during the new-particle formation experiments
at CLOUD that are discussed here were the clusters of type
(NH3)m ⋅ (H2SO4)n ⋅ HSO4- and (NH3)m ⋅ (H2SO4)n ⋅ HSO5-. The latter type gains
relevance in conditions of high [NH3]. Ammonia was present at n≥2 in the HSO5--based clusters but at n≥3 in the
HSO4--based clusters. This suggests that sulfuric acid ion
clusters will cluster more readily (i.e., build more stable clusters) with
NH3 when the charge-carrying component is HSO5- instead of
HSO4-. Most likely, this was the primary reason for the observed
increase of the fraction of HSO5--based clusters at higher values
of [NH3]. A secondary reason was the lower [H2SO4] during
such experiments, which was usually coincidental. The only other difference
to the HSO4--based clusters was an initially higher ratio m / n, as a
direct consequence of the acceptance of NH3 molecules already at n=2. However, the composition of the clusters was otherwise the same, and the
prevalent mechanism of cluster growth was not affected, namely the addition
of a certain number of NH3 molecules per H2SO4 molecule.
Therefore, we conclude that the presence and actions of charge carriers
other than HSO4- do not alter the conclusions of this study.
Interestingly, when n≥9, differences in the ratio m / n disappeared or
became approximately constant, suggesting that at about that size the ion
cluster had become large enough to render any chemical effect of its
charge-carrying component unimportant.
In conclusion, we suggest that HSO5- is a somewhat weaker Lewis
base than HSO4-. Therefore, clusters containing HSO5-
more readily take up bases and less readily acids, as compared with clusters
containing HSO4-, in line with all of our observations.
Furthermore, the highest prevalence of HSO5- in all experiments at
the CLOUD chamber to date has been in the dimethylamine–H2SO4
anion clusters that were produced during later experiments (in 2011 and
2012) following the addition of dimethylamine, a stronger base than NH3
(Almeida et al., 2013).
The role of contaminant levels of amines in the formation and
growth of anion clusters
Throughout our experiments, contaminant levels of amines were below the
limit of direct detection. We estimate the gas-phase contaminant
concentrations of C2H7N between 0.1 and 1 pptv during the CLOUD 2
campaign, and about 0.1 pptv during the CLOUD 3 campaign, but they may have
been even lower. The most abundant contaminant amine observed in anionic
clusters was C2H7N, and the most abundant cluster containing
C2H7N was C2H7N ⋅ (H2SO4)3 ⋅ HSO4- (Fig. 3). Note that
C2H7N ⋅ (H2SO4)2 ⋅ HSO4- was observed, whereas NH3 ⋅ (H2SO4)2 ⋅ HSO4- was not. Nevertheless,
the growth of the clusters at contaminant levels of amines proceeded almost
exclusively by the addition of NH3 and H2SO4 molecules. The
resultant dominant role of NH3 in the growth of the clusters, as
opposed to organic bases (amines or amides), is most likely due to the
differences in respective contaminant-level concentrations ([NH3] about
2 to 4 pptv, [C2H7N] < 1 pptv). Indeed, previous
experimental studies on both positively and negatively charged
dimethylamine–NH3–H2SO4 clusters showed that dimethylamine
molecules would quickly displace NH3 molecules in these clusters
already at low-pptv-level amine concentrations, whereas the opposite
(displacement of dimethylamine by NH3 molecules) does not occur even at
much higher gas-phase NH3 concentrations (Bzdek et al., 2010; Bzdek
et al., 2011).
The effect of higher than contaminant gas-phase concentrations of amines, in
particular of dimethylamine, on the composition of growing clusters and on
particle formation rates was thoroughly investigated in subsequent CLOUD
campaigns (Almeida et al., 2013; Bianchi et al., 2014). A large influence on cluster formation
and particle formation rates was found at dimethylamine concentrations as
low as 3 pptv. Specifically, growing ion clusters consisted of practically
only dimethylamine and H2SO4, and particle formation rates were
significantly enhanced. The enhancement of particle formation rates in those
experiments was due to dimethylamine being a stronger base than NH3 and
consequently forming more stable bonds with H2SO4 molecules, as
has been shown both theoretically (e.g., Bzdek et al.,
2010) and experimentally (e.g., Kurtén et al., 2008).
The same reason can account for the observation (here, as in Almeida et
al., 2013) that dimethylamine binds first to the sulfuric acid trimer anion,
whereas NH3 requires one more H2SO4 in the cluster and binds
first to the sulfuric acid tetramer anion. Specifically,
dimethylamine
competes more successfully than NH3 against HSO4-, which acts
as a strong Lewis base in these clusters.
However, in the experiments presented here, no effect on the rate of
new-particle formation was observed due to the contaminant levels of amines
(< 1 pptv). Also, the composition of the growing ion clusters was
dominated by NH3 and appeared to be unaffected by the occasional
inclusion of an amine.
Interestingly, a similar dominance of NH3 over amines is also apparent
in the composition of base–sulfuric-acid ion clusters that were observed
during new-particle formation in the ambient atmosphere, specifically in the
boreal forest environment in Hyytiälä, Finland (Kulmala et al.,
2013; Schobesberger et al., 2013a). However, it was also shown that other
oxidized organic compounds participate in those atmospheric new-particle
formation events as well from an early stage on (Ehn et al., 2014),
suggesting more complex mechanisms of formation and growth of clusters,
which have not yet been determined in detail.
The composition of NH3–H2SO4 clusters in the CLOUD laboratory experiments
The APi-TOF measurements of NH3–H2SO4 clusters during
particle formation experiments at the CLOUD chamber revealed that these
clusters grow by the accretion of certain numbers of NH3 and
H2SO4 molecules. The measurements covered the range up to clusters
containing about 27 sulfur atoms. This maximum size corresponds to 2.1 nm in
mobility equivalent diameter, when converted according to Ehn et al. (2011) using the bulk density of ammonium bisulfate (1780 kg m-3; and
neglecting a likely involvement of H2O).
The anion clusters were mainly of the form (NH3)m ⋅ (H2SO4)n ⋅ HSO4-. The HSO4-
ion acts as a strong Lewis base, so stable bonds with NH3 molecules are
only possible for n≥3. For larger clusters (n≥4), the cluster
growth was well characterized by the ratio of added NH3 molecules per
added H2SO4 molecule, Δm / Δn, which we found to be
dependent on the ratio [NH3] / [H2SO4] and temperature
(Figs. 4, 6a). At sufficiently high values of [NH3] / [H2SO4], Δm / Δn saturated at just above unity. Note that a ratio of Δm/Δn=1 corresponds to the stabilization of each H2SO4
molecule by an NH3 molecule, as in ammonium bisulfate (whereas Δm/Δn=2 would correspond to the full neutralization of each
H2SO4 molecule by two NH3 molecules, as in ammonium sulfate).
All these observations make it very likely that the binding of molecules via
strong hydrogen bonds between acidic and basic molecules and acid–base
reactions were the dominant mechanism in both the initial formation of these
clusters and their subsequent growth up to > 2 nm. The basic
molecules were the HSO4- ion and NH3 molecules, while the
acidic molecules were H2SO4 molecules. In terms of cluster
composition, therefore, the chemical property of HSO4- appears to
outweigh electrostatic effects due to the electric charge. Consequently, the
enhancements of particle formation rates attributed to NH3 or to a
negative charge (Kirkby et al., 2011) may both be the consequence of
essentially the same process of acid–base stabilization (Kupiainen et
al., 2012).
These findings are in agreement with previous studies that investigated the
structure of the bonds in electrically charged or neutral
NH3–H2SO4 clusters: both theoretical (e.g., Ortega et al.,
2012; DePalma et al., 2012; Ortega et al., 2014) and experimental studies
(e.g., Rozenberg et al., 2011; Froyd and Lovejoy, 2012; Johnson and
Johnson, 2013) have shown that NH3 molecules are bound to
H2SO4 molecules via the transfer of a proton from the acid to the
base (acid–base reaction) in all but the smallest of these clusters. Note
that for simplicity, the chemical formulas used in this paper disregard
these reactions.
Note also that all of the observed clusters were probably hydrated before
their H2O molecules were lost in the sampling process, due to the
abundance of H2O at the conditions in the CLOUD chamber
(e.g., Henschel et al., 2014). The stabilizing effect
of H2O on H2SO4 is much smaller than that of NH3
(e.g., Kurtén et al., 2007a; DePalma et al., 2014), but at least in
the absence of NH3, the contribution of H2O is important
(e.g., Vehkamäki et al., 2002).
Summary of the observed composition of
(NH3)m ⋅ (H2SO4)n ⋅ HSO4- anion clusters for the covered experimental conditions:
varying vapor concentration ratios [NH3] / [H2SO4] (left
vertical axis) and temperatures (horizontal axis and same color scale as for
Fig. 6). Three specific features of cluster composition are presented at
three investigated temperatures each, by a total of nine circles. These
three features are (1) a fraction of 50 % of pure, NH3-free sulfuric
acid clusters, (2) a ratio of added NH3 molecules per added
H2SO4 molecule, Δm/Δn, = 0.5, and (3) Δm/Δn=1. Values and uncertainties are based on the data shown in
Fig. 6. Consequently, the grey shaded area at the bottom covers conditions
allowing the formation of pure binary (NH3-free)
H2O–H2SO4 clusters. The grey shaded area at the top covers
conditions that allow for the formation of clusters with about equal numbers
of NH3 and H2SO4 molecules (as in ammonium bisulfate). The
position of the green box and the color of its dashed edge line mark the
conditions during the measurements in the boreal forest (Hyytiälä).
Shown in brown (right vertical axis and line) is the ratio of saturation
vapor pressures of NH3 and H2SO4 (psat.,NH3/psat.,H2SO4).
In Fig. 7 we summarize the characteristics of the observed composition of
(NH3)m ⋅ (H2SO4)n ⋅ HSO4- anion clusters as a function of vapor concentration ratio
[NH3] / [H2SO4] and temperatures. Pure binary
H2SO4–H2O cluster formation was favored by low values of
[NH3] / [H2SO4] and low temperatures. From a macroscopic point
of view, this temperature dependence is consistent with the differences in
the temperature dependences of the saturation vapor pressures of bulk
H2SO4 and NH3. Namely, the saturation vapor pressure of
H2SO4 decreases more steeply than that of NH3 with decreasing
temperature (Vehkamäki et al., 2002; Hodgman,
1962); therefore the ratio of these pressures (psat.,NH3/psat.,H2SO4) increases with decreasing temperature.
From a molecular-level point of view, the influence of NH3 vs. H2SO4 vapors on the growing clusters is consistent with a barrier
for addition of NH3 to the clusters but barrierless addition of
H2SO4. This has also been deduced from surface-induced cluster
dissociation and quantum chemistry (Bzdek et al., 2013).
Specifically, the near-saturation of the NH3 content of the clusters at
[NH3] / [H2SO4] > 10 suggests that roughly 10
collisions of NH3 with an under-neutralized cluster are necessary to
add the base molecule to a growing cluster, or to overwhelm the evaporation
rate of NH3 from the cluster. The presence of a barrier for the
NH3 uptake would be expected to lead to a slower NH3 uptake at
lower temperatures. Such a temperature dependence is consistent with the
increased value of [NH3] / [H2SO4] required at lower
temperatures to add NH3 molecules to under-saturated
NH3–H2SO4 or H2SO4 clusters (Figs. 6, 7).
At high values of [NH3] / [H2SO4], the ratio Δm / Δn usually exceeded 1, approaching saturation (Figs. 6a, 7). This relationship appeared
independent of the temperature within uncertainties and the investigated
temperature range (-25 to 20 ∘C). However, our data set may not
be sufficiently complete to resolve possible dependencies on temperature. In
this regime of high values of [NH3] / [H2SO4], cation clusters
of the form (NH3)m ⋅ (H2SO4)n ⋅ NH4+ were also observable,
again with Δm/Δn≥1 (Fig. 6a). Note that the anion
clusters tended to feature slightly higher saturation values of Δm / Δn at lower temperatures. This feature could be an indication of
the enhanced evaporation of NH3 molecules from the clusters at higher
temperatures, before or after the sampling. Indeed, results of computer
simulations using the atmospheric cluster dynamics code (ACDC) suggest that
there may be a systematic slight underestimation of the NH3 content of
the experimentally observed clusters in these conditions of relatively
abundant gas-phase NH3 that we cannot exclude (Olenius et al.,
2013b; Olenius et al., 2013a). Those as well as earlier studies
(e.g., Kurtén et al., 2007b) have demonstrated how
at least small NH3–H2SO4 clusters are expected to hold onto a
higher number of NH3 molecules at lower temperatures.
Mass defect diagrams of ion mass spectra during new-particle formation. The NH3–H2SO4 ion clusters are
colored;
other ions are shown in grey. The colors reflect the relative numbers of
acids and bases in the cluster, counting NH3, HSO4-,
HSO5- and SO5- as bases, and H2SO4 as acid,
and not counting NH4+. (a) Positively charged clusters
(NH3)m ⋅ (H2SO4)n ⋅ NH4+ during new-particle formation at CLOUD at
[NH3] / [H2SO4] = 185, temperature (T) = 19 ∘C.
Other ions (in grey) are mostly organic, N-containing contaminants, such as
pyridine, charged by protonation. (b) Negatively charged clusters
(NH3)m ⋅ (H2SO4)n ⋅ HSO4- and (NH3)m ⋅ (H2SO4)n ⋅ HSO5- during new-particle
formation at CLOUD at [NH3] / [H2SO4] = 118, T=19 ∘C. (c) Negatively charged clusters
(NH3)m ⋅ (H2SO4)n ⋅ HSO4- during a new-particle formation event in the boreal forest,
Hyytiälä, at [NH3] / [H2SO4] = 485, T=1 to 5 ∘C. Most of the signal of other ions (in grey) has a more
positive mass defect and likely arises from ions containing mainly a variety
of oxidized organics.
Since both the anion and cation clusters had Δm / Δn slightly
above unity when [NH3] / [H2SO4] ≥10, the clusters became
slightly less acidic as they grew. The mass defect diagrams of the
corresponding ion mass spectra illustrate the details on the increase of the
clusters' base content as they grew (Fig. 8a, b). The anion clusters were
seen to grow first by the addition of acids due to the high basicity of
HSO4-. From the tetramer onwards (#S ≥4; see Sect. 4.2
for exceptions), the anions became chemically (not electrically)
neutralized, as the clusters grew by the addition of slightly more than one
base per acid on average (Fig. 8b). For the cation clusters, NH3 was
already present for a monomer (i.e., NH3 ⋅ H2SO4 NH4+), and the clusters became
slightly more basic from the first step onwards (Fig. 8a). The maximum
observed Δm / Δn ratios were about 1.4 for anion clusters and
1.1 for cation clusters. Therefore the NH3–H2SO4 molar ratio
(m / n) exceeded unity beyond a certain cluster size, as illustrated in Fig. 9.
Note that if the clusters' growth continued to adhere to the observed
Δm / Δn ratios also beyond a mobility size of 2 nm, m / n would
approach these values (grey curves in Fig. 9). And if Δm / Δn
stayed below 1.5, as observed, also for larger clusters, ammonium bisulfate
would remain the favored composition (as opposed to ammonium sulfate).
However, it should also be noted here that for the positively charged
clusters both Fig. 4 and, more clearly, Fig. 9 reveal an apparent
discontinuity at m=n=10, with an increase of the slopes, specifically
an increase of Δm / Δn from 1.0 to about 1.3 for the larger
positively charged clusters.
The average measured NH3 / H2SO4 molar ratio
in the negatively and the positively charged NH3–H2SO4
clusters, plotted against cluster size, which is approximated by the number
of S atoms in the clusters. The secondary abscissa shows the corresponding
mobility-equivalent diameter for these clusters, assuming an
NH3 / H2SO4 molar ratio = 1 and a density of 1780 kg m-3
(i.e., ammonium bisulfate). An NH3 / H2SO4 molar ratio = 2
would correspond to ammonium sulfate. The presented data are
uncertainty-weighted averages over the observations from the CLOUD chamber
experiments in the respective conditions that yielded the maximum NH3
content in the clusters at three different temperatures. Shown as grey
curves are calculated results assuming growth following a constant Δm/Δn, starting from the observed average m at n=4 (anions) or n=2
(cations).
Interestingly, we observed a wider range of the number of NH3 molecules
m for any n in anion clusters than in cation clusters. We hypothesize that
this observation follows from the requirement that NH3–H2SO4
clusters be basic enough to accept an additional proton (therefore becoming
or staying positively charged), whereas they need to be acidic enough to
donate a proton (therefore becoming or staying negatively charged). Sulfuric
acid can form clusters with only itself (plus H2O) much more readily
than can NH3. Therefore, highly acidic (NH3–)H2SO4
clusters can be formed, and they were indeed observed as anion clusters when
[NH3] / [H2SO4] was sufficiently low (e.g., Fig. 2c). For these
clusters Δm / Δn was < 1; that is, they continuously became
more acidic as they grew. The possibility of such relatively acidic anion
clusters leads to the wider range of observed m even at relatively high
[NH3]. On the other hand, the maximum basicity of
NH3–H2SO4 clusters (or their NH3 / H2SO4 molar
ratio, m / n) is apparently limited, even at the highest investigated [NH3]
(Fig. 6a). Only such relatively basic clusters can be observed as cation
clusters.
Note that almost all of our experiments were performed with RH between
37 % and 41 %. Indeed, the data presented in Fig. 5 show that the
observed NH3–H2SO4 clusters contained more NH3 at RH
> 68 %. It remains a task for future studies to thoroughly
investigate how the composition of these clusters changes with changing RH.
It is also worth noting that the APi-TOF was set to measure positively
charged clusters in only few experiments; only for experiments at 19 ∘C did these experiments include conditions with
[NH3] / [H2SO4] < 100.
Comparison to the composition of electrically neutral clusters
from the ACDC model
In this work we hypothesize that the growth mechanism of ionic
NH3–H2SO4 clusters is governed chiefly by hydrogen bonds and
ionic bonds formed by acids and bases and the availability of the gas-phase
precursors NH3 and H2SO4. The electric charge somewhat
increases the stability (Olenius et al., 2013a), but
most importantly it provides an additional base in the form of
HSO4-, whereas the NH4+ ion seems to exhibit neither
strongly basic nor acidic behavior. At relatively high [NH3], both
positively and negatively charged NH3–H2SO4 clusters grow by
adding NH3 and H2SO4 molecules at a ratio Δm / Δn
of about unity, once the cluster is large enough for the effect
of the HSO4- or NH4+ ion to be neutralized in terms of
its basicity or acidity (Figs. 4, 6a, 8a, 8b). Therefore we would expect
electrically neutral clusters to grow by the same ratio Δm / Δn
from the first bond formation onwards. At relatively low [NH3], only
negatively charged clusters were observed in our experiments, growing mainly
by the uptake of H2SO4 molecules (Figs. 4, 6a), whereas positively
charged clusters were not observed. Electrically neutral
NH3–H2SO4 clusters must have been formed under these conditions
as well, since new-particle formation occurred without any ions in the CLOUD
chamber (Kirkby et al., 2011).
Evaporation rates from anion clusters, used for the ACDC
model simulations. Clusters that were seen in the APi-TOF
spectra are shown with the ∗.
Cluster
Highest evaporation rate (s-1)
Evaporating molecule/
-25 ∘C
5 ∘C
19 ∘C
cluster
H2SO4 ⋅ HSO4-
2.24 × 10-22
8.07 × 10-18
4.99 × 10-16
H2SO4∗
(H2SO4)2 ⋅ HSO4-
4.80 × 10-7
1.97 × 10-4
2.12 × 10-3
H2SO4∗
(H2SO4)3 ⋅ HSO4-
9.80 × 10-3
1.48
1.07 × 101
H2SO4∗
(H2SO4)4 ⋅ HSO4-
2.88
2.41 × 102
1.38 × 103
H2SO4∗
NH3 ⋅ HSO4-
2.40 × 1010
1.52 × 1011
3.15 × 1011
NH3
NH3 ⋅ H2SO4 ⋅ HSO4-
7.15 × 108
6.10 × 109
1.42 × 1010
NH3
NH3 ⋅ (H2SO4)2 ⋅ HSO4-
2.03 × 102
1.45 × 104
7.86 × 104
NH3
NH3 ⋅ (H2SO4)3 ⋅ HSO4-
1.31 × 10-2
1.78
1.24 × 101
NH3∗
NH3 ⋅ (H2SO4)4 ⋅ HSO4-
2.56 × 10-4
7.86 × 10-2
7.56 × 10-1
H2SO4∗
(NH3)2 ⋅ (H2SO4)2 ⋅ HSO4-
8.92 × 102
2.06 × 104
7.05 × 104
NH3
(NH3)2 ⋅ (H2SO4)3 ⋅ HSO4-
1.97 × 10-3
4.96 × 10-1
4.44
NH3∗
(NH3)2 ⋅ (H2SO4)4 ⋅ HSO4-
4.31 × 10-5
1.38 × 10-2
1.36 × 10-1
NH3∗
(NH3)3 ⋅ (H2SO4)3 ⋅ HSO4-
3.46
2.20 × 102
1.12 × 103
NH3
(NH3)3 ⋅ (H2SO4)4 ⋅ HSO4-
7.21 × 10-7
3.17 × 10-4
3.52 × 10-3
NH3∗
(NH3)4 ⋅ (H2SO4)4 ⋅ HSO4-
9.21 × 107
8.92 × 108
2.18 × 109
NH3
We could not measure electrically neutral NH3–H2SO4 clusters
in this study. However, we studied their composition using ACDC computer
simulations. ACDC is a kinetic collision and evaporation model addressing
the formation dynamics and concentrations of molecular (charged and neutral)
clusters (McGrath et al., 2012). In this study, we
used ACDC to simulate the steady-state distribution of neutral clusters
(NH3)m ⋅ (H2SO4)n up to m=5 and n=5, as described in Almeida et al. (2013), covering the different
combinations of temperature, [NH3] and [H2SO4] probed during
the experiments. Cluster evaporation rates were calculated from quantum
chemical Gibbs free energies of formation of the clusters (Tables 1–3). The
resulting simulated neutral clusters at steady state had an average NH3
content of up to m=n, dependent on [NH3] / [H2SO4] (Fig. 4).
Note here certain differences in the composition when comparing the combined
results for neutral, positive and negative clusters, in particular for the
smallest ones. For example, the neutral dimers (n=2) are stabilized mostly by
one NH3 ligand (i.e., NH3 ⋅ (H2SO4)2),
which agrees with previous experimental and theoretical findings on the
stabilization of the neutral dimer by NH3 or other bases (e.g.,
Ortega et al., 2014; Jen et al., 2014). On the other hand, the positive
dimers mostly contain two NH3 ligands (i.e., (NH3)2 ⋅ (H2SO4)2 ⋅ NH4+), whereas the anion dimer and trimer (n=1,2) contain no
NH3 at all (i.e., H2SO4 ⋅ HSO4- and
(H2SO4)2 ⋅ HSO4-). Still, all these
compositions are consistent with our assertion that acid–base reactions are
the underlying binding mechanism: the ammonium ion NH4+ (the
conjugate acid of ammonia) acts as a weak acid, accommodating one additional
NH3 compared to the neutral dimer. On the other hand, the anion dimer
and trimer cannot accommodate any NH3 due to the presence of the
bisulfate ion HSO4-, the conjugate base of sulfuric acid, which
acts as a stronger base than NH3, as described above. However, we
expect the addition of more ligands, described by the ratio Δm / Δn, to be independent of the electric charge for cluster sizes
large enough that the acid or base effect of NH4+ or in particular
HSO4- is neutralized.
Evaporation rates from cation clusters, used for the ACDC
model simulations. Clusters that were seen in the APi-TOF
spectra are shown with the ∗.
Cluster
Highest evaporation rate (s-1)
Evaporating molecule/
-25 ∘C
5 ∘C
19 ∘C
cluster
H2SO4 ⋅ NH4+
4.60 × 10-4
3.30 × 10-2
1.77 × 10-1
H2SO4 ∗
(H2SO4)2 ⋅ NH4+
6.41 × 10-2
1.40 × 101
1.17 × 102
H2SO4 ∗
NH3 ⋅ NH4+
8.42 × 10-9
2.36 × 10-6
2.19 × 10-5
NH3∗
NH3 ⋅ H2SO4 ⋅ NH4+
2.71 × 10-6
1.02 × 10-3
1.06 × 10-2
H2SO4∗
NH3 ⋅ (H2SO4)2 ⋅ NH4+
1.96 × 10-4
5.38 × 10-2
4.95 × 10-1
H2SO4∗
NH3 ⋅ (H2SO4)3 ⋅ NH4+
1.93 × 10-1
2.68 × 101
1.88 × 102
H2SO4
(NH3)2 ⋅ NH4+
6.88 × 101
2.75 × 103
1.18 × 104
NH3
(NH3)2 ⋅ H2SO4 ⋅ NH4+
1.57
9.83 × 101
5.04 × 102
NH3 ∗
(NH3)2 ⋅ (H2SO4)2 ⋅ NH4+
6.43 × 10-10
5.41 × 10-7
7.76 × 10-6
NH3∗
(NH3)2 ⋅ (H2SO4)3 ⋅ NH4+
1.01 × 10-1
1.60 × 101
1.07 × 102
H2SO4∗
(NH3)2 ⋅ (H2SO4)4 ⋅ NH4+
5.90
4.36 × 102
2.63 × 103
H2SO4
(NH3)3 ⋅ (H2SO4)2 ⋅ NH4+
4.34 × 101
2.00 × 103
9.10 × 103
NH3∗
(NH3)3 ⋅ (H2SO4)3 ⋅ NH4+
3.57 × 10-8
1.61 × 10-5
1.99 × 10-4
NH3∗
(NH3)3 ⋅ (H2SO4)4 ⋅ NH4+
6.78 × 10-2
8.21
5.44 × 101
H2SO4 ∗
(NH3)4 ⋅ (H2SO4)3 ⋅ NH4+
6.00 × 101
2.73 × 103
1.23 × 104
NH3
(NH3)4 ⋅ (H2SO4)4 ⋅ NH4+
1.11 × 10-4
3.01 × 10-2
2.76 × 10-1
NH3∗
(NH3)4 ⋅ (H2SO4)5 ⋅ NH4+
2.26 × 10-4
5.59 × 10-2
4.92 × 10-1
H2SO4∗
Evaporation rates from neutral clusters, used for the ACDC
model simulations.
Cluster
Highest evaporation rate (s-1)
Evaporating molecule/
-25 ∘C
5 ∘C
19 ∘C
cluster
(H2SO4)2
1.73 × 101
8.23 × 102
3.79 × 103
H2SO4
(H2SO4)3
4.42 × 102
2.15 × 104
1.00 × 105
H2SO4
(H2SO4)4
1.17 × 102
4.54 × 103
1.92 × 104
H2SO4
(H2SO4)5
1.85 × 102
8.15 × 103
3.63 × 104
H2SO4
NH3 ⋅ H2SO4
1.50 × 102
4.74 × 103
1.85 × 104
H2SO4
NH3 ⋅ (H2SO4)2
3.38 × 10-6
1.21 × 10-3
1.25 × 10-2
NH3
NH3 ⋅ (H2SO4)3
3.11 × 10-1
3.02 × 101
1.84 × 102
H2SO4
NH3 ⋅ (H2SO4)4
5.91
6.03 × 102
3.75 × 103
H2SO4
NH3 ⋅ (H2SO4)5
1.87 × 102
1.10 × 104
5.48 × 104
H2SO4
(NH3)2
4.43 × 1012
6.09 × 1012
6.87 × 1012
NH3
(NH3)2 ⋅ H2SO4
4.17 × 104
7.97 × 105
2.56 × 106
NH3
(NH3)2 ⋅ (H2SO4)2
2.61
1.76 × 102
9.30 × 102
NH3
(NH3)2 ⋅ (H2SO4)3
1.37 × 10-5
3.84 × 10-3
3.59 × 10-2
NH3
(NH3)2 ⋅ (H2SO4)4
2.72 × 10-2
5.43
3.92 × 101
H2SO4
(NH3)2 ⋅ (H2SO4)5
6.55 × 101
3.58 × 103
1.96 × 104
H2SO4
(NH3)3
3.60 × 1012
1.31 × 1013
2.18 × 1013
NH3
(NH3)3 ⋅ H2SO4
1.72 × 108
1.72 × 109
4.28 × 109
NH3
(NH3)3 ⋅ (H2SO4)2
2.77 × 104
7.17 × 105
2.59 × 106
NH3
(NH3)3 ⋅ (H2SO4)3
2.24 × 10-4
4.60 × 10-2
3.77 × 10-1
NH3
(NH3)3 ⋅ (H2SO4)4
4.30 × 10-6
2.10 × 10-3
2.45 × 10-2
H2SO4
(NH3)3 ⋅ (H2SO4)5
4.59 × 10-3
7.83 × 10-1
5.95
H2SO4
(NH3)4
9.07 × 1011
2.35 × 1012
3.41 × 1012
(NH3)2
(NH3)4 ⋅ H2SO4
1.36 × 107
1.38 × 108
3.47 × 108
NH3
(NH3)4 ⋅ (H2SO4)2
2.46 × 106
3.33 × 107
9.31 × 107
NH3
(NH3)4 ⋅ (H2SO4)3
4.45 × 103
1.46 × 105
5.81 × 105
NH3
(NH3)4 ⋅ (H2SO4)4
5.87 × 10-1
3.75 × 101
1.94 × 102
NH3
(NH3)4 ⋅ (H2SO4)5
5.97 × 10-4
8.87 × 10-2
6.41 × 10-1
NH3
(NH3)5 ⋅ (H2SO4)4
7.49
8.89 × 102
5.91 × 103
NH3
(NH3)5 ⋅ (H2SO4)5
1.82 × 10-7
1.17 × 10-4
1.52 × 10-3
NH3
We calculated the Δm / Δn ratio for the simulated neutral
clusters in the same way as for the measured data. However, a single neutral
H2SO4 molecule was taken as the starting point for the simulated
neutral clusters, due to the absence of the HSO4- base. The
results are shown as dashed lines in Fig. 6a. The results from the
simulations of neutral clusters agreed with the measurements of charged
clusters in some respects: a maximum Δm / Δn of 1.2 to 1.3 was
reached at high values of [NH3] / [H2SO4], a higher maximum for
lower temperatures, and Δm / Δn decreased when
[NH3] / [H2SO4] < 10. Over the whole range, the
simulations also reproduced the chief dependence of Δm / Δn on
the ratio [NH3] / [H2SO4]. At low values
[NH3] / [H2SO4], however, the simulated neutral clusters gained
NH3 at a much higher rate than the measured negatively charged
clusters. This discrepancy may arise for at least three reasons: (1) H2O
molecules were not included in the model simulations, though they are
abundant at RH = 40 % and may play a more important role at relatively
low [NH3]; (2) small neutral clusters may indeed contain more NH3
than their negatively charged counterparts; (3) there is a barrier for the
uptake of NH3 that is not modeled by ACDC. Reason 1 would imply that
H2O molecules partially take over the role of stabilizing sulfuric acid
clusters from NH3 at relatively low [NH3]. Qualitatively, this
suggestion agrees with the expectation of H2O contributing to
stabilization of sulfuric acid clusters, especially in the absence of
NH3 (e.g., Vehkamäki et al., 2002), and of
these clusters containing more H2O with less NH3
(Henschel et al., 2014). As H2O was absent in the ACDC
simulations, the clusters' NH3 content may thus be over-predicted.
Reason 2 (more NH3 in neutral than in anion clusters) appears plausible
on its own, as it would put the reliance on NH3 of neutral clusters
between that of anion clusters (no NH3 required) and cation clusters
(relatively much NH3 required). Reason 3 implicates a barrier for the
uptake of NH3, but barrierless addition of H2SO4. The same
conclusion was suggested above (Sect. 4.4) and by an independent study
(Bzdek et al., 2013). ACDC assumes that collision
partners instantly arrange to their minimum energy configuration, from which
the new cluster may subsequently break apart. This assumption may be too
simple for conditions of low [NH3] / [H2SO4].
Neutral NH3–H2SO4 clusters were previously investigated
experimentally by Hanson and Eisele (2002), in conditions close to those
in this study, with [NH3] between 100 and 800 pptv, [H2SO4]
between 1 and 3 × 109 cm-3 (40 to 110 pptv), at
temperatures from -8 to +12 ∘C. The resulting
[NH3] / [H2SO4] ranged from about 2 to 13, notably a range
where we obtained few data. In that work, neutral clusters
(NH3)m ⋅ (H2SO4)n, up to n=6, were
ionized by proton transfer to nitrate ions, yielding anion clusters
(NH3)m ⋅ (H2SO4)n-1 ⋅ HSO4-, which were identified and counted using mass spectrometry.
The NH3 content in their ion clusters ranged from m=0 to n-1, and
was unaffected by changes in gas-phase [NH3]. They concluded that the
ionization process may be ineffective for neutral clusters with an NH3
content of m≥n, and also that it may lead to a loss of ligands, in
particular of NH3. The former conclusion agrees with our simulation
results of an NH3 content of small neutral clusters up to m=n (Fig. 4), and it is roughly in line with our observation of anions with m≥n
only starting from about n≥6 (Fig. 8b). The latter conclusion agrees
qualitatively with the experimental and theoretical result in this study
that the most prevalent (simulated) neutral cluster containing n
H2SO4 contains one to three more NH3 ligands than the most
prevalent corresponding (measured) anion cluster, containing n-1
H2SO4 (Fig. 4b). Interestingly, Hanson and Eisele (2002) also
detected trimer anions (n=3) including up to two NH3 ligands,
whereas no trimer anions containing NH3 were found in our study (cf.
Sect. 4.1). This difference is likely due the different production
mechanism for their ion clusters, i.e., ionization of neutral clusters as
opposed to growth of smaller already-charged clusters.
Note that the data presented in this work do not in fact allow conclusions
on the details of the actual growth process of the clusters, but our
discussion of the measured cluster size distributions here may have implied
the assumption of a step-wise addition of single H2SO4 and
NH3 molecules. In the ACDC simulations, > 99 % of the
modeled charged clusters indeed grow by the step-wise addition of single
molecules, due to the small concentrations of the involved clusters.
However, a major fraction of the modeled electrically neutral clusters
formed by the recombination of charged clusters, if [H2SO4] is low
and temperature high (e.g., [H2SO4] ≤ 106 cm-3 at
5 ∘C) (details in Olenius et al., 2013a).
Comparison of NH3–H2SO4 clusters from CLOUD to ambient observations
Negatively charged NH3–H2SO4 clusters are commonly observed
during new-particle formation events at the boreal forest measurement site
in Hyytiälä when using APi-TOF mass spectrometers (Kulmala et
al., 2013; Schobesberger et al., 2013a). A typical anion mass spectrum
obtained from those ambient APi-TOF measurements is presented in Fig. 8c,
with the (NH3)m ⋅ (H2SO4)n ⋅ HSO4- clusters shown colored. The majority of the larger ions
(shown in grey) likely contain mainly a variety of oxidized organics,
sometimes along with H2SO4 molecules (Schobesberger et al.,
2013a; Ehn et al., 2012). However, most H2SO4 molecules in these
anion clusters are found in the (NH3–)H2SO4 clusters. Amines
are present in some of these clusters as well, but most of the larger of
these clusters (n>3) contain only NH3 and H2SO4.
Figures 4b, 6a and 7 include a comparison of the CLOUD results with the
observations of negatively charged NH3–H2SO4 clusters during
new-particle formation events in the boreal forest during springtime. The
ambient measurements were made under conditions comparable to those covered
by the CLOUD experiments, with mean values of 4 ∘C for the
temperature and 47 % for RH. The NH3–H2SO4 clusters from
the boreal forest measurements always showed a high NH3 content,
comparable to clusters observed at CLOUD, and with Δm / Δn > 1. The [NH3] / [H2SO4] ratios measured in the
boreal forest at the same times were relatively high as well, with
[NH3] ranging from 28 to 134 pptv (0.8 to 3.6 × 109 cm-3) and [H2SO4] from 0.5 to 2.1 × 107 cm-3.
In these terms, therefore, the ambient observations fully agree
with the findings from the laboratory experiments at CLOUD (Figs. 4b, 6a,
7). That specific [H2SO4] range is well within the values of
[H2SO4] during new-particle formation events recorded around the
world, those values ranging from about 105 to 108 cm-3 (e.g., Kuang et al., 2008). The [NH3] range
observed during the boreal forest events appears on the low side, as it is
commonly exceeded by measurements of [NH3] at many other locations
(Ziereis and Arnold, 1986; Janson et al., 2001; Riipinen et al., 2007;
Gong et al., 2011; Osada et al., 2011). Therefore, our results suggest that
atmospheric NH3–H2SO4 clusters forming in the warm boundary
layer always feature the maximum NH3 content (i.e., Δm / Δn > 1) as most locations of boundary layer observations are
likely saturated in [NH3] with respect to the growth of
NH3–H2SO4 clusters.
Binary H2O–H2SO4 new-particle formation (i.e., with negligible
contribution of NH3) can only occur in conditions of sufficiently low
[NH3] / [H2SO4] or at low temperatures. Therefore, binary
H2O–H2SO4 new-particle formation must be largely restricted
to the free troposphere, and relatively cold parts are preferred. Low
temperatures can also facilitate appreciable particle formation rates at
atmospherically relevant levels of [H2SO4] without the necessity
of additional participating vapors (Kirkby et al., 2011).
Summary and conclusions
We have presented a comprehensive description of the composition of
NH3–H2SO4 clusters as a function of environmental variables,
in particular concentrations of precursor vapors ([NH3] and
[H2SO4]) and temperature. A wide range of atmospherically relevant
conditions were covered, with [NH3] ranging from < 2 to 1400
pptv, [H2SO4] from 3.3 × 106 to 1.4 × 109 cm-3 (0.1 to 56 pptv), and temperature from -25 ∘C
to +20 ∘C. Our ion cluster measurements covered the size range
between 1 and 2 nm (mobility-equivalent diameters).
We found that the ratio [NH3] / [H2SO4] principally determines
the composition of the measured NH3–H2SO4 ion clusters, with
temperature in a secondary role. Positively charged clusters are only
observed from a sufficiently high ratio [NH3] / [H2SO4]
upwards. From a ratio [NH3] / [H2SO4] of about 10 up to at
least 500, both negatively and positively charged clusters grow by the
addition of on average 1 to 1.4 NH3 molecules per each addition of an
H2SO4 molecule. The resultant NH3 / H2SO4 molar
ratios are remarkably close to that of ammonium bisulfate (with an
NH3 / H2SO4 molar ratio of unity). On the other hand, pure
binary H2O–H2SO4 clusters (without contribution of NH3)
only form at values of [NH3] / [H2SO4] smaller than about 0.1
(depending also on temperature). In our experiments, these binary clusters
were only observed as negatively charged H2SO4 clusters. In the
ambient atmosphere, their formation must be largely restricted to higher
regions of the troposphere, where NH3 concentrations are low.
A detailed comparison of model results probing the growth of negative,
positive and neutral NH3–H2SO4 clusters is presented in
Olenius et al. (2013a). So far, the quantum chemical
data used to calculate the cluster evaporation rates restricted those model
simulations to clusters containing up to 10 molecules, whereas the APi-TOF
measurements could measure negatively and positively charged clusters up to
clusters containing > 50 NH3 and H2SO4 molecules.
The model simulations of neutral clusters and APi-TOF measurements of
charged clusters are consistent and in good agreement with each other for
cases of [NH3] / [H2SO4] > 10. Under these
conditions, electrically neutral NH3–H2SO4 clusters are also
likely to grow principally by adding, on average, 1 to 1.4 NH3
molecules for each added H2SO4 molecule. Note that the
recombination of anion and cation clusters can also contribute to the
population of neutral clusters (Olenius et al., 2013a; Kontkanen et al.,
2013).
In combination, measurement and model results strongly suggest that
acid–base interactions are the dominant clustering mechanism for all
NH3–H2SO4 clusters. These interactions allow for the initial
formation of clusters and facilitate additional inter-molecular bonds as the
clusters grow to 2 nm in size and larger, i.e., sizes that are nowadays
accessible by condensation particle counters
(Lehtipalo et al., 2014). The presence of an
electric charge implies a missing or extra proton, i.e., the creation of a
conjugate base or acid. In particular, the (strong) basic properties of
HSO4- have an important impact on the composition of the small
negatively charged clusters. We identified the HSO5- ion as an
important alternative ion in these clusters for the conditions in the CLOUD
chamber. It appears to be slightly less basic than HSO4- and
similarly affects cluster composition. Besides the stabilizing effect of the
electric charge on the cluster, the property of HSO4- (or
HSO5-) as a Lewis base is probably crucial to the enhancements of
particle formation rates attributed to ions (Kirkby et al., 2011).
In the atmospheric boundary layer, the composition of formed
NH3–H2SO4 clusters will mostly be in the saturation regime
that we observed with respect to the clusters' NH3 content; that is, they
will be seen growing by the addition of on average slightly more than one
NH3 molecule per added H2SO4 molecule. This is because
H2SO4 concentrations in the boundary layer are comparatively low,
mostly at sub-pptv levels, leading to typical vapor concentration ratios
[NH3] / [H2SO4] of larger than 10. Ambient APi-TOF measurements
during new-particle formation events in the Finnish boreal forest indeed
confirm this prediction.
Another general requirement for the formation of NH3–H2SO4
clusters is the sufficiently low abundance of compounds competing with
NH3 in forming clusters with H2SO4. One such class of
compounds has been shown to be amines, in particular dimethylamine (Bzdek
et al., 2011; Almeida et al., 2013). The measurements at the boreal forest
measurement site in Hyytiälä have shown that large
NH3–H2SO4 ion clusters do not usually contain amines
(Schobesberger et al., 2013a). This observation suggests relatively low
amine concentrations in the boreal forest environment. In fact, the
prevalence of NH3 over amines is similar to the observations in the
CLOUD chamber at the presence of contaminant levels of amines, indicating
ambient dimethylamine concentrations in the boreal forest of < 1 pptv.
What remains unsolved is the exact role of NH3–H2SO4
clusters, and NH3 in general, in the initial steps of the boundary
layer events of new-particle formation and growth that are frequently
observed in the Finnish boreal forest and elsewhere
(Kulmala et al., 2004b). Such clusters have been observed,
but laboratory experiments suggest that they are not stable enough to fully
account for the bulk of boundary layer particle formation (Kirkby et al.,
2011). Other experiments conclude that clusters of two to three
H2SO4 molecules plus a mix of basic molecules (NH3 and
amines) can in fact account for particle formation in polluted conditions
(Chen et al., 2012).
At least in clean environments rich in α-pinene, such as boreal
forests, recent experimental evidence hints at an important role of
highly oxidized organic compounds with extremely low volatility in the very
first steps of boundary-layer particle formation (Kulmala et al., 1998;
Schobesberger et al., 2013a; Ehn et al., 2014; Riccobono et al., 2014).
These organic compounds probably feature several functional groups that
facilitate hydrogen bonds with each other and with H2SO4
molecules, in the same way as the bonds between H2SO4 and NH3
(Donahue et al., 2013). However, the inclusion of
dimethylamine in these sub-2 nm clusters has been observed as well
(Riccobono et al., 2014). Our ambient observations in the boreal forest
suggest that a large fraction of H2SO4 molecules in sub-2 nm
clusters are found in clusters with NH3. Therefore, it seems very
likely that mixed sub-2 nm NH3–H2SO4–organics clusters are
also stable and indeed commonly contribute to particle formation in the
boreal forest.
The measurement results presented here substantially extend our knowledge on
how NH3 and H2SO4 interact in detail when forming and growing
clusters under atmospheric conditions. The results are in general agreement
with results from model simulations (e.g.,
Olenius et al., 2013a), as well as with previous experimental work (e.g.,
Hanson and Eisele, 2002; Bzdek et al., 2011; Froyd and Lovejoy, 2012; Bzdek
et al., 2013). Altogether, we are moving closer to gaining a more complete
and detailed understanding of this subject. The most important contribution
of the present study is a detailed examination of cluster compositions under
a wide range of atmospherically relevant conditions, and experimentally
covering the whole sub-2 nm size range for charged clusters. Supported by
simulations of cluster population dynamics, the results also allow for
inferences to be made on electrically neutral NH3–H2SO4
clusters.