Introduction
Understanding the occurrence and composition of ions in the atmosphere is
important because they regulate the electrical properties of the atmospheric
medium, participate in ion-catalyzed and ion–molecule reactions, and
contribute to physicochemical interactions, including ion-induced new
particle formation (NPF) (Schulte and Arnold, 1990).
Ionization in the atmosphere takes place via different routes depending on
the altitude. In the lower troposphere, ions are produced by radioactive
emanation (mainly radon decay and gamma radiation), lightning, and galactic
cosmic rays (GCR), resulting in a production rate of about 2 ion-pairs cm-3 s-1
at sea level. This production rate increases with
altitude, mainly due to increasing GCR intensity, reaching maximum rates of
35–50 ion-pairs cm-3 s-1 at 15 km (Arnold, 2008; Hirsikko et al.,
2011; Smith and Spanel, 1996). The first observations of ions in the free
troposphere and stratosphere were reported by Heitmann and
Arnold (1983). By using ion mass spectrometers at high altitude (either on
aircraft, balloons, or rockets), they found that the main ions in the free
troposphere were complex cluster ions containing H2SO4, H2O,
HNO3, (CH3)2CO, and CH3CN. In the last decade, the
interest in atmospheric ions increased because of the potential impact of
ion–aerosol–cloud interactions on climate (Hirsikko
et al., 2011; Kirkby, 2007; Kirkby et al., 2011, 2016). Therefore, a number
of laboratory and field studies have been conducted aimed at increasing our
understanding of the precise role of ions in new particle formation. High
altitude sites, which are often located in the free troposphere, represent
an interesting region with a low condensational sink and low temperatures,
i.e., conditions that facilitate the formation of new particles. In
addition, the higher concentration of ions at high altitudes could enhance
ion-induced new particle formation. Although several studies have reported
on ions in the free troposphere and their link with new particle formation (Boulon
et al., 2010; Rose et al., 2015; Venzac et al., 2008), they only presented
the total ion concentrations but no chemical composition.
The low concentration of ions in the atmosphere has proven to be an obstacle
in determining their chemical composition. Recent improvements in mass
spectrometer detection limits now enables the measurement of the composition
of ions and ion clusters at atmospheric concentrations and pressure using
the atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF) (Junninen et al., 2010). First
atmospheric measurements with the APi-TOF were reported by Junninen et al. (2010) and
Ehn et al. (2010), both for the boreal
forest site Hyytiälä, in southern Finland. To our knowledge, no
studies have yet been carried out to characterize the diurnal and seasonal
trends and the chemical composition of air ions and ion clusters in the free
troposphere. Here we present 9 months of continuous measurements at the
high altitude research station Jungfraujoch (JFJ, Switzerland), located at 3454 m a.s.l.
The measurements were part of the NUcleation, CLoud and Aerosol
Characterization Experiment (NUCLACE) campaign and complement studies on
detailed new particle formation mechanisms (Bianchi et al., 2016) and
longer-term new particle formation statistics (Tröstl et al., 2016a). The
aim of this work was to characterize the ion composition at this high
altitude site and its possible link to source regions of the air masses and
to new particle formation.
Methods
Site description
The high altitude research station JFJ, Switzerland (3454 m a.s.l.;
46.55∘ N, 7.98∘ E; http://hfsjg.ch) is often
located in the free troposphere. The site is intermittently influenced by
planetary boundary layer (PBL) air masses due to convective conditions and
frontal systems varying from 80 % of the time in summer to 60 % in
spring or autumn and 40 % in winter (Henne
et al., 2009; Herrmann et al., 2015; Zellweger et al., 2003). The JFJ is also a well-known tourist destination, which results in
occasional contamination, for example by cigarette smoke (Fröhlich
et al., 2015). This work shows results from a 9-month campaign of
continuous measurements with a mass spectrometer from August 2013 to April 2014
(time is shown as local standard time (LST), UTC+1). Additionally, two
intensive campaigns were conducted (January–March 2013 and January–March 2014)
when many more instruments were employed for the study of NPF (see Bianchi et al., 2016).
Instrumentation
The main instrument in this study was an APi-TOF (Aerodyne Research Inc. and Tofwerk AG).
The instrument is described in detail by Junninen et al. (2010). The APi-TOF operates in
two stages. The first stage consists of a pressure interface where either
the positive or the negative ions are focused and guided by two quadrupoles
and an ion lens through three pumped chambers. Here the sampled flow is
reduced from atmospheric pressure to ∼ 10-4 mbar. The
second stage consists of a time-of-flight (TOF) mass spectrometer at a
pressure of 10-6 mbar where ions are detected with a mass accuracy
better than 5 ppm. The high sensitivity and resolution of the APi-TOF
(resolving power around 5000 Th Th-1) helps to detect and identify atmospheric
ions in spite of their low concentrations. Air was aspired through a
stainless-steel tube of 1 m length and 25.4 mm inner diameter at 10 L min-1
of which 0.8 L min-1 entered the instrument.
The APi-TOF was set for high sensitivity detection in the mass-to-charge (m / z)
range between 60 and 1200 Th (short flight path in the TOF, V mode)
and it was operated alternately between positive and negative mode,
with ∼ 80 % of the measurements conducted in negative mode.
For the mentioned m / z range it is assumed that the ions are within
the detectable range of the APi-TOF and the relative change in signal of the
individual ions corresponds to the absolute transmission. Data were analyzed
using the software package tofTools, developed by the Department of
Physics at the University of Helsinki. TofTools is implemented in
MATLAB and allows the complete processing of the data, i.e., automatic
averaging, mass calibration, baseline detection, peak deconvolution, and
high-resolution analysis. The noise level is around 0.5 × 10-3 cm-3
below 100 Th and decreases to 0.1 × 10-3 cm-3 at around
500 Th (Junninen et al., 2010). The assignment
of the presented peaks was performed with an accuracy of 2 ppm for the
analysis of special cases and considering isotopic distribution. An example
of the peak fitting is provided in Fig. S1 in the Supplement.
Initially the APi-TOF was installed at the Sphinx observatory at the JFJ
(3580 m a.s.l.) on the upper platform where all the monitoring
instrumentation is located, but no negative ions were measured due to
electrostatic interaction with the building structure, mainly the cupula.
Therefore the APi-TOF was relocated at the JFJ research station (3454 m a.s.l.).
Long-term observations at the Sphinx and the research station show
no important difference in aerosol parameters (see
Bukowiecki et al., 2016), and therefore our measurements should be comparable
with those of the instruments at the Sphinx.
The JFJ is equipped with a suite of permanently operated atmospheric
monitoring instrumentation (see Bukowiecki et al.,
2016). In addition, a nano-scanning mobility particle sizer (nano-SMPS) was
operated to determine the size distribution of freshly nucleated particles
(Tröstl et al., 2016a). Meteorological data (relative humidity,
temperature, wind direction, and global radiation) were obtained from the
station operated at the JFJ by MeteoSwiss and were used at a time resolution
of 10 min.
Transport simulation
To study the origin of different air masses sampled at the JFJ, backward
dispersion calculations were carried out with the Lagrangian particle
dispersion model FLEXPART (LPDM, version 9.02; Stohl et
al., 2005). FLEXPART calculates the trajectories of an ensemble of air
parcels (called particles) through the atmosphere. The model considers
transport by mean flow, turbulence, and subgrid convection. By simulating
thousands of air parcels an LPDM can be used in a quantitative way to derive
source receptor relationships (SRR), establishing the effect of an emission
release from a source on the atmospheric concentration at a receptor. In a
receptor-oriented approach, source sensitivities are derived from backward
simulations, releasing air parcels at the location of the
observation/receptor and following them backward in time. The derived SRRs
provide information about when and where an air mass sampled at the receptor
was in contact with the Earth's surface and potentially took up surface emissions.
Carbon monoxide emissions were also used as a tracer for anthropogenic
activities and therefore PBL influence.
Anthropogenic CO emissions were taken from the EDGAR-v4.2 emission inventory
(http://edgar.jrc.ec.europa.eu/index.php) for the reference year 2008 (the
latest available from the inventory).
Simulations were driven by 3-hourly operational analysis/forecast fields of
the Integrated Forecasting System (IFS) of the European Centre for Medium
Range Forecasts (ECMWF). The horizontal resolution of these inputs was
0.2∘ by 0.2∘ in the Alpine area and 1∘
by 1∘ elsewhere. For the JFJ simulations, 50 000 model particles
were released every 3 h and traced back in the atmosphere for 10 days.
The model output in the form of near surface residence times (referred to as
footprints or source sensitivities) was then used to analyze where sampled
air masses had been exposed to surface fluxes (emissions).
Determination of cloud coverage
In order to distinguish between sunny and cloudy conditions, the cloud cover
was estimated with the clear sky index (CSI) described by Marty
and Philipona (2000). The CSI is defined as the ratio between the apparent
emittance and the theoretical clear sky apparent emittance and is calculated
from the atmospheric long-wave radiation, air temperature, and relative
humidity. As the combination of these three parameters is available
continuously, the presence of clouds at or above the JFJ can be determined
during day and night. A CSI ≤ 1 is considered as clear sky (no clouds),
while a CSI > 1 represents a cloudy sky (overcast). In addition,
cloud coverage was also confirmed by comparing the CSI to pictures from
cameras recording the panoramic view at the JFJ
(http://panocam.skiline.cc/jungfraujoch and http://webcam.switch.ch/jungfraujoch/). These
observations also help identify periods when the JFJ was in cloud.
Results and discussion
First we give an overview of the average composition of positive and
negative ions at the JFJ. Thereafter, a few selected cases of special ion
observations are presented. While this paper is strictly related to ions,
for clarity we mention the ions with the name of the neutral molecule,
e.g., sulfuric acid for HSO4- and nitric acid for NO3-.
For the full period of measurement we observed fluctuations in the total ion
count (TIC). Fluctuations in the TIC were already noted by Boulon et
al. (2010),
who attributed this to the available ion precursors (e.g., radon) and
the strength of the condensation sink. These fluctuations may also be due to
the small mass range of ions measured with the APi-TOF; thus a change in the
size distribution of the ions resulting in a different fraction of ions
outside of the APi-TOF detection range could result in a TIC fluctuation.
Still, this observation should not affect the qualitative analysis presented below.
Representative mass spectra of ions from the Jungfraujoch.
(a) Average daytime spectrum of negative ions from 53 cloudless days between
10:00 and 14:00 LST. (b) Average nighttime spectrum of negative ions from
22 cloudless nights between 22:00 and 02:00 LST. (c) Average spectrum of positive
ions between 10:00 and 14:00 LST on 23 January 2013.
Main ion composition
Negative ions were detected within an m / z range of 60–1200 Th.
Representative mass spectra for clear sky conditions (CSI ≤ 1) are given
in Fig. 1. Figure 1a presents the average spectrum
of 53 clear sky days during daytime (between 10:00 and 14:00 LST), showing sulfuric acid and its clusters (dimer, trimer) as the
main ions. Other important ions are SO5-, as well as the ions of
nitric acid, methanesulfonic acid (MSA), and C3H3O4-
(most probably malonic acid). Figure 1b shows the average spectrum of 22 clear
sky nights (between 22:00 and 02:00 LST). In this case, sulfuric acid and its
clusters were strongly reduced (although they are not completely absent),
and the largest signal is from CH3SO3- followed by
SO5-, nitric acid, and malonic acid (including their clusters).
During both the day and the night, we observed the presence of organics,
identified as C2-18H1-22O2-13, generally with high oxygen
content. In the mass range 120–440 Th, organics occurred mainly in clusters
with NO3- or HSO4-. Halogenated ions were found as well.
Ions from iodine were mainly observed as IO3- and in clusters with
either H2SO4 or MSA, while ions from bromine were detected only as
Br-. These ions were present during day and night, although their
signal was highest during sunrise and sunset. No significant seasonal
variation was observed in the composition of the main ions as shown
in Fig. S2.
Positive ions were identified in the m / z range of 70–300 Th. At
low m / z most of the peaks were identified as protonated amines and
oxygenated organics (C3-14H5-18O1-6H+). At high m / z
(> 120 Th) we found mostly N-containing ions that
could not be unambiguously attributed to amines or oxygenated organics
clustered with a nitrogen-containing ion (either an ammonium or an aminium).
In general, the positive spectrum contained less oxygenated organics compared to the negative mode spectrum, with no significant
significant difference in composition between day and night.
Schulte and Arnold (1990) reported that the
main cation in the middle troposphere (3000–6000 m a.s.l.) was pyridine
(C5H6N+). We also measured this as one of the main cations
together with aniline (C6H8N+) and benzylamine
(C7H10N+). We do not present an average spectrum of the
positive mode since we do not have sufficient data of clear sky days to
generate an unbiased average spectrum. Occasionally, a sequence of peaks
with an increasing number of methylene groups was observed as a clear
pattern in the positive spectrum, mostly after a direct PBL contact (based
on Herrmann et al., 2015); one example
during 23 January 2013 is shown in Fig. 1c. Mostly during summer, some high intensity peaks, mainly at 163.1230 and
192.1383 Th, appeared, with the first one being potentially identified as
nicotine (C10H15N2+). The presence of these ions is most
likely related to tourists smoking on the terrace of the JFJ station (Fröhlich
et al., 2015).
Figure 2 shows the diurnal variations of the main
negative ions normalized to the total negative ion count, averaged for
15 clear sky days (cloudless during the whole day). The figure classifies the
ion counts into seven groups: sulfuric acid (HSO4-,
H2SO4 ⚫ HSO4-, and
(H2SO4)2 ⚫ HSO4-), nitric acid
(NO3- and HNO3 ⚫ NO3-), MSA
(CH3SO3-, CH3SO3H ⚫ CH3SO3-,
CH3SO3H ⚫ NO3-, and
CH3SO3H ⚫ HSO4-), SO5- , malonic acid
(C3H3O4-, C3H4O4 ⚫ HSO4-,
and C3H4O4 ⚫ NO3-), others
(identified ions, e.g., the halogen ions IO3-, Br-), and
non-identified ions. Approximately 40 % of the total ion signal at the JFJ
was identified for this time period. During the daytime (between 08:00 and
18:00 LST) the percentage of identified ions increases to 60 % with the
sulfuric acid group representing around 35 % of the total ion signal.
However, during the nighttime when the rate of sulfuric acid formation is
very low, the charge is redistributed and the “non-identified” category
becomes the most relevant. This group includes mainly the highly oxygenated
organics between 450 and 1200 Th. Such compounds were also observed in the
boreal forest by Ehn et al. (2010), where the
signals of organic ions tended to be stronger during the nighttime (again
due to less competition by the sulfuric acid clusters).
Beside malonic acid, other organic species of low molecular mass were
frequently measured, like C3H5O3-,
C3H3O3- (pyruvic acid), C2HO4- (oxalic
acid), C4H3O4-, and C4H5O4-. Still,
malonic acid was the main organic ion not only during the day but also
during the night and during clear sky and cloudy conditions. Actually, under
cloudy conditions some peaks were less affected, including organics
(e.g., malonic acid) and nitric acid. For the latter, we even measured its water
clusters (e.g., H2ONO3- ⚫ (H2O)2 ⚫ NO3-). In
fact, when we sampled inside clouds (based on CSI and cameras) and during
daytime, most of the ions were composed of organics clustered with
NO3-. Also, all sulfur-containing peaks were absent.
No significant difference was observed for the spectra in different seasons
(winter and summer) except for a more frequent increase in the signals above
450 Th attributed to organics during summertime. Considering that the JFJ
is more frequently under boundary layer influence during summer
(Herrmann et al., 2015), it is expected that vertical transport of air masses
may carry up more organics during this season.
Average diurnal variations of the main negative ions at the JFJ over
15 days of clear sky normalized to the total negative ion count. Ions and clusters
are grouped as follows: Sulfuric acid (HSO4-, H2SO4 ⚫ HSO4-
and (H2SO4)2 ⚫ HSO4-), nitric acid (NO3- and
HNO3 ⚫ NO3-), malonic acid (C3H3O4-,
C3H4O4 ⚫ HSO4- and C3H4O4 ⚫ NO3-), MSA
(CH3SO3-, CH3SO3H ⚫ CH3SO3-,
CH3SO3H ⚫ NO3- and CH3SO3H ⚫ HSO4-),
SO5-, others (identified ions) and unknown (non-identified ions including
ions in the m / z 450–1200 range).
Time series of major ions and meteorological parameters from 6 to 18 November.
Upper panel: global radiation (W m-2), air temperature (∘C),
relative humidity (%) and clear sky index (CSI). Middle and lower panel:
Br-, SO3-, CH3SO3-, HSO4- and SO5-.
Comparison of two different nighttime spectra in negative mode between
22:00 and 02:00 LST. (a) Typical night spectrum, exemplified for the
night from 9 to 10 March 2014. (b) Spectrum with high sulfuric acid
signal in the night from 11 to 12 November 2013.
We frequently detected the presence of sulfuric acid during nighttime
although its rate of formation is expected to be low. Figures 1b and 2
show that the signal of sulfuric acid does not disappear during the night.
In addition, we observed ∼ 35 nighttime events when the
signal increased significantly with also clusters of sulfuric acid up to the
tetramer being present (but never beyond the tetramer).
Figure 3 presents an exemplary time series of some
main ions at the JFJ. From 6 to 11 and from 15 to 18 November a typical
diurnal variation of HSO4- is observed which is affected by the
cloud coverage (CSI < 1, meaning clear sky) and the global radiation.
The period from 11 to 14 November shows cases when sulfuric acid was also
measured during the nighttime. The signal of HSO4- increases due
to photochemical formation around noon on 11 November after a change of wind
direction and subsequent clearing up. High signals are then observed in the
nights to 12 November as well as to 13 November. Although the time trend in
the plot represents only HSO4-, in this particular event the
clusters of sulfuric acid up to the tetramer were observed.
Figure 4 shows this observation in more detail with
a mass defect plot and compares this event with the ion composition observed
during a typical night at the JFJ. The mass defect represents the difference
between the exact and the nominal mass of a compound (Th). The dots
represent the main peaks, where the size of a dot is linearly proportional
to the intensity of the signal and the colors represent the nature of the
ions or clusters. While Fig. 4a presents a typical night (averaged between
22:00 and 02:00 LST), Fig. 4b illustrates the event of unusually high sulfuric
acid signal in the night from 11 to 12 November. During a typical night, the
main ions are composed of CH3SO3- (brown dots) followed by
SO5- (pink dots). Also sulfuric acid (red dots) can be seen and
even the trimer is detected in a typical nighttime spectrum
((H2SO4)2HSO4-, m / z 292.8949). By
contrast, in Fig. 4b the main ion is sulfuric acid followed by
SO5- and CH3SO3-. In this event, also the tetramer
of sulfuric acid is detected ((H2SO4)3HSO4-,
m / z 390.8622). Although Fig. 4a
shows a typical nighttime spectrum, it is important to mention that the
CH3SO3- signal was low in some cases where the main ions were
malonic and nitric acid (NO3-, C3H3O4-,
HNO3 ⚫ NO3-, C3H4O3 ⚫ NO3-).
The SO5- ion was found permanently among the main anions. The
first atmospheric measurements of this ion were reported by
Ehn et al. (2010). They observed a close
correlation between the SO5- and HSO4- signals in the
Finnish boreal forest (R2 > 0.8), with an increase
in signal correlated to global radiation. The same behavior was also
observed at the JFJ (R2 = 0.88), albeit only for daytime measurements (global radiation > 550 W m-2).
However, at the JFJ the signal of SO5- was also present during
clear sky nights, as shown in Fig. 3.
Correlations of SO5- with CH3SO3- and HSO4-.
(a) Scatter plot between SO5- and CH3SO3- ions
during daytime, y = -0.14 + 0.78x and R2 = 0.75.
(b) Scatter plot between SO5- and CH3SO3- ions
during nighttime, y = 0.028 + 1.24x and R2 = 0.87.
(c) Scatter plot between SO5- and HSO4- ions during daytime, y = 0.468 + 3.49x and R2 = 0.88.
Time series from 11 to 16 March illustrating halogen chemistry.
Global radiation, RH and cloud coverage are shown in the top panel. The
decrease in signal of Br- and IO3- during noon in the middle
panel implies photochemical reactions on the precursors of these ions or charge distribution. The
lowest panel shows the anticorrelation of sulfuric acid with CH3SO3-
and SO5-.
Examples of two events with high Br- and IO3- signal
and back trajectories for surface residence time. (a, b) Air masses
from the Atlantic, (c, d) air masses from the Mediterranean. The black
lines denote the time period for the trajectory analyses shown in (b)
and (d).
In addition, we found a remarkably high correlation of SO5- with
CH3SO3- for the full sampling period with a R2 = 0.87
during nighttime (zero global radiation) and R2 = 0.75 during daytime
(global radiation > 550 W m-2).
No special dependencies on boundary layer influence (based on
Herrmann et al., 2015) were found. The
correlation is plotted in Fig. 5 for day and
nighttime. As shown in the same figure, a high correlation between
SO5- and HSO4- is also seen during daytime. Comparing
Fig. 5a and b it seems that the correlation of
SO5- with CH3SO3- during day is offset to higher
SO5-, implying an additional mechanism forming SO5-
during the day. This is further discussed in Sect. 3.3.2 below.
Halogenated species
Naturally occurring halogenated species in the atmosphere are usually linked
to measurements in the marine boundary layer (MBL), especially for chloride
and bromide, which are contained in seawater. Compared to chloride and bromide,
iodide is usually observed at lower signals since it is incorporated as a
nutrient by biological processes. Inorganic iodine originates from the
decomposition of natural iodocarbons such as CH3I and CH2I2
and the inorganic precursors HOI and I2 (Simpson et
al., 2015). Several studies have reported the presence of iodine oxide at
different locations in the MBL. Simpson et al. (2015)
summarized these observations which include measurements at Tenerife,
Tasmania, Cape Verde, West Pacific, and East Pacific. To our knowledge, there
are no reports of bromide or iodide and its oxides in the atmosphere of
continental and high altitude locations similar to the Jungfraujoch (at
3500 m a.s.l. and around 250 km from the nearest coastal region in the Mediterranean).
The halogenated species CF3-, Br-, IO3-,
HNO3 ⚫ IO3-,
CH3HSO3 ⚫ IO3-,
and H2SO4 ⚫ IO3- were detected
regularly at the JFJ. The maximum signal of these ions was generally
observed during 07:00–09:00 and 17:00–21:00 LST, but occasionally high signals
were also detected during nighttime (22:00–04:00 LST).
Figure 6 shows this temporal variation with the
main species, IO3- and Br-, from 11 to 16 March 2014
(for additional time series see also Fig. 7c). The
figure also presents the signals of HSO4-, CH3SO3-,
and SO5- as well as global radiation, RH, and the cloud coverage (CSI)
to provide an overview over the ambient conditions. The clear peaks in
signal, mainly of IO3-, during sunrise and sunset and the strong
decrease during noon suggests the occurrence of efficient photochemical
halogen chemistry. The process could be initiated by the so-called halogen
reservoir species X2, HOX, XNO2, or HX, with X being the halogen atom.
X2,HOX,XNO2,HX⟶OH,HO2,hνX⚫+products
The depletion of the ions is then presumably the result of photolysis or the
reaction of their parent compound with halogen atoms or OH⚫
radicals. Nevertheless, the observed diurnal pattern may also be due to charge
redistribution between iodic and sulfuric acid, as the latter has a strong
diurnal variation (see above) and effectively competes for the limited
charge. However, without measurements of neutral species it is not possible
to discriminate between these two possibilities.
An additional observation in Fig. 6 is the trend
of CH3SO3- which anticorrelates with sulfuric acid and
rather follows the trend of IO3- and Br-. From 12 to 15 March
the solar radiation was strong and the RH low, producing a typical diurnal
cycle of sulfuric acid. However, the CH3SO3- signal decreases
during the peaks of sulfuric acid and only increases around 18:00 LST following
the recovery of IO3- and Br-. SO5- follows a
similar trend as CH3SO3- but less pronounced. This has to do
with additional pathways of SO5- as discussed below. The close
relationship of the time trends of MSA with Br- and IO3-
could indicate a mechanistic connection between these species, for example a
formation of MSA linked with halogen-based chemistry. However, it can also
be simply caused by charge redistribution to sulfuric acid generating a
similar diurnal trend.
The presence of these halogen species suggests that air masses of marine
origin were transported towards the JFJ. Backward dispersion calculations
were carried out with the FLEXPART model (see Sect. 2.3) for days when the
signal of the main ion IO3- was equal to or higher than the 95th percentile
of all its values during this campaign (24 events in total).
During these events other iodine species were also detected, as described
below. The transport simulations revealed that the air masses had an
extended surface residence time over the Atlantic Ocean and occasionally
also over the Arctic, Mediterranean Sea, and continental regions.
Figure 7 shows two events when the halogen signals
increased, specifically for Br- and IO3-. Figure 7a shows the
temporal evolutions of the ion signal on 2 October 2013 while Fig. 7b shows
the surface residence time back trajectories for the same period of time.
Figure 7b is divided into two subplots, i.e., an absolute footprint, τ,
in s m-3 kg-1 (left) and a relative footprint R (%)
(right). The first refers to the amount of time that an air mass stays at
the surface, whereas the second is calculated as the difference between the
mean event footprint and the mean footprint over the whole simulation period
(one year) divided by the mean of these two. The relative presentation
usually assists the identification of special features of a certain
transport situation without being dominated by the generally decreasing
residence time with distance to the site. Areas with negative (positive)
values of the relative footprint correspond to areas with weaker (stronger)
surface sensitivity than the annual average. The event shown in Fig. 7b
represents a case in which the air mass sampled at JFJ was dominated by
above average surface contacts over the Atlantic Ocean. This event
represents a frequently occurring transport of air masses from the Atlantic
Ocean towards the JFJ. Similarly, Fig. 7c shows the time series of elevated
halogen ion signals during an event on 16–17 November 2013 where the air
masses arrived from continental areas with possible influence from the
eastern Mediterranean Sea. This type of transport was less frequent, with a
total of 6 events out of the total 24. Although most of the backward
simulations suggest a marine and coastal origin of the precursors of
halogenated ions, we do not discard the possible contribution from
continental regions as it is also observed from Fig. 7d. A combined footprint of all the events with
high halogen signal at the Jungfraujoch is presented in
Fig. 8. This combined inverse-time calculation
confirms that air masses are transported mainly from the Atlantic Ocean and
even from the US east coast.
The detection of ions and clusters of marine origin is potentially relevant
due to their observed participation in new particle formation
(O'Dowd et al., 2002). However, no new
particle formation events were observed at the JFJ where halogen species
were involved in cluster growth (see below), even though these ions were
detected frequently. This would imply that halogens may be “spectators” at
high altitude, i.e., ions that are present, but do not participate in new
particle formation.
Combined footprint from all events with high halogen signal (IO3-
and Br-).
Iodine species
Besides IO3-, its clusters with H2SO4 and
CH3SO3H and organics (e.g., C7H15N ⚫ IO3-)
were observed. Several other iodine species were also found
at the JFJ. These included I-, IO-, IO2-, and clusters
of I2O3 and I2O5 with ions of sulfuric acid or MSA
(e.g., I2O5 ⚫ CH3SO3-). The observation of
I2O5 in clusters with CH3SO3- and sulfuric acid
confirms the presence of I2O5 in the atmosphere which was proposed
from laboratory and modeling studies (Saunders and Plane,
2005). However, to our knowledge no studies have reported the presence of
I2O5 in the atmosphere up to now. Saunders and
Plane (2005) speculated that I2O5 could be photo-chemically formed
via an oxidation chain of iodine atoms with O3 in the gas phase:
I+O3→IO+O2IO+IO→OIO+IIO+IO→I2O2IO+OIO(+M)→I2O3OIO+OIO(+M)↔I2O4I2O2+O3→I2O3+O2I2O3+O3→I2O4+O2I2O4+O3→I2O5+O2.
In this oxidation chain, the molecules detected at the JFJ as clusters with
CH3SO3-, NO3-, or HSO4- are in
bold. According to quantum chemical calculations by Kaltsoyannis and
Plane (2008),
I2O4 would have a lower stability in the atmosphere
compared to the other iodine-containing species. This may be the reason why
we did not observe this species. Saunders and Plane (2005)
assumed that iodine atoms are formed by a photochemical process. However,
for IO, I2O3, and I2O5 we observed a similar time trend
as for IO3- with a strong decrease during the day (see
Fig. 6). This is further indication of nighttime
halogen chemistry. However, without the measurement of neutral species we do
not have the quantitative concentration data to further elucidate this process.
Dependence of HSO4- and CH3SO3- on relative humidity,
as a function of temperature during the daytime (a, b) and nighttime (c, d).
The signal was normalized to the total ion counts.
Production of sulfur-containing species
In this section we provide some hypotheses to explain the observed formation
of sulfuric acid during the nighttime and the observed correlation between
CH3SO3- and SO5-. Even though our measurements are
restricted to ions, we believe that species such as sulfuric acid and MSA
are formed as neutral channel species. The diurnal cycle of HSO4-
presented in Fig. 2, for example, is a clear
indication that the ion follows the sulfuric acid concentration. Also, the
presence of clusters of the type (H2SO4)1-3 HSO4-
or (CH3SO3H)1-3CH3SO3- (shown in Fig. 4) is
another strong indication of the presence of neutral molecules.
Sulfuric acid during nighttime
The most important gas phase pathway for the production of sulfuric acid
occurs via the reaction of SO2 with the OH radical (Stockwell and Calvert, 1983) and is the
reason for the well-known diurnal cycle of sulfuric acid:
SO2+OH+M→HOSO2+M,
HOSO2+O2→HO2+SO3,HOSO2+O2→HSO5,
SO3+H2O+M→H2SO4+M,
where M is a stabilizing (energy-absorbing) molecule, usually N2 or
O2. However, this reaction cannot explain sulfuric acid formation
during nighttime observed in a total of 35 events at the JFJ with
especially high signals (with clusters up to the tetramer). An alternative
production of sulfuric acid during nighttime can take place through the
Criegee intermediates (CIs); CIs have been observed in field measurements in
a boreal forest (Mauldin III et al., 2012)
and at coastal regions (Berresheim et al., 2014) and
can be formed through the ozonolysis of double bonds containing compounds (alkenes):
O3+alkene→Criegeeintermediate(CI).
About half of the CIs in Reaction (R13) decompose and produce OH. In this
case the SO2 oxidation chain can proceed as shown in Reactions (R10),
(R11a), and (R12) producing sulfuric acid. The other half of the CIs are
stabilized, producing stabilized Criegee radicals (sCI) which decompose over
a much longer lifetime. These sCI are able to oxidize SO2 and therefore
produce sulfuric acid (Mauldin III et
al., 2012). Alkenes can be transported to the free troposphere by injection
of polluted air masses from the planetary boundary layer. Such an injection
also decreases the CO / NOy ratio, which is used as a proxy for the age of
an air mass since boundary layer contact (Zellweger et
al., 2003). The nighttime sulfuric acid signal was not correlated with the
CO / NOy ratio; therefore it is possible that Criegee intermediates are
not the main mechanism for nighttime sulfuric acid formation at the JFJ. We
do not discard, however, the possible transport of isoprene or terpenes from
rural areas not detected by the CO / NOy ratio.
Besides CIs, we also explored the possibility of particle–gas partitioning
from preexisting particles leading to an increase of sulfuric acid in the
gas phase. Figure 9a and c summarize the signal
of sulfuric acid during day and nighttime as function of relative humidity
and temperature. During the day under photolytic production its steady state
concentration is strongly decreasing from low to high RH. At night,
higher signals are observed below 40 % RH and temperatures above
-5 ∘C. Regarding the events when a high signal of sulfuric acid
was observed during night, 14 out the 35 events were characterized by a
significant drop in RH to levels in the range of 3–27 % with an increase
in temperature (never exceeding 5 ∘C). One example of these events
is given in Figs. 3 and 4. For these events we considered the possibility
of stratospheric intrusion as the reason for the drop in RH. During these
events the ozone concentration did not change significantly (65–90 ppbv)
except for five of the events when O3 increased to levels between 103 and 130 ppbv.
However, an O3 concentration in the range of 70–90 ppbv was
observed in the Alpine region by Stohl et al. (2000) during
stratospheric intrusion events; therefore O3 cannot be considered a
strong marker of a stratospheric intrusion at the JFJ. The radon
concentration (Institute of Environmental Geosciences, University of
Basel, http://azug.minpet.unibas.ch) was also investigated in order
to detect a stratospheric intrusion, but no significant variation of radon
was observed.
From the remaining events with high sulfuric acid signal during nighttime,
5 events had an RH between 38 and 62 % and 16 events a RH between 72 and 99 %. These events
were characterized by snowfall; consequently we believe that evaporation of
sulfuric acid from snowflakes occurred in the inlet of the APi-TOF because
of an increase in temperature between ambient conditions and the laboratory.
Therefore, we conclude that all the events of high sulfuric acid during
nighttime that we measured resulted from the evaporation from the particle phase
due to a decrease in relative humidity and/or increase in temperature either
in the atmosphere (events with low RH) or in the instrument (events with
high RH). Indeed, the former process is well known. As an example,
Mauldin III et al. (1999) observed
an increase in the gas phase concentration of H2SO4 with
decreasing RH during evening and nighttime flights over the Pacific. The
measured H2SO4 concentrations were approximately a factor of 10
higher (∼ 1 × 106 to 10–11 × 106 cm-3) when the
aircraft flew in dryer layers of air (RH < 10 % and temperature
∼ -3 ∘C). They confirmed these observations with
models and laboratory measurements showing that H2SO4 evaporates
from the particle phase at low RH. Recently, Tsagkogeorgas et al. (2016)
found in the CLOUD chamber, with experiments at RH between 0.3 and 10 %
and temperatures between -5 and 20 ∘C, that a decrease in RH
and/or an increase in temperature induces particle shrinkage by evaporation
of sulfuric acid, resulting in almost 1 order of magnitude higher
concentrations of sulfuric acid in the gas phase with respect to the background.
Methanesulfonic acid and peroxomonosulfate radical (SO5-)
The peroxomonosulfate radical (SO5-) was first measured in the
laboratory as a product from the reaction of O3- or CO3- with
SO2 (Möhler et al., 1992; Salcedo et al., 2004) according to
CO3-⚫nH2O+SO2→SO3-⚫mH2O+CO2+(n-m)H2O,
SO3-⚫nH2O+O2→SO5-⚫mH2O+(n-m)H2O,
with 0 < n < 2. Reaction (R14) could also take place with
O3- instead of CO3-.
Bork et al. (2013) concluded from quantum chemical
calculations that the distance and the strength of the
O2-SO3- bond resembles more a molecular cluster than a
covalently-bound molecule. A reanalysis of the data from the CLOUD chamber
presented by Schobesberger et al. (2015) reveals the
presence not only of (NH3)m(H2SO4)nHSO5-
clusters but also (NH3)m(H2SO4)nSO5-,
suggesting that SO5- as well as HSO5- are most likely
molecules rather than clusters. The binding of ammonia–sulfuric acid
molecules occurs via strong hydrogen bonds and we believe it is unlikely
that SO5- could be bound by an additional interaction of the type
SO3--O2 with (NH3)m(H2SO4)n.
Reactions (R14) and (R15) could explain the observations of SO5-
during the day and night at the JFJ. The correlation of SO5- with
CH3SO3- shown in the previous section implies another strong
source of SO5- with a common precursor for these two ions. It is
important to note that although we measured ions, the presence of MSA
clusters is a strong indication of neutral chemistry involved in the
formation of MSA (Figs. 1 and 4).
Traditionally, the oxidation of dimethyl sulfide (DMS) is considered the
exclusive source of MSA (Seinfeld and Pandis, 2006). Since DMS
is mainly produced by marine phytoplankton, MSA is also related to marine
emissions. Since oxidation of DMS produces both MSA and SO2, a
correlation between MSA and SO5- could be envisaged based on the
Reactions (R14) and (R15). However, this would imply that the contribution of
anthropogenic SO2 sources is small, in contradiction with studies
showing that the JFJ is influenced by regional sources and PBL air masses
(Herrmann et al., 2015; Bukowiecki et al., 2016; and references therein). Although the production of
MSA derived from DMS is a photochemical process we did not observe a clear
diurnal pattern in the MSA signal similar to sulfuric acid. MSA is often
anticorrelated to sulfuric acid as discussed above (Sect. 3.2 and Fig. 6).
Berresheim et al. (2002) did observe
fluctuating behavior of MSA overlapping the diurnal photochemical formation
at Mace Head (Ireland). They attributed this to a sensitive dependence of
the gas–particle partitioning on RH or temperature. During daytime we
observed elevated MSA in the gas phase at low RH but during nighttime no
clear dependence on RH or temperature (except for very low temperature) was
observed (Fig. 9b and d). Due to the variation
in partitioning of MSA, a high correlation with the independent formation of
SO5- (Reactions R14 and R15) seems questionable. As seen from
Fig. 5c, during sunny days sulfuric acid and
SO5- are well correlated. This fraction of SO5- was
presumably formed from deprotonation of the peroxyradical HSO5. This
radical may be formed from another pathway of Reaction (R11), that is by
addition of O2 to HSO3 (Reaction R11b) rather than H-abstraction
(Gleason et al., 1987). This pathway seems reasonable as also
HSO5- is observed. H2SO5 can be formed from the reaction
HSO5 + HO2. As seen in Fig. 5a the
SO5- produced by this mechanism shifts the MSA–SO5-
correlation to higher SO5- values during the day. Thus, there are
three possible formation pathways of SO5-: (a) Reactions (R.14)
and (R15) occurring all the time, (b) deprotonation of HSO5, or (c) unknown
process correlated with the formation of CH3SO3-. The last
process seems to be not directly influenced by photolysis. The measurement
of the neutral species could shed some more light on this. Previous studies
claim that there might be a missing source of MSA (Bardouki et al., 2003;
Mauldin et al., 2003). Mauldin et al. (2003) and references therein speculate about the oxidation of DMS
involving halogen chemistry or the production of MSA by oxidation of species
other than DMS (e.g., DMSO) to explain MSA measurements during nighttime.
Bardouki et al. (2003) suggest heterogeneous reactions of DMSO on aerosols
as a source of particulate methylsulfonate. Our observations shown in
Fig. 6 could support the hypothesis of a
mechanism involving halogen chemistry. Often MSA decreases concurrently with
the halogen species Br- and IO3- during the day while all three
species are observed during the night. It is also interesting to note that
we often observed methyl bisulfate (CH4O4S-, 110.9758 Th) at
the JFJ, which to our knowledge has not been reported in the atmosphere. We
may speculate that the formation of methyl bisulfate and MSA proceeds via a
similar reaction.
Difference of the negative mode mass spectra between two types of
new particle formation and the average of 21 non-new particle formation
sunny days (average signal of nucleation days - non-nucleation days).
(a) 7 new particle formation events with H2SO4-NH3 clusters
(purple peaks); (b) 19 new particle formation events with HOMs.
New particle formation events
During the whole period of measurement (∼ 9 months) we
identified more than 30 NPF events. From these events,
some were not measured with the APi-TOF due to technical difficulties. In
total, 26 events were registered from which we identified two types of NPF processes: one involving sulfuric acid–ammonia clusters
(H2SO4–NH3, 7 events) and one through highly oxygenated
molecules (HOMs, 19 events). The first of these is well established and has
been observed regularly in the atmosphere (e.g., Zhao et al., 2011). Occasionally, the sulfuric acid–ammonia clusters were
also observed during days with no NPF. In these cases,
the clusters did not grow beyond ∼ 2 nm, obviously because not
sufficient condensable material was present.
The second type of event, triggered by HOMs, was observed more frequently
at the Jungfraujoch, where almost all the measured organic molecules were
highly oxygenated, with an O : C ratio between 1 and 1.25 (see also Bianchi et
al., 2016). Kirkby et al. (2016) showed in
laboratory experiments that new particle formation of HOMs can also proceed
without participation of sulfuric acid. Our data (Bianchi et al.,
2016) confirm that this also happens in the ambient atmosphere, as the HOMs
normally are mostly clustered with NO3- and only very rarely with
HSO4-, suggesting that the major pathway of new particle formation
was through the HOMs while sulfuric acid contributed only to a minor extent
to this new particle formation.
Comparison of ambient and laboratory measurements. (a) Mass defect
plot from the Jungfraujoch on 12 November 2013 from 08:30 to 12:30 LST.
(b) Mass defect plot from a laboratory experiment at the CLOUD chamber
involving H2SO4 and NH3 clusters (adapted from Schobesberger et al., 2013).
Figure 10 presents the difference between the
average spectra of each of these two types of new particle formation events
and the average spectrum of 21 sunny days without new particle formation
(Fig. 10a for the 7 events with sulfuric acid–ammonia clusters and Fig. 10b
for the 19 events with HOMs). For this figure, the spectrum during was deducted from the spectrum during nucleation time. In
the higher mass range of Fig. 10a, ammonia–sulfuric acid peaks are clearly
visible such as
(H2SO4)3NH3HSO4- (m / z 407.8888),
(H2SO4)4(NH3)HSO4- (m / z 505.8562),
(H2SO4)4(NH3)2HSO4- (m / z 522.8827),
(H2SO4)5(NH3)2 HSO4- (m / z 620.8501),
(H2SO4)5(NH3)3HSO4- (m / z 637.8767),
(H2SO4)6(NH3)2HSO4- (m / z 718.8175),
(H2SO4)6(NH3)3HSO4- (m / z 735.8440), and
(H2SO4)6 (NH3)4HSO4- (m / z 752.8706).
However, HOMs at high m / z are also
present, suggesting that these contribute to new particle formation in these
events as well. In Fig. 10b, sulfuric acid–ammonia clusters are clearly
absent. In contrast, elevated signals of compounds/clusters above
m / z ∼ 300 are observed, indicating that cluster
formation of HOMs is driving new particle formation.
Although some other organics are also observed to increase at lower masses,
we consider it unlikely that these molecules contribute to the formation of
new particles due to their relatively high saturation vapor pressure
(Tröstl et al., 2016b). Most likely these molecules
are formed concurrently with the HOMs and contribute to the growth of the
freshly formed particles. Also, we cannot exclude participation of
stabilizing ammonia in the cluster growth of the neutral clusters since this
would not be detected in the anions.
The JFJ data are in excellent agreement with laboratory data reported from
the CLOUD experiment where sulfuric acid–ammonia clusters were observed to
grow by the progressive addition of H2SO4 and NH3 (Almeida et al.,
2013; Kirkby et al., 2011; Schobesberger et al., 2015).
Figure 11 demonstrates with a mass defect plot how
closely the measurements at the Jungfraujoch (Fig. 11a) and the CLOUD chamber
(Fig. 11b) resemble each other. In both cases, pure sulfuric acid clusters
are dominant and the strongest signal was detected for the trimer
(H2SO4)2HSO4-. Although clusters of the type
(H2SO4)m(NH3)nSO5- were measured in the
CLOUD chamber, we did not observe them at the JFJ. A possible explanation
could be the difference in RH. Kurtén et al. (2009)
reported that under atmospheric conditions the hydration of HSO5
increases its lifetime significantly enhancing the probability to act as
nucleation precursor. During the event shown in
Fig. 11 the average RH at the JFJ was only 3 %
(temperature of 1.1 ∘C) in comparison with 40 % RH in the CLOUD
chamber (temperature of 5 ∘C). In the JFJ mass defect plot
clusters involving amines (yellow dots) and iodate (cyan dots) are also
present, but the dominant clusters have the composition
(H2SO4)m(NH3)n⚫HSO4- as in
the CLOUD spectrum. Concentrations of amines must be quite low as they are
strong bases and are expected to bind strongly to acids in the preexisting
aerosol. Indeed, aminium compounds were found in the Jungfraujoch aerosol
(Henning et al., 2003).
The LPDM FLEXPART (see Sect. 2.3) was used
in time-inverse mode to detect the origin of the air masses transported
during all the NPF events in order to determine the SRR. In addition, CO emissions were used as a tracer for
emissions from anthropogenic activities, contained in air vented from the
PBL. The back trajectories of all NPF events were compared to those of
non-event days. We found that all NPF events (i.e., both the
H2SO4–NH3 and HOMs types), were related to an increase in CO
concentration and SRR 12 to 40 h before the arrival of the air mass at
the JFJ, suggesting a PBL contact of the air mass within that time period
(see also Bianchi et al., 2016). For all back trajectories, only a small
increase in CO was found within 6 h before arrival at the JFJ,
suggesting no significant influence from local emissions during this period.
Thus, in a restricted time frame of 1–2 days after PBL contact, precursor
gases are transported to the Jungfraujoch and may trigger NPF if oxidation
of these gases leads to sufficiently high concentration of HOMs and/or sulfuric acid.
Conclusions
We conducted continuous measurements of atmospheric ions for 9 months at
the high altitude research station Jungfraujoch (3454 m a.s.l.). The
positive spectrum was composed mainly of cations of amines, predominantly
pyridine, aniline, and benzylamine, as well as of organic molecules which
showed a regular pattern with increasing CH2 moieties in the mass
spectra. No strong diurnal behavior was observed, which is in agreement with
observations by Ehn et al. (2010). The anion spectrum was usually dominated
by sulfuric acid and its clusters, as well as nitric acid, SO5-,
and CH3SO3-. The latter two were most abundant during nighttime. Likewise, small organic molecules were detected frequently but no
clear dependence on meteorological conditions nor diurnal cycle was found to
be associated with their occurrence. During the total sampling period
malonic acid was the most important organic compound besides MSA. Sulfuric
acid was frequently detected also during nighttime and in some cases the
signal was so high that even the tetramer was detected. The most likely
explanation is that the measured sulfuric acid resulted from evaporation
from the particle phase in the atmosphere under low RH conditions or in the
instrument during events with high RH and snowfall.
We found a remarkably high correlation between CH3SO3- and
SO5- (R2 = 0.87) for the full measurement period.
This correlation was not sensitive to boundary layer influences, as we
observed high correlations at free-tropospheric and boundary-layer-influenced conditions. This correlation points to a common precursor for
these two molecules. Considering DMS as common source would imply that all
SO2 in the JFJ is derived from DMS excluding anthropogenic sources in
contradiction with several studies that show local and PBL influence at the
JFJ. Moreover, the almost permanent occurrence of SO5- and
CH3SO3- during nighttime contradicts the typical
photochemical production of MSA. Therefore, we conclude that an additional
channel for MSA and SO5- (probably HSO5) formation should be considered.
We frequently measured halogenated species with occasional events of high
signals when more species could be detected. Backward transport simulations
linked these events mainly with the Atlantic Ocean as source region although
also continental influence was observed. Besides IO3- we also
measured I2O5, a species not reported so far in the atmosphere.
Their signals as well as that of Br- rapidly decay when the sun rises
and stay low during sunny days, without any other halogenated ions appearing
instead. The mechanism of their formation and diurnal variation of these
ions is unclear. Parallel measurements of neutral halogenated species need
to be performed to elucidate their precursors.
Two types of new particle formation events were identified at the JFJ, one
through sulfuric acid–ammonia cluster formation and a more frequent one
via HOMs which were normally clustered with nitrate
and only occasionally with sulfuric acid. The sulfuric acid–ammonia cluster
formation during these nucleation events at the JFJ compared very well with
laboratory experiments at the CLOUD chamber at CERN, confirming the
relevance of this mechanism for ambient nucleation. New particle formation
of HOMs was evidenced by an enhancement of ions above m / z = 300 Th.
Such new particle formation that is primarily based on HOMs without
significant participation of sulfuric acid was very recently found in the
CLOUD experiment (Kirkby et al., 2016) and confirmed at the JFJ
(Bianchi et al., 2016). New particle formation events take place at the JFJ 1–2 days
after previous air mass contact with the PBL. This time frame appears to be
needed to oxidize organic compounds transported towards the JFJ and
trigger a HOM-based NPF.