AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus GmbHGöttingen, Germany10.5194/amt-8-4001-2015Bisulfate – cluster based atmospheric pressure chemical ionization mass
spectrometer for high-sensitivity (< 100 ppqV) detection of
atmospheric dimethyl amine: proof-of-concept and first ambient data from
boreal forestSipiläM.mikko.sipila@helsinki.fiSarnelaN.JokinenT.https://orcid.org/0000-0002-1280-1396JunninenH.https://orcid.org/0000-0001-7178-9430HakalaJ.RissanenM. P.https://orcid.org/0000-0003-0463-8098PraplanA.SimonM.KürtenA.BianchiF.DommenJ.CurtiusJ.PetäjäT.https://orcid.org/0000-0002-1881-9044WorsnopD. R.Department of Physics, P.O. Box 64, 00014 University of Helsinki, FinlandCenter for Atmospheric Particle Studies, Carnegie-Mellon University, Pittsburgh, PA 15213, USAFinnish Meteorological Institute, Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, FinlandInstitute for Atmospheric and Environmental Sciences, Goethe-University of Frankfurt, Altenhöferallee 1, 60438 Frankfurt am Main, GermanyLaboratory for Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen, SwitzerlandInstitute for Atmospheric and Climate Science, ETH Zürich, 8092 Zurich, SwitzerlandDepartment of Applied Physics, University of Eastern Finland, 70211 Kuopio, FinlandAerodyne Research Inc., Billerica, Massachusetts 01821, USAM. Sipilä (mikko.sipila@helsinki.fi)1October2015810400140112January20159April20154August201517September2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/8/4001/2015/amt-8-4001-2015.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/8/4001/2015/amt-8-4001-2015.pdf
Atmospheric amines may play a crucial role in formation of new aerosol
particles via nucleation with sulfuric acid. Recent studies have revealed
that concentrations below 1 pptV can significantly promote nucleation of
sulfuric acid particles. While sulfuric acid detection is relatively
straightforward, no amine measurements to date have been able to reach the
critical sub-pptV concentration range and atmospheric amine concentrations
are in general poorly characterized. In this work we present a
proof-of-concept of an instrument capable of detecting dimethyl amine (DMA)
with concentrations even down to 70 ppqV (parts per quadrillion, 0.07 pptV)
for a 15 min integration time. Detection of ammonia and amines other than
dimethyl amine is discussed. We also report results from the first ambient
measurements performed in spring 2013 at a boreal forest site. While minute
signals above the signal-to-noise ratio that could be attributed to
trimethyl or propyl amine were observed, DMA concentration never exceeded
the detection threshold of ambient measurements (150 ppqV), thereby
questioning, though not excluding, the role of DMA in nucleation at this
location.
Introduction
Formation of secondary aerosol particles and cloud condensation nuclei in the
atmosphere is initiated by nucleation. The role of sulfuric acid in
nucleation is well established (e.g. Weber et al., 1995; Riipinen et al.,
2007; Sipilä et al., 2010). However, sulfuric acid alone, or with water,
does not nucleate efficiently enough to explain atmospheric nucleation rates
(Kirkby et al., 2011); rather additional vapours are required to stabilize
nucleating clusters. Ammonia (Ball et al., 1999; Vehkamäki et al., 2004;
Kirkby et al., 2011) and amines (Kurten et al., 2008; Berndt et al., 2010,
2014; Erupe et al., 2011; Kirkby et al., 2011) are proposed to act as
stabilizing agents of sulfuric acid clusters in atmospheric new particle
nucleation. Recently, Almeida et al. (2013) showed that dimethyl amine
concentrations below 1 pptV can dramatically enhance formation rates of new
sulfuric acid particles (by several orders of magnitude); further,
concentrations as low as just a few pptV can saturate the nucleation rate at
atmospheric sulfuric acid concentrations. Enhancement of the particle
formation rate is due to dimethyl amine's ability to stabilize molecular
sulfuric acid clusters, minimizing evaporation and enabling further growth
(Almeida et al., 2013). Amines other than dimethyl amine can have a similar
effect on nucleation (Berndt et al., 2014), but no experiments to date have
probed the atmospherically important concentration range from ppqV to a few
pptV.
Atmospheric measurements of amines are rare (Ge et al., 2011; Hanson et al.,
2011; Yu et al., 2012; Freshour et al., 2014). Gas phase concentrations of
these bases are usually low, and reliable measurement of atmospheric amine
concentrations is far from sufficient to evaluate their role in atmospheric
chemistry and physics. For example, from the SMEAR II field station
(Hyytiälä, southern Finland; Hari and Kulmala, 2005), where the
nucleation process has been seriously investigated for two decades, there are
no reliable data for amine concentrations. First attempts to quantify
concentrations of dimethyl or ethyl amine (DMA/EA) and trimethyl or propyl
amine (TMA/PA) were performed by Sellegri et al. (2005), who applied an
ambient pressure protonated water cluster-based chemical ionization mass
spectrometer. Sellegri et al. (2005) reported observations of TMA with the
concentration exceeding 10 pptV. However, that signal is most likely
explained by an isotope of protonated acetone, occurring at the same integer
mass as protonated TMA, making the observation questionable. No other amines
were detected, suggesting that DMA concentrations were below few pptV. Note
that DMA and EA (and also TMA and PA) have identical elemental composition
and can thus not be separated from each other via mass spectrometry (MS).
More recently, amine concentrations at SMEAR II were published by Kieloaho et
al. (2013). Amines collected on phosphoric acid-impregnated fibreglass
filters (through a polytetrafluoroethylene (PTFE) filter) were subsequently
analysed via liquid chromatography electrospray ionization mass spectrometry
(LC-ESI-MS). They reported remarkably high gas phase amine concentrations,
with DMA/EA and TMA/PA concentrations exceeding 100 pptV in autumn.
Concentrations in spring time, relevant for comparison to our present work,
were also reasonably high, up to a few tens of pptV for both DMA/EA and
TMA/PA. This observation is in conflict with Schobesberger et al. (2015) who
measured natural ion cluster distributions at SMEAR II during nucleation and
found much more ammonia than amine composition in bisulfate–sulfuric
acid–base clusters. Based on that observation and targeted laboratory
experiments, Schobesberger et al. (2015) concluded that DMA concentration at
the site should be less than 1 pptV. Obviously, this discrepancy should be
resolved.
Despite some drawbacks, atmospheric pressure chemical ionization mass
spectrometry (APCI-MS) as applied by Sellegri et al. (2005) is a promising
approach for ultrahigh-sensitivity online gas phase amine detection. Nitric
acid has been measured by using bisulfate ion as primary ion (Mauldin et
al., 1998). For acids, such as sulfuric acid, detection limits down to 1 ppqV have been achieved with the APCI-MS technique when the nitrate ion has
been used as the primary ion (e.g. Eisele and Tanner, 1993; Jokinen et al.,
2012). With the APCI-MS technique, interference from compounds in particle
phase is minimized, whereas, in techniques utilizing sample collection and
subsequent analysis (e.g. LC-MS), the separation between particle and gas
phases is difficult.
APCI-MS approaches in use today rely on proton transfer reaction using
protonated water clusters (Hanson et al., 2011) or protonated ethanol or
acetone (Yu et al., 2012). Product ions which are guided through a
differentially pumped section comprising collision dissociation chamber and
an octopole ion guide are subsequently detected by a quadrupole mass
spectrometer (Hanson et al., 2011; Yu et al., 2012). Using the above
technique with protonated ethanol, Yu et al. (2012) reported a limit of
detection (LOD) of 7 pptV for dimethyl amine and from 8 to 41 pptV for a
series of other small alkylamines. Hanson et al. (2011) reported amine
detection at “sub-pptV” levels by means of protonated water cluster
ionization. This sub-pptV measurement range is still above the ppqV range
reachable in the case of NO3- ionization detection of strong acids.
These approaches may also suffer from flaws interfering with reliable amine
detection and quantification: (i) outgassing of amines from gas lines and
surfaces of chemical ionization system, (ii) non-collision limit charging
efficiency, and (iii) uncertainty in identification of the elemental
composition of detected ion due to insufficient mass resolution of the
quadrupole mass spectrometer. Further problems in high-sensitivity amine
measurements can be caused by amine contamination in the zero gas required
for determination of instrument background.
Here we describe a chemical ionization system that utilizes ion-induced
clustering of sulfuric acid and amines or ammonia, with ions detected with
an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF,
Junninen et al., 2010). This approach addresses the above issues that can
complicate amine detection. Instrument response to ammonia and dimethyl
amine was studied by calibrations performed in the CLOUD facility at CERN
(e.g. Kirkby et al., 2011; Almeida et al., 2013). The instrument was used
for quantification of ammonia and dimethyl amine as well as for qualitative
detection of other amines in CLOUD at CERN and at the SMEAR II boreal forest
field station in Hyytiälä, southern Finland, during the PEGASOS
campaign in spring 2013.
Instrument
The instrument uses the nitrate ion atmospheric pressure chemical ionization
(CI) system combined with an APi-TOF (Junninen et al., 2010) as described in
Jokinen et al. (2012). Two modifications were made to the original
instrument (Jokinen et al., 2012). First, due to multiple problems
associated with use and transportation of radioactive materials, the
radioactive 10 MBq Am-241 ion source was replaced by a Hamamatsu (model
L9490) soft (< 9.5 keV) X-ray tube. Second, a system for introducing
gaseous sulfuric acid (H2SO4) in the sample flow was developed. A
schematic representation of the instrument is shown in Fig. 1.
Operation of the chemical ionization ion-induced nucleation inlet is
based on chemical ionization of sulfuric acid, H2SO4 (≡SA) by nitrate ions, NO3-, to form bisulfate ions,
HSO4- (≡SA-), and subsequent formation of bisulfate
ion–sulfuric acid clusters:
NO3-+SA→HNO3+SA-SA-+SA→SA⋅SA-(“dimer”)SA⋅SA-+SA→(SA)2⋅SA-(“trimer”).
In the presence of dimethyl amine (DMA):
(SA)2⋅SA-+DMA→DMA⋅(SA)2⋅SA-.
Further clustering of sulfuric acid takes place:
(SA)2⋅SA-+SA↔(SA)3⋅SA-(“tetramer”)
after which, besides DMA, ammonia (NH3) can also stick to the clusters:
(SA)3⋅SA-+DMA→DMA⋅(SA)3⋅SA-(SA)3⋅SA-+NH3↔NH3⋅(SA)3⋅SA-.
If sufficient sulfuric acid is present in the ambient sample it is possible
that DMA (at very low concentrations) is bound to sulfuric acid. In that
case the following reaction can also occur:
SA⋅SA-+SA⋅DMA→DMA⋅(SA)2⋅SA-.
Clusters formed in reactions (R2)–(R8) can also decompose (evaporate),
specifically in the reduced pressure in the APi interface to the mass
spectrometer. Evaporation rates for reactions (R1)–(R8) at +25 ∘C
have been calculated to be: R2: 2.70 × 10-15, R3:
5.60 × 10-3, R4 and R8: 5.28 × 10-2, R5: 24.1,
R6: 1.89 and R7: 27.4 s-1 (Ortega et al., 2014). In reactions
(R2)–(R4) and (R8), only the forward reaction needs to be considered. Due to
their highly negative formation free energy, clusters formed in these
reactions should be virtually non-evaporating in the 0.1 s residence time of
the CI-system. DMA⋅ (SA3)SA- formed in reaction (R6) should
also be stable in our timescale with a lifetime of the cluster of the order
of 0.5 s at +25 ∘C. However, the most probable fate of DMA⋅
(SA3)SA- is not loss of SA but dissociation to neutral DMA⋅
(SA2) and SA⋅SA-. Therefore, addition of another SA to the
highly stable DMA⋅(SA2)SA- may result in a loss of the DMA
altogether from the ion, especially when the instrument is operated at
temperatures above +25 ∘C.
Schematic of the bisulfate – cluster chemical ionization
atmospheric pressure interface time-of-flight mass spectrometer.
This assumption of stability does not apply to reactions (R5) and (R7), which
complicate the detection of ammonia or amines which do not form stable
adducts with (SA)2SA- similar to reaction (R4). The backward
(evaporation) rates for reactions (R5) and (R7) will also be temperature
sensitive. Thus, stable detection of compounds clustering only with
(SA)3⋅SA- requires precise temperature control of the
instrument.
Presence of (SA)4⋅SA- is unlikely at the
∼+20 ∘C operating temperature of our system (Ortega et al.,
2014) but the clusters formed in reactions (R4), (R6) and (R7) can still add
another SA molecule, forming a reasonably stable cluster. For example, for
NH3⋅(SA)4⋅SA-, the evaporation rate is
2.29 s-1 (+25 ∘C, Ortega et al., 2014). Evaporation rates
for DMA⋅(SA)4⋅SA- are not reported. Attachment of a
fifth sulfuric acid can give ion signals from the bases clustered with the
SA trimer to pentamer (DMA) or with the tetramer or pentamer (NH3).
Further reactions where an additional DMA or NH3 molecule attaches to
cluster can occur but should not significantly affect the cluster
distribution at expected low amine concentrations. These clusters containing
multiple bases are readily detected with TOF-MS (see later).
Besides evaporation in the CI-system, clusters can decompose in energetic
collisions in the electric fields of APi quadrupoles. Also the cluster
temperature will increase as a result of the collisions with the residual
gas molecules thereby increasing the cluster evaporation rates. Detailed
understanding of these effects is a hot topic in MS in
general, but will require significant experimental and modelling efforts.
Collision energies inside APi are not very temperature dependent and thus
once the fields are stable any de-clustering processes should be independent
on environmental conditions. However, since the tuning significantly affects
the ion transmission, fragmentation and evaporation, it is highly important
that instruments are calibrated using the same settings as used in the field
measurements.
Dimethyl (DMA) and/or ethyl amine (EA) and diethyl amine (DEA) form
stable clusters already with bisulfate “trimer”, whereas ammonia, methyl
amine (MA) and trimethyl/propyl amine (TMA/PA) are detected with “tetramer”
or larger clusters.
In our experiment, electric fields inside the APi were optimized by manually
tuning to provide maximum transmission and minimum fragmentation for the
preferred mass range through the APi. In the present experiment we did not
tune the instrument specifically for the purpose but we used the settings
optimized for dimethyl amine–sulfuric acid nucleation experiments
presented in Kürten et al. (2014). Therefore, the sensitivity of the
instrument could still be improved by improving the APi transmission,
especially in the mass range of 300–500 Da.
An overview of sticking preferences of various amines is shown in Fig. 2,
depicting signals observed from laboratory indoor air mixed with a high
concentration (several 1010 molecules cm-3) of sulfuric acid
vapour. While signals from DMA/EA, and diethyl/butyl amine (DEA/BA) are
roughly as abundant with both the “trimer” and “tetramer”, the signals
from NH3, MMA and TMA/PA are larger with the tetramer. This
observation, albeit qualitative, indicates that DMA/EA and DEA/BA can be
detected with higher sensitivity than ammonia and other small amines,
because the “trimer” concentration in the system is significantly higher
than that of the “tetramer” and larger sulfuric acid clusters.
Figure 3 shows the operation of the NO3- CI-system in more detail
(Eisele and Tanner, 1993; Jokinen et al., 2011), including COMSOL
computational fluid dynamical modelling. The system is comprised of a
3/4′′ inlet tube through which the sample is drawn with a
flow rate of 10 L min-1. A cylinder (coaxial to the inlet tube) is held at
-130 V potential, separating the ion production region from the sample tube at
ground potential. Still coaxial to that is an outer cylinder also kept at
-130 V potential. An X-ray source irradiates the space between these two
cylinders through an aluminium window at the outer cylinder surface.
Operation principle of NO3- CI system used to produce
bisulfate–sulfuric acid–base clusters from ambient amines and added
sulfuric acid present in the sample flow. Upper plot represents the flow
profile inside the system. Lower plot depicts the electric potential inside
the system. In the upper panel, the black thick line shows the ion trajectory in the
case where all electric potentials are set to zero and ions “go with the
flow”. In the lower panel, the black thick line shows electric fields guiding
ions toward the centreline of the ion source, allowing ions to mix and
interact with the sample and eventually be transported to the pinhole of the
mass spectrometer. The original concept as presented by Eisele and Tanner (1993)
was adopted by Jokinen et al. (2011) and coupled to the APi-TOF mass
spectrometer.
Sheath gas (ideally cryogenic N2) flows at 20 L min-1 in the
space between the cylinders and carries the ions produced downstream toward
the ion–molecule reaction (IMR) tube. HNO3 vapour added into the sheath
flow promptly converts ions (formed from the X-rays) to
NO3-(HNO3)n,n=0-2 ions or ion clusters. After entering the
IMR region, an electric field (-110 V) between the IMR tube and the ground
potential of the sample tube pushes the ions toward the sample (centreline)
flow. Flows (sheath and sample) and electric field strength are balanced so
that ions do not hit the sample tube wall, rather following an axial
trajectory after entering the sample flow. Ions then interact with the sample
flow for up to 340 ms before the electric field guides the ions into the
0.7 L min-1 flow through the pinhole into the atmospheric pressure
interface of the TOF mass spectrometer.
The upper panel of Fig. 3 shows the flow velocity profile and nitrate ion
trajectory in the absence of any electric potential in the drift tube or in the
ion source. Ions travel close to the wall of the drift tube and exit the
system with excess flow without interacting with the sample flow. In this
case ions do not enter the mass spectrometer. The lower panel of Fig. 3
depicts the electric potential and the nitrate ion trajectory with the
electric field on. The electric field and gas flows guide the ions from the
ion production region to the centreline of the drift tube and eventually
through the pinhole into the APi-TOF. This nitrate ion based CI-APi-TOF has
been used in many recent laboratory and ambient air studies probing
atmospheric chemistry and particle formation (e.g. Mauldin et al., 2012;
Almeida et al., 2013; Ehn et al., 2014; Kürten et al., 2014). It has been
shown to be highly sensitive toward sulfuric acid, sulfuric acid–amine
clusters, and highly oxidized low volatility organics.
For direct amine measurements bisulfate–sulfuric acid clusters need to
be generated first. Therefore, preceding the nitrate CI system, a flow
(1–2 L min-1) saturated in sulfuric acid vapour is mixed with the sample air. The
saturator is a temperature-controlled (+20 ∘C) rotating coaxial
design (Fig. 4). The coaxial design allows a significantly more compact
construction than a cylindrical design, while rotation continuously wets the
walls with liquid sulfuric acid. This minimized unwanted wall effects; e.g.
clean glass surfaces act as a sink to sulfuric acid vapour in contrast to
liquid acid coatings. The glass tube connecting the rotating saturator to
the sample inlet is as short as possible (5 cm). Mixing of sulfuric acid
vapour and the sample gas then takes place in the 20 cm distance before the
sample air enters the drift tube/ion interaction region. The detailed
mixing process is not well known; it most likely involves both small-scale
turbulence, as well as diffusion. Sulfuric acid concentration in the
resulting mixture is in the range of 2–6 × 1010 molecules cm-3. After entering the IMR tube, reactions (R1)–(R8) result in
prompt formation of bisulfate–sulfuric acid–base clusters for mass
spectrometric analysis. Neutral sulfuric acid–base nucleation can occur
both in the sample tube as well as in the IMR tube; in the latter,
ionization of neutral clusters by NO3- or SA- can also
occur. However, since ion-induced clustering is significantly faster (due to the
ion-enhanced collision rate) than neutral cluster formation, neutral
processes likely play a minor role.
Three problems that have previously limited amine measurements (see the Introduction)
are, to a large extent, solved with the present approach. Firstly,
outgassing of amines from flow system walls is effectively prevented by acid
coating of all gas lines and CI-source surfaces. Stainless steel surfaces in
the sheath gas line are also extensively coated with HNO3 added to
sheath gas flow. Such acid coating activates tube walls with respect to base
deposition and prevents desorption. Only the wall of 3/4 inch
diam. 40 cm long inlet tube extending to ambient atmosphere is not actively
acidified. Also the use of cryogenic nitrogen as a sheath gas and stainless
steel surfaces in instrument as well as in gas lines between the nitrogen
Dewar and the instrument help in decreasing the background signals.
The second problem, the non-collision limit charging efficiency, is solved for
DMA by the choice of the ionization method. DMA sticks to sulfuric acid
trimer and tetramer without significant evaporation. This, however, is not
the case with ammonia. Other amines also need to be thoroughly investigated.
The third problem, identification of the atomic composition of detected ion,
is solved by application of high mass resolution TOF mass spectrometer. The mass
resolution of the APi-TOF is ∼ 4000 Th/Th and the mass accuracy is
< 20 ppm. This is facilitated in data post-processing utilizing
high-resolution peak identification and isotopic patterns (tofTools, Junninen
et al., 2010). Combined with high selectivity of the ionization method, many
unwanted compounds are not ionized, or resolved in a very clean mass
spectrum.
Schematic of the rotating sulfuric acid saturator. The saturator is
connected to the sample tube as in Fig. 1.
Sensitivity studies
Sensitivity was studied in the CLOUD experiment at CERN. For a detailed
description of the facility see Kirkby et al. (2011) and Almeida et al. (2013). Briefly, the CLOUD facility is designed for studying nucleation and
growth of secondary aerosol, cloud droplet activation and freezing and the
effect of galactic cosmic radiation on those processes, under precisely
defined laboratory conditions. The CLOUD chamber itself is a 26 m3
electropolished stainless steel tank equipped with a UV-light system and
precise temperature control. Lifetime of condensable gases against wall loss
is in the range of few minutes. Air inside the chamber is prepared from
cryogenic nitrogen and oxygen. Input gases are precisely controlled and the
gas composition is continuously monitored by an extensive suite of analysing
instruments.
DMA and/or NH3 were mixed with the flows of cryogenic oxygen and
nitrogen, prior to entering the chamber. The calculation of the volume mixing
ratios is based on a balance between the flow of DMA into the chamber and its
loss to the chamber walls. While the amount of the inflowing DMA is directly
obtained from the mass flow controller settings, the wall loss rate is
derived from two independent methods. The first one relates the measured wall
loss rate of sulfuric acid to the one of dimethylamine taking into account
that the loss rate is proportional to the square root of the diffusivity for
the different molecules (Crump and Seinfeld, 1981). The second method
involves directly measured decay rates when the flow of DMA is shut off after
a sufficiently long period when DMA was present at high mixing ratios
(several tens of pptv). These measured decay rates were obtained using
nitrate cluster ions for the DMA detection. A detailed description of the DMA
quantification by this method will be given in forthcoming publications. For
the scope of this paper it is sufficient to note that just the relative
change in the DMA signal is required to obtain the wall loss rate, while the
absolute measurement is not a necessity. Overall, the different methods for
determination of the DMA mixing ratios, including also the direct IC
measurement (Praplan et al., 2012), yield consistent results and the error
bars in Fig. 6 indicate the uncertainty.
Mass spectra with [DMA] = 2.22 pptV, the lowest studied [DMA];
and the background signal (in the absence of added DMA) without and after the
baking and flushing of the CLOUD chamber. Integration time is 15 min.
The principal peak is located at 435.92 Th; the first of the isotopes 436.92 Th is
partly overlapping with signal from an unidentified compound. The second isotope
at 437.92 Th is, however, clearly visible. The signal at 437.6 Th originates from a
chlorine-containing substance, whose source is unknown. Lowest detection
limit was defined as three times the background signal at main peak
(435.92 Th) and was found to be 70 ppqV (0.07 pptV). Practically, ultimate
sensitivity is limited by contamination in blank air or on inlet surfaces. A high-resolution fit of DMA is also shown and represents the ideal distribution of
signal in the absence of overlapping signals from other compounds.
Sensitivity for DMA
The CIMS system described in this work was calibrated against the
concentration in the CLOUD chamber with [DMA] ranging from 2.2 to 34.9 pptV.
These experiments included simultaneous generation of sulfuric acid (from
SO2 oxidation) at atmospheric levels to investigate of sulfuric acid–DMA nucleation. Sulfuric acid present in the chamber could affect DMA detection via clustering with the DMA which could slightly affect the
measurement to a pure DMA system. However, since sulfuric acid
concentrations were representative of the atmosphere, the presence of
sulfuric acid makes the calibration system more atmospherically relevant.
In Fig. 5, example mass spectra around mass/charge 436 Th, the mass of the
DMA-tetramer cluster, show signals with [DMA] = 2.22 pptV and with no
added DMA (i.e. chemical background of the system with CLOUD tank filled
with cryogenic N2/ O2 mixture) without special cleaning procedure
and after cleaning the chamber by baking at ∼ 100 ∘C
and flushing with cryogenic air in presence of O3 and OH (UV-light) to
desorb and/or oxidize any contaminants on the chamber wall. The main isotope
at 437.92 Th is clearly visible demonstrating the sensitivity and the
resolution of the method. The isotopic distribution and the exact mass of
the main peak allow unambiguous identification of the atomic composition of
the cluster ion.
Response of bisulfate cluster signal to [DMA] = 0–35 pptV. DMA
concentration in the CLOUD tank was adjusted by adjusting the DMA-containing
flow to the chamber.
The response of the instrument as a function of [DMA] is shown in Fig. 6. DMA is
observed in the clusters with (SA)2-4⋅SA-, with
DMA⋅(SA)3⋅SA- being the most abundant cluster. Summing
up the detected clusters yields the total signal of DMA. The signal is normalized
by (SA)2⋅SA-, the dominant reagent ion at the end of the IMR
tube. The (SA)2⋅SA- should be the ion to which most of the DMA
attach according to reaction (R4) (before the further attachment of sulfuric
acid), since DMA cannot attach to (SA)0-1⋅SA- and larger
clusters (SA)>2⋅SA- are far more sparse. It is possible that a
significant fraction of neutral DMA molecules enter the IMR region bound to
sulfuric acid. In that case, reaction (R8) can play a role as well, and
normalization to sum of bisulfate dimer and trimer might be appropriate.
However, the results in Fig. 6 are independent on the choice of normalization
method.
All detected ions correlate extremely well with the DMA concentration
calculated from the flows injected in the chamber. The correlation between
the normalized sum of detected clusters and DMA concentration is excellent,
R2=0.9994. Such linearity demonstrates the performance of our
instrument and, also, the superior control and performance of the CLOUD
chamber facility. DMA⋅(SA)2-4⋅SA- clusters are also
very stable and, therefore, temperature variations in IMR tube do not alter
cluster distributions or detection efficiency. The limit of detection
(defined as three times the background signal) was found to be 73 ppqV (or
0.07 pptV) for 15 min integration, representing roughly a 10- to 100-fold improvement in comparison to other existing techniques (Hanson et al.,
2011; Yu et al., 2012). It should also be noted that with time-of-flight mass
spectrometer all signals are integrated simultaneously while, in contrast,
with quadrupole MS only one mass is detected at a time. Thus, if several amines
are to be measured, the LOD is unaffected. A calibration coefficient is
obtained from the slope of the linear fit, C=491 pptV. The calibration
coefficient is used to convert the signal to concentration according to
[DMA]=C×∑n=2-4SA-SAnDMASA-SA2.
Sensitivity for ammonia
Sensitivity for ammonia was studied exactly as for DMA. Ammonia easily
evaporates from the “trimer” SA cluster, unlike DMA; and since the
“tetramer” cluster is relatively unstable at near room temperature in the
IMR tube, much lower sensitivity is expected. Calibration results were far
more scattered possibly due to either fluctuating (or poorly defined
NH3 concentration, at low [NH3]) in the CLOUD chamber or
temperature variations in the IMR tube affecting cluster stability.
Instrument temperature was not controlled, e.g. drifted with ambient
temperature in the facility; therefore, temperature-sensitive evaporation
rates would be reflected in ammonia detection sensitivity. If temperature
fluctuations were the cause of variable signals, a LOD of few tens of pptV
could be achieved with proper temperature control. Using only the highest
ammonia concentration (90 pptV) used in calibration experiment, an
approximate calibration coefficient of C=2.9× 104 pptV was
obtained. This coefficient is subject to significant uncertainty, at least a
factor of 5. Major improvements are required before an instrument can be
used for quantitative ammonia measurements. In the case of ammonia, the signal is
normalized to “tetramer” and the equation
[NH3]=C×∑n=3-4SA-SAnNH3SA-SA3
is used for converting the signal to concentration.
Application to field measurements at SMEAR II
Field measurements were conducted at the Smear II boreal forest field
station (Hari and Kulmala, 2005) at Hyytiälä in southern Finland
during the PEGASOS campaign, 1 April–15 June 2013.
Initially ambient air filtered with amine-specific gas mask filter (pumped
using a standard membrane pump) was used to provide the 20 L min-1 sheath air
flow for the ion source. Background signals from amines and ammonia were
unacceptably high, even though the acidified walls of the sheath gas lines
likely scavenged a significant fraction of those bases. Therefore, the
sheath air was promptly substituted with a flow of cryogenic nitrogen.
Figure 7 shows how the change in sheath gas affected the signals, also
indicating the sensitivity to any artificial source of amines, a problem
associated with amine measurements in general.
A portion of time series of ammonia and DMA is shown in Fig. 8, with
concentrations calculated with the calibration coefficients obtained from
the CERN calibration. Backgrounds, measured daily by substituting cryogenic
nitrogen in the sample flow, are not subtracted; rather they are shown in
the shaded blue regions. In the case of ammonia, N2 flushing yielded a
factor of 5–10 decrease in detected signal, corresponding to background
“concentration” of few tens of pptV (using the nominal calibration
coefficient determined above). Average ammonia concentration during the
stable operation period with N2 sheath gas (from 7 May to
2 June, 2013) was approximately 610 pptV, subject to significant
uncertainty of at least a factor of 5 due to the uncertainty (see above) in the
calibration coefficient. Thus, ammonia measurements reported here should
only be taken as a proof-of-concept to monitor ammonia together with DMA and
possibly other amines.
In the case of DMA, signals (always) increased during background N2
flushing, reaching levels corresponding to 250 ppqV (0.25 pptV) in
concentration (Fig. 8, see also Fig. 9a). The source of DMA is likely
somewhere in the stainless steel line, mass flow controller or pressure
regulator between the N2 Dewar and the sample inlet; or, less likely,
from contamination (during flow switching) in the 40 cm long non-acidified
part of the sample tube. Nevertheless, since no meaningful background level
could be determined, the observed ambient signals, corresponding to 40 to
150 ppqV amine concentration, may be all due to chemical background in the
instrument and, thus, not a single data point can be reliably attributed to
DMA concentrations in the ambient atmosphere. An upper limit for DMA
concentration can be obtained if we assume that the only artificial source of
DMA was the zero air fed to the instrument during the background measurement. In
that case the upper limit of [DMA] ranged from a few tens up to approximately
1500 ppqV during the whole measurement period when the instrument worked
stably (7 May to 2 June 2013). However, since we had no calibration standard
available in field measurements, our results should be treated with caution and
considered as a proof-of-concept rather than a solid fact.
During this time period, several nucleation events were observed. Our
observation of low DMA levels indicates that the nucleation process unlikely
involved DMA to a significant extent (Almeida et al., 2013) and other amines
or non-nitrous organics are possibly needed to explain the new particle
formation rates.
Purity of sheath gas is important for high-sensitivity detection.
Cryogenic N2 resulted in 3–5 times lower background than ambient air
filtered with particle filter and amine-specific gas mask filter. Still both
ammonia and dimethyl amine are visible in the spectra, with signals
corresponding to several tens of pptV for ammonia and ∼ 100 ppqV for
dimethyl amine. Data are taken with zero air (N2) fed into the sample
inlet.
Though the DMA concentration was found to be reasonably low, or even
negligible, other amines, TMA or possibly PA, exceeded the detection
(background) threshold. Since no calibration exists for amines other than
DMA, those signals cannot be reliably converted to concentrations. However,
since bisulfate–sulfuric acid–DMA clusters are very stable, it is
likely that other amines are detected with similar or lower sensitivity as
DMA. Thus, we can report the lower limit concentrations for other detected
amines by applying the calibration coefficient obtained for DMA. Example
mass spectra from (a) DMA/EA, (b) ammonia, (c) TMA/PA and (d) a C4-amine
(e.g. DEA) are presented together with corresponding zero measurements in
Fig. 9. Signals from DMA/EA and C-4 amine increase during zero gas
flushing, while ammonia and TMA/PA show clear and moderate reduction upon
zero measurement, respectively, the latter suggesting the presence of TMA/PA
with concentrations larger than a few tens or hundreds of ppqV. However, for
these extremely small signals, exact mass determination is not definitive;
nor could the isotopes be resolved from the background spectrum. Therefore,
the identification of the compound as TMA/PA is not certain.
Portion of a time series measured at SMEAR II boreal forest field
station. While ammonia concentration exceeds the detection threshold
determined by the zero measurement (indicated by the blue shaded regions), the
DMA signal increases when zero air is fed into the instrument, indicating that
DMA concentrations in ambient atmosphere were at maximum 150 ppqV during the
measurement period. Evaluation of ambient concentration is limited by
apparent contamination in “zero air” from N2 flushing of inlet lines. That is, the
[DMA/EA] depicted here represents the upper limit of ambient concentration.
Examples of signals representing ambient measurement (blue) and zero
(black) measurement. Zero gas (cryo N2) contains more gas phase DMA/EA
and a C4-amine (e.g. DEA) than ambient air. Only ammonia and potentially
trimethyl/propyl amine (TMA/PA) signals exceed the signals from zero gas
measurement. Red bars show the exact masses of isotopes and expected isotopic
distribution normalized to the height of the main peak.
Though the role of DMA in particle nucleation seems uncertain, the possible
detection of TMA/PA leaves open the possibility that other amines might
contribute to nucleation. Our result regarding [DMA] agrees with
Schobesberger et al. (2015), suggesting sub-pptV DMA concentrations. However,
our values are in serious conflict with the concentrations of DMA/EA reported
by Kieloaho et al. (2013). The reason could be that the entire signal in
Kieloaho et al. (2013) was due to EA while we were completely insensitive to
EA – for example, we could only detect DMA. However, it would be surprising
that, while we obviously detect several amines, including DMA, MMA, TMA/PA
and DEA (Fig. 2), the sensitivity for EA would be so poor. Another
possibility could be in particle phase amine evaporation from the PTFE filter
used to remove particulate matter from the sample in Kieloaho et al. (2013).
Also, we performed our measurements at the open area of the measurement
container field of SMEAR II, some 10–20 m from the uniform forested area,
while Kieloaho et al. (2013) sampled inside the forest below the canopy.
Amine lifetimes against oxidation and adsorption to particle phase should be
at least a few minutes (even with unity uptake coefficient), and therefore
the small difference in measurement location should not cause such a major
discrepancy.
Conclusions
A bisulfate–cluster based atmospheric pressure chemical ionization
system was developed and integrated into a time-of-flight mass
spectrometer. Calibrations demonstrated that, under ideal conditions, there is a
limit of detection for dimethyl amine (DMA) of less than 70 parts per
quadrillion (ppqV) for a 15 min integration. Sensitivity of the system
for DMA is approximately 10- to 100-fold higher than reported for other
existing methods. Performance results from minimization of amine outgassing
from system surfaces, collision-limited ionization and high mass resolution
of the applied mass spectrometer. Extreme cleanliness of added gas flows is
also crucial.
Besides DMA, other small alkyl amines and ammonia can be detected. Detection
of ammonia with the present system, however, suffers from imperfect
temperature control resulting in varying stability of bisulfate–sulfuric acid–ammonia clusters which is likely reflected in variable
instrument response. Further efforts are required to understand the
clustering dynamics of amines other than DMA and to calibrate the instrument
against well-quantified concentration of these amines. In field measurements
at a boreal forest site, DMA concentration was below ∼ 150 ppqV throughout the whole measurement period in May–June 2013, suggesting
that it is unlikely that DMA played a major role in atmospheric nucleation of
new aerosol particles observed simultaneously at the site. However, since no
calibration standard was available during field measurements, our results
should be treated with caution and considered as a proof-of-concept. Tentative
observations on the existence of other amines (trimethyl and/or propyl amine)
leave open the possibility of amine contribution to new particle formation.
Acknowledgements
We thank CERN for supporting CLOUD with technical and financial resources.
Support of the Academy of Finland (251427, 139656, 264375, Finnish Centre of
Excellence 141135) is acknowledged. Support from the Helsinki Institute of
Physics is acknowledged. This research was partially funded by the European
Commission 7th Framework Programme via PEGASOS (FP7-ENV-2010-265148),
ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure Network),
Marie Curie Initial Training Network “CLOUD-ITN” and “CLOUDTRAIN-ITN”, and
Finnish Funding Agency for Innovation (TEKES) through the APCI project. Tinja Olenius, Oona Kupiainen-Määttä, Kenty Ortega,
Hanna Vehkamäki, Antti-Jussi Kieloaho and Markku Kulmala are acknowledged for
useful discussions. We thank the tofTools team for providing tools for mass
spectrometry analysis. The Finnish Antarctic Research Programme is acknowledged
for providing high-quality facilities for preparing the manuscript.
Edited by: F. Stroh
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