Introduction
Volatile organic compounds (VOCs) play a central role in atmospheric
chemistry by regulating tropospheric ozone and secondary organic aerosol (SOA)
production rates (Goldstein and Galbally, 2007). Global
emissions of non-methane VOCs are dominated by biogenic VOCs (BVOCs) such as
terpenes, where isoprene (C5H8) and monoterpenes (MT;
C10H16) emissions have been shown to be most significant
(Arnold et al., 2009). Recent
studies have shown that sesquiterpenes (SQT; C15H24) may also play
a significant role in secondary organic aerosol production
(Jaoui et al., 2013 and others), but far less is known about
their global emission rates (Guenther et al., 2012; Kanakidou et al., 2005). In one of the few studies with
simultaneous measurements of isoprene, MT, and SQT, Kim et al. (2009) inferred
ecosystem scale fluxes that suggest that SQT fluxes could be as much as
50 % that of MT in deciduous forests (Kim et al., 2009).
In terrestrial regions, the condensable oxidation products of terpenes has
been shown to drive SOA production, particularly in boreal and subtropical
forests (Guenther et al., 2012). In comparison, SOA precursors in the marine boundary layer
have historically been thought to be dominated by dimethyl sulfide (DMS)
emissions, with isoprene and monoterpenes contributing less than 1 % to
SOA mass (Myriokefalitakis et al., 2010). The following
paper focuses on the development of a chemical-ionization procedure for
targeting marine trace gas emissions (e.g., DMS, isoprene, and monoterpenes)
that, following oxidation, may have consequent impacts on aerosol particle
mass loadings and size distributions.
Chemical-ionization mass spectrometry (CIMS) is increasingly utilized as a
fast, sensitive, and selective measurement technique for in situ detection of
reactive trace gas species (Huey, 2007). The
benefits of CIMS are most pronounced when sampling from moving platforms or
during instrumentally tasking sampling methods such as eddy covariance,
where high time resolution is required. Proton transfer mass spectrometry
(PTR-MS) has been used extensively for the sensitive, selective
determination of VOCs providing direct measurements of concentrations and
turbulent fluxes from a variety of mobile platforms
(e.g., Lindinger et al., 1998). PTR-MS is well
suited for the measurement of small molecules such as DMS and isoprene, but
fragmentation of larger volatile organic compounds, such as monoterpenes and
sesquiterpenes, can limit quantitative measurement (Kim et
al., 2009). In parallel, separation techniques (e.g., gas chromatography)
have been utilized to detect BVOCs with good sensitivity with the added
benefit of resolving isobaric interferences at both nominal and fragment
masses (Helmig et al., 2004).
However, low temporal resolution limits their utility for higher time
resolution analyses. In what follows, we extend the early work of
Leibrock and Huey (2000) to
explore the utility of benzene cluster cations,
(C6H6)n+, for the selective, sensitive, and rapid
detection of various SOA precursors via CIMS in the marine boundary layer.
The use of benzene cations as selective reagent ions dates back to at least
the work of Horning et al. (1973), where trace
benzene vapor was introduced into an atmospheric pressure ionization mass
spectrometer. As discussed by Horning, benzene ion chemistry was thought to
proceed through either a charge transfer reaction (R1) if the ionization
energy (IE) for the analyte (A) was less than that of benzene (9.24 eV) or
through a proton transfer reaction (R2) if the gas-phase basicity of the
analyte (B) is greater than that of the phenyl radical:
(C6H6)++A→(C6H6)+A+(C6H6)++B→(C6H5)+BH+
Horning et al. (1973) demonstrated that large, complex organics (e.g.,
testosterone) could be detected with minimal fragmentation as the molecular
ion (M+), while 2,6-dimethyl-γ-pyrone was detected as MH+
following R2. It is thus expected that an array of gas-phase bases (e.g.,
amines) could be detected using this ion chemistry. With specific
application to atmospheric chemistry, Ketkar et al. (1991) revisited benzene ion chemistry for the selective detection of
2-chloroethyl ethyl sulfide.
The most complete and recent work on benzene ion chemistry, with application
to atmospheric measurements, is from two papers by Leibrock and Huey, where
the potential for benzene cations in the detection of isoprene and
monoterpenes, among a suite of other conjugated dienes and aromatics, was
discussed. Leibrock and Huey (2000) demonstrated that select VOCs with
ionization energies lower than that for benzene (9.24 eV) react at or near
the collision limit either via direct charge transfer (R1) forming the charge
transfer product X+ or through a ligand switching reaction involving
the benzene dimer cation (R3) forming the ion-neutral product
X+–(C6H6). It is likely that if ionization proceeds through
R3, the number of molecules that can be detected will be reduced as the
ionization energy for the benzene dimer cation has been measured to be
significantly less (ca. 8.6 eV) than the monomer (Grover et
al., 1987).
(C6H6)2++X→X+(C6H6)+(C6H6)
It follows that benzene ion chemistry should be a sensitive measurement for
a host of molecules of interest to the study of marine air, including
DMS (IE = 8.6 eV), monoterpenes (IE = 8.07 eV,
α-pinene), sesquiterpenes (IE = 8.3 eV, β-caryophyllene),
and their first-generation oxidation products, as well as a host of
atmospheric amines (through R2)
(Al-Joboury and Turner, 1964; Hunter and Lias, 1998; McDiarmid, 1974). The high density of states of
C6H6 and (C6H6)2 may also permit the efficient
formation of weakly bound ion-neutral clusters, as collisional energy from
the ion–molecule reactions (IMRs) can be readily absorbed. However, the extent to
which this is important is governed by the energetics of the specific
IMR.
These advantages are not without potential challenges. These challenges include
the extensive dehydration of alcohols can complicate the interpretation of mass
spectra. Leibrock and Huey showed that the dehydration of 2-methyl-3-buten-2-ol (MBO)
could lead to a positive artifact in the detection of isoprene, despite the fact that
the IE for MBO exceeds 9.24 eV. In addition, benzene ion chemistry is not exceedingly
selective as compared with other ion-molecule chemistries, resulting in the potential
for interferences in low mass resolution instruments, particularly in polluted air masses.
Finally, while it is not expected that benzene cations will directly ionize water, it
has been shown that under high specific humidity, benzene cations may have a series of
attached water molecules (Ibrahim et al., 2005; Miyazaki et al., 2004) that may introduce
a water dependence in the ion chemistry.
In what follows, we describe laboratory characterization experiments and
field observations in the remote marine boundary layer to assess the utility
of benzene cations for the detection of DMS and select BVOCs. Laboratory
experiments were conducted using both a chemical-ionization time-of-flight
mass spectrometer (CI-ToFMS)
(Bertram et al., 2011) and a chemical-ionization quadrupole mass spectrometer (CI-QMS) to
probe IMR mechanisms and adduct stability under different
electric field strengths. Field measurements from the remote North Atlantic
boundary layer were validated against simultaneous measurements of DMS made
by an atmospheric pressure ionization mass spectrometer with an isotopically
labeled standard (APIMS-ILS)
(Blomquist et al., 2010).
Laboratory characterization
Factors controlling primary ion distributions
During laboratory experiments, the CI-QMS and CI-ToFMS ion generation
systems were identical with the exception of the IMR pressure. As shown in
Fig. 1, the benzene ion distribution as measured by CI-QMS was detected
predominantly in the form of (C6H6)2+ at 156 m/Q (Fig. 1a).
The same gas mixture was detected by CI-ToFMS in the form of
C6H6+ at 78 m/Q (Fig. 1b). This suggests that the reagent-ion
distribution primarily exists as n≥2 in the CI-ToFMS IMR as well, but
the field strength of the RF-only quadrupole ion guide in the CDC of the
CI-ToFMS exceeds the benzene dimer binding energy of 15.3 kcal mol-1
(Grover et al., 1987) despite operating at a relatively weak
field strength. Unfortunately, even in the CI-QMS, significant declustering
occurs between the IMR and the mass analyzer. As a result, it is not
possible to comment directly on the distribution of benzene cluster cations
in the IMR and the possibility that the clusters may have attached water
molecules. However, based on the CI-QMS spectra, it is most likely that the
clusters are at least dimers (n≥2) in both instruments.
Dependence on benzene reagent gas concentration
Background mass spectra were examined by overflowing the sample inlet with
UHP zero air while varying the concentration of benzene delivered through
the ionizers of both the CI-ToFMS and the CI-QMS instruments. The relative
humidity at the sample inlet was less than 5 % during these experiments.
On the CI-ToFMS, the benzene monomer accounted for 68–91 % of the total
ion current (Fig. 2) for benzene precursor ion concentrations ranging
between 1 and 45 ppm. Peak benzene ion signal intensities were found at benzene
reagent gas concentrations of approximately 10 ppm. At lower benzene
concentration (< 10 ppm), a wide array of background ions are
prevalent in the mass spectra. Although the zero air cylinders were assumed
to be nominally dry, trace amounts of water (detected as protonated water
clusters) were evident in the sample stream at low benzene concentrations.
At higher benzene concentrations (> 10 ppm) background peaks at
92, 106, and 120 m/Q become an increasing fraction of the total ion current. At
10 ppm benzene, these ions collectively account for 1–3 % of total ion
current. During High Wind Gas Exchange Study
(HiWinGS) underway measurements, the sum of these three peaks
comprised an average of 2.6 % (1σ= 0.8 %) of the total ion
current. These background peaks were observed across all CIMS instruments
and their strength increased with benzene concentration (Fig. 2), suggesting
they are trace contaminants in the benzene cylinders. We provisionally
attribute these peaks to toluene, ethylbenzene, xylene, and
trimethylbenzene.
CI-QMS (a) and CI-ToFMS (b) mass spectra utilizing benzene reagent-ion chemistry while sampling a mixture of dimethyl-1,1,1-d3 sulfide,
isoprene, and α-pinene standards at 40 % relative humidity and room
temperature. The weaker field strength of the CI-QMS transmits clustered
forms of benzene ((C6H6)2+) and dimethyl-1,1,1-d3
sulfide (CH3SCD3-C6H6+). Both are detected at
nominal mass by the CI-ToFMS. Isoprene is detected as a benzene cluster
(C5H8-C6H6+) and α-pinene
(C10H16+) appears at nominal mass on both instruments.
Partitioning of total ion current (TIC) on the CI-ToFMS between
benzene clusters (light blue), O2+ (green), water clusters (dark
blue, n=0 to 3), and background peaks (orange and gray) as a function of
benzene reagent-ion concentration at < 1 % relative humidity.
Molecules corresponding to 92, 106, and 120 m/Q are attributed to
alkyl-substituted benzene molecules: C6H5-CH3+,
C6H4-(CH3)2+, and
C6H4-(CH3)3+, respectively.
Dependence of benzene ion signal intensity on the benzene reagent
gas concentration for both the CI-QMS (a) and CI-ToFMS (b). Nearly all CI-QMS
benzene signal intensity is found at the dimer (156 m/Q) while the CI-ToFMS was
nearly exclusively in the form of the monomer (78 m/Q). On both platforms, the
dominant benzene peak plateau after 10 ppm neutral benzene concentration.
As shown in Fig. 3, the ratio of benzene cation dimer to monomer was
approximately 1:104 on the CI-ToFMS and 3 × 103:1 for the
CI-QMS. The presence of the dimer in the CI-ToFMS mass spectrum provides
additional evidence that the reagent-ion distribution primarily exists as n≥2 in both of the IMRs, but the field strength of the RF-only
quadrupole ion guide in the CDC of the CI-ToFMS exceeds the benzene dimer
binding energy of 15 kcal mol-1 (Grover et al., 1987).
Dependence on ambient humidity
Ambient humidity was varied to probe its impact on ion chemistry and product
ion distributions. In this experiment, benzene reagent gas concentrations
were held at 10 ppm while sampling humid UHP zero air (0–100 % relative
humidity, room temperature). On the CI-ToFMS, benzene monomers remained
> 86 % of the total ion current (Fig. 4a) at 100 % relative
humidity (RH), compared with 91 % at < 1 % RH. Absolute benzene
monomer count rates showed minimal sensitivity to ambient water vapor (Fig. 4b).
Protonated water clusters (H3O+–(H2O)n) were
observed upon the addition of water vapor to sample air. The sum of the ion
current for H3O+–(H2O)n (n= 0–3) ranged from
0.7 to 8.5 % of the total ion current, increasing with relative humidity.
Again, trace amounts of water vapor were detected while sampling nominally
dry UHP zero air (Praxair) due to a small amount of condensation in zero air
cylinders.
(Top) Dependence of primary ion signal intensities and
partitioning between the monomer and dimer on ambient relative humidity
([C6H6] = 10 ppm) on the CI-ToFMS. (Bottom) Absolute benzene ion
signal intensities are shown to be nearly independent of ambient relative
humidity.
Given that the ionization energy of benzene (9.2 eV monomer, 8.69 eV dimer)
(Nemeth et al., 1993) is less than water (12.6 eV) (Page et al., 1988), it
is unlikely that a benzene mediated pathway for the formation of protonated
water clusters is active. It is possible that protonated water clusters may
be formed by direct ionization (Good et al., 1970) due to back diffusion of
ambient water into the Po-210 source, or persistent N2+ from the
ionizer may contribute to direct ionization of water downstream in the IMR.
Similarly, molecular oxygen ions (O2+, 12.0 eV)
(Tonkyn et al., 1991) from ambient
air are observed in trace amounts (< 1.3 % of the total ion
current). However, despite the fact that benzene–water clusters were not
observed in the mass spectra, we have no direct evidence that they are not
present in the IMR and they may impact instrument sensitivity, as described
in Sect. 3.4.
Instrument calibration
Calibration curves were generated using three parallel CIMS systems
(CI-ToFMS, CI-QMS, and CIMS that employs a residual gas analyzer for ion
selection and quantification, CI-RGA) to determine the sensitivity of
benzene cations to dimethyl sulfide, isoprene, and α-pinene. Results
from the first two instruments are included here. Results from the CI-RGA
will be discussed in a forthcoming manuscript. Calibration factors were
determined using atmospherically relevant mixing ratios (0 to 3.2 ppb) of
dimethyl-1,1,1-d3 sulfide (Praxair certified standard, 0.184 × 10 %), isoprene
(Praxair certified standard, 0.500 × 10 %), and α-pinene (Praxair certified standard,
0.497 × 10 %) at specific humidities (q) from 0 to 16.6 g kg-1. The primary
VOC calibration standards were diluted with UHP zero air
(Praxair). Humidified buffer gas was generated by flowing UHP zero air
through three sequential frit bubblers (Ace Glass) containing deionized
water. Flow rates for all VOC standards and both the dry and saturated
dilution flows were measured using mass flow meters or mass flow controllers
(±10 %). A mass closure calculation was performed during portions
of the experiments using measured flow rates and saturation vapor pressures
to ensure the humidified air stream was fully saturated. Bubbler
temperatures were recorded for specific humidity calculations accounting for
cooling at very high flow rates due to gas expansion. All CIMS instruments
were positioned around a central manifold (0.635 cm OD PFA tube and 0.635 cm
PFA Swagelok fittings) and sampled from its own radial branch. Humidified
air was added furthest downstream to minimize the surface area exposed to
condensation. Each inlet branch was the same length to within 5 cm. Excess
flow was directed from the central manifold line to an exhaust at
atmospheric pressure.
Humidity dependence of the laboratory-derived CI-ToFMS
sensitivities to dimethyl-1,1,1-d3 sulfide, α-pinene, and
isoprene. Error bars indicate 1σ as determined from the slope of the
calibration curve.
Benzene cations were demonstrated to be sensitive to isoprene and α-pinene (Fig. 5) in agreement with previous work by Leibrock and Huey (2000), as well as DMS. Normalized sensitivities (normalized counts per
second per ppt or ncps ppt-1) were calculated by adjusting product ion
count rates to a reference reagent-ion signal of 106 cps
(Warneke et al., 2001). Thus, absolute sensitivity
increases with benzene ion current. Calibration factors are reported as the
slope of the linear fit of each calibration curve (1σ= slope
standard deviation). Maximum normalized sensitivities were as follows: DMS
(23.5 ncps ppt-1), α-pinene (41 ncps ppt-1), and isoprene
(8.3 ncps ppt-1). All three VOCs calibration factors exhibited unique
humidity dependences described in more detail in Sect. 3.3.
Adduct declustering
VOC product ions were detected at nominal mass (M+) or as a benzene
adduct (M-(C6H6)+). On both the CI-QMS (Fig. 1a) and CI-ToFMS
(Fig. 1b) isoprene was detected as an adduct (146 m/Q), while α-pinene
appeared at nominal mass (136 m/Q). The former indicates that isoprene
ionization proceeds through a ligand switching reaction. For the latter,
this result suggests that α-pinene may also undergo a ligand
switching mechanism with subsequent disassociation during transmission or,
alternatively, may be ionized via direct charge transfer.
In contrast, dimethyl-1,1,1-d3 sulfide was detected predominately as
M+ (65 m/Q) on the CI-ToFMS and as a benzene–DMS adduct (143 m/Q) on the
CI-QMS. This indicates that DMS ionization proceeds through a
ligand-switching reaction in both instruments, but the adduct dissociates in
the stronger declustering fields of the CI-ToFMS. Despite attempts to
minimize the field strength of the CI-ToFMS, we were not able to observe the
DMS–benzene cluster.
Instrument response to pure, unquantified mixing ratios of a host of other
VOCs, including a sesquiterpene and a monoterpenoid, were assessed with the
CI-ToFMS. In these experiments a small volume (< 1 mL) of pure
β-caryophyllene and eucalyptol standards were deposited into clean
flasks and their vapors were delivered to the CI-ToFMS inlet. Product ions
were identified and normalized (%) to the sum of all feasible products.
As described previously, DMS and α-pinene product ions appear at
M+ (99 and 83 %, respectively), while isoprene is detected
primarily as a benzene adduct (88 %) (Fig. 6). β-caryophyllene was
detected as M+ (204 m/Q) with very limited fragmentation (< 25 %). Since β-caryophyllene has been demonstrated to fragment more
readily that other sesquiterpenes (Kim et al., 2009), these tests suggest
that benzene dimer cations show excellent promise as an accurate chemical-ionization method to target easily fragmented terpene species. Eucalyptol
(1,3,3-Trimethyl-2-oxabicyclo[2,2,2]octane), an oxygenated monoterpenoid
(C10H18O), was also detected as M+ (154 m/Q) with limited
fragmentation (Fig. 6). In the case of eucalyptol, a secondary ionization
reaction was evident at the M-H+ channel
(Maleknia et al., 2007). After accounting for contributions from the carbon-13
isotope, we estimate the ratio of charge transfer to proton transfer product
ions is about 2:1 at ambient RH for eucalyptol.
Impact of ambient humidity on sensitivity
The sensitivity of benzene cluster cations to DMS, α-pinene, and
isoprene standards was shown to be a strong function of specific humidity
(q) (Fig. 5), with peak sensitivity occurring at specific humidities of 6.7 g kg-1 (DMS, 23 ncps ppt-1), 11.7 g kg-1 (α-pinene, 41 ncps ppt-1),
and 11.7 g kg-1 (isoprene, 8.3 ncps ppt-1).
Sensitivity varied with ambient specific humidity on all CIMS platforms. On
the CI-ToFMS, DMS was observed to be least sensitive to changes in ambient
water vapor, while α-pinene sensitivity increased almost linearly
for q values between 0 and 11.7 g kg-1.
Normalized response of the CI-ToFMS to a series of VOC standards.
Significant products (> 5 % of instrument response) are shown
for DMS, isoprene, α-pinene, β-caryophyllene, and eucalyptol
(1,8-cineole). For eucalyptol, the ratio of charge transfer to proton
transfer products was roughly 2:1. Expected contributions from carbon-13
isotopes were accounted for in the calculated proton transfer products.
At present, we do not have a definitive interpretation of the water
dependence. While we do not observe a benzene–water cluster in any of our
instruments, it is likely that the cluster exists in the IMR and is broken
apart in the ion optics between the IMR and the mass analyzer. Results from
Ibrahim et al. (2005) show that the stepwise binding energies for attaching a
water molecule to C6H6+ are on the order of
8.5 kcal mol-1, suggesting that the water molecules are removed via
collisions in the transfer optics. Laboratory studies have shown previously
that benzene–water clusters [C6H6–(H2O)n]+ can be
made efficiently (n= 1–23) (Miyazaki et al., 2004). For large clusters
(n≥ 4) IR spectra of the clusters suggest features that are nearly
identical to protonated water clusters, suggesting exclusion of benzene and
the formation of an attached water cluster. For smaller clusters (n≤ 3), IR spectra were shown to be consistent with a hydrated benzene cation,
where benzene retains over 90 % of the charge (Ibrahim et al., 2005). It
is likely that the sensitivity dependence we observe in these experiments is
related to the aforementioned laboratory studies and depends on the location
of the charge (benzene vs. water) and the size of the benzene–water cluster
(n), both of which are related to q. However, in our experiments it is
almost certain that the reagent ion contains more than one C6H6
molecule limiting the direct comparison between the two studies. In what
follows, we show that standard additions under a range of atmospheric values
for q are required for accurate measurements of mixing ratios in the
atmosphere.
CI-ToFMS performance during the High Wind Gas Exchange Study
The CI-ToFMS utilizing benzene cluster cation ion chemistry was deployed to
the Arctic and North Atlantic aboard the R/V Knorr during the fall of 2013 to
characterize BVOCs in the remote marine boundary layer. Here, we analyze
instrument performance over the 5-week HiWinGS to investigate factors controlling reagent-ion stability and
sensitivity to VOCs in challenging environmental conditions. The atmospheric
implications of these measurements will be discussed in a separate
forthcoming manuscript. Isoprene, α-pinene, and UHP zero air gas
cylinders were used to determine in-field sensitivities and system
baselines, respectively. We validate our DMS measurement against a
collocated DMS measurement made by the University of Hawaii's APIMS-ILS.
Inlet characterization
A low-pressure (200 mbar), rapid-response, high Reynolds number (ca. 2400)
inlet assembly was designed based on the inlet manifold described in
Ellis et al. (2010) and adapted for
measurement of VOC mixing ratios and air–sea exchange from research vessels.
The complete inlet assembly included separate lines for sample, bypass,
baseline, and calibration flows sheathed together by a durable weatherproof
exterior (25 m total length, 18 m weatherproofed, 6 cm outside diameter;
Clayborn Labs). All inlet lines were connected to a glass manifold (Ellis et
al., 2010). Inlet flow rates are set by a critical orifice (ca. 1.2 mm ID,
12.56 sL min-1) set within the 2.54 cm OD cylindrical glass body. Upstream
manifold surfaces were treated with Fluoropel (Cytonix). Downstream of the
orifice (low-pressure side), sample flow is split into a bypass (ca. 2.5 sL min-1) and sample (ca. 10 sL min-1) stream. The bypass branch is in-line with the
manifold intake. The sample flow follows a series of 90 and
180∘ turns, which serve to eliminate super-micrometer sea spray
particles from the sample inlet stream. The manifold was housed in an
insulated, weather-proofed housing and mounted off the foremast. The sample
intake was ca. 20 m above mean sea level (a.m.s.l.) and ca. 17 m aft of the bow
(Yang et al., 2014). The entire inlet sample line and manifold
assembly was heated by a single PID heating circuit (Omega) set to 40 ∘C.
The inlet assembly comprised of a sample (0.95 cm OD FEP), bypass 0.635 cm
OD PFA), baseline (0.95 cm OD FEP), and calibration (0.318 cm OD FEP) line
and ran between the manifold and a temperature-controlled instrument van
housing the CI-ToFMS on the 02 deck. Baseline and calibration gas flows were
controlled by mass flow controllers operated by an automated LabView
program. Ambient sample air was pulled through the sample line to the front
block, an additional differentially pumped chamber upstream of the CI-ToFMS
IMR and held at the inlet assembly pressure (200 mbar). The CI-ToFMS
subsampled from the front block, while the rest of the flow rejoined the
bypass flow downstream of the instrument. Choked flow conditions in the
entire manifold-inlet-front block volume were maintained by a 500 L min-1 scroll
pump (Varian TriScroll 600). Inlet assembly pressures were recorded by a
Pirani pressure gauge on the front block. UHP zero air and VOC standards
were added to the inlet manifold via separate ports located upstream of the
critical orifice (i.e., high-pressure side) to account fully for all losses
to the manifold and inlet surfaces. Instrument baselines were determined by
overflowing the manifold critical orifice with 20 sL min-1 of UHP zero air from
a gas tank (Praxair). VOC sensitivity was assessed with several daily
standard additions and calibrations. The first 3 weeks of the research
cruise utilized isoprene standards before switching to α-pinene
during the last 2 weeks. For both calibrations, a 0.500 ppm primary
standard in N2 (Praxair) was used to achieve a constant 638 ppt mixing
ratio in the inlet.
Inlet time response
Ambient VOC signal decays resulting from zero air additions were examined to
characterize the inlet's time response (Fig. 7). Fitting a double
exponential curve to signal decays yields two characteristic time constants
for the inlet (Ellis et al., 2010). The first
exponential decay time, τ1, is attributed the gas
evacuation time and related to the physical operation and characteristics of
the inlet itself. The second exponential decay time, τ2, is attributed to the equilibration constant of the gas and
its polarity and solubility. For the n=1 protonated water cluster
(H3O+–(H2O)n), τ1 of 3.2 s agreed well with
the volumetric evacuation time of 3.6 s at 12.56 sL min-1. The second
characteristic time, τ2 was long for the protonated water
cluster at 22 s, as expected based on the polarity and high surface affinity
of water vapor. DMS and α-pinene also exhibited fast τ1 time constants,
but τ2 time constants
were also fast (1.2 and 2.5 s, respectively), in contrast to H2O, indicating turbulent flow and
minimal interaction of the analyte with the inlet walls.
Inlet and instrument response to a prompt switch from ambient
sampling to a zero air addition. The first rapid decay is attributed to the
gas evacuation time (τ1) and the second decay is attributed to
the inlet wall-equilibration time (τ2). Best fit estimates of
τ1 for H3O+(H2O), DMS, and monoterpenes ranged
between 2 and 3 s, in agreement with the volumetric evacuation time of
the sample inlet (3.6 seconds). τ2 was determined to be 22, 1.2,
and 2.5 s, respectively.
Probability density functions of benzene signals (% total ion
current, 1 Hz) for ambient and baseline sampling periods. The bimodal
distribution corresponds to abrupt temporal shifts in ion current from
benzene monomers to protonated water clusters. These shifts correspond to
switches in benzene gas cylinders rather than changes in ambient specific
humidity, indicating the bimodal distribution is driven by trace water vapor
contaminants in the benzene gas cylinders.
Summary of in-field CI-ToFMS performance on the HiWinGS research
cruise for select volatile organic compounds. Isoprene and α-pinene
were calibrated against in-field standards. DMS figures of merit utilized
simultaneous independent DMS mixing ratios from the University of Hawaii's
APIMS-ILS. Limit of detection (LOD) was calculated for 1 s averaging
times using normalized sensitivities corresponding to Q = 6 g kg-1.
VOC species
Sensitivity
Background (1σ)
LOD (1 Hz, 3σ)
Dimethyl sulfide
1.0 to 17.9 ncps ppt-1
636 ± 389 cps
25 ppt
Isoprene
0.7 to 1.1 ncps ppt-1
19.5 ± 66 cps
15 ppt
α-Pinene
0.8 to 6.3 ncps ppt-1
192 ± 191 cps
14 ppt
Factors impacting reagent-ion stability
Benzene reagent-ion signal varied between 58 and 89 % of the total ion
current (8.02 × 105 to 6.4 × 106 cps benzene)
during HiWinGS. Probability density functions of benzene counts during
ambient and baseline periods each showed a bimodal distribution but no
difference from each other (Fig. 8). A similar trend in benzene ion current
was observed during standard addition and calibration periods. Benzene ion
signal shifted between the two modes several times over the course of the
research cruise. Step changes in benzene ion signal corresponded to days
when reagent-ion gas tanks were switched. Loss in benzene signal
corresponded to nearly equal increases in protonated water cluster peaks
(0.7 to 39 % total ion current) and vice versa. These abrupt shifts occur
independently of ambient specific humidity or other environmental variables.
We attribute these shifts to trace levels of condensation in the benzene
standard cylinders or water vapor introduced into the gas delivery lines
during cylinder exchange. During the previously described laboratory tests
(Sect. 3.1.2), benzene remains > 86 % and protonated water
clusters peaked at 8.5 % of total ion current at 100 % relative humidity
(Fig. 4a). At typical room temperatures (20–24 ∘C), 100 % relative
humidity is equivalent to specific humidities between 14.6 and 18.8 g kg-1).
Minute average specific humidity during HiWinGS peaked at 13.2 g kg-1,
which suggests the large and abrupt increases in protonated water
cluster signal (and corresponding decreases in benzene) are driven by water
vapor in the reagent-ion generation source rather than ambient humidity. A
less probable explanation for the increase in pronated water cluster signals
may be attributed to diffusion of water into the permeable gas delivery
lines that spanned over 30 m from the gas cylinder rack on the main deck to
the instrument van on the 02 deck. Despite being sheathed, the lines were
invariably exposed to sea spray and condensation during high seas. However,
this is inconsistent with step-function changes in benzene and water signal
over time. Future work will focus on the use of a purified liquid benzene
reservoir that would not need to be exchanged during the course of the
cruise.
In-field calibration and sensitivity
As expected, absolute sensitivity to VOC standard additions varied as a
function of benzene reagent-ion current and ambient water vapor. Calibration
factors were calculated from raw count rates observed during each VOC
standard addition adjusted for ambient contributions calculated by
interpolating values from bracketed sampling periods. DMS calibration
factors were derived from concurrent APIMS-ILS determined mixing ratios
(cps ppt-1),
as discussed in detail below in Sect. 4.5. Mean absolute
sensitivities were 19.4, 2.8, and 6.1 cps ppt-1 for DMS, isoprene, and
α-pinene, respectively.
Instrument baselines are computed from the mean of raw count rates (1 Hz)
measured during valid instrument zero periods throughout the entire campaign
as in (Kercher et al., 2009). Variability in baseline counts
are largely driven by fluctuations in the benzene reagent-ion current.
Detection limits are calculated as in Bertram et
al. (2011) as 3σ of a Gaussian fit for all baseline count rates
over the entire campaign. A conservative estimate of the CI-ToFMS detection
limits, for 1Hz sample averages, were calculated as 152, 13, and 42 ncps for
DMS, isoprene, and α-pinene, respectively (Table 1). Using
sensitivities reported for q = 6 g kg-1, this translates to 1Hz
detection limits of 25, 15, and 14 ppt, respectively.
In-field sensitivities during standard additions as a function of
ambient specific humidity. Dots represent hourly values while squares
represent mean values for a 2 g kg-1 specific humidity bin.
Comparison of α-pinene calibration factors observed
during laboratory experiments (left axis) and during in-field standard
additions (right axis). Similar attenuation with specific humidity was
observed.
After normalizing to the benzene ion current, the humidity dependence of
in-field VOC normalized calibration factors (Fig. 9) is consistent with
laboratory tests (Fig. 4). However, the values of normalized calibration
factors were lower by a factor of 4–7 for each VOC molecule, as is
highlighted for α-pinene in Fig. 10. Possible explanations include
VOC standard degradation, inlet performance (less probable for nonpolar
terpenes), or most likely shifts in sensitivity from tuning instrument
voltages between the two experiments.
In deployments subsequent to the HiWinGS campaign, in-field calibrations
have been performed utilizing continuous standard additions of isotopically
labeled standards (when commercially available) as developed and used on
several previous APIMS-ILS air–sea exchange campaigns
(Bandy, 2002; Blomquist et al., 2010; Thornton, 2002).
Underway dimethyl sulfide comparison
Baseline-adjusted, reagent-ion normalized count rates at 62 m/Q were compared
to DMS mixing ratio measurements provided by the University of Hawaii's
APIMS-ILS. The instruments were housed in separate instrument
vans on the 02 deck of the R/V Knorr. The CI-ToFMS sample manifold was mounted to
the foremast as described in Sect. 4.1. The APIMS-ILS inlet was mounted to
the meteorological mast located at the bow of the ship and ca. 16.3 m a.m.s.l. This is about 17 m fore and 4 m below the CI-ToFMS intake
position.
Dimethyl sulfide baseline-adjusted, normalized count rates were calculated
by subtracting interpolated baseline values from ambient 62 m/Q counts and
scaling to a benzene ion current of 106 cps (Warneke
et al., 2001) (Fig. 11). No adjustments based on ambient humidity were made
due to the lack of an active standard gas calibration. A regression analysis
between the adjusted, normalized CI-ToFMS signal and APIMS-ILS DMS mixing
ratio measurements, for 10 s averages, showed excellent agreement
(R2= 0.97) over extreme wind speeds, low ambient loadings, and
temperatures below freezing (Figs. 11 and 12). This agreement between two
independent DMS measurements lends confidence to the selectivity of both
ionization methods and their robustness in the field. It also indicates both
sampling locations were functionally equivalent and sampled air masses did
not suffer from flow distortion around the superstructure of the ship. As
suggested by laboratory tests, the calibration factor is largely constant
over the humidity range (1.6–12.8 g kg-1) experienced during the
cruise (Fig. 12), indicating robustness to ambient humidity fluctuations.
Regression of background corrected, benzene normalized DMS counts
with DMS concentrations measured by APIMS-ILS. Regression analysis indicates
a campaign-average CI-ToFMS sensitivity of 6.9 ncps ppt-1.
Hourly measurements from the High Wind Gas Exchange Study
(HiWinGS) research cruise. Normalized CI-ToFMS count rates compared well
(R2= 0.80, hourly averages during entire campaign) with dimethyl
sulfide mixing ratios measured by the University of Hawaii's APIMS-ILS. No
corrections were made for ambient humidity.