Geophysical Research Letters
Supporting Information for
First observations, trends and emissions of HCFC-31 (CH2ClF) in the
global atmosphere.
Fabian Schoenenberger1, Martin K. Vollmer1, Matt Rigby2, Matthias Hill1, Paul J.
Fraser3, Paul B. Krummel3, Ray L. Langenfelds3, Tae Siek Rhee4, Thomas Peter5, Stefan
Reimann1
1
Laboratory for Air Pollution and Environmental Technology, Empa, Swiss Federal
Laboratories for Materials Science and Technology, Dübendorf, Switzerland.
2
School of Chemistry, University of Bristol, Bristol, United Kingdom.
3
CSIRO Oceans and Atmosphere Flagship, Aspendale, Australia.
4
Korea Polar Research Institute, KIOST, Incheon, South Korea.
5
Institute for Atmospheric and Climate Science, ETH Zürich, Zürich, Switzerland.
Contents of this file
1. Text SI
2. Figures S1 to S3
3. Table S1
Additional Supporting Information (Files uploaded separately)
1. Tables S2 to S5 and their captions
1
1.
SI Introduction
The supporting information consists of three parts. First, the measured samples and
analytical techniques are described in more detail. In a second part, there are three
figures. Figure S1 shows the results of the molar non-linearity experiment. Figure S2 is a
detailed version of the atmospheric history, covering the period with the highest
resolution of measurement coverage in the Southern and Northern Hemispheres,
depicting the seasonal cycle of HCFC-31. Figure S3 shows an example of a measured
HCFC-31 peak in ambient air on our laboratory Medusa-GCMS at Dübendorf. In a third
part, in Table S1, the most abundant mass-over-charge ratios (m/z) for HCFC-31 in the
literature and on our laboratory mass spectrometer are compared. For additional
information, the following files are submitted separately:
Table S2: Measurement results – Flask samples. The caption is given in that table.
Table S3: Measurement results – In situ observations. The caption is given in that table.
Table S4: Modeled emissions for HCFC-31. The caption is given in that table.
Table S5: Modeled mole fractions for HCFC-31. The caption is given in that table.
2.
SI Analytical Details
2.1
SI Sample Description
The reconstruction of past HCFC-31 abundances in the atmosphere is based on in situ
measurements in Dübendorf (Switzerland), Cape Grim (Tasmania) and several sets of
flask samples. The first set of samples is termed “Northern Hemisphere Mini Archive”,
for which air was collected at Jungfraujoch (Switzerland), Mount Rigi – Seebodenalp
(Switzerland), Mace Head (Ireland), Niwot Ridge (CO, USA), Trinidad Head (CA, USA)
2
and La Jolla (CA, USA). These samples, which are stored at Empa, were collected for
purposes other than archiving. Hence, samples showing non-baseline conditions for other
major halocarbons were rejected. Additionally, eight subsamples from the Cape Grim Air
Archive (CGAA) were used in the present study [Langenfelds et al., 1996; Langenfelds et
al., 2014].
A second set of flasks was collected aboard the South Korean icebreaker Araon, during
the SHIPPO-12 expedition, on a transect through the North Pacific from Incheon (South
Korea) to Nome (AK, USA). These canisters were filled using a diaphragm pump (KNFN-145-ATE, Neuberger). The third set of flasks was filled at the Korean Antarctic
Research Station King Sejong, as part of a long-term flask sampling program which
started in 2007 [Vollmer et al., 2011]. The “Northern Hemispheric Mini Archive”
samples were either filled using oil free diving compressors (SA-6, Rix Industries, USA)
or by applying cryogenic filling methods. During the SHIPPO-12 expedition, canisters
were filled using a diaphragm pump (KNF-N-145-ATE, Neuberger).
The SHIPPO-12 and the King Sejong samples were collected into internally
electropolished stainless steel canisters with volumes ranging from 2L to 6L at a pressure
of ~4 bars. The CGAA subsamples of the cryogenically filled 34 L tanks (Essex
Cryogenics, USA) were transferred into evacuated 4.5 L Essex tanks at CSIRO before
shipping to Empa, where they were analysed. The “Northern Hemisphere Mini Archive”
samples were stored at various pressures in aluminium and internally electropolished
stainless steel cylinders.
3
Continuous in situ measurements are available from the Medusa-GCMS system at Empa
Dübendorf (Switzerland) and from the Cape Grim Baseline Air Pollution Station in Cape
Grim (Tasmania).
2.2
SI Analytical Methods
Flask samples and in situ measurements were analysed on a Medusa sample
preconcentration unit, which is followed by a gas chromatograph (GC, Agilent 6890N)
mass spectrometer (MS, Agilent 5975MS) [Miller et al., 2008]. The Medusa is equipped
with two Hayesep D packed cold traps, which are cooled to -160°C for preconcentration
and heated up to ~100°C for desorption. To enhance peak size and allow for better
measurement precision, most air canisters are analysed by trapping a volume of 4 L,
compared to a standard volume of 2 L. Preconcentrated samples are then
chromatographically separated on a 25m x 0.32mm ID Agilent CP-Porabond Q column
with 5µm film thickness before detection on a selected-ion mode quadrupole mass
spectrometer.
The primary calibration scale (Empa-2013), is based on a gravimetrically produced,
commercially obtained reference gas mixture (Carbagas, Switzerland), which is termed
“HCP-04Carba”. This mixture contains HCFC-31 at mole fractions of 10 ppm (parts-permillion, 10–6) and a number of other substances, not further discussed in this paper.
Additionally, the “boot-strap” compounds methane, CCl2F2 (CFC-12) and SF6 are
contained in “HCP-04Carba” at higher mole fractions. They were used to validate the
accuracy of the dilution process, based on independent primary calibrations, which are
available for these compounds. “HCP-04Carba” was diluted to a final near-two ppt
(parts-per-trillion, 10-12) mixture, using a pressure controlled dilution scheme with
4
purified synthetic air (for further details see Vollmer et al. [2015]). This dilution is termed
EP-001 and serves as the primary calibration standard for all HCFC-31 measurements.
The accuracy for our “Empa-2013 primary calibration scale” for HCFC-31 is estimated to
be at ~6% and is dominated by the accuracy of the primary commercial reference gas
used in the dilution.
The HCFC-31 primary calibration scale is propagated to the flask and in situ
measurements through the use of secondary and quaternary standards. This is necessary
(rather than measuring all samples against the primary standard) due to the relatively
large consumption of standard samples for our measurements. Here we have used
secondary standards (E-094 and E-108) against which the flask samples were directly
measured, and quaternary standards for the Dübendorf in situ air measurements. These
quaternary standards were referenced against the secondary standards. The nomenclature
used here is adapted to the AGAGE protocols, ‘tertiary standards’, missing here, are not
used in our HCFC-31 measurement scheme. For our overall uncertainty analysis below
we also add a ‘propagation’ uncertainty due to the propagation of the scale through this
chain of standards. This uncertainty is however small, and estimated at ~1%.
Ultimately the measurement precisions also contribute to the overall uncertainties of the
reported results. The measurement precision of HCFC-31 on the Medusa-GCMS is
dependent both on trapped volumes and absolute mole fractions in the sample, with
higher chromatographic peak sizes yielding improved precisions. Samples from the
“Northern Hemisphere Mini Archive”, filled before 2009, containing less than 100 ppq
(parts-per-quadrillion, 10–15) of HCFC-31, therefore have a lower mean measurement
precision (1ơ) of 5.7%, while later samples from 2009 - 2015, containing more than 100
ppq of HCFC-31 have a mean measurement precision (1ơ) of 2.8%. Southern
5
hemispheric flask samples with mole fractions below ~100 ppq during the whole
sampling period have been measured with a mean precision (1ơ) of 4.7%. During in situ
measurements, a standard volume of only 2 L was trapped, resulting in a slightly lower
mean measurement precision (1ơ) of 6.0%, based on weekly means.
The accuracy of our reported results is a combination of the calibration uncertainy (~6%),
the propagation uncertainty (~1%), and the measurement precisions (3% - 6%) outlined
above. By combining them as independent uncertainties, we estimate an accuracy of 7% - 9% for our results. For the 12-box model, these uncertainties are added to those deriving
from the model and OH field. Note that when directly comparing sample results from our
measurements, the overall uncertainty reduces because all samples are referenced to the
same Empa-2013 calibration scale.
To integrate the Cape Grim data into our reference system, an aliquot of a whole-air
sample (UAN20140778, G-220) was transferred into a 4.5 L Essex canister at CSIRO in
December 2014 and measured on the Empa-Medusa in January 2015 against a secondary
standard (E-108). This G-220 was measured at Cape Grim against several other standards
thereby allowing to reference the Cape Grim in situ air record to the Empa-2013 absolute
calibration scale.
To determine potential non-linear instrument response for HCFC-31 on our laboratory
Medusa-GCMS instrument, two experiments were conducted. In a first, “volume-based”
nonlinearity experiment, a range of sample volumes (0.15 L - 6 L) of a standard was
measured with each measurement bracketed by that of the same standard using a nominal
2 L sample. When corrected for their sample volumes, these measurements revealed
6
linear system response within the measurement precision. This experiment also showed
that there was no loss of HCFC-31 (no trap breakthrough) even for the 6 L samples.
In a second experiment, the amount of HCFC-31 moles was varied by dilution of a
standard with constant sample volume in contrast to the variable sample volumes in the
first experiments (Figure S1). For this purpose, a subsample of the standard E-094 was
decanted into a 4.5 L, internally electropolished stainless steel container (Essex
Cryogenics), step-wise diluted with HCFC-31-free purified synthetic air and measured
against the undiluted E-094. The dilution factors were determined manometrically
(pressure gauge ‘LEO’, Keller, Switzerland). Each dilution was measured twice using 4 L
samples (to enhance peak sizes for compounds with lower abundances) and bracketed
with measurements of E-094 using the same sample volumes. Using least-square fitting
techniques, a linear function was fitted to the measured mole fractions and the dilution
factors. The slope of the fitted function (136.0 ppq) agreed very well with the HCFC-31
mole fraction in E-094 (135.8 ppq) and the offset at zero was very small (0.8 ppq). Based
on these results and the absence of any systematic offsets in the residuals (Figure S1
panel b) we concluded linear system response within the measurement uncertainties.
Also, it is worthwhile to note that our atmospheric mole fractions were all >30 ppq,
corresponding to a dilution fraction of ~0.2, which is much larger than our largest
dilutions. As an independent check the molar ratios of HCFC-31 to CFC-12 were also
investigated (Figure S2, panel c). CFC-12 is less vulnerable to nonlinearity because of its
larger peak size over the entire dilution range (most nonlinearities of smaller peaks on the
Medusa-GCMS system derive from the choice of peak integration parameters). While the
ratio of HCFC-31 to CFC-12 was preserved in the dilution range of interest (dilution
fraction >0.2), some of the lowest mole fractions showed random larger deviations in
7
these ratios. These indicate larger measurement uncertainties for very small HCFC-31
peaks, which however are well below the mole fraction range used in this study.
References
Langenfelds, R. L., P. J. Fraser, R. J. Francey, L. P. Steele, L. W. Porter, and C. E.
Allison (1996), The Cape Grim air archive: The first seventeen years, 1978-1995,
Bureau of Meteorology and CSIRO Division of Atmospheric Research, Melbourne,
Australia.
Langenfelds, R. L., P. B. Krummel, P. J. Fraser, L. P. Steele, J. Ward, and N. T.
Somerville (2014), Archiving of Cape Grim air, in Baseline Atmospheric Program
Australia 2009-2010, edited by N. Derek, P. B. Krummel and S. J. Cleland, pp. 4445, Burea of Meteorology and CSIRO Marine and Atmospheric Research,
Melbourne, Australia.
Miller, B. R., R. F. Weiss, P. K. Salameh, T. Tanhua, B. R. Greally, J. Mühle, and P. G.
Simmonds (2008), Medusa: A Sample Preconcentration and GC/MS Detector
System for in Situ Measurements of Atmospheric Trace Halocarbons, Hydrocarbons,
and Sulfur Compounds, Analytical Chemistry, 80(5), 1536-1545,
doi:10.1021/ac702084k.
Vollmer, M. K., B. R. Miller, M. Rigby, S. Reimann, J. Mühle, P. B. Krummel, S.
O'Doherty, J. Kim, T. S. Rhee, and R. F. Weiss (2011), Atmospheric histories and
global emissions of the anthropogenic hydrofluorocarbons HFC‐365mfc, HFC‐
245fa, HFC‐227ea, and HFC‐236fa, Journal of Geophysical Research: Atmospheres,
116(D8), D08304, doi:10.1029/2010JD015309.
Vollmer, M. K., T. S. Rhee, M. Rigby, D. Hofstetter, M. Hill, F. Schoenenberger, and S.
Reimann (2015), Modern inhalation anesthetics: Potent greenhouse gases in the
global atmosphere, Geophysical Research Letters, 42(5), 1606-1611,
doi:10.1002/2014GL062785.
8
Figure S1. Non-linearity analysis for HCFC-31 on Medusa-GCMS (Dübendorf). These results show a
linear system behavior within the measurement precision and for the mole fraction range used in this
study (>30 ppq, corresponding to a dilution fraction of >0.2). (a) HCFC-31 mole fraction for a stepwise dilution of the E-094 whole-air standard. (b) Deviation from linear fit, showing the absence of a
systematic system bias. (c) Comparison of molar ratio of HCFC-31 (ppq) to CFC-12 (ppt) for all
diluted samples.
9
Figure S2. Detailed view of main paper Figure 1(A): Atmospheric of HCFC-31. Vertical bars:
measurement precision ( 1 σ) for flask measurements, often smaller than the symbol size;  1 σ for
in situ measurements, referring to weekly means. Solid lines: dry air mole fractions from the
AGAGE 12 box model for the model surface boxes 30°N to 90°N (in blue) and 30°S to 90°S (in red).
10
Figure S3. Mass spectrum of HCFC-31 air sample, collected on Dübendorf Medusa GCMS; 28. April
2013, 23.36h; The figure shows the target peak (m/z: 68) of HCFC-31 (in black) and the qualifier
peak (m/z: 70) (in blue), eluting at 1167s, followed by the CFC-12 qualifier peak (m/z: 68) at 1179s.
The peak represents an atmospheric mixing ratio of 202 ppq. This measurement is based on a volume
of 2 L of preconcentrated air. This chromatogram is obtained from a relatively new column. As the
column ages, both HCFC-31 and CFC-12 retention times increase but that of HCFC-31 at a higher
rate such that peaks are eventually overlapping thereby hindering any further analysis of HCFC-31.
11
(m/z)
68
70
33
49
47
28
51
48
50
67
32
31
35
69
43
29
Abundance
MedusaNIST
GCMS
100.0
100
32.5
33.8
23.3
18.0
20.8
17.3
18.5
5.1
4.5
4.9
4.1
4.7
3.4
1.8
2.5
3.1
2.4
2.1
2.1
2.1
2.5
1.4
1.0
-
Table S1. Comparison of mass spectra for HCFC-31, showing the most abundant mass fragments
(mass over charge ratio (m/z)) from published data (NIST Chemistry webbook) and from the
Dübendorf laboratory Medusa GCMS based on a standard sample. The Medusa-GCMS was run in
scan mode from m/z 47 to m/z 200. Mass fragments not available on our instruments were excluded
to prevent detector damage from potentially high abundances of other major trace gases (e.g. CO 2).
12