Introduction
With increasing efforts to mitigate anthropogenic greenhouse gas emissions,
opportunities to reduce leaks from fossil fuel derived methane (ffCH4)
are of particular importance as they currently account for approximately
30 % of all anthropogenic methane emissions (Kirschke et al., 2013). At
present, technically feasible mitigation methods hold the potential to half
future global anthropogenic CH4 emissions by 2030. Of this mitigation
potential more than 60 % can be realized in the fossil fuel industry
(Hoglund-Isaksson, 2012). However, for effective implementation, sources,
locations and magnitudes of emissions must be well known.
The global increase in the production and utilization of natural gas, of
which methane is the primary component, has brought to light questions in
regards to its associated fugitive emissions, i.e. leaks. Recent estimates
of CH4 leaks vary widely (1–10 % of global production; Allen,
2014) and US inventories of natural gas CH4 emissions have
uncertainties of up to 30 % (US EPA, 2016). In addressing this issue, the ability
to distinguish between biogenic and different anthropogenic sources is of
vital importance. For this reason methane isotopes (δ13CH4) are commonly used to better understand global and local
emissions, as demonstrated in a number of studies (Lamb et al., 1995; Lowry
et al., 2001; Hiller et al., 2014). The discrimination of sources with
relatively close isotopic composition such as oil-associated gas and natural gas,
which can have isotopic signatures separated by only ∼ 4 ‰ (Stevens and Engelkemeir, 1988),
requires precise and reliable
δ13CH4 measurements.
Ethane (C2H6) is a secondary component in natural gas and can be
used as a marker to distinguish between different CH4 sources. Use of
the C2H6 : CH4 ratio provides a robust identifier for the gas
of interest. Recent findings in the US found coal bed
C2H6 : CH4 ratios ranging between 0 and 0.045, while dry and wet
gas sources displayed differing ratios of < 0.06 and > 0.06 respectively (Yacovitch et al., 2014; Roscioli et al., 2015).
Laser spectrometers, especially those based on cavity ring-down spectroscopy (CRDS), are now a common deployment for
site-scale CH4 measurement campaigns (Yvon-Lewis et al., 2011;
Phillips et al., 2013; Subramanian et al., 2015). However, with the advent
of such novel technologies, there is a risk of unknown interference from
laser absorption which can create biases in measurements. Some examples of
this are discussed in Rella et al. (2015) and many others (e.g. Malowany
et al., 2015; Vogel et al., 2013; Nara et al., 2012). Using a CRDS
instrument we show that the presence of C2H6 causes
significant interference to the measured 13CH4 spectral lines,
thus resulting in shifted reported δ13CH4 values. We
propose a method to correct these interferences and test it on measurements
of natural gas samples performed at an industrial natural gas site.
Flow chart illustrating the steps involved to calibrate
C2H6 and δ13CH4. The number in the top right-hand corner corresponds to the subsection in which the methods of each step
are explained in detail.
The CRDS instruments used throughout this study are Picarro G2201-i
analysers (Picarro INC, Santa Clara, USA) which measure gases including
CH4, CO2, H2O, and, although not intended for use by standard
users, C2H6. This model measures in three spectral ranges: lasers
measuring spectral lines at roughly 6057, 6251 and
6029 cm-1 are used to quantify mole fractions of 12CH4,
12CO2 and 13CO2, and 13CH4, H2O and
C2H6 respectively. The spectrograms are fit with two non-linear
models in order to determine concentrations; the primary fit
excludes the model function of C2H6 while the second includes
this function, thus adding the ability to measure C2H6 (Rella et
al., 2015). Such a method for measuring C2H6 concentrations is
crude, thus the uncalibrated C2H6 concentration data are stored in
private archived files which until now have been used primarily for the
detection of sample contamination. The measurements of δ13CH4 and δ13CO2 are calculated using the
ratios of the concentrations of 12CH4, 13CH4,
12CO2 and 13CO2 respectively.
An experimental procedure is presented here which corrects the interference
caused by C2H6 on the retrieval of δ13CH4 using
such a CRDS instrument for application to in situ or continuous measurements
of δ13CH4 strongly contaminated by C2H6, i.e. in
the vicinity of ffCH4 sources. The step-by-step procedure of the
experimental methods developed to quantify the cross sensitivities and the
proposed calibration for δ13CH4 and C2H6
are depicted in Fig. 1 and presented in detail in Sect. 2. Section 3
encompasses a discussion of the results, including an analysis of the
instrumental responses for two spectrometers with an evaluation of the
stability and repeatability of the suggested corrections. Finally, field
measurements were performed at a natural gas compressor station where the
aim was to identify emissions between two natural gas pipelines. In Sect. 5
the importance of the corrections for field measurements is demonstrated by
applying our methods to data retrieved during this period while also
revealing the instruments' potential to measure C2H6.
Methods
The purpose of laboratory tests was to characterize the instruments' response
to concentration changes in gases found at fossil fuel sites (e.g. gas
extraction or compressor stations), specifically, the cross sensitivities of
CO2, CH4 and H2O on C2H6 and of C2H6 on
δ13CH4. Presumably there are additional gases with the
potential for interference; this study focuses on those reported to have a
significant effect on C2H6 and δ13CH4
measurements by Rella et al. (2015). We also define and describe a new
procedure to calibrate both C2H6 and δ13CH4.
In the following chapter the general set-up used for the majority of
experiments is described, after which we enter a more detailed description of
the processes involved in each step.
Description of the gas mixtures used to determine the
cross-sensitivities of the interference of CH4, H2O and CO2
on C2H6 and the interference of C2H6 on δ13CH4. The respective ranges spanned during laboratory tests, and
the typical range at a natural gas site are noted on the right-hand side.
Method
Dilution gas
Working gas
Lab concentration
Typical range
Range
at NG site
H2O interference
< 0.16 % (dry)
Magnesium perchlorate
N/A
Ambient air
0–0.5 % H2O
0–2 % H2O
on C2H6
≥ 0.16 % (wet)
Dilution series
Zero air
0.25–2.5 % H2O
& humidifier
CO2 interference
< 0.16 % (dry)
Dilution series
Zero air
2000 ppm CO2,
0–1500 ppm CO2
400–1000 ppm CO2
on C2H6
≥0.16% (wet)
Dilution series &
1.7 ppm CH4,
< 1 ppb C2H6 and
0–1500 ppm CO2,
humidifier
50 ppb CO in natural air
0.5–1.5 % H2O
CH4 interference
< 0.16 % (dry)
Dilution series &
Zero air
6 ppm CH4, 360 ppm CO2,
0–6 ppm CH4
2–20 ppm CH4
on C2H6
ascarite
310 ppb N2O, < 1 ppb C2H6
and 50 ppb CO in natural air
0–6 ppm CH4, 1 %H2O
≥ 0.16 % (wet)
Dilution series, ascarite
& humidifier
C2H6 interference
Dilution series
Natural air matrix
C2H6 standard
0–1.5 ppm C2H6/
0–0.3 ppm C2H6/
on δ13CH4
(CRDS)
(< 1 ppb C2H6)
of 52 ppm in nitrogen
ppm CH4
ppm CH4
C2H6 calibration
Dilution series
Natural air matrix
C2H6 standard
0–5 ppm C2H6
0.3–3 ppm C2H6
(CRDS & GC)
(< 1 ppb C2H6)
of 52 ppm in nitrogen
General set-up. The dilution and working gas are connected via two
MFCs to two CRDS instruments in parallel. In red is the placement of an
optional glass flask used for the C2H6 calibration only. The flow
is greater than that of the instruments' inlets. Therefore an open split is
included to vent additional gas and retain ambient pressure at the inlets.
Experimental set-up
Method
Each cross sensitivity is measured by creating a gas dilution series
designed to control the concentrations of the gas responsible for the
interference in steps while keeping concentrations of the other gas
components constant (in particular the component subject to interference).
The instrument response was evaluated for a large range of concentrations
and different combinations of gas components. An example of such a
measurement time series can be seen in Fig. S1 in the Supplement. The experimental set-up
used includes two CRDS instruments (Picarro G2201-i) running in parallel in
a laboratory at ambient conditions (25 ∘C, 100 m above sea level; a.s.l). The
instruments were used in iCO2-iCH4 auto switching mode, of which
we consider only the “high precision” mode of δ13CH4
throughout the study. For the dilution series, a working gas is diluted in
steps using a set-up of two mass flow controllers (MFC; El-flow, Bronkhorst,
Ruurlu, the Netherlands), as shown in Fig. 2. A T-junction splits the gas
flow to both instruments; the total flow is greater than the flow drawn into
the instruments. Hence to maintain an inlet pressure close to ambient, the
set-up includes an open split to vent additional gas. In order to assess
variability and error, each experiment is repeated a minimum of three times
consecutively. To detect instrumental drift between experiments, a target
gas is measured before commencing each dilution sequence. An overview of
each targeted cross interference, with information on the gases used and
ranges spanned in laboratory tests, can be found in Table 1.
Gases
Throughout the experiments, four categories of gas were used: a zero air gas
with measured residual concentrations of < 1 ppm CO2, < 30 ppb CH4 , ≈ 170 ppb CO, < 1 ppb C2H6
(Deuste-Steininger,Walldorf, Germany), working gases with variable
concentrations of CO2 and CH4 in a natural air matrix
(Deuste-Steininger,Walldorf, Germany), a C2H6 standard of 52 ppm
in nitrogen (National Physics Laboratory (NPL), Teddington, United Kingdom),
and dried ambient air in 40L aluminium cylinders filled using an oil-free
RIX compressor (RIX industries, Benicia, USA). Details of the gas mixture
used in each dilution series depends on the response targeted within the
experiment. This information can be found in Table 1 and is also discussed in
further detail throughout this chapter.
Determination of C2H6 corrections from H2O, CH4 and CO2 interference
The value of C2H6 based on the standard CRDS data processing
package (hereafter, the raw value) is biased by cross-sensitivities with
H2O, CO2 and CH4. Experiments were conducted at different
constant C2H6 concentrations so that any shifts in the raw
C2H6 are due to the cross sensitivity to other components in the
measured samples. To alter the water vapour content of a sample, the
experimental set-up described in Fig. 2 was modified by incorporating a
humidifier. The humidifier consists of a liquid flow controller (Liqui-flow,
Bronkhorst, Ruurlu, the Netherlands) and a mass flow controller (El-flow,
Bronkhorst, Ruurlu, the Netherlands) fed into a controlled evaporator mixer
(CME) (Bronkhorst, Ruurlu, the Netherlands). The tube departing the CME
contains a gas flow of 2 L min-1 and is heated to 40 ∘C to prevent any
condensation. A short description and diagram of the humidifying bench can
found in Laurent et al. (2015).
The H2O interference on C2H6 was measured by using the
humidifier to vary the H2O content of zero air gas in the range of
0.25–2.5 % H2O, representing the range of real world conditions.
The humidifier set-up cannot reliably reach humidity below 0.2 %
H2O, a range frequently reached when measuring gas cylinders or dried air.
This low range was attained using a H2O scrubber (Magnesium
Perchlorate, Fisher Scientific, Loughborough, UK) connected to the CRDS
instrument inlet while measuring ambient air. As the efficiency of the
scrubber decreases over time, a slow increase of H2O spanning low
concentrations in the range of 0–0.5 % can be observed.
The CH4 interference on C2H6 was measured by creating a
dilution series of variable CH4 content using zero air and a working
gas of 6 ppm CH4, 360 ppm CO2, 310 ppb N2O and 50 ppb CO in
natural air. Methane concentrations ranged from 0 to 6 ppm. To keep other
causes of interference at a minimum, the gas mixture passed through two
scrubbers: the first a CO2 scrubber (Ascarite(ii), Acros Organics,
USA) and the second a H2O scrubber (Magnesium Perchlorate, Fisher
Scientific, Loughborough, UK). As an independent check on the linearity of
the response functions, each dilution sequence was repeated at two
humidities (0 % H2O and 1 % H2O) and four C2H6
concentrations (between 0 and 1.5 ppm).
The CO2 interference on C2H6 was measured with a dilution
series ranging 0–1500 ppm CO2 created by mixing zero air and a working
gas of 2000 ppm CO2, 1.7 ppm CH4 and 50 ppb CO in natural air. Any
interference due to CH4 was accounted for during data processing. This
test was repeated at four water vapour levels (0, 0.5, 1 and
1.5 %) and five C2H6 concentrations (between 0 and 2.5 ppm).
C2H6 calibration set-up
In order to correctly use the C2H6 data from CRDS instruments, the
data must be calibrated to an internationally recognized scale. To achieve
this, the set-up described in Sect. 2.1 was modified to include the filling
of removable samples (1 L glass flasks), the concentrations of which could be
independently verified, as shown in Fig. 2. A gas mixture using the
C2H6 standard and an ambient air cylinder was created via two MFCs
before passing through the flask on its way to the instruments' inlets. Each
step in the dilution series requires an individual flask, which was flushed
for 20 min and then analysed for 10 min with an average precision of
0.02 ppm C2H6 on the CRDS instrument. The flask is subsequently
sealed and removed for analysis on a gas chromatograph (GC) (Chrompack
Varian 3400, Varian Inc, USA) which uses National Physics Laboratory (NPL)
standards and has an uncertainty better than 5 %. The system is described
in more detail in Bonsang and Kanakidou (2001).
In total 17 flasks were filled with gas mixtures spanning from 0 to 5 ppm
C2H6, covering the range expected near a leak of ffCH4
(Gilman et al., 2013; Jackson et al., 2014). In order to calibrate the
linearity of the response at very high concentrations which may be expected
from pure natural gas samples, we conducted a measurement at 100 % of the
C2H6 standard (52 ppm ± 1 ppm).
Determining the correction for δ13CH4
Measured δ13CH4 is altered in the presence of
C2H6. To understand the magnitude of this effect, experiments were
conducted using the method described in Sect. 2.1. The dilution series uses
the C2H6 standard and a cylinder filled with ambient air, i.e.
with a negligible C2H6 mixing ratio (< 1 ppb), to create
concentration values spanning from 0 to 4 ppm C2H6. As there
is only one source of CH4 in the experiment, the addition of
C2H6 should not affect the value of δ13CH4;
hence any change seen is an apparent shift of δ13CH4 due
to C2H6 interference. This concentration range was chosen as it
encompasses a C2H6 : CH4 ratio of 0 to 1, well within the
likely range to be measured from fossil fuel sources (Yacovitch et al.,
2014).
Calibration of δ13CH4
The reported δ13CH4 was calibrated to Royal Holloway
University of London (RHUL) scale using four calibration gases spanning
-25 to -65 ‰ that were created by
different dilutions of pure CH4 and CO2 with ambient air.
The aliquots were measured multiple times by isotope ratio mass spectrometry
(IRMS) at RHUL. The precision for δ13CH4, obtainable with
this IRMS, is reported as 0.05 ‰ – detailed information on
the measurement system can be found in Fisher et al. (2006). The calibration
factor is determined from a linear regression and calibrations were
performed once a day for 3 consecutive days before and after the laboratory
experiments. A target gas was measured regularly to track any drift in
δ13CH4 as an independent check on the calibration
quality.
An example of the results from a H2O interference experiment
spanning the range 0–1 % H2O. The reported C2H6 is altered
due to the addition of water vapour when measuring zero air
(< 1 ppb C2H6). Dark and light blue markers signify the response when dried
and undried ambient air have been measured overnight by the instrument prior
to the experiment respectively. Error bars signify the standard deviation of
each measurement.
Results and discussion
This study focuses on determining a reliable correction and calibration
scheme for a Picarro G2201-i when measuring methane sources with
C2H6 interference. Findings from the experiments described in
Sect. 2 are discussed in detail here.
In order to calibrate δ13CH4 and C2H6 values,
there are a series of corrections that must take place beforehand (see Fig. 1). The initial correction to be applied is on C2H6 due to
interference from CH4, CO2 and H2O. Particular emphasis is
placed on this correction due to the discovery of significant non-linear
behaviour in the presence of H2O, CH4 and CO2 in the sample
gas. Once the C2H6 has been corrected, the calibration of
C2H6 using independent GC measurements, the C2H6
interference correction on δ13CH4 and finally the
calibration of δ13CH4 can be effected.
For our results to be applicable to future studies we examine the
inter-instrument variability and stability over time, compare our results to
current literature and discuss the uncertainties attributed to our results.
Throughout this study we refer to raw, uncorrected C2H6 and
δ13CH4 concentrations as “reported” to highlight that
they may be influenced by interferences and are uncorrected. Within this
section negative C2H6 concentrations are often mentioned. We note
that this is the “reported” C2H6 concentration by the
instrument. Unless otherwise stated, the standard deviation reported is
calculated from 1 min averages and depicted as error bars within
figures.
The discontinuity seen for instrument CFIDS 2072 for two
repetitions denoted by different colours. After the discontinuity at
0.16 % the subsequent slope clearly differs between the two repetitions.
Both instruments display a discontinuity at 0.16 % H2O. Each point
represents a 1 min average, the error bars represent the standard
deviation of the raw data.
Correcting reported C2H6
H2O interference on C2H6
H2O content was found to be the dominating source of interference to
reported C2H6; its presence decreases the reported concentration
of C2H6 with increasing H2O concentration. Furthermore, the
response function exhibits a hysteresis effect, which, although small, can
be considerable when changing from dry to undried air samples (e.g. between
dry calibration gas and undried ambient air). There are two distinct
instrumental responses, depending on whether dried or undried ambient
air are being measured during the night preceding the experiment, which are depicted in Figure 3 by dark and light blue markers respectively. When the CRDS instrument measures dry
air prior to the experiment, a discontinuity is observed at 0.16 %
H2O. Figure 4 shows this effect in more detail; prior to 0.16 %
H2O the response function exhibits a stable linear response. The
correction within this low range was found to be the same for both
instruments, 0.44 ± 0.03 ppm C2H6 / % H2O. After
passing the 0.16 % H2O threshold, the response exhibits a
discontinuity with a magnitude and subsequent slope that are also dependent on the
air moisture beforehand. This is seen in Fig. 4 whereby the discontinuity of
two repetitions (A and B depicted by dark and light blue markers
respectively) differs in magnitude by 0.1 ppm reported C2H6. The
discontinuity occurs when the instrument passes the 0.16 % H2O
threshold, both when moving from dry to wet air and vice versa (see Fig. S2). If measuring undried air before the experiment, the interference due to
H2O can be described well by a linear response (light blue markers in Fig. 3) and potentially causes large biases from the true C2H6. For
example, if measuring at 1 % H2O, both instruments display a change in
reported C2H6 of approximately -0.9 ppm. The response
function calculated for instruments CFIDS 2072 and 2067 differed, showing -0.72 ± 0.03 ppm C2H6 / % H2O and
-1.00 ± 0.01 ppm C2H6 / % H2O with R2 values of 0.98 and 0.99
respectively. The hysteresis effect is evident when measuring with undried
air; the slope was seen to shift after each repetition, in total by
0.1 ppm C2H6 / % H2O.
Relationship between reported C2H6 and concentration
changes of CO2 for instruments CFIDS 2072 and 2067 at varying values of
H2O, at 0 ppm C2H6 (within our instrumental precision). For
each plot the bottom axis indicates the concentration of the targeted gas
(CO2). Plots (a) and (b) are at 0 % H2O, (c) and (d) are
experiments at varying humidities, distinguishable by colour. The legend
denotes repetitions of the experiment. The error bars in each plot denote
the standard deviation of each measurement. The R2 values for the
experiments at 0 % H2O are 0.9 and 0.8 for all other H2O
experiments for both instruments.
CO2 interference on C2H6
For both instruments an increase in the CO2 concentration results in
lower reported values of C2H6, and it is furthermore apparent that
the magnitude of this interference is dependent on air humidity. For a dry
sample gas (H2O < 0.16 % – demonstrated in the left-hand
column of Fig. 5), the interference for both instruments is found to be
highly stable and well characterized by a linear slope of 1 × 10-4 ± 1 × 10-5 ppm C2H6 / ppmCO2
with a R2 value of
0.9. There was no measurable difference in slope at any of the
C2H6 concentrations tested (see Fig. S3). In contrast, for water
vapour levels ≥ 0.5 % H2O (see right-hand column of Fig. 5),
measurements exhibit a higher scatter between repetitions. This is mainly
attributed to a drifting intercept; however the experiments also show a
smaller R2 of 0.8. We calculate a characteristic linear slope of
3.8 × 10-4 ± 1 × 10-5 ppm C2H6 / ppm CO2 and
3.9 × 10-4 ± 1 × 10-5 for ≥ 0.5 % water vapour for
instruments CFIDS 2072 and 2069 respectively. Therefore, when measuring undried ambient
air, the presence of CO2 at a level near 400 ppm will induce a shift in the
reported C2H6 of approximately -0.15 ppm C2H6, whereas
if the air is dried the reported shift is much smaller, at approximately
-0.04 ppm C2H6.
Relationship between reported C2H6 and concentration
changes of CH4 for both instruments at 0 ppm C2H6 (within our
instrumental precision). For each plot, the bottom axis indicates the
increase in concentration of the targeted gas. The vertical bars in each
plot denote the standard deviation of each point. The legend denotes
repetitions of the experiment. Plots (a) and (b) are at 0 % H2O. The
R2 values are 0.4 and 0.6 for instruments CFIDS 2072 and 2067. Plots (c) and (d) show the response at 1 % H2O. These
two plots have a R2 value of 0.2.
CH4 interference on C2H6
The CH4 effect on C2H6, as shown in Fig. 6, is less prominent
by at least an order of magnitude than both the H2O and CO2
interferences. At dried ambient CH4 concentrations a typical change in
reported C2H6 of approximately -0.008 ppm is observed within both
instruments. Dried air experiments show a high scatter of points between
repetitions, and R2 values of 0.4 and 0.6 for instruments CFIDS 2072 and 2067
respectively are calculated. Despite its large uncertainty, the data suggest
that both instruments display a similar response with a statistically significant
slope within the range of C2H6 concentrations tested (see Fig. S3). In light of this we use a weighted mean to calculate a linear response
of 9 × 10-3 ± 2 × 10-3 ppm C2H6 / ppm CH4 for dry
air measurements for CFIDS 2067, and 7 × 10-3 ± 5 × 10-3 ppm C2H6 / ppm CH4
for CFIDS 2072. The results obtained at 1 % H2O
show little correlation (as shown in the right-hand column of Fig. 6), with
both instruments displaying a R2 value of 0.2. An ANOVA test suggests
the slopes are not significantly different from zero; thus we omit a
CH4 correction for this case.
Combining the CO2, CH4 and H2O correction on C2H6
To fully take into account all (known) C2H6 cross-sensitivities,
the corrections to reported C2H6 need to be combined. Due to the
non-linearity of the discontinuity in reported C2H6 at 0.16 %
H2O and its subsequent slope we choose to report correction
coefficients for the two found linear regimes, i.e. for continuous
measurements with sample humidities below 0.16 % and sample humidities
above 0.16 %. Within each range the proposed correction formula is given
as follows:
(C2H6)CORRECTED=(C2H6)RAW+A*(H2O)+B*(CH4)+C*(CO2).
If the humidity is limited to less than 0.16 % before and during
measurements, A= 0.44 ± 0.03 ppm C2H6 / % H2O,
B= 8 × 10-3 ± 2 × 10-3 ppm C2H6 / ppm CH4,
C= 1 × 10-4 ± 1 × 10-5 ppm C2H6 / ppm CO2. Both
instruments demonstrated good agreement for all the correction factors
calculated at < 0.16 % H2O.
Corrections for measurements undertaken at concentrations higher
than or equal to 0.16 % H2O are A= 0.7 ± 0.03 ppm C2H6 / % H2O, B= 0 ppm C2H6 / ppm CH4,
C= 3.8 × 10-4 ± 2 × 10-5 ppm C2H6 / ppm CO2 for
CFIDS 2072 and A= 1 ± 0.01 ppm C2H6 / % H2O, B= 0 ppm C2H6 / ppm CH4,
C= 3.9 × 10-4 ± 2 × 10-5 ppm C2H6 / ppm CO2 for CFIDS 2067.
Summary of C2H6 calibration factors calculated for both
instruments CFIDS 2072 and 2067.
CFIDS 2072
CFIDS 2067
C2H6
Slope
Intercept
Slope
Intercept
Calibration
(ppm)
(ppm)
Feb,15
0.49 ± 0.03
0.00 ± 0.01
Oct,15
0.51 ± 0.01
-0.06 ± 0.04
0.52 ± 0.01
-0.12 ± 0.01
(a) Ethane calibration calculated from measurements of flask
samples by both the GC and CRDS. The x-axis is the corrected C2H6
(C2H6COR) using the corrections described previously. The
y-axis is the C2H6 as measured by a manual GC. The error bars
indicate the standard deviation of each flask measurement, for certain
flasks error bars are smaller than their respective markers. (b) 30 min target measurements over a period of 4 days, from
13 to 16 November 2015. The standard error of
each target is smaller than the plotted marker. The baseline C2H6
is seen to drift with time.
C2H6 calibration
To make use of the corrected C2H6 it should be calibrated to match
an internationally recognized scale. This is achieved by measuring whole-air
samples by CRDS and independently on a calibrated gas chromatograph, as
discussed within Sect. 2. The calibration factor is determined by comparing
the corrected C2H6 resulting from CRDS and C2H6 as
confirmed by the GC and plotted in Fig. 7a. The relationship was found to be
linear throughout the range of 0–5 ppm C2H6 with a slope of
0.505 ± 0.007 and 0.52 ± 0.01 for instruments CFIDS 2072 and 2067 respectively.
The results are reported in Table 2 from which we can see the intercept of
the calibration for instrument CFIDS 2072 shifts between the experiment in February and
that in October, while the slope remains constant throughout the measured time period.
The change in the intercept is attributed to a C2H6 baseline drift
which we have monitored over time using regular target gas measurements;
an example is given in Fig. 7b. To account for this drift and any elevated
baselines (such as that of CFIDS 2067 – see Table 2), a regular measurement
of a working gas is necessary, from which the instrument offset can be
calculated. For the full calibration, we thus suggest using Eq. (2), where D
is the calibration factor (slope) for the instrument, i.e. for CFIDS 2072
D= 0.505 ± 0.007 and Δ (WGS) the baseline drift determined using
the working gas.
(C2H6)calibrated=D*((C2H6)corrected-Δ(WGS))
The various response functions calculated for the δ13CH4 correction due to C2H6.
CFIDS 2072
CFIDS 2067
δ13CH4
Slope
Intercept
Slope
Intercept
Correction
(‰ CH4 / C2H6)
(‰)
(‰ CH4 / C2H6)
(‰)
July,15
+24±2
0.5 ± 0.6
–
–
Nov,15
+23±1
0.2 ± 0.6
+23±1
-2.3 ± 0.7
Nov,15*
+24±1
0.6 ± 0.6
+24±2
-2.5 ± 0.8
* Flask measurement.
During a dilution sequence of ambient gas with C2H6, the
CH4 concentration decreases from its nominal concentration 1948.7 ppb ± 0.32 ,ppb as the contribution from C2H6 is increased. Thus
both 12CH4 and 13CH4 undergo a similar decrease as the
gas is diluted. However, what is observed is an increase in the reported
value of 13CH4, suggesting C2H6 interference. The
12CH4 axis is plotted to the left in light green, whereas the
13CH4 axis is plotted to the right in dark green at a different
scale. Error bars represent the standard deviation, the 12CH4
markers are larger than their associated error bars.
δ13CH4 correction
By measuring the shift of the reported δ13CH4 in
C2H6-contaminated samples, we have observed that the instrument
reports heavier values of δ13CH4 in the presence of
C2H6. The shift is a result of increased reported 13CH4
in samples containing C2H6 (see Fig. 8). This is most likely
caused by the overlapping of spectral lines within the 6029 wave number
region (Rella et al., 2015). We calculate the δ13CH4
correction by taking the slope of Δδ13CH4 (the
difference between the reported δ13CH4 and the initially
reported one of the C2H6-free gas) and the corrected
C2H6 to CH4 ratio. The ratio is used to permit the
calculation of the δ13CH4 response function per ppm
CH4 as the magnitude of interference is dependent on CH4
concentration (Rella et al., 2015). The significance of the interference on
δ13CH4 concentrations is illustrated in Fig. 9; as the
C2H6 : CH4 ratio increases, the change in the reported δ13CH4 increases linearly. Results obtained from tests carried out
throughout the year, for both instruments are noted in Table 3 and plotted
in Fig. 9. The correction equation can be expressed as follows:
(δ13CH4)CORRECTED=(δ13CH4)RAW-E*C2H6CORRECTED/CH4+F,
where E is the slope of the response function and F is the intercept. E and
F are +23.6 ± 0.4 ‰ ppm CH4 / ppm
C2H6 and approximately +0.4 ± 0.2 ‰ for
instrument CFIDS 2072 and +23.3 ± 0.7 ‰ ppm CH4 / ppm C2H6 and approximately -2.4 ± 0.4 ‰ for instrument CFIDS 2067 respectively. These corrections
contain the inherent δ13CH4 offset of the instrument. When
calibrating the δ13CH4 to a known scale (as described in
Sect. 2.5) any instrumental offset will be incorporated within the
calibration. Therefore, the correction equations can be simplified to
(δ13CH4)CORRECTED=(δ13CH4)RAW-E*C2H6CORRECTED/CH4.
Also highlighted in Fig. 9 is the typical measurement range for the majority
of ffCH4 sources related to dry and wet natural gas relative to calibrated
C2H6 / CH4 ratios given on the upper abscissa, whereby dry gas
refers to natural gas that occurs in the absence of condensate/liquid
hydrocarbons (C2H6 : CH4= 1–6 %) while wet gas typically
contains higher concentrations of complex hydrocarbons
(C2H6 : CH4 > 6 %; Yacovitch et al., 2014). It is
clear that within this range the bias on methane isotopic signatures is
significant; dry gas will alter the reported δ13CH4 by
0.8–4 ‰, while wet gas can cause a shift of up to
13 ‰ depending on its C2H6 : CH4 ratio.
δ13CH4 calibration
Full instrument calibrations as described in Sect. 2.4 were performed once
in 2014 and once in 2015. The δ13CH4 values obtained for the
calibration gases by RHUL are measured by IRMS and are therefore not subject
to interferences. The calibration gas aliquots were measured with an average
standard deviation of 0.03 ‰. To calibrate δ13CH4CORRECTED, the δ13CH4CORRECTED was
calculated for each calibration gas and used within the linear regression.
The calibrations were linear with R2 > 0.99 on both occasions
and no change (within our uncertainties) was observed between the two tests.
By measuring an ambient air target regularly, we later detected a shift in
the δ13CH4 baseline. Two further calibrations were
performed in 2016 to assess this incident which confirmed that the offsets
of the linear regressions were significantly shifted, while the slopes
agreed well with previous calibrations. Therefore, to account for a baseline
drift, it is important to measure a target gas regularly and amend the offset
of the calibration equation accordingly.
Typical instrumental performance and uncertainties
In order to characterize the repeatability of the C2H6 measured by
the CRDS instrument, we have measured several targets and monitored the
changes of the reported C2H6 signal over time. The raw signal is a
measurement every 3 s, which displays on average a standard deviation
of 90 ppb. By aggregating the data to 1 or 30 min intervals, the precision
can be improved and a standard deviation of 20 or 8 ppb is reached. Furthermore, the 1 min standard deviation at 52 ppm
C2H6 is 180 ppb. Thus by assuming a linear relationship the
typical performance for 1 min averages is 20 ppb ±0.3 % of
reading.
Of course, there are some substantial uncertainties attributed with the
C2H6 correction and calibration which need to be accounted for
when discussing the uncertainty of the calibrated C2H6
concentrations. With regards to the C2H6 correction for 1 min
averages, if measuring dried ambient air the propagation of uncertainties
are negligible with respect to the raw instrumental precision (20 ppb).
However, if using 30 min averages the uncertainty augments from 8 to
10 ppb. Elevated CH4, CO2 and H2O signals (> 5 ppm,
> 1000 ppm, > 0.2 % respectively) will induce
increased C2H6 uncertainty regardless of aggregation time. After
calibration, the correction factor increases to 21/2 times that of the
corrected C2H6, so at ambient air concentrations calibrated
C2H6 has an uncertainty of 30 ppb.
The effect of C2H6 on reported δ13CH4.
The slopes of reported δ13CH4 vs. the C2H6CORRECTED : CH4 ratio are shown for three tests taken throughout the
course of 1 year. Triangular markers imply whole-air sample measurements,
while square markers are derived from direct measurements. Error bars
indicate the standard deviation. In the presence of C2H6 the
instrument reports heavier values of δ13CH4. The typical
range of (calibrated) C2H6 : CH4 of dry and wet gas are
highlighted in pink and green respectively, corresponding to the top axis.
The repeatability of δ13CH4 for 1 min averages on our
instrument is a standard deviation of 0.66 ‰. The
standard deviation is reduced to 0.29 and
0.09 ‰ by aggregating the raw data for 5 and 30 min respectively. For the correction of δ13CH4 due to
C2H6, error propagation of the factors applied in Eq. (4) must be
taken into account. Therefore, at ambient concentrations, the uncertainty of
a 1 min average will increase to 0.9 ‰.
Generalizability of corrections and calibrations
The experiments in this study were repeated multiple times and performed on
two instruments to better understand how the instrument responses change
over time and how they vary between instruments. The C2H6
correction and calibration, and δ13CH4 correction
experiments were repeated on CFIDS 2072 over the course of a year to determine any
temporal drifts.
The coefficients of the C2H6 correction were examined over a 4-month period. Methane, carbon dioxide and water vapour coefficients for
dried gas displayed no noticeable variation over this time frame. Both
CH4 and CO2 coefficients for undried gas also showed good
stability throughout this period; however the undried H2O coefficient
is seen to vary significantly (±0.1 ppm C2H6 / % H2O).
As discussed previously, the H2O correction is subject to a hysteresis
effect, which makes analysis of its long-term variation difficult. As we did
not find a clear temporal pattern of the variations, we therefore suggest
that this coefficient is not likely to be time dependent.
The calibration of C2H6 was calculated twice within a 9-month
period (see Table 2). No variation of the slope of the response function is
observed within this time frame. The intercept is prone to drift in time as
discussed previously.
The δ13CH4 correction has been examined three times
throughout a 6-month period (see Table 3). The variability of the slope
observed over 6 months is 1 ‰ ppm C2H6 / ppm CH4. Given that the error attribution of each
experiment is approximately ±1 ‰ ppm C2H6 / ppm CH4, this variability is not statistically
significant. The intercepts show good agreement with no variation outside
the expected uncertainties.
The comparison of both CRDS instruments showed good agreement for all
calculated C2H6 correction coefficients, with the exception of the
undried H2O coefficient at > 0.16 % H2O. For this
coefficient we calculate a difference of 0.3 ppmC2H6 / % H2O
between that of CFIDS 2072 and CFIDS 2067. The variance may be the consequence of
spectrometer differences, a long-term hysteresis effect or
differences in their past use (mostly dried samples on CFIDS
2072 and mostly undried samples for CFIDS 2067).
The slopes derived for the C2H6 calibration of both instruments
correspond well, with no significant difference seen between the two. The
intercepts differ by approximately 0.6 ppm, thus suggesting a distinct
difference between intra-instrumental C2H6 baselines.
The slopes of the δ13CH4 correction were found to be in
good agreement between the two instruments. Where the instruments differ is
with regards to their δ13CH4 baselines, thus causing the
observed disparity in intercept (seen in Table 3) of approximately
3 ‰.
To the best of our knowledge, at this time there is only one published study
reporting on a correction due to C2H6 interference on an isotopic
Picarro analyser. Rella et al. (2015) have studied the interference using a
Picarro G2132-i, a high-precision CH4 isotope-only CRDS analyser which
uses similar analysis algorithms and spectral regions to that of the Picarro
G2201-i. Rella et al. (2015) obtained C2H6 correction parameters
of A= 0.658 ppm C2H6 / ppm H2O, B= 5.5 ± 0.1 × 10-3 ppm C2H6 / ppm CH4,
C= 1.44 ± 0.02 × 10-4 ppm C2H6 / ppm CO2 in 2015. Factors B and C for
CH4 and CO2 respectively agree well with the dried air
coefficients attained within this study. The H2O coefficient, as
suggested by Rella et al. (2015) differs from both that of CFIDS 2072 and CFIDS 2067 but
confirms the variability of this factor between instruments when measuring
undried air samples. Lastly, Rella et al. (2015) report a correction factor
for δ13CH4 of 35 ‰ ppm CH4 / ppm C2H6 which indicates a different response to C2H6
contamination of the different instrument series.
Source identification at a natural gas compressor station
In order to quantify the effect of C2H6 contamination in a real
world situation, we have applied the corrections and calibrations discussed
in this paper to measurements taken at a natural gas site, with the aim of
distinguishing emissions between two natural gas pipelines. In the following
section we demonstrate the effect of C2H6 interference on δ13CH4 at a fossil fuel site and discuss the alternative
approach of using calibrated C2H6 : CH4 ratios to distinguish
source signatures, a method which has not been previously tested on a
Picarro G2201-i.
Description of field campaign
Site description
Located in an industrial park in northern Europe, the campaign took place at
a natural gas compressor station in summer 2014. Such stations serve the
distribution of natural gas; their key purpose is to keep an ideal pressure
throughout the transmission pipelines to allow continuous transport from the
production and processing of natural gas to its use. The visited compressor site
comprises two major pipelines with their corresponding compressors.
The two pipelines carry gas of different origins to the site, where after
pressurization, they are combined for further transmission. The site
topography is flat and open with the surrounding area being predominantly farmland and in close proximity to a major road. FFCH4 emissions were
expected to emanate from various sources on site such as the compressors,
methane slip from turbines and fugitive emissions due to the high pressure
of gas (Roscioli et al., 2015). Other possible methane sources in the nearby
region were identified as traffic and agriculture, including a livestock
holding situated less than 500m south-west of the site.
Continuous measurements of CH4, δ13CH4 &
C2H6
Two instruments were utilized for continuous measurements throughout the 2-week field campaign: a CRDS instrument (CFIDS 2072, characterized in detail in
previous sections) and an automatic gas chromatograph with a flame
ionization detector (GC-FID; Chromatotec, Saint-Antoine, France) measuring
VOCs (light fraction C2-C6 hydrocarbons), described in detail in
Gros et al. (2011). They were located at a distance of approximately
200–400 m from the pipelines and compressors.
The air measured by the CRDS instrument was dried consistently to < 0.16 % H2O using a Nafion (Perma Pure LLC, Lakewood, USA). The
δ13CH4 was calibrated using the method described
previously in Sect. 2. Every two days, 20 min measurements of two calibration gases were
made to calibrate the CH4 and CO2 data and to track
any drift in the isotopes. A C2H6 free working gas was measured
every 12 h and used simultaneously as a target gas for the calibration
of CH4 and CO2, and to track any drift in the C2H6
baseline for the calibration of C2H6.
The GC-FID was calibrated at the beginning and end of the campaign using a
certified standard gas mixture (NPL, National Physics Laboratory,
Teddington, UK). The sampling time is a 10 min average every half an
hour; 10 min of ambient air is measured after which the following 20 min are used to analyse the input.
Grab sample measurements of CH4, δ13CH4 & C2H6 in pure natural gas
samples
Grab samples of pure natural gas were taken of both pipelines, with the aim
of characterizing the two differing gas supplies. The 0.8 L stainless steel
flasks were evacuated prior to sampling to a pressure of the order of
10-6 mbar, after which they were filled to ambient pressure when
sampling. The flasks were measured independently in the laboratory with a
manual GC (described in Sect. 2.4) and, after dilution with zero air, by the
CRDS instrument.
Ethane and methane content of two selected peaks. Methane
and ethane 1 min averaged time series is shown in (a) and (b) for Event 1 and (e) and (f) for Event 2.
Miller–Tans plots of the corresponding peaks are shown in (c) and (g),
blue for the corrected δ13CH4 due to C2H6, and
red representing uncorrected δ13CH4. Event 1 includes
elevated C2H6 emissions and thus displays a difference between
the slope before and after C2H6 correction, corresponding to a
shift in isotopic signature. Event 2, with no C2H6 shows no
alteration in slope. The slopes of C2H6 vs. CH4 are shown in
(d) and (h), signifying the C2H6 : CH4 ratio of the
emission. Errors of both the isotopic and C2H6 : CH4 signatures
are calculated from the standard error of the slope.
Impact of C2H6 on δ13CH4 observations at the field site
To quantify the effect of C2H6 interference on δ13CH4 a total of 16 events were selected from the 2-week
field campaign, with criteria defined as a peak exhibiting both
increasing CH4 concentrations and a change in δ13CH4
signature for a minimum of 1 h. Two such events are plotted in Fig. 10.
Event 1 represents the majority of events measured during the field
campaign, in which CH4 and C2H6 are well correlated. This
particular event has a maximum concentration of 11 ppm CH4 and 0.6 ppm
C2H6. On average the selected events have peak concentrations of
5 ppm CH4 and 0.3 ppm C2H6. The methane isotopic signature was
characterized using the Miller–Tans method (Miller and Tans, 2003), in
which δ13CH4* CH4 values are plotted against CH4
to calculate the isotopic signature of the methane source in situations
where the background is not constant. In order to avoid bias stemming from
using ordinary least squared (OLS) regression, the York least squares
fitting method was implemented, thus taking into account both the X and Y
errors (York, 1968). All events excluding one were found to have δ13CH4 signatures characteristic of natural gas, corresponding on
average to -40 ‰. A single event (Event 2 plotted in
Fig. 10) was detected with a δ13CH4 signature of
-59 ‰ ± 1.5 ‰. Such a signature
suggests a biogenic source and, due to the south-westerly wind direction
throughout the event (where the livestock holding is located), suggests the
source is likely to originate from livestock, either as ruminant or manure
emissions.
If the data are left uncorrected, sources containing C2H6
substantially bias the calculated isotopic signature of CH4 events.
This is demonstrated in Fig. 10c where, for Event 1, the slope of points
after C2H6 correction (in blue) is shifted in comparison to the
slope derived from points left uncorrected (in red), signifying a
modification of the δ13CH4 signature. Corrected δ13CH4 suggests a signature
of -40.0 ‰ ± 0.1 ‰, while uncorrected values imply
-37.8 ‰ ± 0.08 ‰. When no
C2H6 is present, i.e. Event 2, there is no disparity between the
raw and corrected δ13CH4 slope, resulting in a δ13CH4 signature of
-59 ‰ ± 1 ‰ for both methods. For the 15 natural-gas-related
events, the average shift induced due to uncorrected data is
2 ‰. Consequently the bias in isotopic signatures due to
C2H6 means that uncorrected data will always overestimate the
source when a simple two end-member mixing model is applied.
Continuous field measurements of ethane
As an independent verification of the CRDS performance we compared two time
series of C2H6 which were measured simultaneously by the CRDS and
GC-FID during the natural gas field campaign by using a co-located air
inlet. The CRDS data were averaged to identical time stamps as the GC-FID,
i.e. a 10 min average every 30 min. From which we calculated a root
mean squared error (RMSE) of 13 ppb. Given the precision of C2H6
measured by the CRDS instrument is 10 ppb for 10 min averages, and the
uncertainty on the GC-FID is 15 %, we conclude that this is an extremely good
agreement.
Furthermore, the flask samples, taken on the 4 July 2014, were
measured by the CRDS to have a C2H6 : CH4 ratio of 0.074 ± 0.001 ppm C2H6 / ppm CH4 and 0.046 ± 0.003 ppm
C2H6 / ppm CH4 for the gas within Pipeline 1 and Pipeline
2 respectively. On the same day gas quality data from the on-site GC recorded a
C2H6 : CH4 ratio of 0.075 ppm C2H6 / ppm CH4 and 0.048 ppm C2H6 / ppm CH4 respectively. Although the
error associated with the later figures is unknown, the strong agreement
between the two verifies our correction and calibration strategy of
C2H6.
Use of continuous observations of C2H6:
CH4 by CRDS
The instruments' capability to now measure interference-corrected and
calibrated C2H6 opens the door for using another proxy for source
apportionment, namely the C2H6 : CH4 ratio (Yacovitch et al.,
2014, Roscioli et al., 2015, Smith et al., 2015). The
C2H6 : CH4 ratio that characterizes each source is determined
by the slope of the C2H6 to CH4 relationship. This method was
applied to the 16 events identified within the natural gas field campaign,
again using the York linear regression method, taking into account both X and
Y error. Two examples of this method are displayed in the bottom panel of
Fig. 10. Event 1, representing a natural gas emission has a measured
C2H6 : CH4 ratio of 0.068 ± 0.002 ppm C2H6 / ppm CH4, suggesting a wet gas source. Biogenic events, such as Event
2, are absent of C2H6 (within our detection limit), thus resulting
in a C2H6 : CH4 ratio of 0 ± 0.2 ppm C2H6 / ppm CH4. Excluding the biogenic event, on average the 15 natural gas
emissions detected have a weighted mean C2H6 : CH4 ratio of
0.069 ppm C2H6 / ppm CH4 with an average event uncertainty of 0.006 ppm C2H6 / ppm CH4. This figure agrees
well with the median value for conventional gas ratios measured by Roscioli
et al. (2015).
If the C2H6 data are left uncorrected and uncalibrated the
C2H6 : CH4 ratio calculated is significantly shifted by
approximately +0.06. The average raw C2H6 : CH4 ratio for
the 15 natural gas events is 0.132 ± 0.007 ppm C2H6 / ppm
CH4, while the biogenic events C2H6 : CH4 ratio
calculated is negative and thus impossible.
Distribution of 16 events according to their C2H6 : CH4 ratios and isotopic signature. The red and purple dashed lines
signify the characterizations of Pipeline 1 and 2 respectively as measured by
the CRDS instrument from flask samples taken on the 4 July 14. For corrected
and calibrated data (square markers), both the isotopic signature and
C2H6 : CH4 ratios identify the biogenic source (bottom-left
point) and suggest the natural gas emissions emanate from Pipeline 1.
Circular markers represent the uncorrected data which does not agree with
the flask sample measurements of pipelines 1 or 2. The error bars indicate
the standard error of the slope calculated from Miller–Tans and
C2H6 vs. CH4 plots for δ13CH4 signature and
C2H6 : CH4 ratio respectively.
Flow chart illustrating the steps and the corresponding equations
to calibrate C2H6 and δ13CH4 as determined from
this study. The coefficients are the mean of both CRDS instruments tested.
We suggest removing H2O from gas samples prior to analysis.
Combined method for CH4 source
apportionment
To distinguish which pipeline the emissions originate from, we compare both
the δ13CH4 signature and the C2H6 : CH4
ratio source apportionment methods. The two pipelines were characterized
from the whole-air samples taken on 4 July 2014; although the gas
within the pipelines is subject to change as incoming gas varies, we assume
here that this did not occur throughout the short duration of the campaign
(24 June to 4 July 2014). The data collected from the
aforementioned 16 events are compiled within Fig. 11, which illustrates the
distribution of δ13CH4 signature vs. C2H6 : CH4 ratios. The results from the flask measurements, i.e.
characteristics of Pipeline 1 and 2, are plotted as dashed purple and red
lines. Both methods clearly identify the biogenic source, seen
as an outlier in the bottom left corner of the plot. Furthermore, both
methods are able to distinguish between the two pipelines. The isotopic
signatures of the natural gas events (on average 40.2 ‰ ± 0.5 ‰) are clustered near the isotopic signature
of Pipeline 1, which has a δ13CH4 signature of 40.7 ‰ ± 0.2 ‰, thus suggesting the
majority of the measured methane is an emission from this pipeline. When
considering the C2H6 : CH4 ratio a similar conclusion may be
drawn as the mean C2H6 : CH4 ratio is 0.069 ± 0.002 ppm
C2H6 / ppm CH4, much like that of Pipeline 1 at 0.074 ± 0.003. A future study will address the shift in measured events to
the left of Pipeline 1 in Fig. 11 by using additional VOC data from the GC-FID
to aid source identification. The uncorrected 16 events are also plotted in Fig. 11 as
circular markers. These are found in the top right-hand corner of Fig. 11
and do not correspond well with either of the pipelines, thus reconfirming
the importance of the corrections.
Concluding remarks
This study focuses on measurements of C2H6 contaminated methane
sources by a CRDS (Picarro G2201-i), with emphasis on correcting δ13CH4 and (although not intended for use by standard users)
C2H6 for cross-interferences before calibration. Our extensive
laboratory tests suggest that CRDS instruments of this model are all subject
to similar interferences (as expected as they scan the same spectral
lines) and that they can have a significant impact on reported
concentrations and isotopic signatures if not accounted for properly when
measuring industrial natural gas sources. For now, we suggest using constant,
instrument-specific correction factors if possible or the ones found in this
study (summarized in Fig. 12). As our study period only encompasses 1 year
it is clear that the stability of the correction over the full life-time
needs to be monitored further. To fully exploit the reported C2H6
data, we suggest drying gas samples to < 0.16 % H2O,
calibrating the instrument and taking frequent measurements of a working gas (or
set of working gases) to monitor and correct for the instrumental baseline
drift.
The results of our field campaign demonstrate the extent of the
interferences of C2H6 on δ13CH4 for a real world
application and also support the validity of our C2H6 correction
and calibration through the comparison with an independently calibrated
GC-FID. In our case, when measuring wet gas emissions we detected an average
shift in isotopic signature of 2.5 ‰ due to
C2H6 interference; however the extent of this bias will vary
according to the contribution of C2H6, therefore affecting each
ffCH4 source to a different degree which can cause problems for source
determination. The results reported here are important for all future work
of CRDS in fossil fuel regions (where sources consist of a
C2H6 : CH4 ratio between 0 and 1 ppm C2H6 / ppm CH4)
to create awareness of such interferences and correct for them accordingly. Our CRDS
instrument is sufficient for measurements of strongly variable
C2H6 sources, where if using calibrated 1 min C2H6
data, concentration variations above 150 ppb are required to achieve a
signal-to-noise ratio of 5. Thus for industrial natural gas sites it offers
a new opportunity to use continuous C2H6 : CH4 observations
as a means of source determination that is independent from δ13CH4
methods. The recently released G2210-i analyser is dedicated to
C2H6 : CH4 ratio measurements and as such achieves a higher
precision, making it suitable for a wider variety of ethane sources.
Finally, we successfully combined both the δ13CH4 and
C2H6 : CH4 ratio source apportionment methods. At the natural
gas compressor site both methods clearly distinguish biogenic sources from
that of natural-gas-based sources. Combining those two independent methods
yields a better fingerprint of the source and spurious C2H6 or
δ13CH4 can be more easily identified. Lastly, by
characterizing both the δ13CH4 and C2H6 : CH4
ratio of our source, we gain insight into the formation and source region of
the gas (Schoell, 1983).