Introduction
Nitrous oxide (N2O) is a greenhouse gas with an atmospheric lifetime
of 131 years and a global warming potential that is approximately 300 times
that of CO2 at a 100-year time horizon (Prather et al.,
2012). At present, the N2O emissions are the most important factor for
stratospheric ozone depletion, and they are expected to remain the largest
factor for this century (Ravishankara et al., 2009; Wuebbles, 2009).
Global observations of the atmospheric N2O mole fraction from networks
such as Advanced Global Atmospheric Gas Experiment (AGAGE), National Oceanic
and Atmospheric Administration's Earth System Research Laboratory
(NOAA/ESRL), Commonwealth Scientific and Industrial Research Organisation's
Global Atmospheric Sampling Laboratory (CSIRO GASLAB) and Réseau
Atmosphérique de Mesure des Composés à Effet de Serre (RAMCES)
showed a mean value of approximately 328 ppb for the Northern Hemisphere and
326.5 ppb for the Southern Hemisphere in 2014 (Lopez et al., 2012;
Schmidt et al., 2014; Thompson et al., 2013). The amplitude of the seasonal
cycle is smaller in the Southern than in the Northern Hemisphere, with a
value of approximately 0.4 ppb at Cape Grim, Tasmania, compared with 0.89 ppb at Mace Head, Ireland (Nevison et al., 2007). The N2O
growth rate in the atmosphere over the last 5 years is, on average,
0.75–0.78 ppb per year. Overall, gradients over the continent or between
maritime and continental air are small (less than 0.6 ppb) and need to be
measured precisely.
Even closer to the sources, at European semi-rural stations, only slight
variations in atmospheric N2O were found on the timescale of days.
Lopez et al. (2012) showed that the mean diurnal cycle at the
semi-urban station Gif-sur-Yvette (France, measurements 7 m a.g.l.) has an
amplitude of 0.96 ppb, whereas at Traînou tall tower (rural area,
measurements up to 180 m a.g.l.) the mean amplitude is only 0.32 ppb. At the
17th WMO/IAEA meeting, 10–13 June 2013 in Beijing, an expert group on
CO2 and other greenhouse gases from the World Meteorological Organization Global
Atmosphere Watch (WMO/GAW) recommended an N2O inter-laboratory
comparability goal of ±0.1 ppb
(http://www.wmo.int/pages/prog/arep/gaw/documents/Final_GAW_213_web.pdf). This ambitious goal has not
yet been reached, as shown recently by Bergamaschi et al. (2015), who
found biases between in situ gas
chromatography (GC) measurements and flask sampling at different
European stations of up to 0.7 ppb. These biases, even when corrected, limit
the precision of N2O emission estimates by inverse models.
High-precision atmospheric N2O measurements in flask measurement
networks and at in situ stations are traditionally measured by GC using an electron capture detector. Methods
incorporating this technique have achieved a typical short-term continuous
measurement repeatability (CMR) of 0.1 to 0.3 ppb (Lopez et al.,
2012; Nevison et al., 2011; Popa et al., 2010; Schmidt et al., 2001). Over
the last few years, new analytical techniques have become commercially available
for obtaining high-precision measurements of atmospheric N2O.
Hammer et al. (2013) described the Fourier transform infrared
(FTIR) absorption, which can reach a long-term repeatability (LTR) for N2O of
0.04 ppb over a 10-month period. More recently, laser-based systems, e.g.,
cavity-enhanced off-axis integrated cavity output spectroscopy (OA-ICOS),
cavity ring-down spectroscopy (CRDS), quantum cascade tunable infrared laser
differential absorption spectroscopy (QC-TILDAS) and difference frequency
generation (DFG)-based systems, were developed and commercialized by
different companies. In this study, we present the first assessment of the
performance of seven N2O analyzers and compare these techniques to
routine instruments, including a GC analyzer (Lopez et al.,
2012) that is used at LSCE (Laboratoire des Sciences du Climat et de
l'Environnement) for ambient air measurements since 2001 and a FTIR that has
been running since 2012. All of the tested instruments have been
characterized for repeatability, long-term stability, linearity, temperature
dependency and spectroscopic cross-interferences with water vapor. The
instrument evaluations were all performed at the ICOS Atmospheric Thematic Center Metrology Laboratory
(ATC MLab) hosted at LSCE in Gif-sur-Yvette, but not all of the instruments
were tested at the same time. Because the ATC MLab was in its creation
phase, there has been an evolution toward the best practices in the test
protocol, which will be detailed below. The evaluations have been performed
between October 2012 and January 2014 over three periods: November–December
2012, May–June 2013 and December 2013–January 2014. While the tests have
been carried out in the frame of ICOS, the results are valid for all groups
and networks doing high-precision atmospheric N2O measurements.
Instrument descriptions
Gas chromatograph instrument, Agilent
The LSCE laboratory at Gif-sur-Yvette is equipped with an automated GC system (HP-6890, Agilent, coupled to a PP1, Peak Performer
Laboratories, up to May 2013; since then, HP-7890A was used with the same PP1
and similar specificities and performances) to analyze the CO2,
CH4, N2O, SF6, CO and H2 mole fractions. This instrument
has been contributing to the RAMCES monitoring network since 2001 and is
used to obtain in situ measurements at Gif-sur-Yvette station for the
analysis of flask samples and the calibration of working standards. It also
serves as a reference instrument for international comparison programs, such
as the WMO Round Robin or the European Cucumbers intercomparison program
(http://cucumbers.uea.ac.uk/). Detailed descriptions of the GC system for
N2O analysis is given by Lopez et al. (2012). The GC is
equipped with a flame ionization detector and a nickel catalyst to
determine the CH4 and CO2 and with an electron capture detector (ECD) for N2O and SF6 analysis. We use a 10 mL sample loop, two
Hayesep-Q columns and Ar/CH4 as a carrier gas to separate N2O and
SF6 from the other compounds of air. Each analysis takes less than 6 min and allows two to six measurements of air samples on an hourly basis.
For the small range of N2O mole fractions in ambient air (324–334 ppb),
the ECD can be corrected for nonlinearity, applying a two-point calibration
strategy with two working standards (322 and 338 ppb). A calibration
frequency of 30 to 45 min was chosen to reach a N2O repeatability of
0.2 ppb. Lopez et al. (2012) described an interference between
SF6 and N2O measurements when SF6 mole fraction exceeds 15 ppt. Therefore, during the comparison period, all of the N2O
measurements made by our routine GC are flagged as not valid when SF6
exceeds 15 ppt. The standard deviation of the quality control gas (i.e., the
“target” gas), which was injected every 2 h, was 0.3 ppb over the
comparison period (October 2012 to January 2014). The GC is used in this
study as the routine reference instrument to compare the instruments'
performance regarding CMR and drift
assessments and for ambient air comparisons.
Fourier transform infrared spectrometer, Ecotech
FTIR spectroscopy is based on the absorption of infrared radiation
(Beer–Lambert law). A polychromatic infrared beam from an infrared source
first passes through a Michelson interferometer, and this modulated beam
then traverses the sample cell. The resulting time-modulated signal is
converted into an infrared spectrum through Fourier transformation. One of
the main advantages of an FTIR analyzer is its ability to record a spectrum
over a broad IR range (1800 to 7500 cm-1), thereby offering
the possibility to measure a large number of species simultaneously. Spectra
are stored and can be analyzed at a later date with a different method to
obtain better data or study new species.
Here, we briefly describe the instrument configuration used during the
comparison. The LSCE purchased this analyzer (built by Ecotech, Australia)
in 2011 from the University of Wollongong (Australia). Detailed descriptions
of similar analyzers used in the atmospheric community are presented by
Griffith et al. (2012) and Hammer et al. (2013).
The instrument used in our laboratory consists of a commercially available
FTIR interferometer (IRcube, Bruker Optics, Germany) with a 1 cm-1
resolution coupled to a 3.5 L multi-pass glass cell with a 24 m optical path
length (PA-24, Infrared Analysis, USA). The cell and the interferometer are
assembled on an optical bench inside a temperature-controlled chamber. An
in situ J-type thermocouple, to monitor the cell temperature, and a pressure
sensor (HPM-760s, Teledyne Hastings, USA) are included in the multi-pass
cell. We used high-purity nitrogen (at least 99.995 vol %) to slowly
purge the interferometer housing and the transfer optics between the cell
and the interferometer. A drying system composed of a 24 in. counter-flow
Nafion dryer (Permapure, Toms River, USA) followed by a chemical dryer
(Mg(ClO4)2) is installed upstream of the cell. The flow is
provided by a pump (MV2NT, Vacuubrand, Germany). The pressure of the cell is
controlled using a built-in mass flow controller mounted at the outlet of
the cell, and the flow is controlled by a mass flow controller installed
upstream of the cell and downstream of the drying system. Our instrument
uses the absorption bands located between 2097 and 2242 cm-1 to provide
the mole fraction of N2O and CO. A calibration with five working standards
is carried out every 2 weeks, and a quality control gas is analyzed every
5 h for 40 min. Due to its size, it takes 5 to 10 min to
empty and flush the cell when changing the type of sample. The sample flow
rate was regulated at 1 ± 0.05 L min-1 and the cell pressure and
temperature were regulated at 1100 ± 0.02 hPa (1.1 × 105 ± 2 Pa) and 32 ± 0.03 ∘C, respectively. The sample measurement
intervals for the target and air measurements are 1 and 3 min,
respectively. The quality control gas showed a N2O standard deviation
of 0.08 ppb for 1 min measurements during the first and third periods of the
comparison (December 2012 and January 2014) and a standard deviation of 0.12 ppb for
1 min measurements during the second period (May 2013).
Cavity ring-down spectrometer/quantum cascade laser
(CRDS-QCL) instruments G5101-I, Picarro
The two CRDS instruments tested in this
study were a loaned prototype, tested from November to December 2012, and
the commercialized model of the G5101-i unit from Picarro (Picarro Inc., CA,
USA) bought by the LSCE in May 2014. The thermal regulation was not yet
fully optimized for the prototype, and it did not possess a water vapor
correction. The CRDS system made by Picarro uses a QCL as a source to measure N2O, δ15Nα,
δ15Nβ and H2O in the mid-infrared region (4.55 µm). The CRDS technique uses an optical cell (48 mL) with three highly
reflective mirrors (Crosson, 2008). Light is injected into the cavity
at the required wavelength from the QCL through a highly reflective mirror
and is measured by a photodetector through a second highly reflective
mirror. The path length inside the cell is about 8 km. The intensity of the
light inside the cell builds up over time by resonance due to a third mirror
mounted on a piezoelectric device, which allows for inter-mirror distance
adjustment. Then, the laser is switched off, and the time constant of the
light intensity decrease is measured. From this cavity decay time, the
concentration is retrieved when knowing the absorption cross section of the
species at the laser wavelength. A measurement interval of less than 10 s is obtained. During the comparison campaign at LSCE, the sample flow
rate is approximately 50 mL min-1. The cell pressure is regulated at
100 ± 0.001 Torr (1.33 × 104 ± 0.13 Pa), and the cell
temperature is set at 40 ± 0.001 ∘C. A calibration with
four working standards is performed every 10 days, and a quality control
gas is analyzed every 5 h for 30 min.
IRIS 4600, Thermo Fisher Scientific (DFG)
The Thermo IRIS 4600 was lent by Thermo Fisher Scientific for our test
campaign from November to December 2012. The instrument measures N2O
and water vapor. It uses DFG laser
technology, which consists of combining two near-infrared telecom lasers
into a nonlinear frequency conversion crystal to reach the mid-infrared
region. The laser continuously sweeps the absorption bandwidth at a rate of
500 Hz (Scherer et al., 2013). This spectrometer measures in
the 4.6 µm N2O and H2O bands, and each sweep provides a near
instantaneous measurement of the two gases. The measurement interval is
adjustable between 0.1 and 10 s. The cell is 40 cm long for a cell size of
approximately 80 mL, which provides an optical path length of 5 m. The
sample flow rate through the cell is 300 mL min-1. The cell pressure
and temperature are regulated at 175 ± 0.002 mbar (1.75 × 104 ± 0.2 Pa) and 37.5 ± 0.002 ∘C, respectively. A
calibration with four standard gases is performed every week, and a quality
control gas is measured every 5 h for 30 min.
OA-ICOS-QCL instruments, Los Gatos Research (OA-ICOS)
Three OA-ICOS-QCLs made by Los Gatos Research (LGR, USA) were provided to
us by the French National Radioactive Waste Management Agency (ANDRA) for a
performance assessment. These three instruments measure N2O, CO and
H2O in the 4.6 µm region. Instruments based on OA-ICOS use a
tunable laser and an optical cavity (408 mL). However, unlike other cavity
methods such as CRDS, this technique is based on directing the laser beam
off axis through the cavity, which, when combined with highly reflective
mirrors, provides a long effective optical path that spatially sweeps the
cavity volume due to spatially separated multi-reflections within the cavity
before the reentrant condition of the optical beam is fulfilled. Moreover,
in contrast to CRDS, the laser beam is not locked on each cavity mode but is
swept over the gas absorption line. The mole fraction can then be obtained
from the measured spectra integrated over the entire absorption feature, the
cell pressure and temperature, the effective optical path length and the
absorption line parameters for each species. These instruments have a
measurement rate of up to 1 Hz with the internal pump and up to 10 Hz with
an optional pump.
One instrument is the standard model of the analyzer, referred to here as
ICOS-SD. The two others are enhanced performance models that incorporate an
improved temperature control of the cavity, referred to here as ICOS-EP38
and ICOS-EP40. The sample flow rate is set for the three analyzers at 300 mL min-1. The cell is
regulated at 85 ± 0.007 Torr (1.13 × 104 ± 0.93 Pa) and 27 ± 0.2 ∘C for the standard model and
45 ± 0.005 ∘C for the enhanced models. For the ICOS-SD, a
calibration is conducted once a week. For the two enhanced models, the
calibration frequency occurs every 2 weeks. For all instruments, a quality
control gas is analyzed every 5 h for 30 min for the ICOS-SD and
every 6 h for 30 min for the two ICOS-EP models.
QCL Mini Monitor, Aerodyne (QC-TILDAS)
The tested Aerodyne's QCL Mini Monitor (Aerodyne Research
Inc., USA) was provided by the Thünen Institut für Agrarklimaschutz
(Braunschweig, Germany) for our test campaign. This instrument is currently
used for eddy covariance measurements.
The instrument uses a quantum cascade tunable infrared laser
differential absorption spectroscopy technique. One QC laser beam is sent through a
Herriott astigmatic multi-pass cavity (0.5 L) with a fixed optical path
length of 76 m. Then, the light is received by a thermoelectrically
cooled infrared detector. This instrument works in the mid-infrared domain
(4.54 µm). The instrument performs an advanced type of wavelength
sweep integration before dropping the laser current below a threshold to
determine the voltage of the detector for zero light. The instrument can
then determine concentrations by fitting the measured spectrum with the
HITRAN database using the cell temperature and pressure. The instrument can
run periodically an auto-background, which consists of acquiring a spectrum
with the sample cell filled with dry nitrogen and is used to normalize
future spectra. This option was not used during this campaign. The
instrument tested had no active control of the cell pressure. During the
tests in our laboratory, we set the sample flow rate at 1 L min-1 and
the pressure at 33 ± 0.2 Torr (4.4 × 103 ± 27 Pa) by using a
valve at the outlet of the cavity and a needle valve at the inlet. The
cell's temperature is regulated at 22 ± 0.03 ∘C. A
calibration is conducted every 2 weeks, and a quality control gas is
measured every 5 h.
Instrument tests
Laboratory description
All tests were performed at the ICOS ATC MLab located at Gif-sur-Yvette, 20 km southwest of Paris. The MLab purpose is to test and validate atmospheric
analyzers, instrumental setups and related components and consumables. The
laboratory is air conditioned and equipped with calibration cylinders, target
gases and inlet lines coupled with drying systems for ambient air
comparisons. The water dependency, which was first tested using the droplet
test, is now evaluated with a dedicated humidifier bench. The ambient air
inlet is located on the roof of the laboratory, 7 m above ground level
(a.g.l.).
For ambient air comparisons, the air is dehumidified, by passing through 335 mL glass traps cooled in an ethanol bath using a cryogenic cooler (Thermo
Neslab CC-65 or HAAKE EK 90). The cooling traps are filled with glass beads
to increase the surface area for water vapor condensation. Depending on the
weather conditions, the cooling traps are typically changed once or twice
per week. This setup dries the air sample down to less than 15 ppm of water.
The time period during which the tests were performed
for all instruments and working specificities. Most instruments used two
sets of calibration cylinders; here, we indicate the most frequently used:
the set filled and calibrated by Max Planck Institute (MPI), spanning a
range from 320 to 360 ppb of N2O, or the set filled by Deuste
Steininger and calibrated at LSCE (DS), ranging from 320 to 345 ppb
N2O.
Instrument
Test period
Cell size
Cell flow rate
Cell temperature
Cell pressure
Main calibration
(mL)
(mL min-1)
(∘C)
(Pa)
set
FTIR
Oct 2012–Jan 2014
3500
1000 ± 50
32 ± 0.03
1.1 × 105 ± 2
MPI
CRDS
Nov 2012–Dec 2012
48
< 50
40 ± 0.001
1.33 × 104 ± 0.13
MPI
DFG
Nov 2012–Dec 2012
80
300
37.5 ± 0.002
1.75 × 104 ± 0.2
MPI
ICOS-SD
Nov 2012–Dec 2012
408
300
27 ± 0.2
1.13 × 104 ± 0.93
MPI
ICOS-EP38
May 2013–Jun 2013
408
300
45 ± 0.005
1.13 × 104 ± 0.93
DS
ICOS- EP40
May 2013–Jun 2013
408
300
45 ± 0.005
1.13 × 104 ± 0.93
DS
QC-TILDAS
Dec 2013–Jan 2014
500
1000
22 ± 0.03
4.4 × 103 ± 667
MPI
In total, we use four sets of calibration cylinders (laboratory standards)
to calibrate all of the N2O analyzers presented in this study. For the
GC measurements, we use two calibration cylinders (Luxfer aluminum
cylinders) filled by Deuste Steininger (Mühlhausen, Germany) in a
synthetic matrix of N2, O2 and Ar. These cylinders have been
calibrated against laboratory primary standards purchased from NOAA/CMDL and
are reported for N2O on the NOAA-2006a scale. For all optical
instruments, three other sets of calibration cylinders are used during the
comparison with a certified concentration of N2O (Table 1) among others
gases such as CO, CH4 or CO2. The first calibration set consists
of five aluminum cylinders (Luxfer), which were filled with ambient air,
spiked and calibrated with a GC-ECD by the Max Planck Institute of Jena,
Germany, on the NOAA-2006a scale (Hall et al., 2007) spanning a
range from 320 to 345 ppb N2O. Because this calibration set has
been routinely used by the FTIR and to test other analyzers in the MLab,
these cylinders were not always available during the second test period and
had to be replaced by another set of calibration cylinders. This second set
consists of six aluminum cylinders filled with a synthetic matrix of 21.0 ± 1 vol %
of O2, 0.93 ± 1 vol % of Ar and
a balance of N2 (Deuste Steininger) and calibrated by our FTIR. This
set spans a range from 320 to 360 ppb N2O. When the QC-TILDAS was
tested, a third calibration set was performed once by the instrument. This
set consists of four aluminum cylinders, spanning a range from 335 to 355 ppm N2O, filled with a synthetic matrix of
21.0 ± 1 vol % of O2, 0.93 ± 1 vol % of Ar and a balance of
N2 (Deuste Steininger) and calibrated by our FTIR. These calibration
sets were analyzed at least every 2 weeks on each analyzer. Calibration
sequences were made by measuring each cylinder for at least 15 min,
including 10 min for flushing the inlet line and instrument cell. The
whole calibration set was analyzed at least three times, with the first run
systematically rejected to ensure a proper flushing of the system. The
target cylinders used for various tests are filled with dry ambient air,
using an oil-free compressor (RIX) and coalescent filters associated with
magnesium perchlorate drying cartridges. These cylinders are analyzed prior
to
and after use by the GC system. All calibration and test cylinders are
equipped with the same type of two-stage nickel-plated brass pressure
regulators (Model 14, Scott Speciality Gases, Breda, the Netherlands).
All of the tests were performed in a temperature-controlled room (22 ± 1 ∘C), except for the temperature-dependence tests, for which the
laboratory temperature was deliberately modified. In general, all analyzers
were tested using similar procedures and during the same time span. We would
have preferred to test all of the instruments at the same time, but due to
constraints in their availability, tests were spread over time from October
2012 to January 2014. During this time span, the test procedures were
improved towards the ICOS standard test protocol, which is now used to test
new instruments in the MLab for the European ICOS atmospheric measurement
network. In some cases, the time period was just too short to perform all
tests. In the paragraphs below, we describe the test procedures in detail
and state when an individual test or analysis diverged from the standard
protocol. Most of the tests presented in the following sections have been
already described by Yver Kwok et al. (2015).
Continuous measurement repeatability calculated as 1
standard deviation over more than 30 h for different averaging times (raw
data, 1 min, 10 min and 1 h averaging). The short-term drift is estimated
with a linear regression over 30 h.
Instrument
1σ(ppb, raw)
1σ
1σ
1σ
Drift
(ppb, 1 min)
(ppb, 10 min)
(ppb, 1 h)
(ppb day-1)
GC
0.16 (5 min)
-
0.113
0.016
–
FTIR
0.15 (1 min)
0.149
0.055
0.026
0.017
CRDS
0.17 (4 s)
0.055
0.026
0.023
-0.034
DFG
0.66 (2 s)
0.159
0.107
0.097
-0.108
ICOS-SD
0.14 (2 s)
0.124
0.114
0.106
-0.185
ICOS-EP38
0.08 (2 s)
0.064
0.062
0.061
0.151
ICOS-EP40
0.10 (2 s)
0.068
0.060
0.054
0.070
QC-TILDAS
0.09 (1 s)
0.075
0.070
0.066
0.046
Continuous measurement repeatability and drift assessment
To determine the CMR (often called
precision by the manufacturer) of the instruments, a single target gas tank
filled at the MLab with dry natural air is measured continuously over a time
period of at least 30 h. The raw data measurement interval of the
analyzers tested varied from 5 min for the GC to 1 s for the QC-TILDAS
analyzer (Table 2). We calculate the standard deviations of 30 h of
continuous measurements of a target gas. Table 2 presents these values for
the raw data at the frequency given by all instruments for 1 min averaged
data (when available), 10 min averaged data and 1 h averaged data.
During this 30 h sequence of target gas measurement, no calibrations were
performed and no drift correction was applied, except for the GC, which
automatically corrects the data with calibration cylinders every 45 min.
From this experiment, we also calculate the drift for each instrument (Table 2). The drift is calculated using a linear regression with the data from the
30 h test. The slope of the regression represents the drift of the
instrument (in ppb h-1, which has been converted to ppb d-1) over
this 30 h period.
For high-frequency measurements (1 to 2 s), the ICOS-EP and QC-TILDAS
analyzers show the best CMR (0.08 to 0.10 ppb). For 1 min averaged data,
the QC-TILDAS, CRDS and ICOS-EP models show very similar standard deviations
of approximately 0.05–0.07 ppb, whereas the standard deviations of the
ICOS-SD, DFG and FTIR are between 0.12 and 0.16 ppb. For the 1 h
averaging data, the GC, FTIR and CRDS are the most precise, with the two
ICOS-EP models and QC-TILDAS 2 times less precise and the ICOS-SD and DFG
5 times less precise. Apart from the GC, whose drift is corrected with
the working standards, the FTIR has the smallest drift, with the CRDS and
QC-TILDAS drifts being slightly higher. The other instruments present a
significant drift of 0.1 ppb day-1 or more. For these instruments, a
standard gas measured several times a day could be used to correct this
drift.
Short-term repeatability assessment. A target tank is
measured 10 times for 15 to 20 min (30 min for the FTIR), alternating
with ambient air (for 5 min). An N2O mean value is calculated by
taking the last 5 min of each analysis. The repeatability is expressed
as the standard deviation (1σ) of these 10 injections. The peak-to-peak value is the difference between the lowest and the highest values of
the 10 analysis.
Instrument
Repeatability
Peak to peak
(ppb, N= 10)
(ppb)
FTIR
0.09
0.26
CRDS
0.03
0.11
DFG
0.17
0.55
ICOS-SD
0.02
0.04
ICOS-EP38
0.02
0.06
ICOS-EP40
0.02
0.06
QC-TILDAS
0.02
0.05
Allan deviation assessment for all instruments. The
two upper panels present the times series for the eight instruments over at
least 30 h. The lower panel presents the Allan deviation for all
instruments from 1 s to 3.104 s (logarithmic scale). The color codes
for the instruments and the test periods are given in the legend. The two
vertical lines in the lower panel correspond to an averaging time of 1 min
and 1 h.
Long-term repeatability computed from the standard
deviation (1σ) of the mean values over the last 5 min of 30
target measurements (N). The peak-to-peak value is the difference between
the lowest and the highest values of the N target measurements. The
calibration frequency gives the mean time between two calibrations. The
calibrations were applied as drift corrections.
Instrument
N
Period
1σ
Peak to
Mean calibration
(min)
(ppb)
peak (ppb)
frequency
GC
1 year
0.29
30 to 45 min
FTIR
30 (7 days)
40
0.07
0.27
20 days
CRDS
30 (12 days)
20
0.07
0.28
11 days
DFG
30 (12 days)
20
0.21
0.86
8 days
ICOS-SD
30 (7 days)
20
0.32
1.00
9 days
ICOS-EP38
30 (7 days)
30
0.25
0.70
13 days
ICOS-EP40
30 (7 days)
30
0.29
0.60
13 days
QC-TILDAS
27 (6 days)
30
0.14
0.44
30 days
The optimal averaging time can be estimated by using Allan standard
deviation plots. These plots can also be used, with LTR
assessment, to estimate the stability of an instrument and decide on a
calibration strategy. Figure 1 shows the time series of the 30 h target
test for each instrument (two upper panels), and the Allan deviation plotted
against the averaging times using a logarithmic scale (lower panel). With
the Allan standard deviation assessment, we can define two main categories.
First is the category of instruments with a high precision for
high-frequency measurements (ICOS-SD, ICOS-EP or QC-TILDAS instruments).
They present their best averaging time for intervals shorter than 5 min and higher variability over longer averaging times. The other
category regroups the instruments with better stability over longer
averaging intervals; the best averaging time is from 10 min to 1 h or
higher (CRDS, FTIR, DFG and ICOS-EP38). Some instruments, such as the
ICOS-EP38, have strong performances in both categories: high precision for
high frequencies and good stability. These two types of performances will
interest different research communities: the high precision for high
frequencies will interest anyone working on short time phenomena (< 1 min), such as eddy covariance studies. The second category will interest
communities working on typically 10 min to hourly averaged data, which is
the case of atmospheric background monitoring stations, such as the ICOS
atmospheric network. It should be noted that all of the instruments tested
at the MLab for this study achieved the CMR specifications given by the
manufacturers.
Short-term repeatability (STR) assessment
Because the CMR test is an assessment of the precision of the instrument
over continuous measurements, the STR assessment
quantifies the ability of one instrument to always reach the same value for
a target gas when alternated with a different sample. For this test, a
target gas is measured 10 times for 15 to 20 min alternating with dry
ambient air measurements for 5 min. From our experience with other
analyzers, 15 to 20 min should be appropriate for all instruments to
stabilize and to provide at least 5 min of stable measurements. Similar
to the CMR assessment, no calibration or drift corrections are applied. A
N2O mean value is then calculated for each injection of the target by
taking the last 5 min of each analysis. The repeatability is expressed
as the standard deviation (1σ) of the 10 injections, and the
results are presented in Table 3.
The STR is approximately the same for all instruments (≈ 0.02 ppb).
Only the FTIR and DFG instruments show higher STR of 0.09 and 0.17 ppb,
respectively. Part of the difference between the FTIR and DFG and the other
instruments can be explained with the CMR, as the FTIR and DFG are the least
precise instruments for small averaging time (1 to 5 min). As a
consequence, when measuring calibration gases, FTIR and DFG
owners would need to increase the measurement time to 20 to 30 min and
then keep the last 10 min to reach a better STR.
Long-term repeatability
The LTR assessment tests quantify the stability of
an analyzer over periods of several days. For each instrument, a target gas
was measured regularly (at least twice a day) alternating with ambient air
for several days in our temperature-controlled laboratory. Depending on the
instrument type and the test period, the target measurements were performed
for a period of 20 min for the instruments that were compared during the
first campaign, 30 min for the second and third campaigns and 40 min for
the FTIR due to its cell size, which needs more time for the stabilization
of the physical parameters. For all instruments, a mean value was calculated
over the last 5 min of each analysis. A calibration was performed every
week or 14 days and was applied as a drift correction, with a linear
interpolation between the bracketing calibrations. Table 4 shows the
standard deviation (1σ) over 30 measurements for all tested
instruments.
The two instruments showing the best LTR are the FTIR and CRDS with a
standard deviation of 0.07 ppb. They are the only two instruments that can
reach the compatibility goal recommended by the WMO. The QC-TILDAS, with a
precision of 0.14 ppb, is just above the recommendations, but the instrument
tested had no pressure control and its pressure needed regular adjustment
during this test. We can expect an improvement of the LTR for the QC-TILDAS
when using a pressure controller. The three ICOS-QCL instruments and the DFG
instrument present a LTR between 0.21 and 0.32 ppb. To meet the WMO
recommendations, the calibration frequency may need to be increased to one
to several calibrations per week. To test this point, the ICOS-EP40 was
re-tested from November to December 2014. During this period, a sequence of
analysis of 1 h of air alternated with 15 min of a target gas was
used. The target gas measurements were separated into two data sets. One was
used as a target gas, and the other was used as a calibration gas, to correct the first data set as a one-point calibration. Different LTRs
were calculated by choosing different frequencies for the calibration
data set. Without any calibrations, the LTR was 0.85 ppb (over 3 weeks),
and with a calibration every 2 days, the LTR was 0.28 ppb. For a calibration
every 12 h, the LTR improved to 0.07 ppb, and for every 2.5 h (one target gas alternated with one calibration gas), the LTR reached
0.03 ppb. Thus, to reach a LTR better than 0.10 ppb for the ICOS-EP40, a
calibration frequency of twice a day is necessary.
Long-term drifts: the drifts between two consecutive
calibrations from the same calibration set are normalized over a time span
of 10 days. The drifts from all consecutive calibrations are then averaged
to obtain a mean drift for all analyzers.
Instrument
Mean drift for
Highest
10 days (ppb)
drift (ppb)
FTIR
0.12
0.23
CRDS
0.07
0.19
DFG
1.02
2.53
ICOS-SD
0.30
0.71
ICOS-EP38
0.76
1.62
ICOS-EP40
0.31
1.08
QC-TILDAS
0.12
0.16
Linearity assessment
Linearity tests for each instrument are plotted in
individual graphs. The upper panel presents the difference between the
certified values of the calibration scale and the values measured for all
the calibrations made during the tests. The cylinders from the MPI scale are
represented with circles, and those from the DS scale are represented by
triangles. Squares for the fourth calibration set are only used once by the
QC-TILDAS. The lower panel presents the residuals from the fit. The color
code for the calibration dates is given in the legend.
For each instrument, linearity assessments were made using calibration tanks
with known N2O mole fractions. As explained in Sect. 3.1, three
calibration sets of four to six different tanks were used during the
campaigns. The mole fraction measured by the instrument compared with the
assigned mole fraction was used to assess the linearity of the instrument.
The linearity assessment for each instrument is displayed in Fig. 2.
All of the analyzers show a linear response curve, which can be described by
a linear fit using several calibration cylinders. To reduce the errors in
the assessment of the calibration cylinders, we recommend the use of at
least three calibration gases, spanning the full atmospheric range. During our
linearity assessment, we looked at the deviation of individual tanks from
the fit curve (Fig. 2, lower panel for each instrument) and used this as
a measure of the linearity of an analyzer. We found typical residuals of up
to ±0.15 ppb for the FTIR, ICOS-QCL, and DFG and ±0.05 for the
CRDS and QC-TILDAS analyzers.
The linear fit of the differences between assigned values minus measured
values plotted against the assigned value (upper panel) show different
slopes, depending on the instrument. Even with the same analyzer model, such
as the ICOS-EP38 and ICOS-EP40, the slopes differ considerably. From the
time evolution of the linear fit function, we can extract further
information about the long-term stability and calibration frequency needed.
For each calibration cylinder, we measure the drift between the consecutive
calibration runs. Then these drifts per day are normalized to drifts per 10
days. Finally we average the drifts from all calibration cylinders to
extract the mean and maximum drift (Table 5). Overall, this study confirms
the results from the much shorter 30 h test presented in Table 2. The
FTIR, CRDS and QC-TILDAS show a mean drift of approximately 0.1 ppb per 10
days, which justifies a calibration frequency of 10–14 days. The ICOS-SD,
ICOS-Eps and DFG show a mean drift over 10 days between 0.3 and 1 ppb, which
suggests that they should be calibrated at least every 3 days or daily
to obtain an equivalent correction of the drift.
Stabilization time: time necessary to reach the final
value (calculated over the last 5 min of an analysis) at either ±0.1
or 2σ ppb (from CMR test; 1 min value) of the final value. The
stabilization time is averaged over at least 24 injections of cylinders from
calibration sets.
±0.1 ppb of final value
±2σ of final value
Instrument
Stab. time
Not reached*
2σ
Stab. time
(min)
(%)
(ppb)
(min)
FTIR
–
70
0.298
10 ± 7
CRDS
11 ± 5
6
0.11
10 ± 6
DFG
17 ± 2
9
0.318
2 ± 3
ICOS-SD
2 ± 1
0
0.248
1 ± 1
ICOS-EP38
2 ± 2
0
0.128
2 ± 2
ICOS-EP40
2 ± 1
0
0.136
2 ± 0
QC-TILDAS
1 ± 1
0
0.125
1 ± 0
* The “not reached” value is the percent of runs that did
not reach ±0.1 ppb of the final value.
Stabilization time
Another important parameter is the time necessary for the instrument to
reach a stable value when changing the sample analyzed. This test is made by
using the calibration runs. The calibration sets the N2O mole fraction
differences between the different samples ranging from 3 to 16 ppb. For each
analysis of a calibration cylinder, the raw data are first averaged over
1 min intervals, and the final values are calculated by averaging the
last 5 min of a 15 to 20 min sequence. We estimated the
stabilization time by examining the time from which all the 1 min
averaged data stay within ±0.1 ppb or ±2σ ppb (see CMR
test for 1 min averaged data, Table 2) of the final value. For all
instruments, the inlet system consisted in pressure regulators (SCOTT MODEL
14 M-14C, nickel-plated brass) installed on each cylinder, connected to a
Valco multi-port valve (VICI) using 2 to 4 m of either
1/4 in. OD Synflex 1300 (EATON) tubing for the FTIR and QC-TILDAS
or 1/16 in. OD stainless steel tubing for the other instruments. A short
length of similar tubing was used to connect the Valco valve to the inlet of
the instruments. It should be noted that such an inlet system did not impact
the time of stabilization as there are nearly no dead volumes and the volume
to flush (mainly the tubing) is not significant in regards to of the flow
rates (short residence time). The stabilization time is a function of the
cell volume and design, dead volume and sample flow rate. The results found
in this study are only valid for the sample flow rates that were considered
and for our inlet systems. Other inlet systems should be mindful of any
possible dead volumes or the influence of the tubing length. We used the
flow rates recommended by the manufacturers, which are documented in Table 1. The amplitude of the concentration change compared to the sample analyzed
previously could also influence the stabilization time, but despite
concentration changes ranging from 3 to 16 ppb no correlation was found
between the two. The values in Table 6 are the stabilization times of all
the instruments obtained by averaging the stabilization times calculated for
at least 24 cylinder runs.
When choosing 0.1 ppb as the criterion for reaching stabilization, the
instruments can be classified into two categories: in the first category,
the stabilization is reached after 1 to 2 min (ICOS and QC-TILDAS); in the second category, the stabilization is reached later or never (FTIR, CRDS and DFG). These last results can be easily
explained by the CMR test (Table 2) because for some instruments, the
±0.1 ppb criterion cannot be reached for 1 min averaged data. To
make a meaningful comparison, a criterion of ±2σ ppb of the
final value was chosen. In this case, the ICOS, DFG and QC-TILDAS
instruments rapidly reach the final values (under 3 min), but the
FTIR and CRDS instruments require much more time to achieve stabilization
(more than 10 min). As a consequence, instrument owners should be
mindful of the time required to reach stabilization to keep only the
relevant data.
Temperature dependence
All tests and measurements described previously were performed in a
laboratory with temperature variations of less than ±1 ∘C.
However, the working conditions at stations where the analyzers will be
installed might not always be as stable. Temperature-dependence tests were
conducted to characterize the sensitivity of the instruments to room
temperature variations. While continuously measuring a target tank, the
temperature of the laboratory was changed. From the laboratory working
conditions (22 ± 1 ∘C), the temperature was varied between
a low temperature (15 to 20 ∘C) and a high temperature (28 to 35 ∘C) before returning to the normal working temperature. The low
and high temperatures were maintained for several hours to allow for
stabilization. Depending on the season or time period when the instrument
was tested, the span of the variation differs between 10 to
17 ∘C. Due to the high gas consumption of the FTIR, the target tank measurements for this
instrument were analyzed not continuously but rather
every 6 h at different temperatures for 3 days, and the last 7 min
of each measurement were kept for this test.
The results of the temperature-dependence test for
each instrument tested. The top panel presents the time series of
concentration (black), the room temperature in the laboratory (orange) and
the temperature in the cell (red). For some instruments, the temperature in
the cell was multiplied by either 10 or 100 to make the variations visible
on the same scale as the room temperature (right axis). The concentration of
N2O is plotted against the room temperature in the lower panel. On the
right of the lower panel, I1 is the slope, I0 is the intercept and R2
is the coefficient of determination of the linear regression.
In Fig. 3, a two-panel plot for each instrument is presented to describe
the N2O response to the room temperature changes. Table 7 summarizes
these results with the room temperature change applied to the instrument,
the type of temperature dependence and its slope when a linear dependence
was found.
Most of the instruments show a significant sensitivity to room temperature
variations. Only the QC-TILDAS instrument and the ICOS-EP38 do not show
significant temperature dependence for N2O, with variation below 0.1 ppb for five degree variations. It should be noted that for the QC-TILDAS
test, the high-frequency variations of N2O at the beginning and near
the end are due to pressure variations inside the cell. The FTIR and CRDS
instruments show a linear dependence to the temperature of -0.04 and
0.05 ppb per ∘C, respectively. The CRDS instrument tested was a
prototype, and thus no correction for temperature was applied at this
stage. Such correction is now built in the commercialized version, and in
May 2014 we had the opportunity to test a newly purchased CRDS analyzer
with temperature correction in the MLab. It shows an improved behavior to
room temperature changes, with a sensitivity to temperature of less than
0.02 ppb of N2O per ∘C. The DFG instrument presents a
dependence that is not significant compared with the relatively large noise.
A larger temperature sensitivity was found for the ICOS-SD with
approximately 2 ppb N2O changes (peak to peak), but the nonlinear
relationship makes it impossible to apply a correction. The ICOS-EP model
does improve the temperature control compared with the standard model, but
it is important to highlight the difference between the instruments:
although instrument ICOS-EP38 presents no significant temperature influence,
instrument ICOS-EP40 shows a temperature dependence of 0.07 ppb N2O per
∘C. To reach the best attainable performance, most instruments
need a temperature-controlled environment, especially the FTIR and ICOS-SD.
If an instrument presents a linear dependence, it is also possible for the
user to add an instrumental specific correction that could be applied to the
final data. In this case, the room temperature needs to be monitored
precisely, and the temperature dependence needs to be determined accurately
by repeating the temperature test two to three times.
Water vapor correction for all instruments except the
FTIR. All the data were averaged over 30 s and separated into bins of 0.05 % of H2O. The dashed lines represent ±0.1 ppb of the dry
value. On the right-hand side of the panels, I0, I1 and I2 are the
coefficients, and R2 is the coefficient of determination of the polynomial regression.
Water vapor correction
Water vapor in the atmosphere can vary from a few ppm to several percent of
volume. Usually, the N2O measurements are presented as a dry mole
fraction, and a drying system is needed for ambient air measurement. Several
of the instruments tested provide water vapor measurements and a correction
function to transfer wet ambient air measurements to the dry mole fraction.
This correction accounts for dilution and spectroscopic effects such as
pressure broadening (Chen et al., 2013). In this study, we test the water
vapor correction applied by the manufacturer of the different instruments.
This test was not performed for the FTIR and GC because the FTIR has its own
built-in drying system, which removes the water vapor to 2–4 ppm, and because
the GC is required to measure dry air only.
Influence of room temperature on N2O.
Instrument
Temperature
Temperature
Temperature
Peak to peak
dependence
range (∘C)
dependence
(ppb)
(ppb ∘C-1)
FTIR
Linear
17 to 34
-0.04
0.84
CRDS
Linear
20 to 31
+0.05
0.73
DFG
Linear
17 to 30
-0.02
1.33
ICOS-SD
No linear dependence
18 to 28
NA*
2.70
ICOS-EP38
No significant dependence
17.5 to 32
NA*
0.60
ICOS-EP40
Linear
17.5 to 32
+0.07
1.11
QC-TILDAS
No significant dependence
15 to 30
NA*
1.19
* Denotes cases where it was not possible to give a value because either there
was no dependence or it was not linear.
This test consists of measuring a high-pressure tank filled with dry natural
air and then injecting a droplet of Milli-Q water (0.2 mL) on a hygroscopic
filter (M&C LB1SS) to humidify the stream. This water droplet humidifies
the gas at approximately 3 % vol of water depending of the room
temperature and sample pressure. After this, the dry natural air from the
high pressure tank dries slowly the filter (droplet evaporation). With this
method, the tanks of dry ambient air were humidified at varying levels, up
to 2–3 % vol of water vapor. However, this method, although easy to
implement, does not offer a steady drying rate over all the H2O range,
resulting in few measurement data over part of the H2O range. In order
to get a better statistical weight on these H2O range parts, the method
is repeated at least three times for all of the instruments. The assessment
of the water vapor correction is made by comparing the values of the wet
target found by the instrument to its dry value. When the QC-TILDAS and the
commercialized model of the CRDS were tested, a new method to characterize
the water vapor correction had been implemented by the MLab. A humidifying
bench is composed of one thermal mass flow controller (F-201CV, Bronkhorst),
to regulate the flow of a tank filled with dry natural air, one liquid mass
flow controller (Mini Cori-Flow M12, Bronkhorst), to regulate the quantity
of Milli-Q water injected in the sample line, and one controlled evaporator
mixer (Bronkhorst) to humidify the target gas by evaporating the water at
40 ∘C while mixing it with the gas. This setup enables a precise
control of the water vapor percentage in the sample analyzed. The target gas
can now be humidified at different H2O levels (up to 5 % vol of water
vapor) with a suitable stability (H2O standard deviation of 100 ppm) as
long as required, allowing long data set averaging and thus improving the
representativeness of the results, especially for noisy analyzers.
The manufacturers Picarro, Los Gatos, Thermo Ficher and Aerodyne provided a
water vapor correction for their instruments. The correction was not yet
implemented in the CRDS prototype tested; therefore, we did not include the
results for this instrument. Figure 4 shows the difference between water
vapor corrected and the dry N2O mole fraction against the concentration
of H2O for each instrument. All data were averaged in 30 s intervals.
This test demonstrates the difficulty of most analyzers to provide a correct
water vapor correction when measuring wet air. Of all of the instruments,
only the QC-TILDAS supplies an accurate water vapor correction: its
corrected wet measurements of N2O did not exceed 0.1 ppb compared with
the dry mole fraction. The ICOS-SD supplies a correction that results in a
N2O difference to the dry value below 0.2 ppb for H2O not
exceeding 1–1.5 % vol. For higher H2O values, the correction
shows larger differences of up to 2 ppb. The two ICOS-EP corrections are not
sufficient, with a N2O difference to the dry value of up to 0.5 ppb for
high water vapor concentrations. The DFG instrument correction is clearly
not suitable, with a difference in the N2O's dry/wet values between
-1.0 and +2.0 ppb. Finally, the commercialized version of the CRDS
supplies a correction that results, similar to the ICOS-SD, in a N2O
difference to the dry value below 0.2 ppb for H2O not exceeding 1 %
vol. However, for higher values of water vapor, the N2O difference
increases to 1.5 ppb. As a result, to achieve the best performances for
high-precision atmospheric N2O measurements, most instruments will need
a drying system prior to the inlet or a careful evaluation of the water
vapor dependence, with the exception of the FTIR, which has a built-in drying
system. While the QC-TILDAS tested here showed a good water correction,
users of this instrument should still test the water correction.
Air comparison between the FTIR and the GC (1 h
averaged data) during the time when the air comparison tests were performed
with the other instruments. The top panel presents the times series of the
GC (red) and the FTIR (black) for the three periods (December 2012, May 2013 and
January 2014). In grey is the difference between the instruments. The colored
frames show the time periods chosen to conduct the air comparison between
the FTIR and the other instruments (see Fig. 6). In the lower panel, three
histograms give the difference for the two instruments for the (a) first test
period, (b) second test period and (c) third test period.
Here, we can only recommend using a drying system for high-precision
N2O measurements with all of the instruments tested. However, if some
stations or laboratories are sufficiently equipped to make their own
instrument-specific water vapor dependency test on a regular basis, wet air
measurements could then be performed.
Ambient air measurement comparisons
All of the instruments that were tested measured at least 100 h of
ambient air during the testing period. The measurements were made at the
MLab, as described in Sect. 3.1. Pumps were used to reduce the residence
time in the air line to avoid time differences between the measurements of
the different instruments. For all instruments, the measurements were hourly
averaged to allow for meaningful comparisons and to reduce the influence of
short time variations. For the three test periods, the GC and the FTIR were
the only instruments that were always present. Figure 5 presents the
comparison between these instruments over the three periods. It can be
observed that although the N2O mole fraction ranged from 325 to 338 ppb, most of the peaks were less than 2 to 3 ppb in height. Although the
mean difference between the instruments is different in each period (-0.21 ppb for the first, 0.01 ppb for the second and 0.14 ppb for the third), it
was constant during each period, with a standard deviation between 0.26 and
0.37 ppb. Because the FTIR showed a smaller standard deviation than the GC
during these periods, it was chosen as the reference instrument for the
comparison with all of the other instruments.
Comparison between the FTIR and the other
instruments: CRDS, DFG, ICOS-SD, ICOS-EP38, ICOS-EP40 and QC-TILDAS (1 h
averaged data). The top panels present the times series of each instrument
(red) and the FTIR (black). In grey is the difference between the
instruments compared. In the lower panels, histograms give the difference
for each comparison. All comparisons were conducted using 100 continuous
hour-averaged air measurements. All data have been automatically corrected
for water vapor using the manufacturer correction.
During the first test period (CRDS, ICOS-SD and DFG), a water trap was used
to dry the air (see Sect. 3.1.), and the dry air measurements were then compared.
During the second period (ICOS-EP38 and ICOS-EP40), there was not enough
common dry air data for the ICOS-Eps and the FTIR to conduct the comparison.
Therefore, we were only able to compare the wet air measurements, which were
corrected for the water vapor by the correction algorithms provided by the
manufacturers (between 0.7 and 1.6 % of water vapor during the period).
For the third period (QC-TILDAS), the instrument only measured wet air, so
the comparison was conducted on the values of the QC-TILDAS with the
manufacturer's water vapor corrections applied (1 % of water vapor or less
during the period). For all comparisons, the 100 h periods were chosen
among the most stable consecutive data available (see Fig. 5 for the
period chosen for each instrument). For all instruments measuring wet air,
we attempted to apply the corrections obtained from the water vapor test to
the dry air values (Sect. 3.8.); however, it either had no effect (QC-TILDAS) or
did not improve the comparison (ICOS-EP). Thus, for all of these
instruments, the air comparison was performed with the dry values given by
the instrument. The data were calibrated by doing an interpolation between
the calibration before and after the comparison period.
Figure 6 presents the relative difference histograms for each instrument
compared with the FTIR. Of the six instruments that were compared with the
FTIR, the ICOS-SD, the ICOS-EP40 and the QC-TILDAS show an offset of the
mean difference with the FTIR of more than 0.10 ppb, whereas the other three
instruments show an offset smaller than 0.05 ppb. However, as observed
previously, there is an offset between the FTIR and GC for the first and
third periods (0.21 and 0.14 ppb). When conducting the comparison with
the GC, the offset with the QC-TILDAS improved to 0.12 ppb, but compared
with the ICOS-SD, CRDS and DFG, the offset increased to 0.25 to 0.38 ppb. As
discussed in Sect. 3.5, increasing the frequency of the calibrations should
decrease the offset for the DFG and the ICOS-EP38. For the QC-TILDAS, the
calibration frequency was once every month, which should be increased to
once every week or 2 weeks for the QC-TILDAS, as indicated by the small
drift time (see Sect. 3.5).
As observed with the different standard deviations in Fig. 6, the CRDS and
the two ICOS-EPs are the instruments that show the best correlation with the
FTIR. For the comparison with the QC-TILDAS, the standard deviation of 0.16 ppb cannot be explained by the drift (only 0.05 ppb for 100 h). The lack
of good pressure control is probably what caused this value. Finally, for
the DFG and the ICOS-SD, the standard deviations are the highest of all
instruments. This is easily explained by the high variability these
instruments have shown, and a calibration every 8–9 days is clearly not
sufficient. Once again, we see the importance of choosing a calibration
frequency adapted to each instrument and its absolute necessity when trying
to compare air measurements from different instruments or, on a larger
scale, networks.
Conclusions
A new standardized protocol to evaluate the performances of trace gases
analyzers was implemented at the ICOS/ATC metrological laboratory in
Gif-sur-Yvette. Yver Kwok et al. (2015) described
the different tests performed for each instrument and showed examples for 47
CO2 analyzers. Because our study, which was dedicated to the evaluation
of N2O analyzers for high-precision atmospheric measurement, was
conducted between October 2012 and December 2014, the experimental protocols
were not fully finalized and have since been continuously improved due to
gains in experience. Though not all of the analyzers were tested in the
exact same way, the tests performed do not differ sufficiently to make
meaningful conclusions impossible.
Most of the analyzers showed a clear dependency to the room temperature.
This needs further investigation and technical improvements by the
manufacturers. As long as the room temperature is still an issue, the
N2O analyzers should be used in an air-conditioned environment, and the
room temperature should be monitored to assess its evolution and the
validity of the measurements. All of the tests demonstrated that the water
vapor correction functions provided by the manufacturer are not sufficient
to analyze wet ambient air. Therefore, we recommend that for high-precision
atmospheric measurements, ambient air should be dried prior to the analysis.
During our initial tests, the calibration strategy was driven too much by
our experiences from CO2 and CH4 analyzers and the wish to have a
similar performance for N2O. With a calibration performed only every
14–21 days (Yver Kwok et al., 2015), some of the
tested N2O analyzers show a significant drift, which cannot be
corrected. In the case of the ICOS-EP40, we tested for possible improvement
when adding a fifth reference cylinder, which is used to correct for
short-term drift. In our case, an injection frequency of 11 h for a
reference gas led to an improvement of the short-term repeatability of the
target gas from 0.85 to 0.07 ppb. Thus, prior to the use of an analyzer,
the calibration strategy should be studied and optimized for the instrument
and station conditions.
This study of seven analyzers shows that new optical techniques have the
potential to replace the gas chromatographic techniques, which were widely
used over the past 20 years for atmospheric measurements of N2O. These
new techniques require much less maintenance at the stations and have lower
operational costs because they do not need consumables, such as carrier gas.
It should be noted that, while we studied only whole N2O without
consideration of possible variations in isotopic composition, all these
optical techniques are sensitive to some degree to isotopic composition and
this dependence has not been assessed. Independent analyses of individual
isotopologues are also not assessed here. Users should be mindful of possible
isotopic dependences until further studies have been made. Achieving the WMO
recommendation for N2O network compatibility of 0.1 ppb is still
challenging but is absolutely needed to characterize the small variability
at continental or coastal stations. This can only be reached at the moment
if the above described recommendations are followed.