AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-10-3273-2017Multi-year comparisons of ground-based and space-borne Fourier transform spectrometers in the high Arctic between 2006 and 2013GriffinDeborahttps://orcid.org/0000-0002-4849-9125WalkerKaley A.kaley.walker@utoronto.cahttps://orcid.org/0000-0003-3420-9454ConwayStephanieKolonjariFeliciaStrongKimberlyhttps://orcid.org/0000-0001-9947-1053BatchelorRebeccaBooneChris D.DanLinDrummondJames R.FogalPierre F.FuDejianhttps://orcid.org/0000-0001-5205-0059LindenmaierRodicaManneyGloria L.WeaverDanhttps://orcid.org/0000-0002-5400-5099Department of Physics, University of Toronto, Toronto, Ontario, M5S 1A7, CanadaDepartment of Chemistry, University of Waterloo, Waterloo, Ontario, N2L 3G1, CanadaDepartment of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, B3H 1Z9, CanadaNorthWest Research Associates, Socorro, New Mexico, USADepartment of Physics, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USAnow at: UCAR Center for Science Education, University Corporation for Atmospheric Research, Boulder, Colorado 80301, USAnow at: Jet Propulsion Laboratory/California Institute of Technology, Pasadena, California 91109, USAnow at: Pacific Northwest National Laboratory, Richland, Washington 99352, USAKaley A. Walker (kaley.walker@utoronto.ca)8September20171093273329418August201619September201629April201723May2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/10/3273/2017/amt-10-3273-2017.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/10/3273/2017/amt-10-3273-2017.pdf
This paper presents 8 years (2006–2013) of measurements
obtained from Fourier transform spectrometers (FTSs) in the high Arctic at
the Polar Environment Atmospheric Research Laboratory (PEARL;
80.05∘ N, 86.42∘ W). These measurements were
taken as part of the Canadian Arctic ACE (Atmospheric Chemistry Experiment)
validation campaigns that have been carried out since 2004 during the polar
sunrise period (from mid-February to mid-April). Each spring, two
ground-based FTSs were used to measure total and partial columns of
HF, O3, and trace gases that impact O3 depletion,
namely, HCl and HNO3. Additionally, some tropospheric
greenhouse gases and pollutant species were measured, namely CH4,
N2O, CO, and C2H6. During the same time period, the
satellite-based ACE-FTS made measurements near Eureka and provided profiles
of the same trace gases. Comparisons have been carried out between the
measurements from the Portable Atmospheric Research Interferometric Spectrometer
for the InfraRed (PARIS-IR) and the co-located high-resolution Bruker 125HR
FTS, as well as with the latest version of the ACE-FTS retrievals (v3.5). The
total column comparison between the two co-located ground-based FTSs,
PARIS-IR and Bruker 125HR, found very good agreement for most of these
species (except HF), with differences well below the estimated
uncertainties (≤6%) and with high correlations (R≥0.8). Partial
columns have been used for the ground-based to space-borne comparison, with
coincident measurements selected based on time, distance, and scaled potential
vorticity (sPV). The comparisons of the ground-based measurements with
ACE-FTS show good agreement in the partial columns for most species within
6 % (except for C2H6 and PARIS-IR HF), which is
consistent with the total retrieval uncertainty of the ground-based
instruments. The correlation coefficients (R) of the partial column
comparisons for all eight species range from approximately 0.75 to 0.95. The
comparisons show no notable increases of the mean differences over these
8 years, indicating the consistency of these datasets and suggesting that
the space-borne ACE-FTS measurements have been stable over this period. In
addition, changes in the amounts of these trace gases during springtime
between 2006 and 2013 are presented and discussed. Increased O3
(0.9%yr-1), HCl (1.7%yr-1),
HF (3.8%yr-1), CH4
(0.5 % yr-1), and C2H6 (2.3%yr-1,
2009–2013) have been found with the PARIS-IR dataset, the longer of the two
ground-based records.
Introduction
Ground-based instruments provide valuable datasets for the
validation of satellite-remote sensing instruments e.g.,. Regular validation of satellite instruments and their retrieval
algorithms is necessary to assess the long-term stability of the measurements
as well as the consistency of these datasets. As such, continuing validation
of the space-borne Atmospheric Chemistry Experiment Fourier Transform
Spectrometer (ACE-FTS) is essential to support its now over 10-year data
record. ACE-FTS started routine measurements in February 2004, followed
quickly by the first of the Canadian Arctic ACE validation campaigns which
continue to this day. These campaigns e.g., comprise ground-based measurements during the
polar sunrise period (from the end of February to early April) at the Polar
Environment Atmospheric Research Laboratory (PEARL) near Eureka, Nunavut
, at approximately 80∘ N, 86∘ W. Two
ground-based Fourier transform spectrometers (FTSs), the Portable Atmospheric
Research Interferometric Spectrometer for the InfraRed (PARIS-IR), and the
high-resolution Bruker 125HR FTS are part of these campaigns. Here, these
datasets are used to compare multiple trace gas species to the space-borne
ACE-FTS v3.5 retrievals between 2006 and 2013 . These
ground-based FTS datasets extend over a long time period and capture many
species, thus contributing to the ongoing validation of the satellite-based
instrument and helping assess whether ACE-FTS measurements have remained
consistent over the last decade.
These multi-year datasets can also help to quantify long-term changes in the
Arctic tropospheric and stratospheric composition due to natural processes
and anthropogenic emissions. Furthermore, the remote location of PEARL means
there are few local pollutant sources, which helps in interpreting these
changes in a global context without the influence of local contributions. The
measurement period of these campaigns, i.e., the polar sunrise period, is of
importance because it is a time during which chemical ozone depletion can
occur. It is also a time that is dominated by highly variable dynamical
conditions due to the polar vortex. The polar vortex is a large-scale cyclone
(low-pressure system) that extends from the upper troposphere to the
stratosphere. It forms in the winter and generally dissipates between late
March and early April as the solar radiation increases . Trace gas
amounts inside and outside the polar vortex are significantly different
e.g.,. For a strong polar vortex, the vortex core is
an isolated air mass and mixing with midlatitude air only occurs around the
outer edge. This leads to strong trace gas gradients across the edge of the
polar vortex e.g.,. As such, it is important
to consider the differences in the instrument viewing geometries with respect
to the location of the polar vortex when comparing the measurements.
Employing a criterion ensuring that similar air masses are considered is
crucial for instrument comparisons in the high Arctic, especially in the
springtime when the polar vortex is at its strongest .
Herein, we focus on the analysis of retrieved partial and total column
values, derived from infrared FTS spectra, for O3 together with
several molecules important in catalytic O3 destruction. These trace
gases are O3, HCl, and HNO3e.g.,. Also retrieved is HF, which is a
stratospheric tracer for dynamics e.g.,. Additionally,
total columns of primarily tropospheric CH4, N2O, CO,
and C2H6 have been measured. All of these trace gases are routinely
derived from the Bruker 125HR spectra and have been used in numerous studies
e.g.,. Retrievals
of O3, HCl, HNO3, HF, N2O, CO,
and C2H6 from PARIS-IR's spectra have also been published and
compared to other instruments in previous studies e.g.,. The PARIS-IR CH4
columns are presented for the first time in this study.
The earlier ACE-FTS retrieval version, ACE-FTS v2.2+updates, of these trace
gases has previously been validated for most species discussed in this study
(O3 in ; HCl in ; HNO3 in ; HF in
; CH4 in ; N2O in ; CO in
). In these studies, partial column comparisons between
ground-based FTSs and ACE-FTS in the Arctic typically show larger differences
than comparisons at lower latitudes. The inclusion of criteria ensuring that
similar air masses are sampled with respect to the polar vortex reduces this
difference seen in the Arctic significantly. Using these additional criteria,
and found much improved mean differences,
between the ACE-FTS (v2.2+updates) and ground-based FTS datasets, for
O3, HCl, HNO3, and HF comparisons by
approximately a factor of 2. These mean differences are comparable to the
typical differences at lower latitudes. While these validation papers have
all used the previous ACE-FTS data version, this study focuses on the ACE-FTS
v3.5 retrievals. The updates for v3.0/3.5 include new microwindows, updated
spectroscopic parameters, and improved temperature and pressure retrievals
. ACE-FTS v3.5 corrects for an error in a priori pressure and
temperature profiles that impacted v3.0 in the period after September 2010
. showed general improvements between
ACE-FTS v2.2 and ACE-FTS v3.0 across all baseline species (O3,
H2O, CH4, N2O, NO2, NO, HNO3,
HCl, HF, CO, CCl3F, CCl2F2,
N2O5, and ClONO2). Some studies have been published that
compare ACE-FTS v3.0/3.5 and ground-based FTS retrievals, including
O3 and NO2 by , several carbon containing
species (including CO and C2H6) by , and
CH4 by .
The aim of this study is to perform a detailed comparison between two
ground-based FTSs and ACE-FTS over multiple years and also to assess changes
in atmospheric composition above Eureka, utilizing 8 years (2006–2013) of
measurements during the polar sunrise period. A comprehensive comparison
between ACE-FTS v3.5 and ground-based FTS measurements of multiple trace
gases is provided, including measurements that were taken inside and outside
the polar vortex. The mean differences of the retrievals from the three FTSs
throughout this time period as well as interannual changes in the total or
partial column differences are determined. For the first time, the stability
of the ACE-FTS dataset is examined over an 8-year time period. For these
comparisons, we will use the same method and criteria for the viewing
geometry as and , which have been shown to
improve the comparison between ground- and satellite-based instruments in the
Arctic. Also, the interannual variability of the eight trace gases is
discussed and changes in the trace gas columns near Eureka are investigated
between 2006 and 2013.
This paper is organized as follows. Subsequent to this introduction, the
measurement site, the instruments, and the retrieval procedures used in this
study are described. The third section discusses the comparison methodology
and results of the ground-based intercomparisons between PARIS-IR and Bruker
125HR. The following section focuses on the methodology and results of the
ACE-FTS comparison results. The measurement series and trends from PARIS-IR
measurements are presented in the fifth section. This is followed by the
conclusions and highlights of our results.
Instrumentation and datasetsMeasurement site
The ground-based measurements were taken at the Canadian Network for the
Detection of Atmospheric Change (CANDAC), PEARL Ridge Laboratory
(80.05∘ N, 86.42∘ W; 610 m a.s.l.), in Eureka,
Nunavut . This laboratory is located 15 km away from the
Eureka Weather Station (79.98∘ N, 85.93∘ W; 0 m a.s.l.)
and over 400 km away from the closest settlement. This remote
location minimizes the influence of locally polluted air on the atmospheric
observations. It is also a good location for measuring Arctic springtime
ozone depletion, since the core of the polar vortex can be above Eureka,
which is only ∼ 1100 km from the North Pole.
Ground-based measurements at PEARL have been carried out as part of the
Canadian Arctic ACE validation campaigns during the polar sunrise period
(typically from late February to early April) since 2004. As part of this
campaign project, two ground-based FTSs, PARIS-IR and the CANDAC Bruker
125HR, were operated simultaneously during the 2007–2013 campaigns to measure
total as well as partial columns of the eight target species. These two
instruments are located side by side in the PEARL Ridge Laboratory and share
a solar beam from the same sun tracker installed on the roof above. During
the campaigns, the satellite-based ACE-FTS took routine measurements in the
high Arctic and provided profiles of over 30 trace gases. Details of these
instruments and their datasets are given below.
PARIS-IR
PARIS-IR is based on the design of the ACE-FTS
. It was built by ABB Inc. in 2003 and has been part of the
Canadian Arctic ACE validation campaigns since 2004. It records atmospheric
solar absorption spectra between 750 and 4400 cm-1 at a maximum
spectral resolution of 0.02 cm-1, equivalent to a maximum optical path
difference (MOPD) of ±25 cm. Since the 2006 campaign, the instrument
has been operated in a consistent way and at its maximum spectral resolution.
Interferograms are recorded using two liquid-nitrogen-cooled detectors,
mercury cadmium telluride (HgCdTe) and indium antimonide (InSb) detectors,
which are configured in a sandwich arrangement, and a zinc selenide (ZnSe)
beam splitter. The entire spectral range (750–4400 cm-1) is measured
simultaneously for each observation, since no narrow-band filters are used.
No apodization is applied to the spectra. Each measurement is recorded
approximately every 7 min and consists of 20 co-added spectra
. All eight species of interest are measured every
7 min throughout the campaign period, whenever there are favourable
weather conditions.
Bruker 125HR
The CANDAC Bruker 125HR is a high-resolution
ground-based FTS, operated to produce atmospheric solar absorption spectra.
During the sunlit period, it measures mid-infrared atmospheric solar
absorption between 600 and 4300 cm-1 at a maximum resolution of
0.0035 cm-1 (equivalent to a MOPD of 257 cm)
. It was installed at PEARL in July 2006 and is part of
the Network for the Detection of Atmospheric Composition Change (NDACC,
http://www.ndsc.ncep.noaa.gov/). These spectra are recorded with either a
HgCdTe or InSb detector using a potassium bromide (KBr) beam splitter. Seven
narrow-band filters are used and no apodization is applied to the spectra.
During each campaign, spectra were recorded approximately every
4–8 min and are comprised of either two or four co-added spectra.
Therefore, subject to favourable weather conditions and depending on the
filter sequence, each species is measured approximately every 30 min.
All eight species of interest are retrieved from the Bruker 125HR spectra.
Ground-based retrieval algorithm
The same retrieval algorithm has been utilized to estimate
the total column amounts of trace gases from the solar absorption spectra
recorded by both ground-based FTSs. The retrieval technique applied is based
on an optimal estimation method (OEM) . This
is an iterative process, wherein a calculated spectrum is fitted to the
observed one by adjusting the target trace gas profile. Single or multiple
microwindows, typically each with a width between 0.3 and
1.0 cm-1, are employed in the retrieval process.
Table lists the microwindows used and interfering trace gases
taken into account for the retrieval of each gas. These are consistent for
both instruments and follow the recommendations from
the InfraRed Working Group of NDACC (IRWG, http://www.acom.ucar.edu/irwg/).
Example of a total column averaging kernel (AVK, solid lines) and
sensitivity (dashed-dotted lines) of the retrieval are shown for each of the
eight species for PARIS-IR (red) and the Bruker 125HR (blue). If no
dashed-dotted line is shown in the figure, the sensitivity and total column
averaging kernel are too similar to distinguish the difference on the plot.
Summary of the parameters used for the eight trace gas retrievals
(retrieval microwindows and interfering species)
with an estimate of the total uncertainty of the retrieval (± in %),
degrees of freedom for signal (DOFS), and the root-mean-square (RMS) DOFS values (in %) used to filter the dataset of the
PARIS-IR and the Bruker 125HR retrievals. In each retrieval, a single or multiple microwindows are fitted
simultaneously as listed in the table below. For the calculation of
the total uncertainty and contributions, see the description given
in the text.
Atmospheric profiles have been retrieved from the spectra with the SFIT4
version 0.9.4.4 retrieval package
(https://wiki.ucar.edu/display/sfit4/Infrared+Working+Group+Retrieval+Code,+SFIT)
and the HIgh-resolution TRANsmission database (HITRAN) 2008 spectroscopic
database . Total column amounts were calculated within SFIT4
from the retrieved volume mixing ratio (VMR) profiles. These columns are
estimated by integrating the retrieved VMR profiles and the atmospheric
density between the ground and the top of the atmosphere for the total
columns, or over given altitude ranges specified for the partial columns. Due
to the lower spectral resolution of PARIS-IR compared to the Bruker 125HR,
two different altitude grids have been used for the retrieval. The retrievals
from PARIS-IR spectra have been performed on a 29-layer grid (from the ground
(0.61 km) to
100 km) and those for the Bruker 125HR on a 47-layer grid (from the
ground (0.61 km) to
120 km). It has been shown using SFIT2 that this prevents
non-physical oscillations in the retrieved profile for the lower-resolution
FTS but only results in a very small difference in the total columns
(between 0.1 and 0.6 % depending on the retrieved gas) .
We have confirmed that this is still valid for SFIT4 by testing the retrieval
on both 29-layer and 47-layer grids. For the retrieval of CH4 from
the Bruker 125HR spectra, the retrieval strategy that incorporates a
first-order Tikhonov constraint to the state vector as recommended by
has been used to prevent non-physical oscillations of the
retrieved profiles. This was not necessary for the PARIS-IR CH4
retrieval since no oscillations occurred in the retrieved profiles with the
standard retrieval technique.
The retrieval algorithm requires input meteorological parameters, which are
used in the radiative transfer calculation. Daily pressure and temperature
profiles (versus altitude) are calculated from National Centers for
Environmental Prediction (NCEP,
ftp://ftp.cpc.ncep.noaa.gov/ndacc/ncep/) profiles interpolated to PEARL
and are used to approximately 1.0 mbar (∼ 45 km). Above this
altitude, the monthly mean pressure and temperature profiles from the Whole
Atmosphere Chemistry Community Model WACCM,
https://www2.acd.ucar.edu/gcm/waccm; v6 for Eureka are used. The
inversion procedure of the OEM requires a priori information for the gases
involved in the retrieval (target and interfering species) to stabilize the
solution. This a priori knowledge refers to the VMR profile and its
variability. The a priori profiles used for the retrieval of trace gases are
the mean of a 40-year run (1980–2020) of WACCM v6 for Eureka, as recommended
by NDACC/IRWG. Only one single a priori profile for each species is used for
the entire retrieval of the dataset for both FTSs. This provides consistency
within the retrievals and ensures that variability in the dataset results
from the measurements. The forward model used in SFIT is a radiative transfer
model that is utilized to generate a modelled absorption spectrum from this a
priori information based on the daily pressure and temperature information,
as well as the location of the measurement site.
The characterization of the information content of the retrieval is the
primary benefit of the OEM approach. The averaging kernel
A is a matrix that characterizes this information
. The total column averaging kernel (solid lines) and the
sensitivity (dashed-dotted lines) of the retrieval for each of the eight
species for PARIS-IR (red) and the Bruker 125HR (blue) are shown in Fig. . The total column averaging kernel is estimated by the sum of
the columns of A at each altitude, and the sensitivity equals the
sum of the rows of A at each altitude. The sensitivity presents
the fraction of the retrieved value that is derived from the measurement
rather than the a priori for a given altitude . A sensitivity
of 1 indicates that 100 % of the information at this altitude results
from the measurement. Note that in some cases no dashed-dotted line is
visible in the figure, since the sensitivity and total column averaging
kernel are very similar and the difference cannot be seen. It follows from
Fig. that the O3 retrieval is sensitive from the
surface to approximately 40 and 50 km for PARIS-IR and the Bruker
125HR, respectively. The retrieval of O3 and the following species
are primarily sensitive (with a sensitivity that is at least 0.1) in the
stratosphere in the range given (and are, therefore, referred to in the
following sections as “stratospheric species”): HCl from 10 to
40 km (60 km for the Bruker 125HR); HNO3 from 10 to
40 km; and HF from 10 to 40 km (50 km for the
Bruker 125HR). Retrievals for the other species (N2O, CH4,
CO, and C2H6) are mainly sensitive in the troposphere and
lower stratosphere and are referred to in this study as “tropospheric
species”. The retrieved columns of N2O and C2H6 for both
instruments are mainly sensitive from the surface to almost 30 and almost
20 km, respectively. The CO and CH4 retrievals from PARIS-IR
are primarily sensitive between the surface and approximately
20–30 km, with significantly smaller sensitivity in the stratosphere
than in the troposphere. The sensitivity for these species is different for
the retrievals from the Bruker 125HR, which are sensitive in the troposphere
as well as in the stratosphere up to approximately 40 and
80 km for CH4 and CO, respectively. The altitudes for
which the retrievals are most sensitive are later used (in
Sect. ) to determine the range of the partial columns for the
comparison between the ground-based and satellite-borne instruments. Because
a different retrieval technique has been used to determine the Bruker 125HR
CH4, the total column averaging kernel (as shown in
Fig. ) is 1 at all altitudes. The degrees of freedom for signal
(DOFS) are a measure of the vertical information of the retrieved profile. It
is defined as the trace of the averaging kernel matrix A. The DOFS of each retrieved species used in this study
can be found in Table . For quality assurance, a root-mean-square DOFS filter
(RMS/DOFS; see Table ) has been applied to the retrieved datasets, as
presented in .
The retrieval uncertainties are derived with SFIT4 by employing the method
described by and are listed in Table . The
total uncertainties (given in Table ) consist of the measurement
error, the uncertainties of the line width and line intensity parameters of
the retrieved trace gas from HITRAN 2008 (where values are
unavailable from HITRAN, 20 % has been used; see Table ),
and uncertainties caused by the temperature and solar zenith angle (SZA)
uncertainty (see Table ). The measurement error is based on
the signal-to-noise ratio of the observed spectra and is determined by SFIT4,
based on the algorithm described in and
. The SZA error is based on the average change in the SZA
during the time it takes to perform a measurement. The random temperature
error (Table ) is based on the difference between the NCEP
temperature profiles and “measured profiles” created by averaging the
twice-daily radiosonde profiles. The systematic temperature error is based on
the NCEP temperature error profile (Table ). The total
uncertainty has then been determined by adding all errors in quadrature. The
smoothing error is not included in the total error . The
average total uncertainty of each species is listed in Table .
The estimated uncertainty for HNO3 is significantly larger than that
of other species. This is due to the line intensity error and temperature
broadening being unavailable in HITRAN 2008. Therefore, we used empirically
estimated 20 % uncertainty for HNO3 line parameters in the
measurement uncertainty analysis. This creates a large systematic error for
HNO3 and results in a total error of 19 %.
Inputs used for the error budget to estimate the total retrieval
uncertainties. The line width error (discussed in the text) is the combined
uncertainty of the pressure and temperature broadening. The same
uncertainties have been used to estimate the errors for PARIS-IR and the
Bruker 125HR, with the exception of the SZA that is dependent on the
measurement duration for each FTS. Details can be found in the text.
SpeciesO3HClHNO3HFCH4CON2OC2H6Systematic error (fractional value) Line intensity0.050.0150.20.0350.0750.0350.0350.04Pressure broadening0.0350.0150.0750.0150.0750.0150.0350.04Temperature broadening0.0750.150.20.0150.150.0350.0750.04Temperature uncertaintybetween 0.49 and 1.44 K depending on altitude Random error Temperature uncertaintybetween 9.0 and 0.63 K depending on altitude SZA uncertainty from change over measurement0.075∘ (PARIS-IR) and 0.06∘ (Bruker 125HR) ACE-FTS
ACE-FTS was launched on board the Canadian satellite SCISAT on 12 August
2003. The satellite has a circular low-Earth orbit (650 km) with an
inclination of 74∘ and therefore measurements cover tropical,
midlatitude, and polar regions over the course of 1 year .
ACE is equipped with two instruments, ACE-FTS and the Measurement of Aerosol
Extinction in the Stratosphere and Troposphere Retrieved by Occultation
(MAESTRO) . Its mission goals include improving our
understanding of polar ozone chemistry; thus every year during the Arctic
sunrise period, ACE takes measurements over the high Arctic, near Eureka. The
observation technique is solar occultation at sunrise and sunset. This work
will focus on the FTS, which covers the spectral region between 750 and
4400 cm-1 with a spectral resolution of 0.02 cm-1
(which is identical to PARIS-IR). ACE-FTS has a vertical sampling of
1.5–6 km, varying with the orbit and, based on its field-of-view, a
vertical resolution of about 3–4 km. This is much
higher than the vertical resolution of both ground-based FTSs, which
typically retrieve partial or total columns with DOFS varying between 1 and
4.5 (see Table ). The retrievals from ACE-FTS infrared spectra
provide profiles for over 30 atmospheric
trace gases as well as the meteorological variables of temperature and
pressure .
In this study, the VMR as well as the temperature and pressure profiles are
taken from the latest version, ACE-FTS v3.5 . The retrievals
are based on the same global-fit method with a Levenberg–Marquardt nonlinear
least-squares fitting algorithm as used in the version 2.2+updates processing
described by . The range of the measurements is from the top
of the clouds to the top of the atmosphere (at approximately 150 km).
For clear-sky conditions the lower altitude range is approximately
5 km, depending on the season and location of the measurements.
Derived meteorological parameters
In the high Arctic, particularly during the highly variable Arctic
springtime, it is important to ensure that the air masses observed by the two
instruments being compared are similar. Therefore, characterizing the viewing
geometry with respect to polar vortex dynamics is essential to perform a
robust instrument comparison in the high Arctic. We use the scaled potential
vorticity sPV; as calculated by, as well as the
temperatures along the line of sight for each instrument to compare the
similarity of the air masses sampled by the instrument with respect to the
polar vortex. The ground-based line-of-sight calculations are described in
and the sampling of the meteorological fields in
. The sPV profiles along the line of sight have been derived
from GEOS version 5.2.0 analyses (GEOS-5) . This provides
information on whether measurements were taken outside or inside the polar
vortex. Within this study, the edge of the polar vortex is defined to be
between 1.2×10-4 and
1.6×10-4s-1, which is consistent with the discussion in
and with that used by and
.
Comparison between the two ground-based FTSsMethodology
Throughout the multi-year Canadian Arctic ACE validation campaigns, efforts
were made to provide the best possible conditions for instrument comparisons.
Thus, as previously mentioned, the two ground-based FTSs made coincident
measurements, sharing the same solar beam. During the Arctic sunrise period,
the SZA is quite large (typically ranging between 75 and 90∘) and
consequently the locations of the air masses sampled by the FTSs vary
significantly throughout the day with the changing SZA. For a meaningful
comparison, a temporal constraint is therefore necessary. We have chosen the
temporal difference between the measurements of the two FTSs to be less than
30 min to restrict the difference in distance along the line of sight to
approximately 50 km. Note that a stricter time constraint did not
lead to a better comparison between the two instruments. In the case when
more than one PARIS-IR observation matched the coincidence criterion (which
happens regularly since PARIS-IR observations are taken every 7 min), the
mean of all the coincident PARIS-IR measurements was used to compare with one
Bruker 125HR retrieval.
Comparison of PARIS-IR and the smoothed Bruker
125HR total columns for all trace gases in this study, averaged between 2007
and 2013. N is the number of coincident pairs involved in this calculation.
The third column (TC diff) represents the mean differences between the total
columns of the two FTSs (in %) along with the 1σ standard deviation
and the standard error (1σ/N; in brackets). The correlation
coefficient (R) and the slope of the regression plot (slope), along with
the uncertainty of the slope, are shown in columns 4 and 5, respectively.
PARIS-IR versus Bruker 125HR total columns for each of the trace
gases used in this study, showing the correlation before (cyan triangles) and
after smoothing (red dots). The line of best fit is shown as a thin grey line
for the unsmoothed total columns and as a thick black line for the smoothed
total columns. The dashed black line represents the one-to-one line as a
reference. Slopes and correlation coefficients are given in Table .
As previously mentioned, the observed trace gas amounts can vary considerably
depending on whether air masses are measured inside or outside the polar
vortex. Thus, we have additionally included a criterion that restricts the
difference of the sPV at 20 km along the line of sight of the two
instruments, so that the maximum difference cannot exceed
0.3×10-4s-1. Note that this criterion
is included as a precaution and a significant difference of the sPV for the
two instruments does not occur frequently within the maximum temporal
difference of 30 min.
As described in Sect. and , the
spectral resolution of the two FTSs is different. The instrument resolution
can affect the retrieved total columns, since the retrieval from measurements
with a higher-resolution instrument is typically less influenced by the a
priori profile and has larger DOFS . The retrievals from
PARIS-IR spectra therefore typically result in fewer DOFS than the retrievals
from the Bruker 125HR (see Table ). The different resolutions are
accounted for here by smoothing the VMR profiles following the method
described in . Accounting for the difference in vertical
resolution between the two instruments has been addressed in numerous
publications e.g.,. The smoothed profile
xsmooth is estimated by
xsmooth=xa+A⋅(xh-xa),
where the profile, xh, was retrieved by the spectrometer
having higher vertical resolution (Bruker 125HR), and is linearly
interpolated onto the lower-resolution instrument (PARIS-IR) retrieval grid
and smoothed with the PARIS-IR averaging kernel, A, and a priori
profile, xa. The total or partial columns for these
smoothed profiles have been calculated by integrating the smoothed VMR
profile, xsmooth, and the atmospheric density throughout
the altitude range. This is consistent with the total column calculation
method used within SFIT4.
Results and discussion
The results of the comparisons of the total columns between PARIS-IR and the
Bruker 125HR for all measurements satisfying the coincidence criteria (as
defined above) are shown in Table for the comparisons between
2007 and 2013. The total column differences were calculated as
([PARIS-Bruker]/[0.5 × (PARIS+Bruker)]) for individual pairs and
then averaged. Note that the Bruker 125HR was installed in Eureka during the
summer of 2006 and thus there are no coincident measurements for 2006.
Figure shows the correlation of the total column measurements
from the two instruments during the campaigns between 2007 and 2013. The
figure displays both smoothed (red dots) and unsmoothed (cyan triangles)
total columns for the Bruker 125HR and the slopes of each regression plot
(the regression analysis assumes errors on both axes). The black line is the
regression plot for the smoothed total columns and the thin grey line is that
for the unsmoothed total columns. The one-to-one correlation line is included as
a reference (black dashed line).
Mean differences for the stratospheric species estimated for each
year (late February to early April) between PARIS-IR and ACE-FTS (blue), the
Bruker 125HR and ACE-FTS (cyan), and PARIS-IR and the Bruker 125HR
using total columns (yellow) and partial columns (red) for the comparison.
The error bars display the standard error of the mean differences. The number
displayed below each bar represents the number of pairs. The combined
retrieval uncertainty from PARIS-IR and the Bruker 125HR is shown as a red
line. The lower panel illustrates the average sPV along the line of sight at
20 km of the pairs that are compared. The cyan solid and dashed lines
represent the inner and outer polar vortex edge, respectively.
Same as Fig. but for the tropospheric species.
For most species (O3, HNO3, CH4, and N2O), as
can be seen in Fig. , the differences between the Bruker 125HR
smoothed and unsmoothed total columns are very small and typically less than
1 %, a negligible amount compared to the total retrieval uncertainty (see
Table ). The difference between the HCl, CO, and
C2H6 is somewhat larger, between 3 and 4 %. The differences
between the smoothed and unsmoothed columns are relatively large
∼9% for HF, for which the total column retrievals from
PARIS-IR have DOFS of approximately 1, whereas the Bruker 125HR HF
retrievals have twice as many DOFS (see Table ). This suggests
that the PARIS-IR columns are more influenced by the a priori than the Bruker
125HR retrievals. Thus, for this species, it is important to consider the
different vertical resolutions of the retrieval from the two FTSs. Although
the differences between the smoothed and unsmoothed Bruker 125HR retrievals
are very small (less than 1 %) for O3, HNO3, CH4,
and N2O, an approach has been taken that utilizes only the smoothed
columns for the higher-resolution instrument to provide consistent analysis.
Therefore, only differences between PARIS-IR and the smoothed Bruker 125HR
total columns are discussed in the following.
The correlation is excellent for O3, HCl, HNO3, and
CO, with correlation coefficients R≥0.95 and the slopes of the
regression plot between 0.93 and 1.13 (N=685 to 1623) (see
Table ). This is also apparent in the mean differences, which are
all small (see Table ) compared to the combined retrieval
uncertainty (based on the total uncertainties of the retrieval from each
instrument that are added in quadrature; see Table ). These
combined retrieval uncertainties are ±6.1 % for O3,
±3.1 % for HCl, ±26.9 % for HNO3, and
±5.0 % for CO.
The comparison between the PARIS-IR and Bruker 125HR total columns is very
good for CH4, N2O, and C2H6, with mean differences
that are smaller than the combined retrieval uncertainty (±10.5 % for
CH4, ±5.1 % for N2O, ±6.7 % for
C2H6). However, PARIS-IR and Bruker 125HR total column retrievals of
CH4 and N2O are not very well correlated, with a correlation
coefficient R∼0.5. This low correlation is likely due to the lack of
variability observed compared to the retrieval uncertainty of the total
columns of these gases, as the total columns only vary by approximately
10 % around 3.5×1019 and 5.5×1018 molec cm-2 for CH4 and N2O, respectively.
This variation is within the combined total retrieval uncertainty for
CH4 and about half as much as the combined retrieval uncertainty of
N2O. It should be noted that the correlation is higher, with a
correlation coefficient R>0.85 for CH4 and N2O, if the
partial columns (using the same altitude range as for the PARIS-IR and
ACE-FTS comparison in the following section; see Table ) are
considered for this comparison since the variation of the partial columns is
higher than the total retrieval uncertainty. This is likely due to the fixed
altitudes of the partial columns (see Table ) and, as such,
these partial columns can be influenced by subsidence inside the polar
vortex.
The PARIS-IR and Bruker 125HR HF total columns have a high
correlation, but the slope of the regression plot (0.63±0.02)
suggests a negative bias between the HF datasets, which can be seen in
the mean difference of the total columns and is larger than the combined
retrieval uncertainty of the two datasets (±4.5 %), suggesting that
the PARIS-IR HF retrievals underestimate the amount of HF in
the atmosphere. This bias is mainly apparent for large HF amounts with
total columns greater than 3.0×1015moleccm-2.
The lower HF columns lie very close to the one-to-one correlation line.
This negative bias of the PARIS-IR HF retrieval has been seen
previously by , who compared it to another ground-based FTS in
Eureka (Environment Canada Bomem DA8). The absorption lines of HF are
quite narrow and the observation can be problematic with a ground-based
instrument like PARIS-IR due to its limited spectral resolution. Generally,
due to the limited DOFS of PARIS-IR's HF retrieval, the retrieved
columns tend to be closer to the a priori
(∼1.6×1015moleccm-2) and, therefore,
issues arise in retrieving high HF amounts. Relaxing the covariance
matrix constraint within the PARIS-IR retrieval resulted in oscillations of
the retrieved HF profile and was not able to resolve this issue.
Following this discussion of the mean differences for the entire dataset
between 2007 and 2013, next we focus on individual years during this time
period. Little variation of the differences was found and they were within
the combined retrieval uncertainty in most years for most species (except for
HF). These yearly mean differences of the smoothed total columns
together with the standard error can be found in Figs. and
for the stratospheric and tropospheric species, respectively
(yellow bars, for the PARIS-IR and Bruker 125HR total column comparison). The
number of pairs compared varies for each year and is displayed above or
below the bars in the figure. The difference in numbers of coincident pairs
is mainly due to the different weather conditions for each year. For example
in 2009 and 2010, there were many days of sunshine and little to no cloud
cover, and measurements could be taken almost every day throughout the
campaign. There is interannual variation of the retrieval differences between
PARIS-IR and the Bruker 125HR, however, these are within the combined
retrieval uncertainties for most species during most years. For the
HCl comparison, 2 years (2010 and 2013) are outside the combined
retrieval uncertainty. For the comparisons of N2O, CO, and
C2H6, 1 year in each case is outside the combined retrieval
uncertainty. Overall, no significant degradation of the comparison could be
found over the 7-year period for any of the eight retrieved species.
To conclude, we found that after accounting for the different resolutions by
smoothing, the mean differences between PARIS-IR and the Bruker 125HR total
columns are below 4 % and within the estimated combined retrieval
uncertainties for all species, with the exception of HF. These
differences and correlation coefficients are comparable or slightly better
for some species compared to previous side-by-side instrument comparisons for
PARIS-IR (that used SFIT2) in Eureka e.g., and
at other locations in North America e.g.,.
Comparison between ACE-FTS and the ground-based FTSsMethodology
As described in the introduction, instrument comparisons between ground- and
satellite-based remote sensing instruments in the high Arctic, especially
during the springtime when the Sun rises, are challenging, primarily due to
the polar vortex. Here we apply a comparison method similar to
for the stratospheric species (O3, HCl,
HNO3, and HF), which can have quite different concentrations
inside to outside the polar vortex. Measurements are considered coincident if
they were recorded within 12 h of each other and when the distance at
specific altitudes along the line of sight (between 14 and
40 km) is less than 1000 km. Additionally, for those same
altitudes, the difference in sPV between the measurements was restricted to a
maximum of 0.3×10-4s-1 and the difference of the
temperature was limited to less than 10 K to ensure that similar air
masses are observed by both instruments. While these criteria filter
dissimilar air masses in most cases, this may still allow occasionally for
dissimilar air masses to be compared right near the edge of the polar vortex.
For the retrievals of the tropospheric trace gases from the ground-based FTSs
(CH4, N2O, CO, and C2H6), an effort was made
to compare partial columns with a lower boundary as far into the troposphere
as possible. This lowers the number of ACE-FTS occultations that can be
compared but improves the comparison. As for the stratospheric species
comparison, measurements are considered coincident if they were recorded
within 12 h of each other. However, to have enough observations to compare,
a less strict criterion has been used, where only the distance along the
line of sight is considered at 14 km. Including the difference in sPV
criterion along the line of sight at 14 km did not impact the comparison,
since none of the differences in sPV were found outside this criterion. Furthermore,
we varied the distance criterion between a maximum of 500 and
1000 km along the line of sight at 14 km. As will be discussed in
detail in the following section, the tighter distance criterion improves the
correlation between the partial columns for the tropospheric species.
In order to compare the space-borne to the ground-based FTS measurements,
partial columns have to be considered. This is because ACE-FTS measurements
are not made in the lower troposphere. The partial column altitude ranges are
specific to each species and are based on where the ground-based instrument
retrievals are the most sensitive (see Sect. and
Fig. ) and on the observation lower-altitude limit of ACE-FTS.
The altitude ranges of the partial columns are slightly different for each
ground-based FTS, since the instruments' resolutions, and therefore the
sensitivities, are not the same. These ranges for each partial column are
listed in Tables to , for each instrument and
species.
Comparison of ACE-FTS v3.5 and PARIS-IR partial columns (2006–2013)
for the stratospheric gases presented in this study. N is the number of
coincident pairs used in this calculation. The third column gives the
altitude range of the partial columns used for this comparison, the
fourth column shows the mean distance
between the observed air masses from the instruments along the line of sight
at 20 km. The mean time between the observations is displayed in the
fifth column and the mean beta
angle of the ACE observations in the sixth column. The seventh column (PC diff) represents the mean
differences in the partial columns between the FTSs (in %) along with the
standard error. The correlation coefficient (R) and the slope of the
regression plot (slope), along with the 1σ uncertainty of the slope,
are shown in columns 8 and 9, respectively.
Smoothing has been applied to ACE-FTS profiles with either the PARIS-IR or
the Bruker 125HR averaging kernels in a similar manner as described in
Sect. . ACE-FTS profiles have been interpolated from its
1 km altitude grid to the 29- and 47-layer altitude grids used for the
PARIS-IR and Bruker 125HR retrievals, respectively. In the lower troposphere,
where no ACE-FTS measurements are available, the ground-based a priori values
are used for the calculation. These interpolated ACE-FTS VMR profiles are
then smoothed with the averaging kernel and the a priori of the comparison
instrument, as described in Eq. (). The partial columns are
then calculated from the smoothed profiles, based on the partial column
altitude ranges (see Tables –). The included
a priori values, in the lower troposphere, are not considered in this partial
column calculation. This method is consistent with other validation studies
that have compared satellite-based instruments to ground-based FTSs
e.g.,. The partial column
differences were calculated as ([GB-ACE]/[0.5 × (GB+ACE)]),
where GB is the ground-based instrument, either PARIS-IR or the Bruker 125HR
as applicable. If more than one ground-based measurement is coincident with a
particular ACE-FTS occultation, the mean of all coincident ground-based
measurements was used to calculate the difference.
Results and discussion
The mean partial column differences for the stratospheric species between ACE-FTS and PARIS-IR (from 2006
to 2013) are shown in Table and
between ACE-FTS and the Bruker 125HR (from 2007 to 2013) in
Table . For each of the stratospheric species, approximately
120 and 100 satellite occultations were found to be coincident with PARIS-IR
and the Bruker 125HR measurements, respectively. The smaller number of
coincident measurements with the Bruker 125HR is partly due to the shorter
time period (2007–2013) for the comparisons, but also because individual
species are not measured as often as with PARIS-IR. The number of coincident
measurements varies annually and due to a data processing gap for ACE-FTS in
2012, no coincident occultations were found that year. Scatter plots of the
partial column comparisons between the ACE-FTS and ground-based datasets for
the stratospheric species can be found in the Supplement
(Figs. S1 and S2).
Same as Table , but for the comparison of ACE-FTS
v3.5 and the Bruker 125HR partial columns (2007–2013).
Very good agreement was found between ACE-FTS and both ground-based FTSs for
O3 and HCl. The correlations for these gases are excellent
(see Tables and ). The mean differences for
the comparison to both ground-based FTSs for the O3 partial columns
are within the total retrieval uncertainties of ±3.5 % (PARIS-IR) and
±5.0% (Bruker 125HR), respectively (see Table ). The
mean differences for the HCl comparisons are within the total
retrieval uncertainty of ±2.5 % from PARIS-IR and are 0.5 %
larger than the total retrieval uncertainty of ±1.9 % for the Bruker
125HR.
The ground-based HNO3 partial columns are in good agreement with
ACE-FTS (see Tables and ) and are negligible
compared to the very large total retrieval uncertainty of ±19%. The
correlation between ACE-FTS and the ground-based partial columns is high.
The ACE-FTS HF partial columns agree well with the Bruker 125HR, for
which the mean difference is approximately half of the Bruker 125HR total
retrieval uncertainty of ±3.5 %. The correlation of those partial
columns is high. The comparison between ACE-FTS and PARIS-IR HF
partial columns is not as good, since the mean difference is more than twice
PARIS-IR's total retrieval uncertainty (±2.5 %). Large differences
are mainly observed when high HF concentrations are measured by
ACE-FTS. Additionally, although the slope of the regression plot is close to the
one-to-one line, the correlation between the partial columns is relatively low.
This negative difference of the PARIS-IR HF retrieval, especially for
high HF columns, is consistent with the bias found from comparison of
the total columns to the Bruker 125HR (see Sect. ).
Same as Table , but for the comparison of the
tropospheric species for ACE-FTS v3.5 and PARIS-IR partial columns
(2006–2013). Different coincidence criteria are used. Compared here is the
maximum distance between the observed air masses at 14 km along the
line of sight for both instruments.
Next we consider the variation of the mean differences for each individual
year between 2006 and 2013. The yearly differences are displayed in
Figs. and for the stratospheric and
tropospheric species, respectively: the ACE-FTS and PARIS-IR comparisons are
shown as blue bars and the ACE-FTS and Bruker 125HR comparisons are displayed
as cyan bars. Also displayed are the partial column comparisons between
PARIS-IR and the Bruker 125HR (red bars) to better understand the impacts of
comparing partial columns rather than total columns. The largest difference
between using partial columns and total columns can be seen for HF,
where the partial column differences are approximately twice as large
compared to the total column differences. This is due to the low vertical
resolution of the PARIS-IR HF retrieval, for which the partial columns
have generally less than 1 DOFS (∼ 0.8) and are therefore more strongly
influenced by the a priori profile. The impact of the total column versus the
partial column comparison is not as significant for O3, HCl,
and HNO3, where the partial columns contain more information from the
measurement.
Same as Table , but for the comparison of ACE-FTS
v3.5 and the Bruker 125HR partial columns (2007–2013).
Also shown in Figs. and are the standard
errors of the yearly mean differences (shown as error bars) and the number
of pairs (e.g., number of ACE-FTS occultations) used to estimate these
differences (listed below or above the bars). Variation of the annual mean
differences are apparent for the ground-based versus ACE-FTS comparisons.
Generally, for all four stratospheric species, the absolute mean differences
do not appear to increase between 2006 and 2013. Furthermore, it was found
that comparisons made with measurements taken inside the polar vortex are
different to the comparisons made with measurements taken outside the polar
vortex, especially for O3. The yearly average and standard deviation
of the sPV along the line of sight at 20 km is displayed in the lower
panel of Fig. , together with the inner and outer polar vortex
edge as a reference. In 2009 and 2013, on average, measurements that are used
for the comparison were taken at the edge of the polar vortex. For
measurements in those years, the O3 and HNO3 comparisons seem
to result in larger differences. In 2007 and 2011, all measurements that are
used for the comparison were taken inside the polar vortex, where on average
ACE-FTS's O3 partial columns were larger than the ground-based ones.
The reverse was seen in all other years when the comparison was made
primarily outside the polar vortex. In 2011, the mean difference for
HCl between ACE-FTS and the Bruker 125HR seems to be very large,
but only two occultations were compared to the ground-based
measurements. Furthermore, the partial columns from both instruments were
approximately 4 times smaller than in previous years
(1.3×1015moleccm-2), which impacts the
percentage difference; the absolute difference of those columns was
approximately 0.2×1015moleccm-2, which is comparable
with other years. The comparison of HF between ACE-FTS and PARIS-IR
seems to be worse in 2007 and 2011 compared to previous years. The PARIS-IR
HF partial columns were larger
(∼2.3×1015moleccm-2) than in other years
(∼1.8×1015moleccm-2) and tended to be closer to
the a priori (∼1.6×1015moleccm-2) (see
Sect. ).
As mentioned in Sect. , different coincidence criteria were
used for the tropospheric species and the impact of the maximum distance
criteria between the measurements (at 14 km along the line of sight) was
investigated. Tables and summarize the
ACE-FTS and PARIS-IR and ACE-FTS and Bruker 125HR comparisons, respectively,
for coincident measurements at a maximum distance of 1000 and of
500 km. A maximum distance of 500 km shows significantly
improved correlation compared to using 1000 km between the satellite-
and ground-based instruments' partial columns. For this, the correlation
coefficient R increases on average from 0.6 to 0.8. However, this
distance criterion has a relatively small impact on the mean differences for
CH4 and N2O. As such, the mean differences for those species
using a stricter distance criterion are within the standard error of the
differences found with a 1000 km distance criterion. Since the mean
differences are quite similar for the two different distance criteria, but
the correlation is significantly improved, only the mean differences,
correlation coefficients, and correlation slopes at a maximum distance of
500 km are considered in the following discussion. This is consistent with
many other validation papers which have used 500 km as a limit for
coincident measurements for tropospheric species in the high Arctic
e.g.,. Tightening the distance criteria any further
results in very few measurements that are selected for the comparisons. It
should also be noted that the different altitude ranges (listed in
Tables and ) selected for the partial column
comparison between ACE-FTS and PARIS-IR and ACE-FTS and the Bruker 125HR
contribute to differences in the number of pairs that are compared
(approximately one third less for the Bruker 125HR). Scatter plots of the
partial column comparisons between the ACE-FTS and ground-based datasets for
the tropospheric species can be found in the Supplement (Figs. S3
and S4).
The CH4 partial column datasets agree well. The mean differences
between ACE-FTS and PARIS-IR and ACE-FTS and Bruker 125HR partial columns
are well within the estimated total retrieval uncertainty of the ground-based
instruments (±6.8 % for PARIS-IR and ±8.0 % for the Bruker
125HR). There is a high correlation between the instruments' partial columns
for the comparisons with both ground-based instruments.
The N2O partial columns of ACE-FTS agree well with those of the
Bruker 125HR, for which the mean difference is approximately half of its
total retrieval uncertainty (±3.7%), with a high correlation between
those partial columns. The mean difference for the ACE-FTS and PARIS-IR
comparison is larger than the estimated total retrieval uncertainty of
PARIS-IR (±3.5%), but the correlation between the partial columns
is high.
ACE-FTS partial columns of CO and C2H6 are only compared to
the Bruker 125HR. The PARIS-IR partial columns of those species have less
than 0.5 DOFS (∼0.3), which is not ideal for an instrument comparison
. The correlation of the partial columns for both species is
quite high and the slope of the regression plot is close to the one-to-one line.
However, the mean differences are quite large (7.1% for CO and
20% for C2H6). Note that the mean difference for CO is
more than twice as large (16.7±3.3%) in 2010 compared to all other
years. Excluding 2010 from the comparison leads to a mean difference of
3.28% that is within the total retrieval uncertainty (±3.5%). In
2010, a number of slightly enhanced CO columns were observed by both
ground-based FTS instruments near Eureka that were not observed by the
ACE-FTS and could be a local enhancement.
Looking at the annual variability of the instruments' mean differences (see
Fig. ), relatively small year-to-year variability, that is
within the ground-based total retrieval uncertainty, can be seen for the
CH4 and N2O partial column comparisons. As noted above, the
CO difference in 2010 is significantly larger than in all other years,
likely due to a localized enhancement. The C2H6 annual partial
column differences vary between 6 and 34 %.
To conclude, very little bias was seen between ACE-FTS and both ground-based
FTSs for the comparison of the stratospheric species (except for the
comparison to PARIS-IR HF). There is a negative bias for the
comparison between the HF partial columns from PARIS-IR and ACE-FTS,
which is consistent with the bias seen in the ground-based comparisons (see
Sect. ). The differences between ACE-FTS and the Bruker 125HR
for O3, HCl, HNO3, and HF between 2006 and 2013
using SFIT4 and ACE-FTS v3.5 are consistent with for
2007 and 2008 using SFIT2 and ACE-FTS v2.2+updates. For stratospheric
species, the distance criterion of 1000 km is sufficient, but
the comparison for tropospheric species is improved when the distance is
limited to 500 km. ACE-FTS v3.5 CH4, N2O, and
CO partial columns compare well to the ground-based retrievals. The
mean differences found for the tropospheric species (with ACE-FTS
v2.2+updates) are comparable with and for
N2O and CH4, respectively, and are improved by more than
15 % for CO compared to . This improvement could
be due to the latest retrieval version of ACE-FTS (v3.5) and also to the
ground-based retrieval algorithm (SFIT4) that has been used for this study.
We found that with the new retrieval algorithm SFIT4 and latest NDACC/IRWG
recommendations, CO has a higher sensitivity in the lower stratosphere
compared to previous retrievals. Furthermore, it was shown that the
comparison between the two ground-based instruments did not degrade over this
time period. The mean differences change slightly each year for all species
but did not increase over time.
Evolution of the stratospheric and tropospheric species during Arctic springtime, 2006–2013
The dataset displayed in Fig. shows the springtime campaign
average, late February to early April (and standard deviation), obtained from
the PARIS-IR dataset for each year. This dataset consists of yearly
springtime average total column measurements between 2006 and 2013 for the
eight species, as well as the yearly springtime average sPV at 20 km
along the line of sight. The red solid lines represent the line of best fit
(first-order polynomial fit) and the black dashed lines display the standard
deviation of the fit. The outer and inner edges of the polar vortex are
marked in the lower panel by cyan dashed and solid lines, respectively. The
yearly variation of the stratospheric species (O3, HCl,
HNO3, and HF) is highly influenced by the dynamics of the
stratosphere and the strength of the polar vortex. Note that the dataset
shown in Fig. has not been filtered for observations taken
inside or outside of the polar vortex. What is immediately apparent is that
O3, HCl, and HNO3 columns are very low in 2011. In this
year, measurements were mainly sampled inside a strong polar vortex, as can
be seen in the lower panel of Fig. as well as in
. The vortex remained near Eureka for the whole month of
March, so ground-based observations were mainly taken inside the vortex
see Fig. 5f in. A strong and cold vortex is typically
associated with chemical O3 depletion and denitrification
, and we see the averages for O3, HCl, and HNO3
are low in 2007 and significantly lower in 2011 than in all other years. The
location of air sampled by the instruments with respect to the polar vortex
has a high interannual variability over Eureka between 2006 and 2013.
Figure shows the averages of the stratospheric species
outside the polar vortex, when the sPV is less than
1.2×10-4s-1 at 20 km along the line of sight
(see Sect. ). The polar vortex near Eureka was not strong
until the middle of March in 2007, while it remained strong until the end of
March in 2011. Therefore, the air mass outside the polar vortex has been
measured for a few days in early March 2007 and in late March to early April
2011. Thus, there is the potential for a temporal sampling influence as
measurements in 2007 and 2011 are taken in different months, approximately
1 month apart. The dataset inside the polar vortex is not shown here due
to the strong interannual variability of the polar vortex and the chlorine
activation processes.
In examining PARIS-IR's 8-year dataset, we can estimate whether the
changes seen in the dataset over this time period are statistically
significant. To determine whether or not it is possible to assess a trend, a
number of factors need to be considered: the time period of the dataset, the
magnitude of the trend wo, the variability σ, and the
autocorrelation ϕ of the noise of the dataset. This is described in
detail in . The minimum number of years, n*, needed to
observe a trend, can be estimated by
n*=3.3⋅σ|wo|⋅1+ϕ1-ϕ2/3.
Using the slopes of the lines of best fit of Figs. and
for each species, we can determine whether or not a trend
can be detected in our dataset based on the number of years compared to the
estimated minimum number of years n*, computed from Eq. ().
The total columns of all stratospheric species sampled outside the polar
vortex in the springtime (Fig. ) show an increase between
2006 and 2013. The lines of best fit ± standard deviation indicate an
increase of 0.9±1.2%yr-1 for O3, 1.7±0.8%yr-1 for HCl,
1.7±0.7%yr-1 for HNO3, and
3.8±1.4%yr-1 for HF. Using the
method above, the minimum number of years required to detect a trend of these
magnitudes from this dataset is approximately 5 years for O3, 6 years
for HCl, 7 years for HF, and 9 years for HNO3. With
this 8-year dataset, trends are likely detected in HCl and
HF in the atmosphere of the high Arctic (outside the polar vortex)
from PARIS-IR measurements. Although it seems there are enough years
available to detect a trend in O3, it should be noted that the
uncertainty of the increase seen in O3 is larger than the actual
estimated increase. However, recent increasing stratospheric O3 (in
the tropics and midlatitudes) has been previously reported by
using satellite and ozonesonde observations, and our findings for high
latitudes are consistent with their results. The magnitude of the increase of
HCl at northern high latitudes is consistent with
and is assumed to be due to atmospheric circulation changes in the Northern
Hemisphere. These changes occurred after 2005/2006 and are possibly on a
short timescale . The increase of HF is likely due
to the increase of COF2 that has been discussed in .
A longer dataset is necessary to be able to observe a trend for HNO3.
Yearly springtime campaign averages (blue dots) and 1σ
standard deviation (error bar) between 2006 and 2013 obtained from the
PARIS-IR total column retrievals for all eight trace gases used in this
study. The sPV at 20 km along the line of sight is shown in the lower
panel, together with the inner (solid cyan line) and outer (dashed cyan line)
edge of the polar vortex. The red solid lines represent the lines of best fit
and the black dashed lines display the 1σ standard deviation of the
fit. The best fit between 2009 and 2013 is displayed for C2H6 as a
dark blue line.
Springtime campaign averages (and 1σ standard deviation)
between 2006 and 2013 obtained from the PARIS-IR total column retrievals for
all stratospheric species for measurements that were taken outside the polar
vortex (sPV <1.2×10-4s-1 at 20 km along the
line of sight). The colour and symbol scheme is the same as in
Fig. .
For the tropospheric species, no sPV filter has been applied, since the
influence of the polar vortex is not as significant in the troposphere.
Looking at Fig. , it seems that CH4 and C2H6
are increasing each year since 2006. Between 2006 and 2013, the CH4
columns increased by approximately 0.5±0.1%yr-1, and the
C2H6 columns increased by approximately
1.6±0.2%yr-1. However, C2H6 has started to
increase at a higher rate since 2009 as can be seen from Fig. . Between 2009 and 2013, C2H6 increased by
2.3±0.5%yr-1 based on our dataset. CO appears to
be decreasing slightly over the time period between 2006 and 2013, by
approximately -0.8±0.6%yr-1. N2O seems
relatively constant in the Arctic spring between 2006 and 2013 and the slight
increase seen over this time period is well within the standard deviation
with a slope of 0.3±0.3%yr-1. The minimum number of
years to detect a trend from these datasets, based on ,
is 8 years for CH4 and C2H6, 9 years for CO, and
more than 10 years for N2O. Based on the available data and Eq.
, we can detect a trend for CH4 in the high Arctic
between 2006 and 2013. For C2H6, CO, and N2O our
dataset is not long enough to observe a trend. Increasing C2H6
starting in 2009 has also been reported in previous studies . In the Arctic, have found increasing
C2H6 of approximately 3 ± 1 % yr-1 near Eureka and
Thule, Greenland, between 2009 and 2014. A decrease of CO has been
reported above the high Arctic station in Kiruna by ground-based FTS
observations of -0.61±0.16%yr-1. These
results are consistent with our observations. For CH4, no recent
changes in the Arctic have been reported yet, but at lower latitudes
increasing CH4 has been found. observed a
0.3 % yr-1 increase of CH4 between 2007 and 2011 in
Garmisch and Zugspitze, Germany, from ground-based FTS measurements. This is
similar to the CH4 increase that we found in the high Arctic near
Eureka (0.5%yr-1, 2006–2013).
To conclude, we have found that with our dataset we can detect a trend for
HCl, HF, O3, and CH4 near Eureka between 2006
and 2013. Total columns of all these species are increasing over this time
period.
Summary and conclusions
We have presented 8 years of measurements between 2006 and 2013 in the
high Arctic, with the purpose of providing a detailed comparison between the
two ground-based FTS instruments and the space-borne ACE-FTS as well as
examining atmospheric composition change over this period. In total, eight
atmospheric gases have been utilized and assessed, namely O3,
HCl, HNO3, HF, CH4, N2O, CO, and
C2H6.
Side-by-side instrument comparisons were carried out for the two ground-based
FTSs at PEARL during the Canadian Arctic ACE validation campaigns, from 2007
to 2013. With respect to the smoothed total columns, the instrumental
differences are well within the estimated combined retrieval uncertainties
and below 6 % for most species (except HF) and the retrieved columns
are highly correlated (with a correlation coefficient R>0.85) for the two
FTSs. Our results are comparable with ground-based side-by-side comparisons
with PARIS-IR, such as those reported by and
. Smoothing the retrieved profiles of the Bruker 125HR with
PARIS-IR's averaging kernel provides a more accurate comparison and has been
done for all species. However, this is only significant for gases with low
DOFS such as C2H6 and HF. The comparison also showed that
HF total columns are slightly underestimated by PARIS-IR versus the
Bruker 125HR HF columns. Overall, these comparisons contribute to the
satellite validation effort for ACE-FTS in the high Arctic with the latest
retrieval algorithm SFIT4. It was further found that the comparisons did not
degrade during this time period.
The partial column comparisons between ACE-FTS v3.5 and the two ground-based
FTSs were carried out over this 8-year period. For O3,
HCl, HNO3, and HF, coincidence criteria including sPV
and temperatures along the line of sight were employed. The resulting mean
biases are smaller than 4 % and within the estimated uncertainty of the
ground-based retrieval for all species for the Bruker 125HR comparison. The
mean bias between ACE-FTS and PARIS-IR was within approximately 6 % for
all species. Our results have shown that the correlation between the datasets
is significantly improved (by ∼0.2) when the maximum distance is
limited to 500 km for the comparisons of tropospheric species. For
CH4, N2O, and CO, the biases are smaller than
3.5 % and less than the ground-based total retrieval uncertainty for the
comparison between ACE-FTS and the Bruker 125HR. The PARIS-IR CH4 and
N2O columns agree well with ACE-FTS, with differences of 3.0
and 6.6 %, respectively. The mean differences of the ACE-FTS and the
Bruker 125HR C2H6 partial columns are ∼20%; however, a
high correlation (with a correlation coefficient R=0.75) between these
datasets was found. Overall, the results show that ACE-FTS 2006–2013
retrievals are consistent with ground-based observations, even in 2013, a
decade after the instrument was launched. No increasing mean differences of
the yearly comparisons were found over this time period. The long-term
ground-based FTS measurements continue to contribute to the validation of the
trace gas amounts retrieved from measurements from the ACE-FTS instrument
on board SCISAT.
During this entire time period (2006–2013), increasing O3
(0.9%yr-1), HCl (1.7%yr-1),
HF (3.8%yr-1), CH4
(0.5 % yr-1), and C2H6 (2.3%yr-1,
2009–2013) have been found near Eureka in the springtime. These results were
compared to previously published measurements from different datasets and at
different locations. Overall, our estimated increases are consistent with the
values reported by for O3, for
HCl, and for CH4. As such, our
results from the ground-based PARIS-IR dataset complement their findings by
showing that these increases are also apparent in the high Arctic.
The PARIS-IR data are available from
https://eureka.physics.utoronto.ca/PARIS-IR-Data-2006-2013 or upon
request from the corresponding author (kaley.walker@utoronto.ca). The Eureka
FTIR data used in this paper are available from the NDACC archive:
ftp://ftp.cpc.ncep.noaa.gov/ndacc/station/eureka/hdf/ftir/. The ACE-FTS
Level 2 data used in this study can be obtained via the ACE-FTS website
(registration required), http://www.ace.uwaterloo.ca, or upon request
from the corresponding author (kaley.walker@utoronto.ca).
The authors declare that they have no conflict of
interest.
The Supplement related to this article is available online at https://doi.org/10.5194/amt-10-3273-2017-supplement.
Acknowledgements
The Canadian Arctic ACE validation campaigns are funded by the Canadian Space
Agency (CSA), Environment and Climate Change Canada (ECCC), the Natural
Sciences and Engineering Research Council of Canada (NSERC), and the Northern
Scientific Training Program. CANDAC and PEARL are supported by the Atlantic
Innovation Fund/Nova Scotia Research Innovation Trust, Canadian Foundation
for Climate and Atmospheric Sciences, Canada Foundation for Innovation, CSA,
ECCC, Government of Canada International Polar Year funding, NSERC, Ontario
Innovation Trust, Polar Continental Shelf Program, and the Ontario Research
Fund. The Atmospheric Chemistry Experiment (ACE) is mainly supported by the
CSA and NSERC. We would like to acknowledge the ACE campaign and CANDAC
operators: Ashley Harrett, Alexei Khmel, Paul Loewen, Oleg Mikhailov and
Matt Okraszewski,
as well as Keeyoon Sung, Emily McCullough, and Joseph Mendonca
for maintaining and operating the ground-based FTSs. The authors would like
to thank the staff at the Eureka Weather Station and CANDAC for the
logistical, on-site support provided at Eureka, and the launch of many radio-
and ozone-sonde balloons for us. We are also very grateful to William Daffer
from JPL, who carried out the derived meteorological parameters calculations.
Edited by: Gabriele Stiller
Reviewed by: three anonymous referees
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