10 research outputs found
An open-path observatory for greenhouse gases based on near-infrared Fourier transform spectroscopy
Monitoring the atmospheric concentrations of the greenhouse gases (GHG) carbon dioxide (CO2) and methane (CH4) is a key ingredient for fostering our understanding of the mechanisms behind the sources and sinks of these gases and for verifying and quantitatively attributing their anthropogenic emissions. Here, we present the instrumental setup and performance evaluation of an open-path GHG observatory in the city of Heidelberg, Germany. The observatory measures path-averaged concentrations of CO2 and CH4 along a 1.55 km path in the urban boundary layer above the city. We combine these open-path data with local in situ measurements to evaluate the representativeness of these observation types on the kilometer scale. This representativeness is necessary to accurately quantify emissions, since atmospheric models tasked with this job typically operate on kilometer-scale horizontal grids.
For the operational period between 8 February and 11 July 2023, we find a precision of 2.7 ppm (0.58 %) and 18 ppb (0.89 %) for the dry-air mole fractions of CO2 (xCO2) and CH4 (xCH4) in 5 min measurements, respectively. After bias correction, the open-path measurements show excellent agreement with the local in situ data under atmospheric background conditions. Both datasets show clear signals of traffic CO2 emissions in the diurnal xCO2 cycle. However, there are particular situations, such as under southeasterly wind conditions, in which the in situ and open-path data reveal distinct differences up to 20 ppm in xCO2, most likely related to their different sensitivity to local emission and transport patterns.
Our setup is based on a Bruker IFS 125HR Fourier transform spectrometer, which offers a spacious and modular design providing ample opportunities for future refinements of the technique with respect to finer spectral resolution and wider spectral coverage to provide information on gases such as carbon monoxide and nitrogen dioxide.</p
Respiration driven CO2 pulses dominate Australia's flux variability
The Australian continent contributes substantially to the year-to-year
variability of the global terrestrial carbon dioxide (CO2) sink. However, the
scarcity of in-situ observations in remote areas prevents deciphering the
processes that force the CO2 flux variability. Here, examining atmospheric CO2
measurements from satellites in the period 2009-2018, we find recurrent
end-of-dry-season CO2 pulses over the Australian continent. These pulses
largely control the year-to-year variability of Australia's CO2 balance, due to
2-3 times higher seasonal variations compared to previous top-down inversions
and bottom-up estimates. The CO2 pulses occur shortly after the onset of
rainfall and are driven by enhanced soil respiration preceding photosynthetic
uptake in Australia's semi-arid regions. The suggested continental-scale
relevance of soil rewetting processes has large implications for our
understanding and modelling of global climate-carbon cycle feedbacks.Comment: 28 pages (including supplementary materials), 3 main figures, 7
supplementary figure
Long open-path measurements of greenhouse gases in air using near-infrared Fourier transform spectroscopy
In complex and urban environments, atmospheric trace gas composition is
highly variable in time and space. Point measurement techniques for trace
gases with in situ instruments are well established and accurate, but do not
provide spatial averaging to compare against developing high-resolution
atmospheric models of composition and meteorology with resolutions of the
order of a kilometre. Open-path measurement techniques provide path average
concentrations and spatial averaging which, if sufficiently accurate, may be
better suited to assessment and interpretation with such models. Open-path
Fourier transform spectroscopy (FTS) in the mid-infrared region, and
differential optical absorption spectroscopy (DOAS) in the UV and visible,
have been used for many years for open-path spectroscopic measurements of
selected species in both clean air and in polluted environments. Near
infrared instrumentation allows measurements over longer paths than mid-infrared FTS for species such as greenhouse gases which are not easily
accessible to DOAS.In this pilot study we present the first open-path near-infrared
(4000–10 000 cm−1, 1.0–2.5 µm) FTS measurements of CO2,
CH4, O2, H2O and HDO over a 1.5 km path in urban Heidelberg,
Germany. We describe the construction of the open-path FTS system, the
analysis of the collected spectra, several measures of precision and
accuracy of the measurements, and the results a four-month trial
measurement period in July–November 2014. The open-path measurements are
compared to calibrated in situ measurements made at one end of the open
path. We observe significant differences of the order of a few ppm for
CO2 and a few tens of ppb for CH4 between the open-path and point
measurements which are 2 to 4 times the measurement repeatability, but we cannot
unequivocally assign the differences to specific local sources or sinks. We
conclude that open-path FTS may provide a valuable new tool for
investigations of atmospheric trace gas composition in complex, small-scale
environments such as cities
First continuous measurements of δ<sup>18</sup>O-CO<sub>2</sub> in air with a Fourier transform infrared spectrometer
The continuous in situ measurement of δ<sup>18</sup>O in atmospheric CO<sub>2</sub>
opens a new door to differentiating between CO<sub>2</sub> source and sink
components with high temporal resolution. Continuous
<sup>13</sup>C–CO<sub>2</sub> measurement systems have already been commercially available for some time,
but until now, only few instruments have been able to provide a continuous
measurement of the oxygen isotope ratio in CO<sub>2</sub>. Besides precise
<sup>13</sup>C/<sup>12</sup>C observations, the Fourier transform infrared (FTIR)
spectrometer is also able to measure the <sup>18</sup>O / <sup>16</sup>O ratio in
CO<sub>2</sub>, but the precision and accuracy of the measurements have not yet
been evaluated. Here we present a first analysis of δ<sup>18</sup>O-CO<sub>2</sub> (and δ<sup>13</sup>C-CO<sub>2</sub>) measurements with the
FTIR analyser in Heidelberg. We used Allan deviation to determine the
repeatability of δ<sup>18</sup>O-CO<sub>2</sub> measurements and found that it
decreases from 0.25‰ for 10 min averages to about
0.1‰ after 2 h and remains at that value up to
24 h. We evaluated the measurement precision over a 10-month period
(intermediate measurement precision) using daily working gas measurements
and found that our spectrometer measured δ<sup>18</sup>O-CO<sub>2</sub> to
better than 0.3‰ at a temporal resolution of less than
10 min. The compatibility of our FTIR-spectrometric measurements to
isotope-ratio mass-spectrometric (IRMS) measurements was determined by
comparing FTIR measurements of cylinder gases and ambient air with IRMS
measurements of flask samples, filled with gases of the same cylinders or
collected from the same ambient air intake. Two-sample <i>t</i> tests revealed
that, at the 0.01 significance level, the FTIR and the IRMS measurements do
not differ significantly from each other and are thus compatible. We
describe two weekly episodes of ambient air measurements, one in winter and
one in summer, and discuss what potential insights and new challenges
combined highly resolved CO<sub>2</sub>, δ<sup>13</sup>C-CO<sub>2</sub> and δ<sup>18</sup>O-CO<sub>2</sub> records may provide in terms of better understanding
regional scale continental carbon exchange processes
Comparisons of continuous atmospheric CH4, CO2 and N2O measurements – results from a travelling instrument campaign at Mace Head
A 2-month measurement campaign with a Fourier transform infrared analyser as a travelling comparison instrument (TCI) was performed at the Advanced Global Atmospheric Gases Experiment (AGAGE) and World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) station at Mace Head, Ireland. The aim was to evaluate the compatibility of atmospheric methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) measurements of the routine station instrumentation, consisting of a gas chromatograph (GC) for CH4 and N2O as well as a cavity ring-down spectroscopy (CRDS) system for CH4 and CO2. The advantage of a TCI approach for quality control is that the comparison covers the entire ambient air measurement system, including the sample intake system and the data evaluation process. For initial quality and performance control, the TCI was run in parallel with the Heidelberg GC before and after the measurement campaign at Mace Head. Median differences between the Heidelberg GC and the TCI were well within the WMO inter-laboratory compatibility target for all three greenhouse gases. At Mace Head, the median difference between the station GC and the TCI were -0.04 nmol mol(-1) for CH4 and -0.37 nmol mol(-1) for N2O (GC-TCI). For N2O, a similar difference (-0.40 nmol mol(-1)) was found when measuring surveillance or working gas cylinders with both instruments. This suggests that the difference observed in ambient air originates from a calibration offset that could partly be due to a difference between the WMO N2O X2006a reference scale used for the TCI and the Scripps Institution of Oceanography (SIO-1998) scale used at Mace Head and in the whole AGAGE network. Median differences between the CRDS G1301 and the TCI at Mace Head were 0.12 nmol mol(-1) for CH4 and 0.14 mu mol mol(-1) for CO2 (CRDS G1301 - TCI). The difference between both instruments for CO2 could not be explained, as direct measurements of calibration gases show no such difference. The CH4 differences between the TCI, the GC and the CRDS G1301 at Mace Head are much smaller than the WMO inter-laboratory compatibility target, while this is not the case for CO2 and N2O
Comparisons of continuous atmospheric CH4, CO2 and N2O measurements - results from a travelling instrument campaign at Mace Head
A 2-month measurement campaign with a Fourier transform infrared analyser as
a travelling comparison instrument (TCI) was performed at the Advanced Global
Atmospheric Gases Experiment (AGAGE) and World Meteorological Organization
(WMO) Global Atmosphere Watch (GAW) station at Mace Head, Ireland. The aim
was to evaluate the compatibility of atmospheric methane (CH4), carbon
dioxide (CO2) and nitrous oxide (N2O) measurements of the routine
station instrumentation, consisting of a gas chromatograph (GC) for CH4
and N2O as well as a cavity ring-down spectroscopy (CRDS) system for
CH4 and CO2. The advantage of a TCI approach for quality control is
that the comparison covers the entire ambient air measurement system,
including the sample intake system and the data evaluation process. For
initial quality and performance control, the TCI was run in parallel with the
Heidelberg GC before and after the measurement campaign at Mace Head. Median
differences between the Heidelberg GC and the TCI were well within the WMO
inter-laboratory compatibility target for all three greenhouse gases. At Mace
Head, the median difference between the station GC and the TCI were
−0.04 nmol mol−1 for CH4 and −0.37 nmol mol−1 for
N2O (GC-TCI). For N2O, a similar difference
(−0.40 nmol mol−1) was found when measuring surveillance or working
gas cylinders with both instruments. This suggests that the difference
observed in ambient air originates from a calibration offset that could
partly be due to a difference between the WMO N2O X2006a reference scale
used for the TCI and the Scripps Institution of Oceanography (SIO-1998) scale
used at Mace Head and in the whole AGAGE network. Median differences between
the CRDS G1301 and the TCI at Mace Head were 0.12 nmol mol−1 for
CH4 and 0.14 μmol mol−1 for CO2 (CRDS G1301 –
TCI). The difference between both instruments for CO2 could not be
explained, as direct measurements of calibration gases show no such
difference. The CH4 differences between the TCI, the GC and the CRDS
G1301 at Mace Head are much smaller than the WMO inter-laboratory
compatibility target, while this is not the case for CO2 and N2O