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 (CO$_2$) and methane
(CH$_4$) 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 instrumen-
tal setup and performance evaluation of an open-path GHG
observatory in the city of Heidelberg, Germany. The obser-
vatory measures path-averaged concentrations of CO$_2$ and
CH$_4$ 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 ob-
servation types on the kilometer scale. This representative-
ness is necessary to accurately quantify emissions, since at-
mospheric 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 CO$_2$ (xCO$_2$) and
CH$_4$ (xCH$_4$) in 5 min measurements, respectively. ... mehrAfter
bias correction, the open-path measurements show excellent
agreement with the local in situ data under atmospheric back-
ground conditions. Both datasets show clear signals of traffic
CO$_2$ emissions in the diurnal xCO$_2$ cycle. However, there are
particular situations, such as under southeasterly wind condi-
tions, in which the in situ and open-path data reveal distinct
differences up to 20 ppm in xCO$_2$, most likely related to their
different sensitivity to local emission and transport patterns.
Our setup is based on a Bruker IFS 125HR Fourier trans-
form spectrometer, which offers a spacious and modular de-
sign 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.