Abstract. We thoroughly evaluate the performance of a multi-species, in situ Fourier transform infrared (FTIR) analyser with respect to high-accuracy needs for greenhouse gas monitoring networks. The in situ FTIR analyser is shown to measure CO2, CO, CH4 and N2O mole fractions continuously, all with better reproducibility than the inter-laboratory compatibility (ILC) goals, requested by the World Meteorological Organization (WMO) for the Global Atmosphere Watch (GAW) programme. Simultaneously determined δ13CO2 reaches reproducibility as good as 0.03‰. Second-order dependencies between the measured components and the thermodynamic properties of the sample, (temperature, pressure and flow rate) and the cross sensitivities among the sample constituents are investigated and quantified. We describe an improved sample delivery and control system that minimises the pressure and flow rate variations, making post-processing corrections for those quantities non-essential. Temperature disequilibrium effects resulting from the evacuation of the sample cell are quantified and improved by the usage of a faster temperature sensor. The instrument has proven to be linear for all measured components in the ambient concentration range. The temporal stability of the instrument is characterised on different time scales. Instrument drifts on a weekly time scale are only observed for CH4 (0.04 nmol mol−1 day−1) and δ13CO2 (0.02‰ day−1). Based on 10 months of continuously collected quality control measures, the long-term reproducibility of the instrument is estimated to ±0.016 μmol mol−1 CO2, ±0.03‰ δ13CO2, ±0.14 nmol mol−1 CH4, ±0.1 nmol mol−1 CO and ±0.04 nmol mol−1 N2O. We propose a calibration and quality control scheme with weekly calibrations of the instrument that is sufficient to reach WMO-GAW inter-laboratory compatibility goals.

Abstract. Different carbon dioxide (CO 2 ) emitters can be distinguished by their carbon isotope ratios. Therefore measurements of atmospheric δ 13 C(CO 2 ) and CO 2 concentration contain information on the CO 2 source mix in the catchment area of an atmospheric measurement site. This information may be illustratively presented as the mean isotopic source signature. Recently an increasing number of continuous measurements of δ 13 C(CO 2 ) and CO 2 have become available, opening the door to the quantification of CO 2 shares from different sources at high temporal resolution. Here, we present a method to compute the CO 2 source signature (δ S ) continuously and evaluate our result using model data from the Stochastic Time-Inverted Lagrangian Transport model. Only when we restrict the analysis to situations which fulfill the basic assumptions of the Keeling plot method does our approach provide correct results with minimal biases in δ S . On average, this bias is 0.2 ‰ with an interquartile range of about 1.2 ‰ for hourly model data. As a consequence of applying the required strict filter criteria, 85 % of the data points -mainly daytime values -need to be discarded. Applying the method to a 4-year dataset of CO 2 and δ 13 C(CO 2 ) measured in Heidelberg, Germany, yields a distinct seasonal cycle of δ S . Disentangling this seasonal source signature into shares of source components is, however, only possible if the isotopic end members of these sources -i.e., the biosphere, δ bio , and the fuel mix, δ F -are known. From the mean source signature record in 2012, δ bio could be reliably estimated only for summer to (−25.0 ± 1.0) ‰ and δ F only for winter to (−32.5 ± 2.5) ‰. As the isotopic end members δ bio and δ F were shown to change over the season, no year-round estimation of the fossil fuel or biosphere share is possible from the measured mean source signature record without additional information from emission inventories or other tracer measurements.

Abstract.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 (CH 4 ), carbon dioxide (CO 2 ) and nitrous oxide (N 2 O) measurements of the routine station instrumentation, consisting of a gas chromatograph (GC) for CH 4 and N 2 O as well as a cavity ring-down spectroscopy (CRDS) system for CH 4 and CO 2 . 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 CH 4 and −0.37 nmol mol −1 for N 2 O (GC-TCI). For N 2 O, 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 N 2 O 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 CH 4 and 0.14 µmol mol −1 for CO 2 (CRDS G1301 -TCI). The difference between both instruments for CO 2 could not be explained, as direct measurements of calibration gases show no such difference. The CH 4 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 CO 2 and N 2 O.

Abstract. We investigate different methods for estimating anthropogenic CO 2 using modeled continuous atmospheric concentrations of CO 2 alone, as well as CO 2 in combination with the surrogate tracers CO, δ 13 C(CO 2 ) and 14 C(CO 2 ). These methods are applied at three hypothetical stations representing rural, urban and polluted conditions. We find that, independent of the tracer used, an observation-based estimate of continuous anthropogenic CO 2 is not yet feasible at rural measurement sites due to the low signal-to-noise ratio of anthropogenic CO 2 estimates at such settings. The tracers δ 13 C(CO 2 ) and CO provide an accurate possibility to determine anthropogenic CO 2 continuously, only if all CO 2 sources in the catchment area are well characterized or calibrated with respect to their isotopic signature and CO to anthropogenic CO 2 ratio. We test different calibration strategies for the mean isotopic signature and CO to CO 2 ratio using precise 14 C(CO 2 ) measurements on monthly integrated as well as on grab samples. For δ 13 C(CO 2 ), a calibration with annually averaged 14 C(CO 2 ) grab samples is most promising, since integrated sampling introduces large biases into anthropogenic CO 2 estimates. For CO, these biases are smaller. The precision of continuous anthropogenic CO 2 determination using δ 13 C(CO 2 ) depends on measurement precision of δ 13 C(CO 2 ) and CO 2 , while the CO method is mainly limited by the variation in natural CO sources and sinks. At present, continuous anthropogenic CO 2 could be determined using the tracers δ 13 C(CO 2 ) and/or CO with a precision of about 30 %, a mean bias of about 10 % and without significant diurnal discrepancies. Hypothetical future measurements of continuous 14 C(CO 2 ) with a precision of 5 ‰ are promising for anthropogenic CO 2 determination (precision ca. 10-20 %) but are not yet available. The investigated tracer-based approaches open the door to improving, validating and reducing biases of highly resolved emission inventories using atmospheric observation and regional modeling.

We thoroughly evaluate the performance of a multi-species, in-situ FTIR analyser with respect to high accuracy needs for greenhouse gas monitoring networks. The in-situ FTIR analyser measures CO₂, CO, CH₄ and N₂O mole fractions continuously, all with better reproducibility than requested by the WMO-GAW inter-laboratory compatibility (ILC) goal. Simultaneously determined δ¹³CO₂ reaches reproducibility as good as 0.03‰. This paper focuses on the quantification of residual dependencies between the measured components and the thermodynamic properties of the sample as well as the cross-sensitivities among the sample constituents. The instrument has proven to be linear for all components in the ambient range. The temporal stability of the instrument was investigated by 10 months of continuously collected quality control measures. Based on these measures we conclude that for moderately stable laboratory conditions weekly calibrations of the instrument are sufficient to reach WMO-GAW ILC goals

Abstract. The continuous in situ measurement of δ 18 O in atmospheric CO 2 opens a new door to differentiating between CO 2 source and sink components with high temporal resolution. Continuous 13 C-CO 2 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 2 . Besides precise 13 C/ 12 C observations, the Fourier transform infrared (FTIR) spectrometer is also able to measure the 18 O / 16 O ratio in CO 2 , but the precision and accuracy of the measurements have not yet been evaluated. Here we present a first analysis of δ 18 O-CO 2 (and δ 13 C-CO 2 ) measurements with the FTIR analyser in Heidelberg. We used Allan deviation to determine the repeatability of δ 18 O-CO 2 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 18 O-CO 2 to better than 0.3 ‰ at a temporal resolution of less than 10 min. The compatibility of our FTIR-spectrometric measurements to isotoperatio 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 t 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 2 , δ 13 C-CO 2 and δ 18 O-CO 2 records may provide in terms of better understanding regional scale continental carbon exchange processes.

Abstract. 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.

Within the CO2 time series measured at the Environmental Research Station Schneefernerhaus (UFS), Germany, as part of the Global Atmospheric Watch (GAW) program, pollution episodes are traced back to local and regional emissions, identified by δ13C(CO2) as well as ratios of CO and CH4 to CO2 mixing ratios. Seven episodes of sudden enhancements in the tropospheric CO2 mixing ratio are identified in the measurements of mixing/isotopic ratios during five winter months from October 2012 to February 2013. The short-term CO2 variations are closely correlated with changes in CO and CH4 mixing ratios, achieving mean values of 6.0 ± 0.2 ppb/ppm for CO/CO2 and 6.0 ± 0.1 ppb/ppm for CH4/CO2. The estimated isotopic signature of CO2 sources (δs) ranges between −35‰ and −24‰, with higher values indicating contributions from coal combustion or wood burning, and lower values being the result of natural gas or gasoline. Moving Keeling plots with site-specific data selection criteria are applied to detect these pollution events. Furthermore, the HYSPLIT trajectory model is utilized to identify the trajectories during periods with CO2 peak events. Short trajectories are found covering Western and Central Europe, while clean air masses flow from the Atlantic Ocean and the Arctic Ocean.