Tall tower measurements with laser isotope spectrometry to investigate urban CO2 emissions in Vienna

Cities contribute significantly to global carbon dioxide (CO₂) emissions, and it is important to understand and accurately measure these emissions in order to effectively mitigate climate change. Current methods for estimating emissions, such as emission inventories, can be very uncertain at the scale of individual cities. Measurement methods that involve analyzing local atmospheric CO₂ levels and the respective stable carbon isotopic composition of CO₂ can provide additional, independent information on local emissions, particularly in terms of source contributions from combustion of different fossil fuels and natural respiration. As part of the Vienna Urban Carbon Laboratory (VUCL), a cavity-ring-down laser isotope spectrometer (G2201-i, Picarro Inc., USA) has been operating on a radio tower in Vienna’s city centre since May 2022 to measure atmospheric mixing ratios of CO₂ and stable carbon isotopic composition of CO₂ (δ¹³C) 144 m above the surface.

The overall objective here is to establish an analysis framework to best utilize these measurements in combination with tall-tower eddy covariance measurements for the identification and quantification of local CO₂ emission emitters in Vienna. Initial analysis of the half-hourly CO₂ concentrations and fluxes between May and Dec 2022 show that a night-time increase of measured CO₂ concentrations are followed by an early morning peak, due to a nocturnal build-up of surface-level CO₂ that is followed by an upward flush of CO₂ in the morning. The δ¹³C of CO₂ (based on keeling plot analysis) suggests that fluxes from natural respiration are dominant over the night. In the afternoon, the δ¹³C of CO₂ sources decreases, which may be due to an increased contribution from sources with isotopically depleted CO₂, such as traffic emissions and small-scale stationary methane combustion. We also observed higher concentrations CO₂ that are isotopically depleted, during the summer when winds came from the area southeast of the tower, which has more industrial and refinery activity. In addition to these initial results from keeling plot analysis, our presentation will also include results from the ongoing winter measurements, where we expect to see indications of enhanced methane combustion for space heating. Furthermore, results from ongoing tests of other analysis methods for identifying emitting sources (e.g., application of the miller trans model method, analysis of the data at higher temporal resolutions) will be presented.