Tobias Schmitt and colleagues present an 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. They compare the open-path measurements to 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.
They find that the open-path measurements are in 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.
Schmitt, T. D., Kuhn, J., Kleinschek, R., Löw, B. A., Schmitt, S., Cranton, W., Schmidt, M., Vardag, S. N., Hase, F., Griffith, D. W. T., and Butz, A.: An open-path observatory for greenhouse gases based on near-infrared Fourier transform spectroscopy, Atmos. Meas. Tech., 16, 6097–6110, https://doi.org/10.5194/amt-16-6097-2023, 2023.