Towards long open-path FTIR spectrometry of CO2 and CH4 in an urban environment

Quantifying sources and sinks, as well as photochemical activity of trace gases in the lower troposphere, requires accurate measurements of the concentrations of the species of interest. While there exist in-situ measurement techniques, which are highly accurate, point-like measurements tend to suffer from insufficient representativeness, especially true for high-gradient environments, e.g., an urban setting. Hence, measuring those concentrations averaged on the length scales of a few kilometers is desirable. Further, quantifying emissions requires combining the concentration measurements with regional transport models, which cover a comparable spatial resolution.

Here, we present a long open-path setup that aims at delivering concentration averages on these scales in the urban boundary layer. Our setup is based on an IFS 125 HR Fourier transform spectrometer, which is a commercially available, high precision spectrometer for the IR to near UV regime. We use an artificial light source, which is modulated within the interferometer of the instrument. The modulated beam is then sent towards an array of retro-reflectors 1.5 km away. Sending of the initial beam and collecting of the reflected light is accomplished by a single telescope, which is coupled to the instrument via an optical fiber. The collected light is measured using an InGaS detector. In a first feasibility study, we aim at measuring CO₂ and CH₄ above Heidelberg to quantify the achievable precision and accuracy. Here we present our setup, first measurements, and ongoing developments.