Toward CO2 and CH4 measurements by ground-based observations of surface-scattered sunlight: Instrumentation and experiments

Precise knowledge of sources and sinks in the carbon cycle is desired to understand its sensitivity to climate change, and to account and verify man-made emissions. An important role herein play extended sources like urban areas. While in-situ measurements of carbon dioxide (CO₂) and methane (CH₄) are highly accurate but localized, satellites measure column-integrated concentrations over an extended footprint. Our innovative measurement technique aims at determining CO₂ and CH₄ concentrations on the scale of a few kilometers near the ground, and therefore fills the sensitivity gap between in-situ and satellite measurements.

 

Our development starts out from the EM27/SUN Fourier transform spectrometer, which is a reliable, mobile and commercially available spectrometer for the measurement of CO₂ and CH₄ column densities using direct sunlight spectra. We increased the radiometric sensitivity of the instrument by enhancing optical throughput and replacing the detector module by a thermoelectrically cooled detector. This enables the measurement of surface scattered sunlight spectra in the range of 4000 - 11000 cm^-1 under various viewing directions. Our setup is independent of sun position and exhibits a high sensitivity to the concentrations in the lower boundary layer, due to the near ground horizontal path component.

 

Here, we report progress on our instrumental developments, as well as first retrievals of column averaged CO₂ and CH₄ mole fractions from ground-scattered sunlight spectra recorded with this setup. We present the instrument modifications and extensions to the experimental setup: A Lambertian reflector allows for reference measurements without horizontal path component, a coaligned camera enables specific targeting and the motorized tracking system facilitates automated alternation between multiple targets. We characterize the setup with a spectral resolution of 0.54 ± 0.03 cm^-1 a signal to noise ratio above 200 for solar zenith angles < 85°, and precision of 1.8 ppm and 9 ppb regarding the inferred column averaged CO₂ and CH₄ mole fractions obtained from retrievals with a simple radiative transfer model, neglecting atmospheric scattering.