A portable reflected-sunlight spectrometer for measuring atmospheric CO2 and CH4: Accounting for aerosols

Mapping the greenhouse gases carbon dioxide (CO2) and methane (CH4) above source regions, such as urban areas, can deliver insights into the distribution and dynamics of local emission patterns. To this end, we conduct ground-based measurements in the reflected-sun geometry, where a NIR spectrometer in an elevated position points downward at shallow viewing angles and observes reflected sunlight from a target area. From the spectra, we infer CO2 and CH4 concentrations integrated along a long (>10 km) horizontal path. Measurements in this viewing geometry are particularly sensitive to concentrations in the planetary boundary layer. We deployed our portable reflected-sun prototype instrument (termed EM27/SCA) [1] during one month in April/May 2022 on Mt. Wilson above the Los Angeles basin to perform side-by-side measurements with the stationary CLARS-FTS [2]. We find a relative precision of 0.36%–0.55% for CO2 and CH4 slant column densities and good consistency with simultaneous CLARS-FTS measurements. However, we also identify the necessity to account for radiation scattered into the ray path when performing the quantitative analysis of recorded spectra. Here, we present the instrument performance as well as our approach to account for atmospheric scattering effects. Our retrieval algorithm is based on the RemoTeC radiative transfer and retrieval algorithm, previously employed for solar backscatter satellite measurements. We showcase its performance by simultaneously inferring aerosol optical thickness, CO2 and CH4 from EM27/SCA observations.   References:[1] Löw, B. A., et al.: A portable reflected-sunlight spectrometer for CO2 and CH4, Atmos. Meas. Tech., 16, 5125–5144, https://doi.org/10.5194/amt-16-5125-2023, 2023.[2] Fu, D., et al.: Near-infrared remote sensing of Los Angeles trace gas distributions from a mountaintop site, Atmos. Meas. Tech., 7, 713–729, https://doi.org/10.5194/amt-7-713-2014, 2014.