Urban regions are major sources of carbon dioxide (CO2) and methane (CH4). Although their emissions are substantial, the quantification is challenged by the complex spatial distribution of sources. While in-situ measurements of CO2 and CH4 are highly accurate but localized, satellites measure column-integrated concentrations over an extended footprint with a long revisit period. Groundbased remote sensing using reflected sunlight can fill the sensitivity gap between both methods. For this purpose, a spectrometer is positioned above a source region, pointing towards ground targets at a shallow angle. It records solar absorption spectra containing information on CO2 and CH4 abundances integrated along the light path. Because of the extended quasi-horizontal light path component, the measurements are sensitive to near-ground concentrations and representative of urban enhancements. By repeatedly scanning through multiple targets, the method enables the mapping of temporal and spatial source patterns.

In this talk, I present the EM27/SCA, the first portable ground-based Fourier-transform spectrometer (FTS) designed for urban greenhouse gas mapping using near-infrared spectra of ground-scattered sunlight. The performance of the remote sensing setup is demonstrated through an instrument deployment on Mt. Wilson looking into the Los Angeles basin, USA. Besides reasonable precision and good agreement to the stationary CLARS-FTS, the measurements highlight that aerosol scattering is the major error source. I show that this challenge can be addressed by employing a retrieval algorithm, which explicitly accounts for aerosol scattering and mitigates the scattering-induced bias.