Quantifying subaerial fluxes of CO2 is key in a diverse range of applications, including carbon capture and storage sites, emissions from urban areas and industrial sites such as oil refineries, or forecasting volcanic eruptions. All of these have one thing in common: they represent spatially extended sources with a generally unknown spatial distribution of CO2 concentration. The conventional approach to measure CO2 fluxes is to first measure CO2 concentrations in situ at several points and estimate 2D CO2 concentration profiles. Along with the plume transport speed, the concentration profiles can then be used to compute CO2 fluxes. Active remote sensing of CO2 concentrations offers crucial advantages over in situ probing, including a spatially comprehensive measurement, a safe measurement distance, and faster acquisition, which enables real-time monitoring. This makes it also a viable complement or alternative to fence-line monitoring at industrial sites. In the last few years, technology has advanced sufficiently to allow for the realization of robust and portable remote sensing platforms that are relatively inexpensive and user friendly. Within the frameworks of the European Research Council CO2Volc and proof-of-concept CarbSens projects, such a remote sensing platform has been developed to probe CO2 emissions. It may be operated from a fixed location on the ground, from moving platforms (e.g., cars), or be airborne. The kit was used to probe CO2 concentrations and perform a feasibility test to obtain a tomographic 2D image of the subaerial CO2 distribution inside the Solfatara crater, part of arguably the most hazardous volcano in the world: Campi Flegrei near Naples, Italy. The methodology could be applied directly to industrial applications, including quantifying fugitive CO2 at storage and industrial sites. An unmanned aerial vehicle portable kit is envisaged.

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