2-D tomography of volcanic CO₂ from scanning hard-target differential absorption lidar: the case of Solfatara, Campi Flegrei (Italy)

Abstract

Solfatara is part of the active volcanic zone of Campi Flegrei (Italy), a densely populated urban area where ground uplift and increasing ground temperature are observed, connected with rising rates of CO2 emission. A major pathway of CO2 release at Campi Flegrei is diffuse soil degassing, and therefore quantifying diffuse CO2 emission rates is of vital interest. Conventional in situ probing of soil gas emissions with accumulation chambers is accurate over a small footprint but requires significant time and effort to cover large areas. An alternative approach is differential absorption lidar, which allows for a fast and spatially integrated measurement. Here, a portable hard-target differential absorption lidar has been used to acquire horizontal 1-D profiles of column-integrated CO2 concentration at the Solfatara crater. To capture heterogenic features in the CO2 distribution, a 2-D tomographic map of the CO2 distribution has been inverted from the 1-D profiles. The scan was performed one-sided, which is unfavorable for the inverse problem. Nonetheless, the result is in agreement with independent measurements and furthermore confirms an area of anomalous CO2 degassing along the eastern edge as well as the center of the Solfatara crater. The method may have important implications for measurements of degassing features that can only be accessed from limited angles, such as airborne sensing of volcanic plumes. CO2 fluxes retrieved from the 2-D map are comparable, but modestly higher than emission rates from previous studies, perhaps reflecting an increase in CO2 flux or a more integrated measurement or both.