There have been substantial advances in the ability to monitor the activity of hazardous volcanoes
in recent decades. However, obtaining early warning of eruptions remains challenging, because the
patterns and consequences of volcanic unrests are both complex and nonlinear. Measuring volcanic
gases has long been a key aspect of volcano monitoring since these mobile fluids should reach the
surface long before the magma. There has been considerable progress in methods for remote and
in-situ gas sensing, but measuring the flux of volcanic CO2—the most reliable gas precursor to an
eruption—has remained a challenge. Here we report on the first direct quantitative measurements
of the volcanic CO2 flux using a newly designed differential absorption lidar (DIAL), which were
performed at the restless Campi Flegrei volcano. We show that DIAL makes it possible to remotely
obtain volcanic CO2 flux time series with a high temporal resolution (tens of minutes) and accuracy
(<30%). The ability of this lidar to remotely sense volcanic CO2 represents a major step forward
in volcano monitoring, and will contribute improved volcanic CO2 flux inventories. Our results also
demonstrate the unusually strong degassing behavior of Campi Flegrei fumaroles in the current
ongoing state of unrest
