Reply to Kern, C. The Difficulty of Measuring the Absorption of Scattered Sunlight by H2O and CO2 in Volcanic Plumes: A Comment on Pering, et al. “A Novel and Inexpensive Method for Measuring Volcanic Plume Water Fluxes at High Temporal Resolution”, Remote Sens. 2017, 9, 146

We would like to thank our colleague, Christoph Kern, for his comment [1] on our recent paper [2],
which provides a valuable adjunct to that published piece. In the comment, Kern details the difficulty
of measuring water vapour in volcanic plumes at relatively low altitudes, especially considering the
importance of in-plume scattering effects [2]. In particular, Kern [1] suggests that our image-based
assessments of plume water amounts at Vulcano Island and Mt. Etna may in fact be more related
to in-plume scattering, rather than in-plume water vapour column amounts. This said, we would
respectfully argue, that as per the work of others, e.g., [3,4], that an empirical relationship between
water and measured in-plume scattering can be established, from which trends in flux data can be
determined, provided that sufficiently regular calibrations are performed. This was indeed the key
message of the article, and in our case calibration was employed. As Kern remarks, the high ambient
concentrations of CO2 and H2O in volcanic plumes do present key challenges to remote sensing of
these species in volcano plumes. One key mitigating step is to measure plumes at higher altitude,
where the overlying atmospheric column of these species will be reduced. Indeed, the possibility of
measuring plume water vapour in this scenario has recently been rather elegantly demonstrated, in the
case of Sabancaya volcano in Peru, one of the highest sources of volcanic degassing on the planet