Application of the LI‐COR CO 2 analyzer to volcanic plumes: A case study, volcán Popocatépetl, Mexico, June 7 and 10, 1995

Volcanic CO 2 emission rate data are sparse despite their potential importance for constraining the role of magma degassing in the biogeochemical cycle of carbon and for assessing volcanic hazards. We used a LI‐COR CO 2 analyzer to determine volcanic CO 2 emission rates by airborne measurements in volcanic plumes at Popocatépetl volcano on June 7 and 10, 1995. LI‐COR sample paths of ∼72 m, compared with ∼1 km for the analyzer customarily used, together with fast Fourier transforms to remove instrument noise from raw data greatly improve resolution of volcanic CO 2 anomalies. Parametric models fit to background CO 2 provide a statistical tool for distinguishing volcanic from ambient CO 2 . Global Positioning System referenced flight traverses provide vastly improved data on the shape, coherence, and spatial distribution of volcanic CO 2 in plume cross sections and contrast markedly with previous results based on traverse stacking. The continuous escape of CO 2 and SO 2 from Popocatépetl was fundamentally noneruptive and represented quiescent magma degassing from the top of a magma chamber ∼5 km deep. The average CO 2 emission rate for January‐June 1995 is estimated to be at least 6400 t d −1 , one of the highest determined for a quiescently degassing volcano, although correction for downwind dispersion effects on volcanic CO 2 indicates a higher rate of ∼9000 t d −1 . Analysis of random errors indicates emission rates have 95% confidence intervals of ∼±20%, with uncertainty contributed mostly by wind speed variance, although the variance of plume cross‐sectional areas during traversing is poorly constrained and possibly significant.