Direct solar FTIR measurements of CO2 and HCl in the plume of Popocatépetl Volcano, Mexico

Abstract

Volcanic CO2 emissions inventories have great importance in the understanding of the geological carbon cycle. Volcanoes provide the primary pathway for solid-earth volatiles to reach the Earth’s atmosphere and have the potential to significantly contribute to the carbon-climate feedback. Volcanic carbon emissions (both passive and eruptive degassing) included in inventories, largely stem from patchy surface measurements that suffer from difficulties in removing the atmospheric background. With a 27-year-long ongoing open-vent eruption, Popocatépetl ranks as one of the highest permanent volcanic CO2 emitters worldwide and provides an excellent natural laboratory to design and experiment with new remote sensing methods for volcanic gas emission measurements. Since October 2012, infrared spectra at different spectral regions have been recorded with a solar occultation FTIR spectrometer. The near-infrared spectra allow for high precision measurements of CO2 and HCl columns. Under favorable conditions, the continuous observations during sunrise allow the reconstruction of a plume cross-section of HCl and the estimation of the emission flux using wind data. Despite that the detection of CO2 is more challenging, on April 26th, 2015 we captured a volcanic plume under favourable wind conditions which allowed us to reconstruct from this particular event a CO2 emission rate of 116.10 ± 17.2 kg/s. The volcanic HCl emission on this event was the highest detected during the 2012-2016 period. An annual average CO2 emission estimate of (41.2 ± 16.7) kg/s ((1.30 ± 0.53) Tg/yr) could be determined from a statistical treatment of the detected CO2 and HCl columns in the IR spectra, and their corresponding molecular ratios, during this period. A total of 25 events were used to derive a mean CO2/HCl molecule ratio of 11.4 ± 4.4 and an average HCl emission rate of (3.0 ± 0.3) kg/s could be determined. The CO2 emissions of Popocatépetl were found to be around 0.32% of the total anthropogenic CO2 emissions reported in the country and 3.6% of those corresponding to the Mexico City Metropolitan Area (MCMA). CO2 emissions from the Popocatépetl volcano can be considered to play a negligible role in the global CO2 budget, but should be taken into account.