Direct solar FTIR measurements of CO$_2$ and HCl in the plume of Popocatépetl Volcano, Mexico

Volcanic CO$_2$ 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 CO$_2$ 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 CO$_2$ 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. ... mehrDespite that the detection of CO$_2$ 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 CO$_2$ 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 CO$_2$ 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 CO$_2$ 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 CO$_2$/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 CO$_2$ emissions of Popocatépetl were found to be around 0.32% of the total anthropogenic CO$_2$ emissions reported in the country and 3.6% of those corresponding to the Mexico City Metropolitan Area (MCMA). CO$_2$ emissions from the Popocatépetl volcano can be considered to play a negligible role in the global CO$_2$ budget, but should be taken into account.