1 Introduction
Located in Central Mexico and in the vicinity of highly populated urban centers, Popocatépetl is an active stratovolcano only 50 km SE from the Mexico City Metropolitan Area (MCMA) and 30 km SW from Puebla. Large pyroclastic flows have reached the MCMA in former epochs (Siebe et al., 1996) and after 70 years of dormancy the volcano reawakened in 1994, starting its current eruption. The current activity is characterized by permanent passive degassing, interrupted by periodic dacitic dome growth episodes accompanied by strombolian-type activity and Vulcanian explosions.
Popocatépetl is known to have one of the largest contributions of volcanic gas emissions worldwide (Gerlach, 1991), with a total SO2-emission between 1993 and 2001 that is estimated to surpass the accumulated SO2-emissions from the climatic 1991 eruption of Pinatubo (Delgado et al., 2001). In current years, Popocatépetl has been estimated to be among the top ten emitters, with around 2.8% of the total global volcanic SO2 emissions (McLinden et al., 2016). The SO2 plume of Popocatépetl has occasionally been detected by air quality monitoring stations in Mexico City (Raga et al., 1999; de Foy et al., 2009) and Puebla (Juarez et al., 2005) and, under certain meteorological conditions, it can impact regional air quality. Ash emissions from the volcano also affect local aviation.
Ground-based ultraviolet (UV) remote sensing techniques have shown to be effective in the measurement of SO2 gas emissions from this active volcano from a safe distance Fickel and Delgado Granados, (2017); Platt et al. (2018); Galle et al. (2010); Delgado et al. (2001); Grutter et al. (2008); Campion et al. (2012); Campion et al. (2018); Schiavo et al. (2019). Additionally, other volcanic trace gases such as HCl, HF, SiF4 have been detected in the Popocatépetl plume by passive infrared (IR) spectroscopy and give valuable information about this volcano (Love et al., 1998; 2000; Goff et al., 2001; Stremme et al., 2011; 2012; Taquet et al., 2017; 2019).
One of the most abundant volcanic gases, carbon dioxide (CO2) is difficult to measure with remote sensing techniques because the high atmospheric CO2 background concentration means that the relative contribution of volcanic plumes to the measured total atmospheric column is generally very small. On the other hand, in situ measurements are often associated with risk and are generally temporally sparse and spatially localized. Nevertheless, the CO2 signal from the Popocatépetl plume was detected in the past by passive IR spectroscopy (Goff et al., 2001) during extraordinarily strong CO2 emission episodes of 1998, for which the high CO2/SO2 ratios were interpreted to reflect a possible episodic assimilation of limestone during magma migration (Goff et al., 2001). These events were associated with very variable CO2/SO2 ratios and therefore the reported CO2 emission rate is most likely not a representative mean value. Emission rates of up to 100 Gg/d (36.5 Tg/yr) were calculated during this exhalation (Goff et al., 2001).
Aiuppa et al. (2019) combined the estimated CO2/SO2 molecular ratio of 8.2 (Aiuppa et al., 2017), which was based on earlier successful measurements Goff et al. (2001), together with the satellite-based SO2 flux estimate by McLinden et al. (2016) and Carn et al. (2017) in order to estimate an average CO2 emission from Popocatépetl of 9.284 Gg/day, 3.4 Tg/yr or 107 kg/s. However, the assumed volcanic CO2 emissions from earlier measurements of the CO2/SO2 ratio and the extrapolation to quiescent average out-gassing phases using more recent SO2 emission estimates could overestimate the CO2 contribution of Popocatépetl to the global carbon cycle Werner et al. (2019).
It is important to measurements (Goff et al., 2001), together CO2 during various days in the typical and more frequent passive degassing state of the volcanic activity in order to obtain more statistically solid results and gain knowledge of the role these prolonged volcanic emissions play in the local, regional and global carbon budget. In recent years, the remote sensing technique using solar absorption high-resolution spectroscopy in the near-infrared has gained sufficient precision to detect enhancements of the CO2 total column of less than one percent (Wunch et al., 2011). Instruments with lower spectral resolution (Gisi et al., 2012) using a similar measurement configuration have been successful in detecting CO2 enhancements in the volcanic plume of Mount Etna (Butz et al., 2017) during field campaigns of short duration. In that study, the CO2/HCl ratios and those of other molecular ratios showed high temporal variability.
In this work, we measured the volcanic gas composition from a fixed site during an extended time period using a high-resolution Fourier Transform Infrared (FTIR) spectrometer that has been used to document not only the atmospheric variability of CO2 (Baylon et al., 2017), but also ozone (Plaza-Medina et al., 2017) and various gases of volcanic origin (Taquet et al., 2019). This station, the Altzomoni Atmospheric Observatory, forms part of the Network for Detection of Atmospheric Composition Change (NDACC) (De Mazière et al., 2018). In this paper we include information about the site, instrumentation (Section 1.1), and about our measurement strategy (Section 2). Further on, we describe how the volcanic gas emission rate is reconstructed from the given information (Section 3.1). For a particular day (26 April 2015), we present the retrieval strategy (Section 2.2) and how to deal with the airmass dependence (Section 2.2), how to improve the precision of the derived molecular ratio (Section 2.3) and calculate the cross-section (Section 3.2) and emission rate (Section 3.3). In that section, we also describe how we combine the CO2/HCl molecular ratios and the HCl emission rates to obtain a statistically more significant estimation of the total CO2 emission (Section 3.5), to provide then a discussion of the principal uncertainties (Section 3.4). The results are finally presented in Section 3.6, discussed in Section 4, and the conclusions are provided in Section 5.
1.1 Site, instrument, and measurements
Our measurements are taken from the Altzomoni Atmospheric Observatory (19.1187°N, 98.655°W, 3,985 m a.s.l.) using a high-resolution FTIR spectrometer from Bruker, model IFS HR120/5, that contributes to NDACC (De Mazière et al., 2018). The high-altitude station is located 11 km away from the crater of Popocatépetl, as can be seen in Figure 1. A solar tracker (Gisi et al., 2011) directs the radiation of the Sun to the interferometer, which measures spectra with a 0.02 cm−1 resolution (optical path difference = 45 cm), as commonly used in the Total Carbon Column Observing Network (TCCON) (Wunch et al., 2011). In the near-infrared spectral region (NIR), an aperture of 0.8 mm is chosen, which results together with the 418 mm focal length of the focusing and collimating mirror in an external and internal field of view of 2 mrad (approx 0.1°). Thus, the field of view diameter is less than 1/5 that of the Sun (the Sun has a size of approx 10 mrad or 0.5°.). The internal divergence and resulting self-apodization are not critical for the used spectral resolution. The remotely-operated FTIR instrument is started manually by an operator and then programmed to continuously measure a sequence of filters and detectors covering the range from 600 cm−1 (17 μm) to 10,000 cm−1 (1 μm). For these measurements in the NIR region, both CaF2 and KBr beam splitters are utilized and an InGaAs detector records the signal of the interferograms (4,000 - 10,000 cm−1). Unlike the MCT and InSb detectors used for the mid-infrared (MIR), the InGaAs detector is not cooled by liquid nitrogen (LN2). All applied detectors are able to record the interferograms in direct current mode making it possible to monitor the solar intensity variations and correct them before the Fourier transformation is performed (Keppel-Aleks et al., 2007). These intensity variations originate from thin clouds, or specifically in our case, from the volcanic plumes that move across the field of view during a single measurement (time of the forward and backward scan is 30-40 s). The correction, which removes smoothed intensity fluctuations of the interferogram outside of the center burst, located at zero path difference, is realized in this work using routines from the “CALPY” software package (Kiel et al., 2016) and slightly improve the precision, Supplementary Section S2.
Measurements are possible almost every morning, but rising clouds typically prevent measurements in the afternoon. As shown in Figure 2, predominant wind directions at pressure level 500 hPa (around 6 km a.s.l.) are west to east and east to west. However for the detection of the plume, wind towards the north (WD=180°) is necessary. Figure 2 shows the wind speed and direction on all days with concurrent HCl measurements. The wind data (500 hPa) are taken for the time at which the highest by PROFFIT96 calculated HCl vertical column of the corresponding day was measured. Of all measurement days, 14% (approx once per week) show a maximum HCl column greater than 1.5E17 Molec/cm2. This value is twice the median value μ (7.6E16 Molec/cm2) and ad hoc chosen to classify days with a clear volcanic signal in the measurements. The corresponding wind direction frequency distribution for days on which a volcanic plume was detected shows a distribution around the wind directions towards North. Applying different, stricter criteria for the classifying of days with volcanic event detection, as 3 or 4 times of the median value results in a subset of 10% or only 6% of all days being chosen, but the relative distribution of wind directions shows a similar pattern and is valid for the subset of 25 measurement days with a HCl slant columns 1E18 Molec/cm2 which have been chosen for the analysis of the CO2/HCl ratio Section 3.5. The PROFFIT96 output is the vertical column assuming a horizontal homogeneous atmosphere. Therefore it is simpler to classify the days using the vertical columns. However, the signal in the absorption spectrum is proportional to the slant column, which indicates whether we could expect to find volcanic CO2 in the spectra of this day. Most plume intersections are actually recorded by high solar zenith angles (sza 60°) and therefore the slant column is mostly greater than twice the vertical column, please see Supplementary Material.
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