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SO<sub>2</sub> emissions from Popocatépetl volcano: emission rates and plume imaging using optical remote sensing techniques

SO2 emissions from Popocatépetl volcano: emission rates and plume imaging using... Atmos. Chem. Phys., 8, 6655–6663, 2008 Atmospheric www.atmos-chem-phys.net/8/6655/2008/ Chemistry © Author(s) 2008. This work is distributed under the Creative Commons Attribution 3.0 License. and Physics SO emissions from Popocatepetl ´ volcano: emission rates and plume imaging using optical remote sensing techniques 1 1 2 3 4 1 5 M. Grutter , R. Basaldud , C. Rivera , R. Harig , W. Junkerman , E. Caetano , and H. Delgado-Granados Centro de Ciencias de la Atmosfera, ´ Universidad Nacional Autonoma ´ de Mexico, ´ Mexico Department of Radio and Space Science, Chalmers University of Technology, Sweden Technische Universitat ¨ Hamburg-Harburg, Germany Institut fur ¨ Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe, Germany Instituto de Geof´ ısica, Universidad Nacional Autonoma ´ de Mexico, ´ Mexico Received: 17 March 2008 – Published in Atmos. Chem. Phys. Discuss.: 23 April 2008 Revised: 15 August 2008 – Accepted: 19 September 2008 – Published: 18 November 2008 Abstract. Sulfur dioxide emissions from the Popocatepetl ´ 1 Introduction volcano in central Mexico were measured during the MI- LAGRO field campaign in March 2006. A stationary scan- Volcanic emission of gases and particles can contribute sig- ning DOAS (Differential Optical Absorption Spectrometer) nificantly to the chemistry of the atmosphere, its aerosol bud- was used to monitor the SO emissions from the volcano get and thus to the radiative forcing both in the regional and the results were compared with traverses done with a and global scales. Depending on how far these emissions COSPEC from the ground and a DOAS instrument on board are transported prior to deposition, these emissions can have an ultra-light aircraft. Daytime evolutions as well as day-to- important environmental effects. 15–21 Tg of SO are in- day variation of the SO emissions are reported. A value of jected into the atmosphere every year due to active volca- 2.45±1.39 Gg/day of SO is reported from all the daily av- noes worldwide (Halmer et al., 2002), either continuously by erages obtained during the month of March 2006, with large passive degassing or from short-lived eruptions. These emis- variation in maximum and minimum daily averages of 5.97 sions account for approx. 7.5–10.5% of the total global sulfur and 0.56 Gg/day, respectively. The large short-term fluctu- emission which has as major contributors the burning of fos- ations in the SO emissions obtained could be confirmed sil fuels, oxidation of oceanic dimethyl sulfide and biomass through 2-D visualizations of the SO plume measured with burning. SO sinks are dominated by oxidation and deposi- a scanning imaging infrared spectrometer. This instrument, tion processes and its lifetime can range from a few days to a based on the passive detection of thermal radiation from the couple of weeks, reacting mostly with OH to form H SO or 2 4 volcanic gas and analysis with FTIR spectrometry, is used being removed by clouds and aerosols. For volcanic plumes for the first time for plume visualization of a specific vol- with high water vapor content or low-altitude volcanoes near canic gas. A 48-h forward trajectory analysis indicates that the coast, this reaction might occur over hours and even tens the volcanic plume was predominantly directed towards the of minutes. During large eruptions, however, SO can be in- Puebla/Tlaxcala region (63%), followed by the Mexico City jected to higher altitudes and form longer-lived stratospheric and Cuernavaca/Cuautla regions with 19 and 18% occur- aerosols. ◦ ◦ rences, respectively. 25% of the modeled trajectories go- Popocatepetl ´ (19.02 N, 98.62 W, 5465 m a.s.l.) is a high ing towards the Puebla region reached altitudes lower than emission rate, passively degassing active volcano located 4000 m a.s.l. but all trajectories remained over this altitude 60 km south-east of Mexico City. This stratovolcano is part for the other two regions. of the Tran-Mexican volcanic belt. After being in a dormant period for nearly 70 years, it began significant fumarolic and seismic activity with a moderate eruption in December 1994 (Siebe et al., 1996). A new episode of activity began in March 1996 with pyroplastic flows and strong explosions Correspondence to: M. Grutter that occurred in June of 1997. Ash columns reached 15 km ([email protected]) a.s.l. In December of 2001, another strong eruption produced Published by Copernicus Publications on behalf of the European Geosciences Union. 6656 M. Grutter et al.: SO emissions from Popocatepetl ´ lava flows reaching the timberline and ignited fires 4.5 km 1998; Goff et al., 2001). The broad range of application of from the crater (Delgado-Granados et al., 2001; Macias and these methods as well as the ongoing work will be explained. Siebe, 2005). Frequent ash and gas emissions have continued irregularly 2 Methodologies since the reawakening of this volcano. SO emission rates have been estimated beginning early February 2004 with sev- Out of the many techniques available to analyze the com- eral COSPEC instruments, averaging 2.2 and 3.4 Gg/d in position of volcanic plumes, the spectroscopic remote sens- studies performed during the 23 December 1994–28 January ing methods are preferred due to the potential hazard in ap- 1995 (Galindo et al., 1998) and 30 January 1995–30 June proaching an active volcano for sample taking. Other ad- 1995 (Delgado-Granados et al., 2001) time periods. Other vantages include the high temporal and spatial resolution averages have been reported in the past: 2.0 Gg/d in 1994, which can be achieved from the different measurement con- 1.6 Gg/d in 1995, 15 Gg/d in 1996 and occasionally above figurations and instrument platforms from which the spec- 50 Gg/d in 1997 (Goff et al., 1998). As reported by these troscopic analysis is performed (Oppenheimer et al., 1998). authors, the total discharge of volatile gases through Novem- The COSPEC (Barringer Research correlation spectrome- ber of 1996 was similar to yearly amounts released by Mount ter) has been the most widely used instrument for volcanic Etna. plume surveillance. It is configured to measure SO column The Mexican national inventory for the year 1999 re- concentrations using sky UV radiation and estimating emis- ported annual emissions of 1871 and 735 Gg/y from the sions by combining plume cross-section and wind velocity Popocatepetl ´ and Colima volcanoes, respectively (INE- information. The instrument has been widely described by SEMARNAT, 2006), which can be translated into an aver- many authors (see for example Stoiber et al., 1983) and is age daily emission of 5.13 Gg/d from Popocatepetl ´ alone. therefore not explained here. Differential Optical Absorption This value is comparable to 6.65 Gg, which is the amount Spectrometers (DOAS) are now being implemented as more of SO emitted every year anthropogenically by the Mexico modern, light and versatile instruments. Possible platforms City Metropolitan Area from both point and mobile sources, include ground-based measurements, aircraft and satellites as has been officially reported for 2004 (SMA-GDF, 2006). (Eisinger and Burrows, 1998; Afe et al., 2004). The impor- This value, however, does not include the emissions from tance of complementing satellite observations with ground- important industries like power plants and refineries located based instrumentation has been made evident, particularly just outside the metropolitan area. For example, the Tula when attempting to quantify volcanic emissions from space industrial complex alone, located 60 km north of Mexico (Matiella Novak et al., 2008). City and 130 km NW of the volcano, emits around 0.43 Gg/d (158 Gg/y) as estimated from optical remote sensing mea- 2.1 DOAS surements carried out in 2003 (de Foy et al., 2007). Thus, the Popocatepetl ´ volcano has been an important source of DOAS is a widely used technique for the continuous mea- emissions during this past decade releasing similar or higher surement of atmospheric gases both in active and passive configurations (Platt et al., 1979; Platt, 1994). It is based amounts of SO than all the anthropogenic sources in the on the spectral analysis of the differential absorption by central region of Mexico. The objective of this investigation was to study the emis- molecules in the ultraviolet and visible part of the spectrum. sions of SO from the Popocatepetl ´ volcano during the MI- The broader extinction of UV light due to other processes LAGRO field campaign (Fast et al., 2007) and to examine such as scattering on air molecules and aerosol particles is its possible interaction with the Mexico City plume. The cancelled during the DOAS retrieval and thus not taken into measurement of the emissions of SO from Popocatepetl ´ is account. In this investigation, scattered sunlight was used important in the context of a megacity field study such as as the radiation source and the differential absorption of the MILAGRO, where sulfate production and its radiative and SO gas was analyzed and used to obtain differential slant chemical impacts are to be characterized in detail (Graf et al., columns as has been described elsewhere (Bobrowski et al., 1997). More commonly, the monitoring of gases and their 2003; Galle et al., 2003; Lee et al., 2005). relative ratios in volcanic plumes has aimed at a better under- Passive DOAS measurements were made by collecting the standing and forecasting of eruptive processes since changes scattered UV light with a narrow field-of-view (<20 mrad) in the magmatic activity are reflected in both the quantity and telescope. This consists of a convex lens (f=100 mm), a chemical composition of the emissions. Particular emphasis bandpass optical filter (Hoya U330) blocking visible light has been placed on the relative abundances of emitted gases with wavelengths higher than 360 nm to reduce stray light, such as HCl, HF, H S, SiF , CO , BrO, ClO, among oth- and a 200μm diameter quartz optical fiber. The light is an- 2 4 2 ers. Both optical remote sensing methods employed in this alyzed with a spectrometer (Ocean Optics, model S2000), at investigation to characterize SO in the infrared (FTIR) and a resolution of <0.6 nm between 280–420 nm. This device ultraviolet (DOAS) wavelength regions have been used for employs a UV holographic grating and a 2048 element CCD this purpose before (Love et al., 1998; Oppenheimer et al., detector. The instruments described below use their own Atmos. Chem. Phys., 8, 6655–6663, 2008 www.atmos-chem-phys.net/8/6655/2008/ M. Grutter et al.: SO emissions from Popocatepetl ´ 6657 acquisition and control interfaces, although the same spec- the scanning mirror is sequentially set to all positions within tral evaluation software DOASIS (Kraus, 2001) was used in the area of interest and both the video images and recorded both configurations. spectra are graphically displayed on the PC. The size and the direction of the area of interest to be measured, as well as the 2.1.1 Scanning DOAS step size (i.e. the angle between adjacent fields of view) can be varied by the operator. A complete description and speci- A scanning DOAS instrument was placed at Tlamacas station fication of this system, which has been successfully deployed ◦ ◦ (19.06 N, 98.63 W, 4000 m a.s.l.), located on the northern mostly for industrial pollution monitoring, can be found in flank of the volcano and 4.7 km from the crater. The instru- (Harig and Matz, 2001; Harig et al., 2002; Grutter et al., ment is equipped with a turning mirror and a housing hold- 2008). ing a quartz window for environmental protection. A stepper In the scanning mode, an area of interest within the video motor turns the 45 mirror which is oriented so that the light image as well as a step size is chosen. The instrument, ca- coming from the quartz window is directed towards the fo- −1 pable of measuring 6 spectra/s at 4 cm resolution, will se- cusing lenses and into the optical fiber. The elevation angle is quentially record a spectrum at each position such that an scanned from a fixed position perpendicular to the direction image of 45×30 pixels will take about 3 min to be com- of the plume propagation. A full scan of the plume is accom- pleted. The radiation measured by the spectrometer contains plished every 1–6 min from which an SO emission rate is the spectral signatures of the background atmosphere and the calculated using the slant column of every scanned position, gas cloud and the atmosphere in each position in the area of plume height, wind direction and wind speed information. A interest. detailed description of the instrument and spectra evaluation The primary objective from these passive IR observations can be found in (Edmonds et al., 2003; Galle et al., 2003; was to visualize the evolution of the SO plume. This can McGonigle et al., 2003). be accomplished by analyzing the spectra and fitting the fea- tures of the expected atmospheric gases at each position. The 2.1.2 Airborne DOAS procedure for identification of SO within the area of inter- est follows the steps described in (Harig et al., 2002). In The FZK-ENDURO Ultra-light aircraft (Junkermann, 2005) the first step, the spectra of the brightness temperatures of was used as the platform to perform downwind plume tra- a field of view with and without the plume are calculated. verses with a portable DOAS instrument. The aircraft was The first spectrum of each row of the image is used as the stationed at Hermanos Serdan ´ Airport (PBC at 19.16 N, background spectrum which is subtracted from the plume 98.37 W, 2244 m a.s.l.) near Huejotzingo, Puebla. The spectra. The reference spectra of the target gas, H O and telescope was mounted above the wing looking towards the other interfering gases are then fitted to the resulting spec- zenith and the spectra were continuously recorded after take- trum using a least-squares fitting procedure. Reference spec- off using a LabView® based interface that couples each ac- tra with different column densities are calculated by convo- quisition with a longitude-latitude fix from a GPS receiver. lution of high-resolution transmittance spectra using the HI- The software is designed to automatically set the acquisition TRAN database (Rothman et al., 2005) with an instrumental time of the spectrometer according to the light intensity. User line function (Harig, 2004). The fitting procedure includes defined parameters along with dark and background spectra an approximation of the baseline. In the next step, the contri- are entered prior to each measurement along the trajectory. butions of all fitted signatures (i.e. interferences, atmospheric The traverses were planned so as to fly around the volcano species, and baseline) except the signature of the target com- with a radius of ∼18 km, which is the permissible distance pound are subtracted from the measured spectrum. from the crater regulated by local aviation officials. In this work, the spectral range between 1095 and −1 1250 cm was selected for the analysis of SO . Spectra of 2.2 Scanning Imaging Infrared Spectrometry H O and ozone were used as interference species in the fit- A scanning imaging infrared spectrometer for visualizing the ting procedure. The column densities of SO that were used ◦ ◦ SO plume was placed at Altzomoni (19.12 N, 98.65 W, for the calculation of the reference spectra used in the fitting 4000 m a.s.l.) which is 11 km NNW of Popocatepetl ´ at the procedure were 800, 5000 and 18 000 ppm m. In order to de- flanks of Iztacc´ ıhuatl volcano. This location allows for an cide if the target compound is present, the coefficient of cor- adequate view of the gas plume at a safe distance from the relation between the corrected spectrum, i.e. the result of the active volcano. The system (SIGIS) (Harig et al., 2002) subtraction, and a reference spectrum is calculated. The cal- is based on the combination of a modified Michelson in- culation is sequentially performed for three different column terferometer (Bruker, Opag 22), a telescope (7.5 mrad), an densities of the target compound. In this final step, a color is azimuth-elevation-scanning mirror, a video camera and a assigned according to the maximum coefficient of correlation computer for control, data analysis, and display of the results. obtained in the fitting procedure and plotted at each position The video image is used as reference and to control the posi- on top of the video image, as presented in Fig. 4. tion of the scanning mirror. For visualization of gas clouds, www.atmos-chem-phys.net/8/6655/2008/ Atmos. Chem. Phys., 8, 6655–6663, 2008 6658 M. Grutter et al.: SO emissions from Popocatepetl ´ NARR 500-550 h Pa Table 1. Occurrences of the wind direction at crater height from the RS 500-550 hPa NARR and radiosonde (RS) datasets classified by regions during March 2006. Region WDR range % Occurrence Degrees NARR Frw Trajectory RS-500 HPa Mexico City 90–173 14.1 19.0 15.7 Metropolitan Area Puebla, Tlaxcala 173–310 64.1 62.9 64.7 Cuautla, Cuernavaca 310–90 21.8 18.1 19.6 Fig. 1. 3-hourly wind speed data from NCEP (NARR) model output as well as 6 and 18 h (LST) radiosonde data (RS) from SMN for the month of March 2006. All data is averaged over the 550–500 hPa range. 3 Results and discussion 19.4 0 5 10 15 20 25 30 Measurements of the SO emissions from Popocatepetl ´ were SO [ppm m] -99.0 -98.8 -98.6 -98.4 Longitude carried out during the month of March 2006. Apart from the scanning DOAS, which automatically measures slant column 0 10 20 30 40 50 60 70 cross-sections of the plume from the Tlamacas fixed site, tra- SO [ppm m] 19.2 Mexico 2 verses done with COSPEC from the ground and a portable Tlaxcala City DOAS instrument on board the ultra-light aircraft were per- 0 km formed. Calculating the emission rates from any of these Iztaccíhuatl 180 km techniques requires knowing the plume velocity and direc- 19.2 tion at the time the measurement was done. This was derived 19.0 Altzomoni from estimates of the wind speed at the altitude of the vol- PUEBLA canic emission at ∼5400 m a.s.l. The National Weather Ser- Tlamacas vice’s National Center for Environmental Prediction (NCEP) Popocatépetl 19.0 runs a series of operational computer analyses and forecasts. Cuerna- 18.8 Their North American Regional Reanalysis (NARR) prod- vaca ucts include meteorological fields such as u- and v- wind components, temperature and humidity on a 32 km grid 8 Cuautla 18.8 99.0 98.8 98.6 times 98.4 a day (Mesinger et al., 2006). The three-dimensional Longitud wind data from NARR was used to calculate the propagation velocity of the plume and the forward trajectories starting from Popocatepetl ´ at 500 hPa using the scheme proposed by 120 (Krishnamurti and Bounoua, 2000). The terrain effects are 80 taken into account in the trajectory computation through the 40 2006-03-21 60 eta vertical coordinate as well as the vertical velocity field in 0 20 40 60 80 100 120 140 160 180 areas where there is convection. However, no other convec- distance travelled [km] tion effects such as washout and wet deposition were consid- ered for the 48-h forward trajectories that were generated for Fig. 2. Map of the region around the Popocatepetl ´ volcano showing 00:00 to 21:00 UTC in three hour intervals. the results from a DOAS measurement made on board an ultra-light 20 Since the altitude of the volcanic plume varies and is not aircraft. The color scaled line represents the slant column of SO known with precision for every individual measurement, the (ppm×m) measured along the path flown on 18th March 2006. Blue 3-hourly model outputs for the 550 and 500 hPa layers, corre- dashed lines separate the main regions described in Table 1. sponding to approximate altitudes between 5100 and 5900 m 0 20000 40000 60000 80000 100000 120000 a.s.l.,140000 were averaged 160000 and are shown in Fig. 1. This altitude range is thought to contain the plume above the measurement Distancia [m] Evidence that the emissions of the Popocatepetl ´ volcano site at Tlamacas (4.7 km downwind) most of the time. Ra- can be influencing the particle formation in the Mexico City diosonde data taken in Mexico City from the Servicio Mete- area has been presented elsewhere (Raga et al., 1999). In or- orologico ´ Nacional (SMN, station 76679 located at 19.4 N, der to estimate the probability that volcanic emissions would 99.196 W, 2303 m a.s.l.) in this pressure range are also plot- affect neighboring urban areas during the MILAGRO field ted in Fig. 1. For consistency and since the radiosonde data is campaign, a frequency analysis was produced based on the assimilated by the NARR, the continuous dataset from NCEP NARR time series (500–550 hPa) for the month of March. A was used for all emission calculations throughout this work. geographical division was established defining three major Atmos. Chem. Phys., 8, 6655–6663, 2008 www.atmos-chem-phys.net/8/6655/2008/ SO [ppm/m] Latitud WSP (m/s) SO [ppm m] 28 Feb 2 Mar 4 Mar 6 Mar 8 Mar 10 Mar 12 Mar 14 Mar 16 Mar 18 Mar 20 Mar 22 Mar 24 Mar 26 Mar 28 Mar 30 Mar 1 Apr Latitude M. Grutter et al.: SO emissions from Popocatepetl ´ 6659 DOAS 160 Table 2. Daily averages and standard deviation of SO emissions U-Light COSPEC calculated from the ground-based DOAS instrument given the num- AVG_SO2 ber of observations. COSPEC and airborne-DOAS emission calcu- lations are instantaneous values from single traversals. Date Ground-based scanning DOAS U-Light DOAS COSPEC 2006 No. of samples Daily Avg. Std. Dev. [Gg/d] 4-Mar 18 0.09 8-Mar Fig. 3. Sulfur dioxide emission rates from the Popocatepetl ´ volcano 9-Mar 44 1.84 0.42 5.23 10-Mar 48 2.53 0.82 calculated from the ground-based scanning DOAS measurements 11-Mar 42 3.53 2.29 (red dots), from traverses done with a mobile DOAS on board an 12-Mar 46 1.94 0.73 ultra-light aircraft (dark blue) and with a COSPEC instrument (light 13-Mar 28 0.99 0.30 14-Mar 51 4.42 3.42 blue) in 2006. 15-Mar 19 0.78 0.27 17-Mar 18 0.63 0.46 18-Mar 28 3.41 1.85 2.71 19-Mar 49 5.97 2.70 basins containing the largest nearby metropolitan areas: 20-Mar 48 2.70 0.74 21-Mar 49 2.68 1.79 2.51 Mexico City Metropolitan Area (MCMA), Puebla/Tlaxcala 22-Mar 38 3.96 1.94 and Cuautla/Cuernavaca. For this purpose, lines where 23-Mar 5 1.93 0.26 7.23 24-Mar 18 2.36 0.61 drawn from the position of the crater in the directions 130, 25-Mar 32 2.29 1.18 270 and 353 as presented in Fig. 2, taking into account the 26-Mar 15 2.74 1.42 surrounding mountains as physical barriers and major urban- 27-Mar 32 1.05 0.38 28-Mar 5 0.79 0.38 ized centers of the above-mentioned regions. The corresponding wind direction ranges considered for this analysis are presented in Table 1. This table contains the percent occurrences from instantaneous wind direction during a specific event. Data is presented from only three datasets from NARR at the position of the crater, the final flights since not all the flight-patterns were relevant to this position of the forward trajectory calculated for 48 h and the investigation and the instrument performance and weather radiosonde 500 hPa wind speed data which fall under this conditions were not always favorable. The result from one of criterion. The results show that the volcanic plume was pre- these experiments, corresponding to 18th March, is graph- dominantly directed towards the Puebla/Tlaxcala region with ically presented in Fig. 2. The trajectory of the flight is an occurrence of about 63%, followed by the Mexico City marked with a colored line expressed in column density and Cuernavaca/Cuautla regions with approximately 19 and (ppm×m) of SO , starting south east of Iztacc´ ıhuatl volcano 18%, respectively. Three dimensional plots generated for all and ending at the Puebla airport for landing after 180 km. trajectories during the month of March revealed that 25% of the modeled trajectories going towards the Puebla/Tlaxcala 3.2 Ground-based DOAS measurements region reached altitudes below 4000 m a.s.l., while all the tra- jectories towards the other two regions remained above this altitude most of the time. An automated scanning DOAS was operated continuously from the Tlamacas site as described in Sect. 2.1. SO emis- The March mean wind field at 500mb is representative of the boreal winter over Central Mexico. The westerly winds sions were calculated using the NARR dataset throughout the are dominant form the middle to upper atmosphere. Dur- month of March for consistency. The emission results have ing the summer the weaker and moister easterly winds span been filtered so that only those data where the volcanic plume the lower atmosphere up to 500 hPa due to northward dis- was completely crossed during a full scan of the instrument placement of the trade winds. The winter pattern is modified is included. These emission values from the ground-based by cold fronts (northerly winds) and the summer patterns is DOAS instrument are plotted as red dots in Fig. 3 for the modulated by the local convection and mesoscale convective month of March 2006. It is important to note that since this systems. method requires dispersed light from the sun, only daytime values are reported. Also in this plot, the emission calcula- 3.1 Ultra-light aircraft measurements tions obtained from the three traverses done from the ultra- light aircraft (blue) and two emission calculations from the SO emissions were calculated from selected DOAS mea- COSPEC instrument are included. Ground-based traverses surements performed on board an ultra-light aircraft in order with COSPEC are performed routinely twice a month by CE- to 1) compare them with the results from the ground-based NAPRED (Centro Nacional de Prevencion ´ de Desastres) in instrument and 2) determine the plume position and width collaboration with the Instituto de Geof´ ısica of UNAM. The www.atmos-chem-phys.net/8/6655/2008/ Atmos. Chem. Phys., 8, 6655–6663, 2008 SO (kg/s) 8 Mar 9 Mar 10 Mar 11 Mar 12 Mar 13 Mar 14 Mar 15 Mar 16 Mar 17 Mar 18 Mar 19 Mar 20 Mar 21 Mar 22 Mar 23 Mar 24 Mar 25 Mar 26 Mar 27 Mar 28 Mar 29 Mar 6660 M. Grutter et al.: SO emissions from Popocatepetl ´ average of all measurements from the ground-based DOAS to visualize the plume shape and monitor its temporal evo- instrument, reported as 31.7 kg/s, is represented in the graph lution. The instrument, described in Sect. 2.2, was placed at as a horizontal line. Altzomoni and was able to detect the SO signature in the infrared spectra collected from a distance of 11 km from the The individual averages for every day measured during the month of March 2006 are presented in Table 2. Instanta- crater. This detection is represented in a two-dimensional im- neous emission determinations from traverses performed on age according to the coefficient of correlation R with respect selected days from the ground and from the air are also tab- to a reference spectrum of SO as seen in the example shown ulated for comparison. Values obtained from the instrument in Fig. 4 for 16 March. on board the ultra-light aircraft agree with the daily averages These images include the black and white video image of from the ground based DOAS falling within their standard the volcano at the beginning of each scan and the false color deviations on both days where the data is available. On the image of the SO plume. For clarity, only spectra with R- other hand, the emission calculation from the COSPEC mea- values greater than 0.97 are plotted in order to better separate surements performed on the 9th and 23rd are 2.8 and 3.7 the plume shape from the background. In this particular case, times higher than the daily average reported from the ground- the duration of each scan was 3 min and 14 s, although the based DOAS. Unfortunately, no direct comparison between scan time generally varied depending on the size of the area the COPSPEC and DOAS instruments was done in a single chosen. It is evident from these observations that the plume transect but a previous work aiming specifically at this (Elias changes significantly from scan to scan. There are two poten- et al., 2006) shows that the retrieved slant columns should tial reasons for this. The first reason is that changes in wind not differ by more than 10% from properly calibrated instru- speed lead to a variability of the number of molecules present ments. Thus, a poor calibration of the COSPEC instrument along the optical path if the emission rate is constant. The is not discarded but more probable causes for the observed second reason is that the emission rate of the gas is not con- discrepancies are explained below. stant. The presence of these “puffs” or events of higher emis- It was observed from the wind trajectories calculated from sion can be investigated by analysis of consecutive scans. NARR data, that often the wind direction changes and makes abrupt turns along the path of the plume. Although a geomet- 4 Conclusions rical correction is taken into account for the flux calculation, this considers a linear propagation between the origin of the Knowing Popocatepetl’ ´ s SO emission source strength is im- emission and the position at which the SO -peak was ob- 2 portant to assess its potential contribution to the atmospheric served. If, however, the wind direction at the position where chemistry, aerosol formation and its radiative implications in the plume is crossed has changed and is considerably differ- the central region of Mexico. Optical remote sensing meth- ent from the one used for the correction, the cross-section ods were deployed for this purpose during the MILAGRO and thus the flux will be overestimated. Another possibility international field campaign. An average of 2.45±1.39 Mg would be for a difference in radiative transfer. It has been of SO were released every day to the atmosphere dur- shown that measured SO slant-column densities decrease ing the month of March 2006 as determined by a passive with distance to the plume due to UV scattering (Mori et al., DOAS instrument continuously measuring from the ground 2006). If the scanning DOAS is measuring a plume close and confirmed by traverses done with a similar instrument to the horizon while the zenith-looking COSPEC measure- from an ultra-light aircraft. A frequency analysis of the ment has a shorter distance to the plume, then the flux from 48-h forward trajectories suggests that the emissions from the DOAS would be underestimated. The geometrical factor Popocatepetl ´ were transported towards Puebla/Tlaxcala ap- and thus overestimation of the COSPEC flux is, nevertheless, proximately 63% of the time during the month of March thought to be the most probable cause for the discrepancies. 2006. At this altitude the wind direction towards the Mex- This is due to the fact that the traverses are performed at long ico City Metropolitan Area, located only 60 km NW of the distances (20–40 km) from the source and wind is expected crater, accounted for only 19% of the occurrence during this to change directions as was commonly seen in the wind tra- period and none of these trajectories crossed below 4000 m jectories. a.s.l. The observed mean wind field at 500 hPa is typical for Two days with particularly high emission of SO were the month of March (de Foy et al., 2008) and is representative the 14th and 19th, with daily averages reaching 4.42 and only for the boreal winter over Central Mexico. 5.97 Gg/d, respectively. The lowest activity was recorded on The SO emitted by the volcano originates at an altitude the 13, 15, 17 and 28th, all with emissions below 1 Gg/d. well above the planetary boundary layer and is not expected to impact the metropolitan areas directly most of the time 3.3 Plume visualization unless strong convective conditions are present. This could A large variability in the SO emission is evident from the be observed only in the case of the Puebla/Tlaxcala direc- DOAS measurements shown above. A scanning imaging in- tion, where 25% of the calculated trajectories cross at some frared spectrometer was deployed on selected days in order point below an altitude of 4000 m a.s.l. However, sulfate Atmos. Chem. Phys., 8, 6655–6663, 2008 www.atmos-chem-phys.net/8/6655/2008/ M. Grutter et al.: SO emissions from Popocatepetl ´ 6661 Fig. 4. SO plume visualization of the Popocatepetl ´ volcano by passive infrared spectroscopy during 17th March 2006. White numbers indicate the local time. containing particles formed as a result of these emissions are would eliminate much of the uncertainty in flux estimation) more likely to interact with the urban pollution as found in and 5) can be used to measure other gases like HCl, HF, SiF a previous study (Raga et al., 1999). It would be important (although at higher spectral resolutions) and report their rela- to further investigate the fate of these emissions by modeling tive abundances. Work is in progress to achieve this and also not only their trajectories, but also the chemical and physical to determine column densities of SO from the measured IR transformations along their path. spectra which would provide an alternative method for esti- mating emissions. A scanning imaging infrared spectrometer was used to vi- sualize the dispersion of the sulfur dioxide plume and inves- Acknowledgements. This project was partly funded by CONACyT tigate the large fluctuations observed in the emissions. The (grant # 41531), UNAM (PAPIIT # IN113306) and IMK-IFU. The thermal infrared radiation of the emitted gases was collected NOVAC (Network for Observation of Volcanic and Atmospheric and used to detect the SO emission band from a distance of Change) project is acknowledged for providing the ground-based 11 km. Two-dimensional images of the detected SO sig- DOAS measurements used in this study. We are particularly nature were generated to determine the plume shape and grateful for the assistance of the National Park Service and its monitor plume evolution. These observations confirm that staff (A. Lopez, A. Tagle and J. Rodriguez). D. Baumgardner is gaseous emissions from the volcano are not continuous but acknowledged for his collaboration and support during this project. appear rather as “puffs”. This spectroscopic technique, used Edited by: L. Molina for the first time for plume visualization of a specific volcanic gas using its thermal radiation, represents important progress for the surveillance of volcanic activity since 1) it can oper- ate day or night, 2) can handle cloudy conditions for gas de- References tection as long as the clouds are not between the instrument and the plume, 3) can visualize shape, direction and evolu- Afe, O. T., Richter, A., Sierk, B., Wittrock, F., and Burrows, J. tion of a volcanic plume, 4) can allow for the determination P.: BrO emission from volcanoes: A survey using GOME and of the plume’s velocity by analyzing sequential images (this SCIAMACHY measurements, Geophys. Res. Lett., 31, 24, 2004. www.atmos-chem-phys.net/8/6655/2008/ Atmos. Chem. Phys., 8, 6655–6663, 2008 6662 M. 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SO<sub>2</sub> emissions from Popocatépetl volcano: emission rates and plume imaging using optical remote sensing techniques

M., Grutter,; R., Basaldud,; C., Rivera,; R., Harig,; W., Junkerman,; E., Caetano,; H., Delgado-Granados,
Atmospheric Chemistry and Physics – Nov 18, 2008
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Atmos. Chem. Phys., 8, 6655–6663, 2008 Atmospheric www.atmos-chem-phys.net/8/6655/2008/ Chemistry © Author(s) 2008. This work is distributed under the Creative Commons Attribution 3.0 License. and Physics SO emissions from Popocatepetl ´ volcano: emission rates and plume imaging using optical remote sensing techniques 1 1 2 3 4 1 5 M. Grutter , R. Basaldud , C. Rivera , R. Harig , W. Junkerman , E. Caetano , and H. Delgado-Granados Centro de Ciencias de la Atmosfera, ´ Universidad Nacional Autonoma ´ de Mexico, ´ Mexico Department of Radio and Space Science, Chalmers University of Technology, Sweden Technische Universitat ¨ Hamburg-Harburg, Germany Institut fur ¨ Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe, Germany Instituto de Geof´ ısica, Universidad Nacional Autonoma ´ de Mexico, ´ Mexico Received: 17 March 2008 – Published in Atmos. Chem. Phys. Discuss.: 23 April 2008 Revised: 15 August 2008 – Accepted: 19 September 2008 – Published: 18 November 2008 Abstract. Sulfur dioxide emissions from the Popocatepetl ´ 1 Introduction volcano in central Mexico were measured during the MI- LAGRO field campaign in March 2006. A stationary scan- Volcanic emission of gases and particles can contribute sig- ning DOAS (Differential Optical Absorption Spectrometer) nificantly to the chemistry of the atmosphere, its aerosol bud- was used to monitor the SO emissions from the volcano get and thus to the radiative forcing both in the regional and the results were compared with traverses done with a and global scales. Depending on how far these emissions COSPEC from the ground and a DOAS instrument on board are transported prior to deposition, these emissions can have an ultra-light aircraft. Daytime evolutions as well as day-to- important environmental effects. 15–21 Tg of SO are in- day variation of the SO emissions are reported. A value of jected into the atmosphere every year due to active volca- 2.45±1.39 Gg/day of SO is reported from all the daily av- noes worldwide (Halmer et al., 2002), either continuously by erages obtained during the month of March 2006, with large passive degassing or from short-lived eruptions. These emis- variation in maximum and minimum daily averages of 5.97 sions account for approx. 7.5–10.5% of the total global sulfur and 0.56 Gg/day, respectively. The large short-term fluctu- emission which has as major contributors the burning of fos- ations in the SO emissions obtained could be confirmed sil fuels, oxidation of oceanic dimethyl sulfide and biomass through 2-D visualizations of the SO plume measured with burning. SO sinks are dominated by oxidation and deposi- a scanning imaging infrared spectrometer. This instrument, tion processes and its lifetime can range from a few days to a based on the passive detection of thermal radiation from the couple of weeks, reacting mostly with OH to form H SO or 2 4 volcanic gas and analysis with FTIR spectrometry, is used being removed by clouds and aerosols. For volcanic plumes for the first time for plume visualization of a specific vol- with high water vapor content or low-altitude volcanoes near canic gas. A 48-h forward trajectory analysis indicates that the coast, this reaction might occur over hours and even tens the volcanic plume was predominantly directed towards the of minutes. During large eruptions, however, SO can be in- Puebla/Tlaxcala region (63%), followed by the Mexico City jected to higher altitudes and form longer-lived stratospheric and Cuernavaca/Cuautla regions with 19 and 18% occur- aerosols. ◦ ◦ rences, respectively. 25% of the modeled trajectories go- Popocatepetl ´ (19.02 N, 98.62 W, 5465 m a.s.l.) is a high ing towards the Puebla region reached altitudes lower than emission rate, passively degassing active volcano located 4000 m a.s.l. but all trajectories remained over this altitude 60 km south-east of Mexico City. This stratovolcano is part for the other two regions. of the Tran-Mexican volcanic belt. After being in a dormant period for nearly 70 years, it began significant fumarolic and seismic activity with a moderate eruption in December 1994 (Siebe et al., 1996). A new episode of activity began in March 1996 with pyroplastic flows and strong explosions Correspondence to: M. Grutter that occurred in June of 1997. Ash columns reached 15 km ([email protected]) a.s.l. In December of 2001, another strong eruption produced Published by Copernicus Publications on behalf of the European Geosciences Union. 6656 M. Grutter et al.: SO emissions from Popocatepetl ´ lava flows reaching the timberline and ignited fires 4.5 km 1998; Goff et al., 2001). The broad range of application of from the crater (Delgado-Granados et al., 2001; Macias and these methods as well as the ongoing work will be explained. Siebe, 2005). Frequent ash and gas emissions have continued irregularly 2 Methodologies since the reawakening of this volcano. SO emission rates have been estimated beginning early February 2004 with sev- Out of the many techniques available to analyze the com- eral COSPEC instruments, averaging 2.2 and 3.4 Gg/d in position of volcanic plumes, the spectroscopic remote sens- studies performed during the 23 December 1994–28 January ing methods are preferred due to the potential hazard in ap- 1995 (Galindo et al., 1998) and 30 January 1995–30 June proaching an active volcano for sample taking. Other ad- 1995 (Delgado-Granados et al., 2001) time periods. Other vantages include the high temporal and spatial resolution averages have been reported in the past: 2.0 Gg/d in 1994, which can be achieved from the different measurement con- 1.6 Gg/d in 1995, 15 Gg/d in 1996 and occasionally above figurations and instrument platforms from which the spec- 50 Gg/d in 1997 (Goff et al., 1998). As reported by these troscopic analysis is performed (Oppenheimer et al., 1998). authors, the total discharge of volatile gases through Novem- The COSPEC (Barringer Research correlation spectrome- ber of 1996 was similar to yearly amounts released by Mount ter) has been the most widely used instrument for volcanic Etna. plume surveillance. It is configured to measure SO column The Mexican national inventory for the year 1999 re- concentrations using sky UV radiation and estimating emis- ported annual emissions of 1871 and 735 Gg/y from the sions by combining plume cross-section and wind velocity Popocatepetl ´ and Colima volcanoes, respectively (INE- information. The instrument has been widely described by SEMARNAT, 2006), which can be translated into an aver- many authors (see for example Stoiber et al., 1983) and is age daily emission of 5.13 Gg/d from Popocatepetl ´ alone. therefore not explained here. Differential Optical Absorption This value is comparable to 6.65 Gg, which is the amount Spectrometers (DOAS) are now being implemented as more of SO emitted every year anthropogenically by the Mexico modern, light and versatile instruments. Possible platforms City Metropolitan Area from both point and mobile sources, include ground-based measurements, aircraft and satellites as has been officially reported for 2004 (SMA-GDF, 2006). (Eisinger and Burrows, 1998; Afe et al., 2004). The impor- This value, however, does not include the emissions from tance of complementing satellite observations with ground- important industries like power plants and refineries located based instrumentation has been made evident, particularly just outside the metropolitan area. For example, the Tula when attempting to quantify volcanic emissions from space industrial complex alone, located 60 km north of Mexico (Matiella Novak et al., 2008). City and 130 km NW of the volcano, emits around 0.43 Gg/d (158 Gg/y) as estimated from optical remote sensing mea- 2.1 DOAS surements carried out in 2003 (de Foy et al., 2007). Thus, the Popocatepetl ´ volcano has been an important source of DOAS is a widely used technique for the continuous mea- emissions during this past decade releasing similar or higher surement of atmospheric gases both in active and passive configurations (Platt et al., 1979; Platt, 1994). It is based amounts of SO than all the anthropogenic sources in the on the spectral analysis of the differential absorption by central region of Mexico. The objective of this investigation was to study the emis- molecules in the ultraviolet and visible part of the spectrum. sions of SO from the Popocatepetl ´ volcano during the MI- The broader extinction of UV light due to other processes LAGRO field campaign (Fast et al., 2007) and to examine such as scattering on air molecules and aerosol particles is its possible interaction with the Mexico City plume. The cancelled during the DOAS retrieval and thus not taken into measurement of the emissions of SO from Popocatepetl ´ is account. In this investigation, scattered sunlight was used important in the context of a megacity field study such as as the radiation source and the differential absorption of the MILAGRO, where sulfate production and its radiative and SO gas was analyzed and used to obtain differential slant chemical impacts are to be characterized in detail (Graf et al., columns as has been described elsewhere (Bobrowski et al., 1997). More commonly, the monitoring of gases and their 2003; Galle et al., 2003; Lee et al., 2005). relative ratios in volcanic plumes has aimed at a better under- Passive DOAS measurements were made by collecting the standing and forecasting of eruptive processes since changes scattered UV light with a narrow field-of-view (<20 mrad) in the magmatic activity are reflected in both the quantity and telescope. This consists of a convex lens (f=100 mm), a chemical composition of the emissions. Particular emphasis bandpass optical filter (Hoya U330) blocking visible light has been placed on the relative abundances of emitted gases with wavelengths higher than 360 nm to reduce stray light, such as HCl, HF, H S, SiF , CO , BrO, ClO, among oth- and a 200μm diameter quartz optical fiber. The light is an- 2 4 2 ers. Both optical remote sensing methods employed in this alyzed with a spectrometer (Ocean Optics, model S2000), at investigation to characterize SO in the infrared (FTIR) and a resolution of <0.6 nm between 280–420 nm. This device ultraviolet (DOAS) wavelength regions have been used for employs a UV holographic grating and a 2048 element CCD this purpose before (Love et al., 1998; Oppenheimer et al., detector. The instruments described below use their own Atmos. Chem. Phys., 8, 6655–6663, 2008 www.atmos-chem-phys.net/8/6655/2008/ M. Grutter et al.: SO emissions from Popocatepetl ´ 6657 acquisition and control interfaces, although the same spec- the scanning mirror is sequentially set to all positions within tral evaluation software DOASIS (Kraus, 2001) was used in the area of interest and both the video images and recorded both configurations. spectra are graphically displayed on the PC. The size and the direction of the area of interest to be measured, as well as the 2.1.1 Scanning DOAS step size (i.e. the angle between adjacent fields of view) can be varied by the operator. A complete description and speci- A scanning DOAS instrument was placed at Tlamacas station fication of this system, which has been successfully deployed ◦ ◦ (19.06 N, 98.63 W, 4000 m a.s.l.), located on the northern mostly for industrial pollution monitoring, can be found in flank of the volcano and 4.7 km from the crater. The instru- (Harig and Matz, 2001; Harig et al., 2002; Grutter et al., ment is equipped with a turning mirror and a housing hold- 2008). ing a quartz window for environmental protection. A stepper In the scanning mode, an area of interest within the video motor turns the 45 mirror which is oriented so that the light image as well as a step size is chosen. The instrument, ca- coming from the quartz window is directed towards the fo- −1 pable of measuring 6 spectra/s at 4 cm resolution, will se- cusing lenses and into the optical fiber. The elevation angle is quentially record a spectrum at each position such that an scanned from a fixed position perpendicular to the direction image of 45×30 pixels will take about 3 min to be com- of the plume propagation. A full scan of the plume is accom- pleted. The radiation measured by the spectrometer contains plished every 1–6 min from which an SO emission rate is the spectral signatures of the background atmosphere and the calculated using the slant column of every scanned position, gas cloud and the atmosphere in each position in the area of plume height, wind direction and wind speed information. A interest. detailed description of the instrument and spectra evaluation The primary objective from these passive IR observations can be found in (Edmonds et al., 2003; Galle et al., 2003; was to visualize the evolution of the SO plume. This can McGonigle et al., 2003). be accomplished by analyzing the spectra and fitting the fea- tures of the expected atmospheric gases at each position. The 2.1.2 Airborne DOAS procedure for identification of SO within the area of inter- est follows the steps described in (Harig et al., 2002). In The FZK-ENDURO Ultra-light aircraft (Junkermann, 2005) the first step, the spectra of the brightness temperatures of was used as the platform to perform downwind plume tra- a field of view with and without the plume are calculated. verses with a portable DOAS instrument. The aircraft was The first spectrum of each row of the image is used as the stationed at Hermanos Serdan ´ Airport (PBC at 19.16 N, background spectrum which is subtracted from the plume 98.37 W, 2244 m a.s.l.) near Huejotzingo, Puebla. The spectra. The reference spectra of the target gas, H O and telescope was mounted above the wing looking towards the other interfering gases are then fitted to the resulting spec- zenith and the spectra were continuously recorded after take- trum using a least-squares fitting procedure. Reference spec- off using a LabView® based interface that couples each ac- tra with different column densities are calculated by convo- quisition with a longitude-latitude fix from a GPS receiver. lution of high-resolution transmittance spectra using the HI- The software is designed to automatically set the acquisition TRAN database (Rothman et al., 2005) with an instrumental time of the spectrometer according to the light intensity. User line function (Harig, 2004). The fitting procedure includes defined parameters along with dark and background spectra an approximation of the baseline. In the next step, the contri- are entered prior to each measurement along the trajectory. butions of all fitted signatures (i.e. interferences, atmospheric The traverses were planned so as to fly around the volcano species, and baseline) except the signature of the target com- with a radius of ∼18 km, which is the permissible distance pound are subtracted from the measured spectrum. from the crater regulated by local aviation officials. In this work, the spectral range between 1095 and −1 1250 cm was selected for the analysis of SO . Spectra of 2.2 Scanning Imaging Infrared Spectrometry H O and ozone were used as interference species in the fit- A scanning imaging infrared spectrometer for visualizing the ting procedure. The column densities of SO that were used ◦ ◦ SO plume was placed at Altzomoni (19.12 N, 98.65 W, for the calculation of the reference spectra used in the fitting 4000 m a.s.l.) which is 11 km NNW of Popocatepetl ´ at the procedure were 800, 5000 and 18 000 ppm m. In order to de- flanks of Iztacc´ ıhuatl volcano. This location allows for an cide if the target compound is present, the coefficient of cor- adequate view of the gas plume at a safe distance from the relation between the corrected spectrum, i.e. the result of the active volcano. The system (SIGIS) (Harig et al., 2002) subtraction, and a reference spectrum is calculated. The cal- is based on the combination of a modified Michelson in- culation is sequentially performed for three different column terferometer (Bruker, Opag 22), a telescope (7.5 mrad), an densities of the target compound. In this final step, a color is azimuth-elevation-scanning mirror, a video camera and a assigned according to the maximum coefficient of correlation computer for control, data analysis, and display of the results. obtained in the fitting procedure and plotted at each position The video image is used as reference and to control the posi- on top of the video image, as presented in Fig. 4. tion of the scanning mirror. For visualization of gas clouds, www.atmos-chem-phys.net/8/6655/2008/ Atmos. Chem. Phys., 8, 6655–6663, 2008 6658 M. Grutter et al.: SO emissions from Popocatepetl ´ NARR 500-550 h Pa Table 1. Occurrences of the wind direction at crater height from the RS 500-550 hPa NARR and radiosonde (RS) datasets classified by regions during March 2006. Region WDR range % Occurrence Degrees NARR Frw Trajectory RS-500 HPa Mexico City 90–173 14.1 19.0 15.7 Metropolitan Area Puebla, Tlaxcala 173–310 64.1 62.9 64.7 Cuautla, Cuernavaca 310–90 21.8 18.1 19.6 Fig. 1. 3-hourly wind speed data from NCEP (NARR) model output as well as 6 and 18 h (LST) radiosonde data (RS) from SMN for the month of March 2006. All data is averaged over the 550–500 hPa range. 3 Results and discussion 19.4 0 5 10 15 20 25 30 Measurements of the SO emissions from Popocatepetl ´ were SO [ppm m] -99.0 -98.8 -98.6 -98.4 Longitude carried out during the month of March 2006. Apart from the scanning DOAS, which automatically measures slant column 0 10 20 30 40 50 60 70 cross-sections of the plume from the Tlamacas fixed site, tra- SO [ppm m] 19.2 Mexico 2 verses done with COSPEC from the ground and a portable Tlaxcala City DOAS instrument on board the ultra-light aircraft were per- 0 km formed. Calculating the emission rates from any of these Iztaccíhuatl 180 km techniques requires knowing the plume velocity and direc- 19.2 tion at the time the measurement was done. This was derived 19.0 Altzomoni from estimates of the wind speed at the altitude of the vol- PUEBLA canic emission at ∼5400 m a.s.l. The National Weather Ser- Tlamacas vice’s National Center for Environmental Prediction (NCEP) Popocatépetl 19.0 runs a series of operational computer analyses and forecasts. Cuerna- 18.8 Their North American Regional Reanalysis (NARR) prod- vaca ucts include meteorological fields such as u- and v- wind components, temperature and humidity on a 32 km grid 8 Cuautla 18.8 99.0 98.8 98.6 times 98.4 a day (Mesinger et al., 2006). The three-dimensional Longitud wind data from NARR was used to calculate the propagation velocity of the plume and the forward trajectories starting from Popocatepetl ´ at 500 hPa using the scheme proposed by 120 (Krishnamurti and Bounoua, 2000). The terrain effects are 80 taken into account in the trajectory computation through the 40 2006-03-21 60 eta vertical coordinate as well as the vertical velocity field in 0 20 40 60 80 100 120 140 160 180 areas where there is convection. However, no other convec- distance travelled [km] tion effects such as washout and wet deposition were consid- ered for the 48-h forward trajectories that were generated for Fig. 2. Map of the region around the Popocatepetl ´ volcano showing 00:00 to 21:00 UTC in three hour intervals. the results from a DOAS measurement made on board an ultra-light 20 Since the altitude of the volcanic plume varies and is not aircraft. The color scaled line represents the slant column of SO known with precision for every individual measurement, the (ppm×m) measured along the path flown on 18th March 2006. Blue 3-hourly model outputs for the 550 and 500 hPa layers, corre- dashed lines separate the main regions described in Table 1. sponding to approximate altitudes between 5100 and 5900 m 0 20000 40000 60000 80000 100000 120000 a.s.l.,140000 were averaged 160000 and are shown in Fig. 1. This altitude range is thought to contain the plume above the measurement Distancia [m] Evidence that the emissions of the Popocatepetl ´ volcano site at Tlamacas (4.7 km downwind) most of the time. Ra- can be influencing the particle formation in the Mexico City diosonde data taken in Mexico City from the Servicio Mete- area has been presented elsewhere (Raga et al., 1999). In or- orologico ´ Nacional (SMN, station 76679 located at 19.4 N, der to estimate the probability that volcanic emissions would 99.196 W, 2303 m a.s.l.) in this pressure range are also plot- affect neighboring urban areas during the MILAGRO field ted in Fig. 1. For consistency and since the radiosonde data is campaign, a frequency analysis was produced based on the assimilated by the NARR, the continuous dataset from NCEP NARR time series (500–550 hPa) for the month of March. A was used for all emission calculations throughout this work. geographical division was established defining three major Atmos. Chem. Phys., 8, 6655–6663, 2008 www.atmos-chem-phys.net/8/6655/2008/ SO [ppm/m] Latitud WSP (m/s) SO [ppm m] 28 Feb 2 Mar 4 Mar 6 Mar 8 Mar 10 Mar 12 Mar 14 Mar 16 Mar 18 Mar 20 Mar 22 Mar 24 Mar 26 Mar 28 Mar 30 Mar 1 Apr Latitude M. Grutter et al.: SO emissions from Popocatepetl ´ 6659 DOAS 160 Table 2. Daily averages and standard deviation of SO emissions U-Light COSPEC calculated from the ground-based DOAS instrument given the num- AVG_SO2 ber of observations. COSPEC and airborne-DOAS emission calcu- lations are instantaneous values from single traversals. Date Ground-based scanning DOAS U-Light DOAS COSPEC 2006 No. of samples Daily Avg. Std. Dev. [Gg/d] 4-Mar 18 0.09 8-Mar Fig. 3. Sulfur dioxide emission rates from the Popocatepetl ´ volcano 9-Mar 44 1.84 0.42 5.23 10-Mar 48 2.53 0.82 calculated from the ground-based scanning DOAS measurements 11-Mar 42 3.53 2.29 (red dots), from traverses done with a mobile DOAS on board an 12-Mar 46 1.94 0.73 ultra-light aircraft (dark blue) and with a COSPEC instrument (light 13-Mar 28 0.99 0.30 14-Mar 51 4.42 3.42 blue) in 2006. 15-Mar 19 0.78 0.27 17-Mar 18 0.63 0.46 18-Mar 28 3.41 1.85 2.71 19-Mar 49 5.97 2.70 basins containing the largest nearby metropolitan areas: 20-Mar 48 2.70 0.74 21-Mar 49 2.68 1.79 2.51 Mexico City Metropolitan Area (MCMA), Puebla/Tlaxcala 22-Mar 38 3.96 1.94 and Cuautla/Cuernavaca. For this purpose, lines where 23-Mar 5 1.93 0.26 7.23 24-Mar 18 2.36 0.61 drawn from the position of the crater in the directions 130, 25-Mar 32 2.29 1.18 270 and 353 as presented in Fig. 2, taking into account the 26-Mar 15 2.74 1.42 surrounding mountains as physical barriers and major urban- 27-Mar 32 1.05 0.38 28-Mar 5 0.79 0.38 ized centers of the above-mentioned regions. The corresponding wind direction ranges considered for this analysis are presented in Table 1. This table contains the percent occurrences from instantaneous wind direction during a specific event. Data is presented from only three datasets from NARR at the position of the crater, the final flights since not all the flight-patterns were relevant to this position of the forward trajectory calculated for 48 h and the investigation and the instrument performance and weather radiosonde 500 hPa wind speed data which fall under this conditions were not always favorable. The result from one of criterion. The results show that the volcanic plume was pre- these experiments, corresponding to 18th March, is graph- dominantly directed towards the Puebla/Tlaxcala region with ically presented in Fig. 2. The trajectory of the flight is an occurrence of about 63%, followed by the Mexico City marked with a colored line expressed in column density and Cuernavaca/Cuautla regions with approximately 19 and (ppm×m) of SO , starting south east of Iztacc´ ıhuatl volcano 18%, respectively. Three dimensional plots generated for all and ending at the Puebla airport for landing after 180 km. trajectories during the month of March revealed that 25% of the modeled trajectories going towards the Puebla/Tlaxcala 3.2 Ground-based DOAS measurements region reached altitudes below 4000 m a.s.l., while all the tra- jectories towards the other two regions remained above this altitude most of the time. An automated scanning DOAS was operated continuously from the Tlamacas site as described in Sect. 2.1. SO emis- The March mean wind field at 500mb is representative of the boreal winter over Central Mexico. The westerly winds sions were calculated using the NARR dataset throughout the are dominant form the middle to upper atmosphere. Dur- month of March for consistency. The emission results have ing the summer the weaker and moister easterly winds span been filtered so that only those data where the volcanic plume the lower atmosphere up to 500 hPa due to northward dis- was completely crossed during a full scan of the instrument placement of the trade winds. The winter pattern is modified is included. These emission values from the ground-based by cold fronts (northerly winds) and the summer patterns is DOAS instrument are plotted as red dots in Fig. 3 for the modulated by the local convection and mesoscale convective month of March 2006. It is important to note that since this systems. method requires dispersed light from the sun, only daytime values are reported. Also in this plot, the emission calcula- 3.1 Ultra-light aircraft measurements tions obtained from the three traverses done from the ultra- light aircraft (blue) and two emission calculations from the SO emissions were calculated from selected DOAS mea- COSPEC instrument are included. Ground-based traverses surements performed on board an ultra-light aircraft in order with COSPEC are performed routinely twice a month by CE- to 1) compare them with the results from the ground-based NAPRED (Centro Nacional de Prevencion ´ de Desastres) in instrument and 2) determine the plume position and width collaboration with the Instituto de Geof´ ısica of UNAM. The www.atmos-chem-phys.net/8/6655/2008/ Atmos. Chem. Phys., 8, 6655–6663, 2008 SO (kg/s) 8 Mar 9 Mar 10 Mar 11 Mar 12 Mar 13 Mar 14 Mar 15 Mar 16 Mar 17 Mar 18 Mar 19 Mar 20 Mar 21 Mar 22 Mar 23 Mar 24 Mar 25 Mar 26 Mar 27 Mar 28 Mar 29 Mar 6660 M. Grutter et al.: SO emissions from Popocatepetl ´ average of all measurements from the ground-based DOAS to visualize the plume shape and monitor its temporal evo- instrument, reported as 31.7 kg/s, is represented in the graph lution. The instrument, described in Sect. 2.2, was placed at as a horizontal line. Altzomoni and was able to detect the SO signature in the infrared spectra collected from a distance of 11 km from the The individual averages for every day measured during the month of March 2006 are presented in Table 2. Instanta- crater. This detection is represented in a two-dimensional im- neous emission determinations from traverses performed on age according to the coefficient of correlation R with respect selected days from the ground and from the air are also tab- to a reference spectrum of SO as seen in the example shown ulated for comparison. Values obtained from the instrument in Fig. 4 for 16 March. on board the ultra-light aircraft agree with the daily averages These images include the black and white video image of from the ground based DOAS falling within their standard the volcano at the beginning of each scan and the false color deviations on both days where the data is available. On the image of the SO plume. For clarity, only spectra with R- other hand, the emission calculation from the COSPEC mea- values greater than 0.97 are plotted in order to better separate surements performed on the 9th and 23rd are 2.8 and 3.7 the plume shape from the background. In this particular case, times higher than the daily average reported from the ground- the duration of each scan was 3 min and 14 s, although the based DOAS. Unfortunately, no direct comparison between scan time generally varied depending on the size of the area the COPSPEC and DOAS instruments was done in a single chosen. It is evident from these observations that the plume transect but a previous work aiming specifically at this (Elias changes significantly from scan to scan. There are two poten- et al., 2006) shows that the retrieved slant columns should tial reasons for this. The first reason is that changes in wind not differ by more than 10% from properly calibrated instru- speed lead to a variability of the number of molecules present ments. Thus, a poor calibration of the COSPEC instrument along the optical path if the emission rate is constant. The is not discarded but more probable causes for the observed second reason is that the emission rate of the gas is not con- discrepancies are explained below. stant. The presence of these “puffs” or events of higher emis- It was observed from the wind trajectories calculated from sion can be investigated by analysis of consecutive scans. NARR data, that often the wind direction changes and makes abrupt turns along the path of the plume. Although a geomet- 4 Conclusions rical correction is taken into account for the flux calculation, this considers a linear propagation between the origin of the Knowing Popocatepetl’ ´ s SO emission source strength is im- emission and the position at which the SO -peak was ob- 2 portant to assess its potential contribution to the atmospheric served. If, however, the wind direction at the position where chemistry, aerosol formation and its radiative implications in the plume is crossed has changed and is considerably differ- the central region of Mexico. Optical remote sensing meth- ent from the one used for the correction, the cross-section ods were deployed for this purpose during the MILAGRO and thus the flux will be overestimated. Another possibility international field campaign. An average of 2.45±1.39 Mg would be for a difference in radiative transfer. It has been of SO were released every day to the atmosphere dur- shown that measured SO slant-column densities decrease ing the month of March 2006 as determined by a passive with distance to the plume due to UV scattering (Mori et al., DOAS instrument continuously measuring from the ground 2006). If the scanning DOAS is measuring a plume close and confirmed by traverses done with a similar instrument to the horizon while the zenith-looking COSPEC measure- from an ultra-light aircraft. A frequency analysis of the ment has a shorter distance to the plume, then the flux from 48-h forward trajectories suggests that the emissions from the DOAS would be underestimated. The geometrical factor Popocatepetl ´ were transported towards Puebla/Tlaxcala ap- and thus overestimation of the COSPEC flux is, nevertheless, proximately 63% of the time during the month of March thought to be the most probable cause for the discrepancies. 2006. At this altitude the wind direction towards the Mex- This is due to the fact that the traverses are performed at long ico City Metropolitan Area, located only 60 km NW of the distances (20–40 km) from the source and wind is expected crater, accounted for only 19% of the occurrence during this to change directions as was commonly seen in the wind tra- period and none of these trajectories crossed below 4000 m jectories. a.s.l. The observed mean wind field at 500 hPa is typical for Two days with particularly high emission of SO were the month of March (de Foy et al., 2008) and is representative the 14th and 19th, with daily averages reaching 4.42 and only for the boreal winter over Central Mexico. 5.97 Gg/d, respectively. The lowest activity was recorded on The SO emitted by the volcano originates at an altitude the 13, 15, 17 and 28th, all with emissions below 1 Gg/d. well above the planetary boundary layer and is not expected to impact the metropolitan areas directly most of the time 3.3 Plume visualization unless strong convective conditions are present. This could A large variability in the SO emission is evident from the be observed only in the case of the Puebla/Tlaxcala direc- DOAS measurements shown above. A scanning imaging in- tion, where 25% of the calculated trajectories cross at some frared spectrometer was deployed on selected days in order point below an altitude of 4000 m a.s.l. However, sulfate Atmos. Chem. Phys., 8, 6655–6663, 2008 www.atmos-chem-phys.net/8/6655/2008/ M. Grutter et al.: SO emissions from Popocatepetl ´ 6661 Fig. 4. SO plume visualization of the Popocatepetl ´ volcano by passive infrared spectroscopy during 17th March 2006. White numbers indicate the local time. containing particles formed as a result of these emissions are would eliminate much of the uncertainty in flux estimation) more likely to interact with the urban pollution as found in and 5) can be used to measure other gases like HCl, HF, SiF a previous study (Raga et al., 1999). It would be important (although at higher spectral resolutions) and report their rela- to further investigate the fate of these emissions by modeling tive abundances. Work is in progress to achieve this and also not only their trajectories, but also the chemical and physical to determine column densities of SO from the measured IR transformations along their path. spectra which would provide an alternative method for esti- mating emissions. A scanning imaging infrared spectrometer was used to vi- sualize the dispersion of the sulfur dioxide plume and inves- Acknowledgements. This project was partly funded by CONACyT tigate the large fluctuations observed in the emissions. The (grant # 41531), UNAM (PAPIIT # IN113306) and IMK-IFU. The thermal infrared radiation of the emitted gases was collected NOVAC (Network for Observation of Volcanic and Atmospheric and used to detect the SO emission band from a distance of Change) project is acknowledged for providing the ground-based 11 km. Two-dimensional images of the detected SO sig- DOAS measurements used in this study. We are particularly nature were generated to determine the plume shape and grateful for the assistance of the National Park Service and its monitor plume evolution. These observations confirm that staff (A. Lopez, A. Tagle and J. Rodriguez). D. Baumgardner is gaseous emissions from the volcano are not continuous but acknowledged for his collaboration and support during this project. appear rather as “puffs”. This spectroscopic technique, used Edited by: L. 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