.................(1)
A. JUÁREZ
Facultad de Ciencias Físico-Matemáticas, Universidad Autónoma de Puebla, México
C. GAY
Centro de Ciencias de la Atmósfera, UNAM, Ciudad Universitaria D. F. 04510, México
Y. FLORES
Facultad de Ciencias Físico-Matemáticas, Universidad Autónoma de Puebla, México
Corresponding author: A. Juárez; e-mail: ajuarez@fcfm.buap.mx ]]>
Received September 18, 2003; accepted August 17, 2004
RESUMEN
En este trabajo se reportan mediciones de contaminantes atmosféricos en la ciudad de Puebla, incluyendo las registradas durante el período caracterizado por una intensa actividad del volcán Popocatépetl, entre diciembre de 2000 y enero de 2001. Aplicamos un modelo de dispersión gaussiano para calcular el impacto de las emisiones volcánicas de compuestos de azufre en las mediciones de estos compuestos en las estaciones de la Red de Monitoreo Atmosférico de la ciudad de Puebla. Los datos muestran que durante el período analizado, las emisiones volcánicas afectaron la calidad del aire incrementando los índices de PM10, CO y compuestos de azufre. Además, los resultados del modelo gaussiano de dispersión del aire para los compuestos de azufre, explican las mediciones de la estación Tecnológico para los días con intensa actividad volcánica y viento viniendo del volcán hacia la ciudad de Puebla.
ABSTRACT
In this work we report measurements of atmospheric pollutants in Puebla City, including those registered during the period characterized by intense volcanic activity from Popocatépetl volcano between December 2000 and January 2001. We used a gaussian air dispersion model to calculate the impact of sulfur compounds from volcanic emissions on the measurements of these compounds in the stations belonging to Puebla City Atmospheric Monitoring Network. The data show that during the analyzed period, this volcanic emissions affected the air quality, increasing the indexes of PM10, CO and sulfur compounds. Also, the results of applying a gaussian air dispersion model to these sulfur compounds explains the measurements from Tecnológico station for days with intense volcanic activity and wind coming from the volcano to Puebla City.
Key words: Volcanic activity, sulfur compounds, atmospheric pollution, gaussian model.
]]> 1. Introduction
Volcanic activity results in the emission of air pollutants such as sulfur dioxide (SO2) and particulate matter below ten micrometers in diameter (PM10), known as criteria air pollutants, as well as H2S, HC1, HF, and H2SO4(Goff et al., 1998; Williams-Jones et al., 2000).
The emission of sulfur gases from the Popocatépetl volcano (PV) has been monitored closely on an ongoing basis since the beginning of the activity in December 1994 (Delgado-Granados et al. 1997; Andres et al. 1998; Delgado-Granados et al. 2001; Heiken, 1993).
Within the radius of influence of PV (sixty kilometers approximately), lies the Metropolitan Area of México City, one of the biggest cities in the world. In addition, inside this perimeter there is another important urban zone named Puebla City (Delgado Granados et al., 2001; Goff et al., 1998), with circa two million inhabitants.
Considering its geographical location, the metropolitan area of Puebla is the most important and nearest to the PV. Due to the Popocatepetl's intense volcanic activity and wind direction, Puebla received ash rain and gaseous emissions during December 2000 - January 2001. According to the Centro Nacional de Prevención de Desastres data (National Center for Disaster Prevention), in 84% of those 62 days, the wind direction was towards the Northeast-East-Southeast from the volcano's crater (CENAPRED, 2002).
Taking into account the above mentioned, it was considered important to determine the impact that the volcanic emissions had over the air quality of Puebla City and calculate the influence of air pollutants as sulfur compunds in the concentrations measured by the Atmospheric Monitoring Network (AMN) of Puebla State's stations.
2. Measurements
Puebla City is located in a valley surrounded by the volcanoes Popocatépetl, Ixtaccíhuatl and Malinche. The mean altitude of the city is 2179 meters above sea level and it is located between 18° 40' 53" and 19° 13' 48" of North latitude, and 98° 0' 24" and 98° 14' 48" West longitude.
Taking into account the number of residents, Puebla is the fourth most populates city in the country; it has an intense vehicular traffic as well as an important industrial zone in its metropolitan area (Herrera, 2000).
]]> The Atmospheric Monitoring Network of Puebla State, which started working in August 2000, has four stations distributed in Puebla City as shown in Figure 1.The AMN of Puebla city has devices to analyze gases (Thermo Environmental Instruments), and to measure meteorological parameters (Climatronix) and particles (Andersen); all of which have been certified by the Environmental Protection Agency (EPA) through reference methods and/or equivalents. The AMN provides data about concentrations of criteria air pollutants, H2S and meteorological parameters.
In Figures 2 to 4 (2, 3, 4), we show the results for some criteria air pollutants in the four stations, for the period from November 2000 to June 2002. In the graphics, the monthly means, maxima, average and the Mexican Official Norm (MON), for each pollutant are shown. The monthly maxima values show the highest levels during periods of volcanic activity indicating its influence on the air quality of Puebla.
Specifically, the daily maxima for the period between December 2000-January 2001 (the period with the maximum PV activity), for the four stations are shown in Figures 5 to 7 (5, 6, 7)for CO, SO2, and PM10. In all cases, it is clear that the concentrations increased during periods of volcanic activity, reaching values above the norm in certain specific dates.
3. Gauss' diffusion equation applied to the PV emission of sulfur compounds
The gaussian diffusion model (Nevers, 1997; Wark-Warner, 1998) allows us to estimate the concentration of pollutants as a function of the wind direction, considering a source point X (x, y, z). In its general form, the concentration is given by the following equation:
.................(1)
where c = pollutant's concentration (mass/volume), k = coefficient of turbulent dispersion (area/ time), t = dispersion time of the pollutants flow, Q = rate of emitted gases, Dt = duration of the emission, and H = plume's effective height.
The plume's effective height is given by the following relation H = h + Dh, where h is the chimney's height (in our case the PV's crater) and Dh is the height that PV's flow reaches, and is calculated using Holland's equation (Wark-Warner, 1998):
]]>
......................................(2) where Vs = escape speed of the gases from the chimney (m/s), Ts= temperature of the chimney's gas (C°), Ta = environmental temperature (C°), d = diameter of chimney's exit (m), u = wind speed at the chimney's height (m/s) and P = atmospheric pressure (atm).
4. Restrictions when applying the gaussian model
The gaussian model for a point source assumes that the pollutant concentrations in any point are stabilized and do not depend upon time. The pollutants are dispersed in the wind direction and are swept away with the same speed (Nevers, 1997; Wark-Warner, 1998). In order to apply the model, some requirements need to be assumed as following (Perry-Green, 1984; Wark-Warner, 1998):
1. The gas density is higher than that of air.
2. Possible accumulations of other substances are dismissed.
3. The wind speed is higher than 0.5 m/s, which means that there is no turbulence.
4. The location must not suffer the effects due to the evaporations from the sea into the atmosphere.
]]> 5. Possible reactions of the gas inside the plume are dismissed.6. Complex topography is absent, e.g. irregular soils and high elevations surrounding the issuing focus.
In our study, and taking into account the existent conditions for the emissions of the Popocatépetl volcano, all this restrictions were observed.
Applying this model and considering the PV emanations, the concentrations of SO2 (in mg/m3) were calculated as a function of the distance from the origin of the source. In this way, and using the parameters shown in Table 1, we obtained the concentrations of the pollutants in the transversal plane as well as in the X direction (where the wind is directed).
The results for the dispersion of SO2 obtained using the parameters above, are shown in the following tables. In both cases we have considered a rectangle of 52 km x 42 km and the Northeast wind direction respective to the PV crater. Table 2 presents the view in the XY plane at 2000 m of altitude above surface level. Table 3 presents the concentrations in the XZ plane.
In Table 3, the emission source is situated in the second column (from left to right). This table shows that at 20800 meters from the origin of the source (in the X direction), the pollutants made contact with the surface, it also shows that the maximum concentration in that level occurred at 46 800 meters.
Also considering Table 3, 52000 meters away from the origin of the source (the distance from which the Tecnológico station is located), there is a concentration of 263 µg/m3. Converting this concentration to parts per million, we obtained the value of 0.09 ppm. We can consider this quantity the contribution of the PV to the measurements for SO2 at the Tecnológico station.
On the other hand, taking into account the recorded data by the AMN on January 29, 2001, a maximum concentration of 0.18 ppm is found for SO in the Tecnológico station. Application of the gaussian model suggests that the contribution by the PV would be around the 50% of the total concentration that was recorded for that particular day. This is consistent with historical measurements by the AMN during winter season for that station.
Additionally, we estimate the arrival time of the pollutants flow from the PV to the Tecnológico station as 2 hours; emissions from the PV when the wind direction is towards Puebla City rapidly affect the city's air quality.
]]> 5. Discussion
According to Figures 2 to 4 (2, 3, 4), the Mexican Official Norm (MON) for the PM10 air pollutants has been surpassed for some periods in the four stations located in Puebla City. In those figures, a seasonal behavior is observed for carbon monoxide with important increases during the winter period and with an outstanding value over the winter 2000-2001. During the same periods, PM10 had the same behavior.
On the other hand, the PV's volcanic activity during recent years has been characterized by emissions of ash and gases, especially during the months of December and January.
Figure 8 shows the isopleths associated to Table 2.
The wind rose for the months of December 2000 and January 2001 is shown in Figure 9. In accordance with this figure, for the majority of days for this period, the wind flew in a Northeast-East-Southeast direction, towards the region in which the metropolitan area of Puebla is located.
This behavior explains the fact that during December 2000-January 2001 the concentration of sulfur compounds, PM10 and carbon monoxide increased in comparison to a similar period a year later (December 2001-January 2002), as shown in Figures 10, 11, 12.
According to Figures 5, 6, 7 and 9, the days when there was intense volcanic activity and the wind blew towards Puebla City, there was a considerable increase in the indexes of pollutants such as PM10, carbon monoxide and sulfur dioxide. This is in line with the hypotheses that volcanic activity affects the air quality of Puebla City.
An additional supporting argument is found in the Table 4, where the number of times that the MON was exceeded for the different criteria pollutants is shown during both periods, in the four stations of the AMN.
]]>6. Conclusions
According to the analysis, there is a general increase in the concentration of criteria pollutants during volcanic activity, locally affecting Puebla City.
The SO2 measurements at the Tecnológico station, Figure 6, shows an increase in concentration during the days when the wind direction came from PV. The high concentrations recorded for this pollutant at this station, besides being attributed to the volcanic activity, could also be attributed to the chemical industry established in that zone.
Taking into account the historically measured concentrations at the Tecnológico station for SO2 and adding the other factors considered in this work, there is congruence with the measured data during days of intense volcanic activity.
In this work we make clear how volcanic emissions influence air quality at Puebla City. Our results show that the pollutants analyzed increased during periods of volcanic activity.
References
Andres, R. J., A. C. Kasgnoc, 1998. A time averaged inventory of sub aerial volcanic sulfur emmisions. J. Geophys. Res. 103, 25251-25261. [ Links ]
CENAPRED, 2002. http://www.cenapred.unam.mx/mvolcan.html: Monitoreo volcánico. [ Links ]
Csanady, G. T., 1973. Turbulent diffusion in the environment. Seidel, Derdrecht, Hollans, 248, p. (IAEA safety guide 1983, No. 50-SG-S3). [ Links ]
Delgado-Granados, H., 2001. Large volcanic-SO2 fluxes: COSPEC measurements at Popocatépetl Volcano (México). In: Hickson C., Stix, J. (Eds.). The COSPEC Cookbook. Geological Survey of Canada (in press). [ Links ]
Delgado-Granados, H. and L.Cárdenas González, 1997. Passive degassing at Volcán Popocatépetl México): 2.6 x 106 tons of SO2 released in 617 days of activity. Volcanic Activity and the Environment. Abstracts. IAVCEI, Puerto Vallarta, p. 49. [ Links ]
Goff, F., C. Janik, H. Delgado, C. Werner, D. Counce, J. A. Stimac, C. Siebe, S. P. Love, S. N, Williams, T. Fischer, and L. Johnson, 1998. Geochemical surveillance of magmatic volatiles al Popocatépetl Volcano, México. Geol. Soc. Amer. Bull. 110: 695-710. [ Links ]
Heiken, G.., 1993. Volcanic ash: what it is and how it forms. US Geol. Sur. Bull., 2047: 39-45. [ Links ]
Herrera M. A., 2000. Una propuesta para la ubicación de estaciones de monitoreo ambiental. Tesis de Maestría pp. 54-56; Facultad de Ingeniería, Universidad Autónoma de Puebla. [ Links ]
Nevers N., 1997. Ingeniería de control de contaminación ambiental. Ed. Mc Graw Hill. 548-562, México, D. F. [ Links ]
Perry R. H. and D. Green, 1984. Perry's chemicals engineer's handbook. 6th. edition, McGraw-Hill. [ Links ]
Wark, K and C. Warner, 1998. Contaminación del aire. Origen y control. Ed. Limusa. pp. 327-328, México, D. F. [ Links ]
Williams-Jones, G., J. Stix, M. Heiligmann, A.Charland, B. Sherwood-Lollar, N. Arner, G. V. Garzon, J. Barquero, and E. Fernández, 2000. A model of diffuse degassing at three subduction-related volcanoes. Bull. Volcanol. 62, 130-142. [ Links ] ]]>