The Science of the Total Environment 287 Ž2002. 167᎐176 Basic statistics of PM 2.5 and PM 10 in the atmosphere of Mexico City a , E. Ortiz a , M. Ruiz a , J. Chow b, E. Vegaa,U , E. Reyes a , G. Sanchez ´ J. Watsonb, S. Edgertonc a Instituto Mexicano del Petroleo, Cardenas Num. ´ Eje Central Lazaro ´ ´ ´ 152, Col. San Bartolo Atepehuacan, Delegacion D.F., Mexico ´ Gusta¨ o A. Madero 07730, Mexico, ´ b Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512-1095, USA c Pacific Northwest National Laboratory, Richland, WA, USA Received 12 February 2001; accepted 16 July 2001 Abstract The high levels of fine particulate matter in Mexico City are of concern since they may induce severe public health effects as well as the attenuation of visible light. Sequential filter samplers were used at six different sites from 23 February to 22 March 1997. The sampling campaign was carried out as part of the project ‘Investigacion ´ sobre Materia Particulada y Deterioro Atmosferico-Aerosol and Visibility Evaluation Research’. This research was a ´ cooperative project sponsored by PEMEX and by the US Department of Energy. Sampling sites represent the different land uses along the city, the northwest station, Tlalnepantla, is located in a mixed medium income residential and industrial area. The northeast station, Xalostoc, is located in a highly industrialized area, Netzahualcoyotl is located in a mixed land use area, mainly commercial and residential. Station La Merced is located in the commercial and administrative district downtown. The southwest station is located in the Pedregal de San Angel, in a high-income neighborhood, and the southeast station located in Cerro de la Estrella is a mixed medium income residential and commercial area. Samples were collected four times a day in Cerro de la Estrella ŽCES., La Merced ŽMER. and Xalostoc ŽXAL. with sampling periods of 6 h. In Pedregal ŽPED., Tlalnepantla ŽTLA. and Netzahualcoyotl ŽNEZ. sampling periods were every 24 h. In this paper the basic statistics of PM 2.5 and PM 10 mass concentrations are presented. The average results showed that 49, 61, 46, 57, 51 and 44% of the PM 10 consisted of PM 2.5 for CES, MER, XAL, PED, TLA and NEZ, respectively. The 24-h average highest concentrations of PM 2.5 and PM 10 were registered at NEZ Ž184 and 267 ␮grm3 . and the lowest at PED Ž22 and 39 ␮grm3 .. The highest PM 10 correlations were between XAL-CES Ž0.79., PED-TLA Ž0.80.. In contrast, the highest PM 2.5 correlations were between U Corresponding author. Tel.: q525-333-6867; fax: q525-333-6935. E-mail address: evega@imp.mx ŽE. Vega.. 0048-9697r02r$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 1 . 0 0 9 8 0 - 9 168 E. Vega et al. r The Science of the Total En¨ ironment 287 (2002) 167᎐176 CES-PED Ž0.74., MER-CES Ž0.73. and TLA-PED Ž0.72., showing a lower correlation than the PM 10 one. The results of the PM 10 from 12.00 to 18.00 h at CES and MER presented the highest variability and also the highest median concentrations, meanwhile XAL showed them from 06.00 to 12.00 h. The highest variability and median concentrations of PM 2.5 were from 06.00 to 12.00 h for the three stations. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Particulate matter; Particulate matter measurements; Mexico City; Data management 1. Introduction Mexico City is one of the largest metropolitan areas in the world, with nearly 20 million inhabitants within the city and an annual growth rate of 3.3%. The basin of Mexico City has an area of approximately 1300 km2 and is naturally open to the north and enclosed by mountains, 1000 m in height above the basin, to the south, east and west. As most cities located in valleys and surrounded by mountains, it has air pollution problems, especially for ozone and suspended particles. Mexico City is a dry region of moderate climate with a diurnal pattern of winds blowing from the northwest and the northeast. The rainy season lasts from June to October. The industrial area comprises more than 30% of the whole national industry and is located in the northern sectors of the city. In October 1990 the Integral Program against Atmospheric Pollution ŽPICCA, 1990. was promulgated in Mexico City. This document emphasized, for the first time, the importance of measuring the concentration of particulate matter. During the past 6 years, the environmental authorities have deployed an automated network for atmospheric monitoring ŽRAMA., which routinely measures particulate matter in specific sites of the city. The Mexican total suspended federal NOM ŽMexican Official Standard. particle for 24 h Ž260 ␮grm3 . is frequently exceeded in Mexico City ŽDOF, 1994.. There have been some PM 10 values exceeding the mobile average 24-h NOM of 150 ␮grm3 standard. Fig. 1 shows that PM 10 at long-term measurement sites in Mexico City have exceeded the established mobile average 24-h standard during the last 5 years. Although there is no Mexican standard for PM 2.5 , the proposed NAAQS US 24-h average standard of 65 ␮grm3 was used to Fig. 1. Percentage of exceedence of the PM 10 standard of 150 ␮grm3 24-h average, from January 1995 to March 2000 at six sites. E. Vega et al. r The Science of the Total En¨ ironment 287 (2002) 167᎐176 compare the values of fine particles observed in Mexico City during the sampling period of the IMADA-AVER study. High concentrations of particulate matter usually result from a combination of primary and secondary contributions. Primary particles are directly emitted from sources, while secondary particles form in the atmosphere from gaseous emissions of sulfur dioxide, oxides of nitrogen, ammonia, and heavy organic gases Žof typically eight or more carbon atoms.. Secondary aerosol formation may occur under stagnant air conditions, after gaseous emissions from different sources have mixed and aged, and when pollutants generated on previous days accumulate or are recycled by winds and carry over above nighttime surface inversions ŽVega et al., 1995.. There have been some controls since 1990, which have resulted in major reductions of some specific pollutants such as SO 2 emissions. Emission inventories are inadequate to estimate the magnitude of these reductions. However, these strategies are generally thought to be significant in decreasing the concentration of primary particles, NO x and carbon monoxide in ambient air. Sulfur in diesel fuel has been reduced from 0.5 to 0.05%. Gasoline-powered vehicles were required to have catalytic converters after 1990, and unleaded fuel was introduced to provide cleaner emissions. Within Mexico City, many old diesel buses and trucks have been replaced with new vehicles operating more modern, cleaner engines. These efforts have assuredly attenuated the emissions engendered by urban growth, but hourly PM 10 concentrations exceeding 500 ␮grm3 are still measured at some monitoring sites. A continuous haze blankets the city, especially during winter, and there is great concern among residents about the effects of suspended particles on health. With the most obvious emitters controlled, more information is needed about the composition, size, and spatial and temporal variabilities of suspended particles before additional measures can be taken. The spatial, temporal, size and chemical characteristics of specific emission sources are needed to allow their contributions to ambient particulate matter concentrations to be 169 distinguished from each other. These, coupled with accurate emission inventories are needed to provide accurate inputs to air quality models. The major weakness of air quality modeling in Mexico is the inaccurate quantification and limited resolution of emissions. During the winter Ž23 February to 22 March. of 1997 a sampling campaign was carried out as part of the project ‘Investigacion ´ sobre Materia Particulada y Deterioro Atmosferico-Aerosol and Visi´ bility Evaluation Research’ ŽIMADA-AVER.. This research was a cooperative project sponsored by PEMEX through the Instituto Mexicano del Petroleo ŽIMP. and by the US Department of Energy ŽDOE. through national laboratories and universities ŽEdgerton et al., 1999.. This campaign was used to determine the nature and causes of high concentrations of PM 2.5 and PM 10 and to provide accurate and precise inputs to air quality models. The objective of this paper is to characterize PM 2.5 and PM 10 with as much detail as possible, spatially and temporally, for a 1-month period in the Mexico City basin. Sampling sites ŽFig. 2. encompass residential, industrial and mixed settings. The northwest station, Tlalnepantla, is located in a mixed medium income residential and industrial area. The northeast station, Xalostoc, is located in a highly industrialized area, and historically has shown the highest concentrations of total suspended particles recorded in Mexico City ŽVega et al., 1998.. Netzahualcoyotl is located in a mixed land use area, mainly commercial and residential. Station La Merced is located in the commercial and administrative district downtown. The southwest station is located in the Pedregal de San Angel, in a high-income neighborhood, and the southeast station located in Cerro de la Estrella is a mixed medium income residential and commercial area. The stations northeast and southeast are located west of the Texcoco Lake. Table 1 provides more detail on the characteristics and major source influences for each site. 2. Methodology The sequential filter samplers ŽSFS. equipped 170 E. Vega et al. r The Science of the Total En¨ ironment 287 (2002) 167᎐176 Fig. 2. Map showing the PM 2.5 and PM 10 measurement sites. with a Sierra᎐Andersen medium-volume PM 2.5 and PM 10 inlets ŽAtlanta, GA. were collocated at each of the six sampling sites within Mexico City. Samples of PM 2.5 and PM 10 were collected four times a day in Xalostoc, La Merced and Cerro de la Estrella with sampling periods of 6 h. In Tlalnepantla, Netzahualcoyotl and Pedregal sampling periods were every 24 h. Teflon-membrane filters of 47 mm in diameter ŽGelman Scientific, Ann Arbor, MI. with 2-␮m pore size were used to collect samples and subsequently analyzed for mass by gravimetry ŽChow, 1995.. The Teflon filters were stabilized for 6 weeks to a relative humidity of 25᎐35% at constant temperature of 21.5" 0.5⬚C before weighing. The data were validated according to three levels of quality assurance ŽChow et al., 1996.. Level Ž0. designates data as they come off the sequential filter samplers; level Ž1. invalidates measurements affected by malfunctions of the samplers, and adjusts values for quantifiable calibration; and level Ž2. applies consistency tests based on known physical relationships between variables. 3. Results and discussion The data collected from the sequential filter sampler were used to calculate the 24-h average PM 2.5 and PM 10 mass concentrations. In Xalostoc, Cerro de la Estrella and La Merced the 6-h measurements from the sampler were aggregated to match the 24-h sampling intervals. In Tlalnepantla, Netzahualcoyotl and Pedregal stations sampling periods were 24 h. According to three levels of validation for quality assurance, 95% of PM 2.5 and PM 10 mass concentrations were used to calculate the basic statistic. Only 5% of PM 2.5 and PM 10 mass concentrations were discarded according to any of the three levels of validation. The basic 24-h mass concentration statistics for PM 2.5 and PM 10 are presented in Fig. 3. The highest median values were observed in the NE at XAL ŽPM 2.5 s 42.5 ␮grm3 and PM 10 s 99.8 ␮grm3 . and NET ŽPM 2.5 s 43.9 ␮grm3 and PM 10 s 89.4 ␮grm3 .. Central, SE and NW portions of the basin showed more mid-range median concentrations, CES ŽPM 2.5 s 29.3 ␮grm3 and PM 10 s 59.0 ␮grm3 ., MER ŽPM 2.5 s 33.5 E. Vega et al. r The Science of the Total En¨ ironment 287 (2002) 167᎐176 171 Table 1 Characteristics and locations of the sampling sites Site name Site code Site description Major source influence Tlalnepantla ŽNW. TLA Mixed industryresidential Xalostoc ŽNE. XAL Netzahualcoyotl ŽNE. NEZ La Merced ŽC. MER Cerro de la Estrella ŽSE. CES Pedregal ŽSW. PED Located on Av. Toluca and Atlacomulco, in Tlalnepantla, State of Mexico. The sampling site is located at 8 m height, in front of a water deposit and surrounded by lightly traveled roads. Industrial and residential area with nearby electronic manufacturing, corn milling, and metal fabricating facilities. Located inside a vehicular service center on the 14.5-km Mexico᎐Pachuca motorway and Hierro Street, in Ecatepec, State of Mexico. The sampling site is 4.3 m in height with nearby heavily traveled paved and unpaved roads with old and new gasoline and diesel vehicles. Highly industrialized area. Located on Netzahualcoyotl and Xochiaca Avenues, State of Mexico. It is a commercial and residential area. Heavily traveled paved and unpaved roads with old and new gasoline and diesel vehicles. The site is near a landfill and 10 km south of Texcoco Lake. Located on Congreso de la Union Avenue in La Merced. The sampling site is located 8 m height. Heavily traveled paved curbed surface streets with light duty vehicles and modern heavy-duty diesel buses. Residential, commercial and services area with nearby green areas. Located inside a water treatment plant on San Lorenzo Avenue in Iztapalapa. It is a heavily traveled paved and unpaved road. Several small stone-cutting operations nearby. Heavy old and new vehicle traffic. Residential, commercial and service area. Located in the middle of a park on Canada ˜ street and Crater Avenue in San Angel. The sampling site is 4 m high and the site is surrounded by volcanic stone. It has paved roads lightly traveled. Residential area. ␮grm3 and PM 10 s 48.4 ␮grm3 . and TLA ŽPM 2.5 s 30.2 ␮grm3 and PM 10 s 60.1 ␮grm3 .. Pedregal in the SW showed the lowest concentrations 21.9 ␮grm3 and 39.4 ␮grm3 for PM 2.5 and PM 10 , respectively. This is consistent with the heaviest density of industrial activity and mobile sources being in the NE portion of the Mexico City basin. The highest variability in day-to-day PM 2.5 mass concentration was observed at Netzahualcoyotl Strong mobile and road dust influence. Mixed commercialresidential. Mixed mobile ŽLDV and HDV., residential and commercial. Strong mobile and road dust influence with mixed residential and commercial sources. Mixed residential and mobile Žlight duty vehicles.. Ž183.7 ␮grm3 ., with a maximum mass concentration of 83.9 ␮grm3. It can be seen that median values for PM 10 mass concentration varied across sites from 50 to 100 ␮grm3. Pedregal and Tlalnepantla showed the lowest variation and maximum concentrations were much less at these sites relative to the industrial area. Xalostoc, Netzahualcoyotl, La Merced and Cerro de la Estrella showed the highest variability. Netzahualcoyotl Ž267.3 and 172 E. Vega et al. r The Science of the Total En¨ ironment 287 (2002) 167᎐176 Fig. 3. Basic statistic for PM 2.5 and PM 10 mass concentrations. 246.7 ␮grm3 ., La Merced Ž157.9 ␮grm3 . and Cerro de la Estrella Ž253.0 ␮grm3 . experienced the highest maximum PM 10 concentrations. PM 2.5 and PM 10 mass concentrations for 6and 24-h sampling periods were compared. The results indicate that there is considerable sub-24-h variability at all sites which is not captured by the 24-h integrated sample. The increased temporal resolution will be necessary to identify those sources most relevant to ambient particulate matter levels in the Mexico City basin ŽFig. 4.. In general, the results showed that the PM 2.5 and PM 10 mass concentrations have a similar temporal pattern with median PM 10 concentrations being twice as much or more than PM 2.5 mass. It can be observed that for all sampling sites the 6- and 24-h average PM 2.5 mass concentrations were similar, though 6-h averages could be "30% of the 24-h average. 4. Time series of PM 2.5 and PM 10 mass concentrations The time series of the average 24-h PM 2.5 and PM 10 mass concentrations were analyzed for the six sampling sites. As can be seen, the highest concentrations of PM 2.5 and PM 10 were measured at Netzahualcoyotl, Xalostoc and Cerro de la Estrella and the lowest concentrations were at Pedregal with an average concentration of 21.6 and 39.4 ␮grm3 for PM 2.5 and PM 10 , respectively. The period from 3 to 6 March showed the highest values for all six sampling sites and both size fractions ŽFig. 5.. Although there is no Mexican standard for PM 2.5 , the proposed USA 24-h average standard of 65 ␮grm3 was exceeded four times during the study. It was exceeded three times in Netzahualcoyotl with 83.9, 183.7 and 68.4 ␮grm3 for the 4th, 5th and 6th of March, respectively, and once in Xalostoc with 66.7 ␮grm3 Ž4 March.. The Mexican NOM 24-h PM 10 standard of 150 ␮grm3 was exceeded three times in Netzahualcoyotl Ž267.3, 189.0 and 246.7 ␮grm3 ., three times in Xalostoc site Ž154.7, 181.4 and 153.2 ␮grm3 ., and once in Cerro de la Estrella Ž253.0 ␮grm3 .. These results are consistent with the prevailing meteorological conditions during the sampling campaign. High concentrations of particulate matter were observed during weak high-pressure systems and also when mixing layer heights were the lowest, less than approximately 1000 m. E. Vega et al. r The Science of the Total En¨ ironment 287 (2002) 167᎐176 173 Fig. 4. Average of 6 and 24 h for PM 2.5 and PM 10 mass concentrations. Note: 24-h averages can underestimate peaks by f 30%. 174 E. Vega et al. r The Science of the Total En¨ ironment 287 (2002) 167᎐176 Fig. 5. Time series of PM 2.5 and PM 10 mass concentrations at six sampling sites. 5. Ratios of PM 2.5 r PM 10 mass Table 2 Average 24-h ratios of PM 2.5rPM 10 mass at six sampling sites To evaluate the consistency of the PM 2.5 and PM 10 mass measurements, the mass ratios were calculated for each to ensure that PM 2.5 mass did not exceed PM 10 mass. These mass ratios are presented in Tables 2 and 3. Table 2 shows the average ratios of PM 2.5 and PM 10 mass; the number Ž N . of valid pairs of PM 2.5 and PM 10 concentrations from a total of 29 samples is also shown. The PM 2.5rPM 10 mass ratios were the lowest Ž- 0.5. at Cerro de la Estrella, Xalostoc and Netzahualcoyotl sites, indi- Site Ratio N TLA XAL NEZ CES PED MER 0.51" 0.22 0.46" 0.11 0.44" 0.13 0.49" 0.12 0.57" 0.12 0.61" 0.14 24 28 18 28 26 25 cating that PM 10 minus PM 2.5 , known as coarse particles, were a large portion of PM 10 at these sites. The average PM 2.5rPM 10 mass ratio in Table 3 Average 6-h ratios of PM 2.5 rPM 10 mass at three sampling sites Site XAL CES MER PM2.5 rPM10 0᎐06.00 h 06.00᎐12.00 h 12.00᎐18.00 h 18.00᎐24.00 h 0.48" 12 0.52" 14 0.59" 13 0.48" 14 0.55" 16 0.64" 15 0.49" 19 0.45" 14 0.62" 14 0.42" 10 0.46" 12 0.54" 12 E. Vega et al. r The Science of the Total En¨ ironment 287 (2002) 167᎐176 Tlalnepantla was 0.51" 0.22 indicating than PM 2.5 mass ratio constituted more than 50% of PM 10 at this site. Average PM 2.5rPM 10 mass ratios were the highest Ž) 0.57. at La Merced and Pedregal, which is consistent with greater transport of fine particles of both primary and secondary origin. Table 3 shows the results of the ratios of PM 2.5rPM 10 mass concentration at three sites with 6-h sampling resolution, G n s 22. It can be observed that La Merced showed the highest average PM 2.5rPM 10 mass ratio for all sampling periods. Xalostoc showed the lowest ratios for all sampling periods, indicating that the PM 2.5 fraction was a small part of the PM 10 . This is consistent with the 24-h average period, on the other hand, in La Merced and two sampling periods Žmorning and midday. of Cerro de la Estrella, the PM 2.5rPM 10 ratio was higher. Cluster analyses and correlation coefficients among sites were obtained for PM 2.5 and PM 10 mass. The results showed that major source influences for both primary and secondary particulate matter in Xalostoc were different from the others, this is probably due to the fact that this site is mainly an industrial area and the emissions differ from the vehicular source. It was also found that for PM 2.5 the highest correlation was between CES-PED Ž0.74., MER-CES Ž0.73. and TLA-PED Ž0.72.. When correlating PM 10 , Netzahualcoyotl was different from all the other sites. This may be due to this site being strongly influenced by different unpaved roads and, the nearby Texcoco Lake, which is a dry and deforested area and a major source of resuspended dust. The highest correlation was between TLA-PED and CES-XAL with 0.80 and 0.79, respectively. These results can be explained, as they are consistent with the prevailing meteorological conditions during the sampling campaign. Winds from the NE in TLA converged in PED and those come from the SE to the NE in XAL and the central area in MER. 6. Conclusions Temporal and spatial variations of PM 2.5 and 175 PM 10 mass concentrations were studied for 1 month in 1997 with good spatial coverage of monitors. Data may reflect specific meteorological conditions to the sampling period and the conclusions of this paper are therefore specific to the study period, and their applicability to other time periods is not known. The results showed that: 䢇 䢇 䢇 䢇 䢇 Most of the PM 2.5 and PM 10 mass concentrations Ž95%. were used to calculate the basic statistics according to three levels of validation for quality assurance. Some outliers for PM 2.5 were found at Cerro de la Estrella, La Merced and Tlalnepantla and only one outlier for PM 10 in La Merced was identified. One highest maximum PM 2.5 concentration was found at Netzahualcoyotl and one for PM 10 in Cerro de la Estrella, La Merced and Netzahualcoyotl. The 6-h sampling period better represents the temporal variability and maximum concentrations than does the 24-h sampling period. The highest concentrations of PM 2.5 and PM 10 were measured at Netzahualcoyotl, Xalostoc and Cerro de la Estrella and the lowest concentrations were at Pedregal. High concentrations of particulate matter were observed during weak high pressures systems and also when mixing layer heights were the lowest, approximately less than 1000 m. Xalostoc station does not represent the exposure of the community as it is located near an industrial area. The 24-h mobile average PM 10 standard was exceeded a total of seven times, three times in Netzahualcoyotl, three times in Xalostoc and one time in Cerro de la Estrella. The US 24-h PM 2.5 standard was exceeded three times in Netzahualcoyotl with 83.9, 183.7 and 68.4 ␮grm3 and once in Xalostoc with 66.7 ␮grm3. The ratio of PM 2.5rPM 10 mass showed that coarse particles made up a large portion of PM 10 at Cerro de la Estrella, Xalostoc and Netzahualcoyotl sites showing that dust is an important source of coarse particles, mainly from paved, unpaved and the dry Texcoco Lake. On the other hand, PM 2.5 constituted E. Vega et al. r The Science of the Total En¨ ironment 287 (2002) 167᎐176 176 44᎐61% of PM 10 mass indicating that combustion processes are important. Acknowledgements The Mexican Petroleum Institute under the project ‘Estudio integral para entender los problemas de contaminacion por ´ atmosferica ´ partıculas en la ZMCM y su impacto regional y ´ global utilizando modelos de calidad del aire’ supported this research. The authors wish to acknowledge the Desert Research Institute for the analyses of samples. References Chow JC. Critical review ᎏ measurement methods to determine compliance with ambient air quality standards for suspended particles. 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