Elsevier

Atmospheric Environment

Volume 38, Issue 8, March 2004, Pages 1219-1229
Atmospheric Environment

Commuters’ exposure to PM2.5, CO, and benzene in public transport in the metropolitan area of Mexico City

https://doi.org/10.1016/j.atmosenv.2003.11.008Get rights and content

Abstract

A survey was carried out to measure commuters’ exposure to PM2.5, CO, benzene, and the chemical composition of PM2.5 on different routes and modes of transport in Mexico City. PM2.5 (n=62), CO (n=54) and benzene (n=22) are presented from morning (6:30–9:30 a.m.) and evening (17:30–20:30) rush hours on minibuses, buses and Metro (underground or subway system). Three routes were selected from a previous commuters’ exposure study covering some of the most important thoroughfares of the valley. For PM2.5, mass concentration was determined for all the samples. Nitrates, sulphates, inorganic elements and carbon fraction were analysed. CO was sampled using electrochemical sensors and 6-l canisters with flow controller devices were used to collect integrated samples for benzene. Minibuses had a slightly higher geometric mean PM2.5 concentration in the morning than other modes of transport, but the ranking of geometric mean PM2.5 by mode of transport is opposite in the evening and the variability within modes is approximately double the difference between modes. The highest single measurement was a concentration of 137 μg m−3 on a bus during an evening rush hour. The main component identified in PM2.5 was carbon. Carbon monoxide levels in this study were approximately 3 times lower than those found in a commuter exposure study conducted in 1991. A strong association was shown between wind speed and PM2.5 exposure in minibuses (r2=0.50) and buses (r2=0.54). The relationship between wind speed and CO exposure was strong only in minibuses (r2=0.52).

Introduction

The Metropolitan area of Mexico City is one of the biggest and most important developing urban areas with serious air pollution problems. Mexico City has a vehicle fleet of approximately 3.2 million vehicles and an atmosphere containing a complex cocktail of pollutants covering an area where more than 20 million people work, live, and commute every day. The transport sector (Colvile et al., 2001), has been identified as a major source of air pollution. In 1994, the trips per person registered were 20.6 million in the metropolitan area of Mexico City (SETRAVI, 2002). The modal distribution of the transport sector in the city has shifted from medium (buses) and high capacity (Metro, light rail and trolleybus) to low-capacity motor vehicles (minibus, combis, taxis and private cars) in the past 15 years. For low-capacity vehicles, minibus shows the highest shift from carrying 6% of commuters in 1986 to 55% in 2000 (SETRAVI, 2002). In order to decrease this trend, the local authority is implementing a scheme to replace old minibuses with new buses.

Concentrations of the pollutants PM2.5, CO and benzene are indicators of health effects of pollution originating from inefficient combustion. In Mexico City, PM2.5 has been measured and chemically characterised using different state of the art techniques at the urban scale (Aldape et al., 1996; Miranda et al., 1998; Edgerton et al., 1999; Chow et al., 2002). The potential health effects caused by PM2.5 have also been investigated, including mortality (Borja-Aburto et al., 1998; Castillejos et al., 2000) and cardiovascular dysfunctions in different cohorts (Holguin et al., 2001). For CO (Riveros et al., 1998) and benzene (Bravo et al., 2002; Meneses et al., 1999; Ortiz et al., 2002), some studies have been conducted to evaluate levels in different microenvironments as well as at the urban scale level to understand the complexity in the formation of air pollutants in the city. However, the exposure during morning and evening rush hours have not been evaluated for PM2.5, CO and benzene simultaneously. In Mexico City, only one study has been conducted to evaluate CO levels of commuters in different transport modes (Fernandez-Bremauntz and Ashmore, 1995). In Europe, studies evaluating commuters’ exposure for one or more pollutants (Kingham et al., 1998) have been carried out within different means of transportation, measuring levels of PM2.5 (Adams et al., 2001a) and elemental carbon (see Adams et al 2002), CO (Clifford et al., 1997), and VOCs (Van Wijnen et al., 1995). In the United States, several studies have conducted measurements in transport microenvironments, mainly in private cars. They evaluated one or more of the pollutants selected for this study (Brice and Roesler, 1966; Chan et al (1991a), Chan et al (1991b); Ott et al., 1994; Rodes et al., 1999). In Asia, exposure in public transport was evaluated in Hong Kong (Chan and Liu, 2001; Chan et al., 2002a) and China (Chan et al., 2002b) but these studies did not measure the same pollutants and modes of transport as this study. Chemical speciation of particles has been conducted only in a few studies (Adams et al., 2002; Zielinski et al., 1997), to identify potential sources and determinants of commuters’ exposure (Adams et. al., 2001b). The overall aim of this study was to quantify the levels of commuter exposure to PM2.5 and its chemical composition, CO and benzene during morning and evening rush hours, focusing on three public transport modes on three different routes in Mexico City.

Section snippets

Selected public transport and corridors

The campaign methods were based on a combination of two studies conducted to assess the personal exposure of transport users in Mexico City (Fernandez-Bremauntz and Ashmore, 1995) and London (Adams et al., 2001a). The public transport modes chosen for the study were minibuses, buses and the Metro. According to local authorities, these means of transportation represent 78% of the modal distribution in Mexico City (SETRAVI, 2002). Measurements focused more on minibuses than buses and Metro

Descriptive statistics

Descriptive statistics are shown in Table 1 for selected pollutants divided by mode of transport and time of day. The levels of the selected pollutants divided by mode of transport presented minimal differences. Buses and minibuses had the highest concentrations for all the pollutants. Morning rush commuting periods showed the highest values for all the pollutants mainly in minibuses. On the Metro, the exposure of commuters was similar for morning and evening journeys for CO and benzene. PM2.5

PM2.5

The PM2.5 levels measured in buses in this study (AM=71 μg m−3) were double than those measured in central London (AM=33 μg m−3) (Adams et al., 2001a). Exposure levels to PM2.5 in buses in Guangzhou, China (AM=145 μg m−3) (Chan et al., 2002b) were 50% higher than the level found in this study. For minibuses, no studies have evaluated commuters’ exposure to PM2.5. The PM2.5 concentration of commuters in China in the subway (AM=44 μg m−3) was lower than in Mexico City, and in London higher (AM=247.2 μg m−3

Conclusions

Commuters’ exposure to PM2.5, CO and benzene was measured in minibuses, buses and Metro in Mexico City during morning and evening rush hours. Minibuses measured the highest concentrations for all the pollutants during morning rush hours. For PM2.5, chemical speciation was carried out for all modes of transport. Carbon was identified as the main component of the total composition of PM2.5. Carbon monoxide was lower by a factor of between 2.2 and 3.8 in 2002 compared with a campaign conducted in

Acknowledgements

The authors would like to acknowledge the information and logistical field support from Hilda Martı́nez Salgado (INE-SEMARNAT), Rafael Ramos Villegas, Roberto Muñoz Cruz, Cristina Ortuño Mojica and Vicente Pérez Nuñez (RAMA, GDF). The great effort provided by the field team, Mariana Toledano Zamora, Manuel Ramos Charines, Felipe Angeles López and Ismael León Dı́az, as well as the support offered by Beatriz Cárdenas González, and Teresa Ortuño Arzate (CENICA, INE-SEMARNAT). Jorge Gómez would

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