Project-Study paper, University of Salzburg, Austria (Dr. W. Hofmann) Authors: Maricela Yip and Pierre Madl
14th Dec. 2000 |
1. Introduction- picture galery Mexico City’s air has gone from among the world’s cleanest to among the dirtiest in the span of a generation. Novelist Carlos Fuentes first novel took place here in 1959 and was entitled "Where the air is clear" - a title he has said is ironic considering the city’s now –soupy environment. The average visibility of some 100 km in 1940s is down to about 1.5 km. Snow-capped volcanoes (Popocatepetl, Ixtacihuatl, and Paricutin) that were once parts of the landscape are now visible only rarely (fig.1.2). And levels of almost any pollutant like nitrogen dioxide (NO2) now regularly break international standards by two to three times. Levels of ozone (O3), a pollutant that protects us from solar radiation in the upper atmosphere but is dangerous to breathe, are twice as high here as the maximum allowed limit for one hour a year and this occurs several hours per day every day (fig.1.1). |
![]() Fig.1.2: Mexico City on a clear day (50kB) |
Facts and figures of Mexico city: The Mexican Republic is formed by 31 states and the Federal District (fig.1.3). Among the states of this country there is one called the State of Mexico, which is situated in the center of the country surrounding the Federal District. The Federal District (DF) and some counties of the State of Mexico form the Mexico City Metropolitan Area (ZMCM). The Federal District is divided into 16 counties called Delegaciones, all of them constitute the ZMCM. From the total of 121 counties for all of Mexico, only 16 of them are considered part of the ZMCM (fig.1.4) |
![]() Fig.1.3: Republic of Mexico (100kB) |
2a. Climatic Parameters - Global Factors The climatic conditions of the state of Mexico are quite diverse; they range from a semiarid belt in the far North to a rather tropical environment in the South. Although its elevation is high, Mexico City's location at 19° north latitude provides it with a temperate climate throughout the year. The climate is generally dry, but thunderstorms are frequent and intense from June through October. Winters are slightly cooler than summers and have a more semiarid character - see table. |
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The global wind-distribution charts reveal the general trends among the most distinct seasons (January
and July average - see fig.2.1). During the winter months a very persistent high pressure system resides over the south-eastern Pacific of the northern hemisphere. This enables a weak flow of moderately tempered air (synoptic flow from the south) into the highlands of Mexico. The geographical conditions of Mexico City with its northern opening traps the air that are pushed by turbulent flow (as a result of the synoptic flow) from the north towards the southerly located mountain chain favoring an inversion zone. As mentioned previously, the location of heavy industry at the northern outskirts of the City keeps pushing their exhaust gases into the metropolitan area, which further aggravates the already tense situation present due to exhaust emissions of the automotive fleet. The summer months, experience a stronger synoptic flow from the south, intensive sunshine, and the absence of inversed atmospheric strata by lifting the trapped masses of air and thereby cleansing the daily accumulating toxic cocktail. And still, often the winds crossing the southern mountain chain run over the cushion of firmly residing air in the valley. Like in winter, the resulting vortex further below the mountain chain pushes the air back from the northern end into the valley towards the south. The only easing effect during the summer months are the wash-out effects due to rain and lifting of the air strata during intense sunshine. |
![]() Fig.2.1: The Horseshoe latitudes are dominated by high-pressure systems (320kB) |
3. Air Pollution in Mexico City - Sources and Effects Before going deeper into Mexico City's pollution problem, it is worth considering the great smog of London in 1952/53 and the resulting effects on its population. In December of 1952, London experienced an unusually cold winter conditions. In response, the people of London burnt large quantities of coal in their grates. Smoke was pouring from the chimneys of their houses and becoming trapped beneath the inversion of an anticyclone that had developed over the southern parts of the British Isles during the first week of December (fig.3.1). |
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Trapped, too, beneath this inversion were particles and gases emitted from factory chimneys in the London area, along with pollution, which the winds from the east had brought from industrial areas on the Continent. The total number of deaths in Greater London in the week ending the 6th December of 1952 was 2,062, which was close to normal for the time of year. The following week, the number was 4,703. The death rate peaked at 900 per day on the 8th and 9th and remained above average until just before Christmas. Mortality from bronchitis and pneumonia increased more than sevenfold as a result of the fog. It should not, however, be complacent. The air of Mexico City contains other types of pollutants, mostly of vehicle exhausts. Among these pollutants are carbon monoxide, nitrogen dioxide, ozone, benzenes and aldehydes. They are less visible than the pollutants of yesteryear but are more or less toxic, causing eye irritation, asthma and bronchial complaints. |
![]() Fig.3.1: History - London and the great smog of 1952 (55kB) |
Effects of Pollutant on the Urban Population: According to the nature of the pollutant, concentration
levels and the period of exposure, the effects of pollution can range from a little irritation to acute sickness or even
to premature death. To evaluate the effects of pollutants, two approaches are used: i) Epidemiological studies, based on the measurable effects on the health of people when naturally exposed to a particular pollutant. The exposure time for human experiments is usually limited because of possible damage to health. Epidemiological studies can help to evaluate chronic, long-term effects. Nitrogen present in the air and as an impurity in fuels convert to nitric oxide in exhaust gases. In similar way, other trace impurities can give rise to a variety of pollutant gasses in emission. The presence of chlorine and sulfur in fuels results in the emission of gaseous chlorine and sulfur compounds. Emission Inventory of ozone precursors in the ZMCM (Percentage in Pollutant Weight, 1995). The emission inventory is the basic instrument of diagnosis and planning, and offers a rational basis for decision-making. From the ZMCM emission inventory, motor vehicles make the greatest contribution to the emission of ozone precursors (55% HC and 71% NOX), followed by thermo-electric power plants (15% NOX), services (38% HC), industry (10% NOX and 3% HC), and the rest from other human activities and natural sources (fig.3.2). |
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Although pollutant emissions have been reduced in the Mexico City metropolitan area (ZMCM), approximately 4
million tons per year are emitted at the present time (data of 1998). According to local census data, the main source
of most pollutants is the internal combustion engine (75%), followed by natural sources (12%), services (10%) and
industries (3%). Sulfur dioxide is related to industrial activity, while carbon monoxide, nitrogen dioxide and
hydrocarbons arise mainly from transport emissions. The main sources of sulfur dioxide (SO2) are industries (57%), followed by internal combustion engines (27%) and services (16%). Some particles emissions in the city are due to natural sources (erosion). |
![]() Fig.3.2: Main Pollutants in D.F. |
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HSO3 + O2 → HSO5 → HO2 + SO3 SO3 + H2O → H2SO4 |
![]() Fig.3.3: Effects of SO2 (60kB) |
In reference, the Ozone Protection Act for European countries (e.g. in Austria it went into effect on May 1st of 1992), group ozone concentration into three categories - according to the average mean of 3-hours - see table. |
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![]() Fig.3.5: Effects of NOX (50kB) |
In the atmosphere, nitric oxide is oxidized to nitrogen dioxide, which is a major constituent of smog: |
(w/n the combustion engine) |
Light with a wavelength of 400 nm is at the violet end of the visible spectrum; so when it is absorbed, the remaining transmitted light appears yellow-orange (fig3.6). |
![]() Fig.3.6: 2NO + O2 → 2NO2 (90kB) |
·O· + O2 + M → O3 + M |
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Many reaction paths lead to the formation of PAN. In one common path, an ·O· atom first abstracts an ·H atom from a molecule of unburned hydrocarbons in fuel (HC’s - member of the VOC-family): |
.... to close the loop or reacts further with HC's to .... CH4 + ·O· → · CH3 + ·OH |
Hydroxyl radicals react with many substances, including fuel molecules that were only partly oxidized in an engine; i.e. acetaldehyde, CH3CHO (another member of the VOC-family) , reacts with the highly reactive hydroxyl radical to produce yet another radical: |
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Although there is no air quality standard for PAN, high PAN concentrations are a good indicator of the organic oxidizing capacity of the urban air mass, since they are directly connected to organic peroxy radical formation and nitrogen dioxide (both ozone-forming precursors). During the field study mentioned above, PAN was typically 90% of the total PANs observed, with the rest of the PAN's being approximately 9% peroxy-propionyl nitrate (PPN), and approximately 1% peroxy-butyl nitrate (PPB). Maximum values for PAN, PPN, and PPB were 35, 6, and 1 ppb, respectively. These high levels of PAN's trap an appreciable amount of the nitrogen dioxide, thus slowing the reaction of OH with NO2 to form nitric acid and subsequently ammonium nitrate aerosols (see Particular Matter - further below). Nitric Acid (HNO3): NOX in combination with humid air (water aerosols) react to form nitric acid (HNO3), that, if inhaled, exerts corrosive properties to the nasal mucus, the trachea of the lungs and the alveolar tissue. Ultimately resulting in respiratory problems and severe attacks of coughing - see table. |
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NO2 + O3 → NO3 + O2 | |
NO3 + NO2 → N2O5 NO3 + NO → 2NO2 |
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NO + OH· → HNO2 NO2 + OH· → HNO3 |
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NO + NO2 + H2O → 2HNO2 HNO2 + h·f (<400nm) → NO + OH· | |
OH· + CH3CHO → H2O + CH3CO OH· + CO → H + CO2 As an initial simplification, methane is regarded as an alkane. The radical product of the above reaction becomes involved in subsequent reactions that oxidise NO to NO2 and regenerate OH radicals simultaneously. Methane (CH4) oxidation proceeds as follows: CH3O2· + NO → CH3O· + NO2 2CH3O· + O2 → 2HCHO + H2O 2CH3O2· + O2 → 2HCHO + HO2· HO2· + NO → NO2 + OH· Water (H2O) can also react photo-chemically or with ozone, hydrogen (H) or oxygen (O) to regenerate hydroxyl radicals. Formaldehyde (HCHO) can photo-dissociate into H and formylium (HCO+) or react with O2 to give hydro-peroxyl radical (HO2·) and carbon monoxide (CO). The reactions above can be summed up to show the importance of the presence of hydrocarbons in generating nitrogen dioxide in photochemical smog: This equation indicates that the oxidation of nitric oxide has not used ozone. Thus the presence of alkane such as methane in the polluted urban air provides a way in which nitric oxide can be oxidised without consuming ozone. The attack of the OH on acetaldehyde (CH3CHO) yields the acetyl radical (CH3CO·), which oxidises along the following path, yielding the methyl radical (CH3·) mentioned above: CH3COO2 + NO → CH3CO2 + NO2 CH3CO2 → CH3· + CO2 The atomic hydrogen produced in the reactions above, or from the photo-dissociation of formaldehyde can react with HO2· to produce OH· that can in turn initiate further attack on organic compounds, or it can form a hydro-peroxyl radical: (where M as a third party collision partner, e.g. N2, able to absorb kinetic energy) Thus, aldehydes also provide effective ways of oxidising NO to NO2. This nitrogen dioxide photolyses to produce further O3 and NO for reoxidation. While there are loss processes in the cycles, the build up of ozone throughout the day can thus be explained. |
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![]() Fig.3.7: Effects of VOC's (80kB) |
Effects of Carbon-monoxide - see table. |
4.a Energy Production and Consumption |
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Most of Mexico City's energetic requirements are met by fossil fuel derivatives; i.e. petroleum. It is a complex mixture of organic compounds, mainly hydrocarbons, with smaller quantities of other organic compounds containing nitrogen, oxygen, sulfur, and other trace elements. The usual first step in the refining or processing of petroleum is the separation of the crude oil into fractions on the basis of their boiling points (fig.4.1). Only certain fractions are taken from that mixture. The fractions that boil at higher temperatures are made up of molecules with larger numbers of carbon atoms per molecule. The fractions collected in the initial separation require further processing to yield a usable product. In the case of gasoline, modifications must be made to render it suitable for use as a fuel in automobile engines. Similarly, the fuel oil fraction may need additional processing to remove sulfur before it is suitable for use in an electrical power station or domestic heating system. |
![]() Fig.4.1: Extraction and processing of natural gas and oil (145kB) |
National Oil Production: The production of national oil company PEMEX (Petróleos Mexicanos),
represents the main energy production of the country and amounts to about 4.5% of total world production. The chart (fig.4.2.) shows the rather flat (constant) rate of production over the last 20 years. Composition of the produced oil-fractions within the fuel sector: The main commercial fuels are LPG (liquefied petroleum gas), gasoline (petrol), diesel and fuel oil (production figures for 1995, 1996 and 1997 in thousands of barrels per day, were obtained from PEMEX. Consumed Oil Products: The volume of internal purchases of oil products and natural gas are shown in the plot. It is important to mention that oil production from sources in Mexico is not only for national consumption but a significant volume is also exported world-wide. Distribution of Energy Consumption in Mexico (1995): The main consumers of energy produced in Mexico are transportation and industry, as showed in the following graphics; the domestic sector does contribute considerably as most electrical energy is produced by coal-combustion reactors (fig.4.3). Type of Fuel Consumed by Sector at the ZMCM (1994): Energy consumption distribution is similar to the rest of the country, with transportation as the main energy consuming sector. Energetic Consumption by Type of Transportation (1989): Private vehicles accounted for 78% of energy consumption, followed by 9% on collective taxis, 7% on suburban transport, 4% on public buses, 1% on the Metro system, and finally, less than 1% on trams (fig.4.4). Number of Vehicles in Mexico City. Being the biggest urban population center in the world, it spreads over a large area, thus has very demanding transportation requirements. 84% of its inhabitants use public transport, which accounts for 7% of the total number of vehicles on the roads. Private vehicles make up 71%. This situation creates the following urban problems: heavy vehicle circulation in urban areas, crowds of pedestrian in the downtown area, overloaded roads, intense pollution due to fossil fuel powered and lowered efficiencies of public services. |
![]() Fig.4.2: Oil production / consumption (125kB)
![]() Fig.4.3: Fuel consumption (55kB)
![]() Fig.4.4: Vehicle consumption (60kB) |
The Internal Combustion Engine: Internal combustion engines are devices that generate work from combustion reactions. Combustion products under high pressure produce work by expansion through a turbine or piston (Carnot-cycle). The combustion reactions inside these engines are not necessarily neutralizing or complete and air pollutants are produced. There are three major types of internal combustion engine in use today:
Diesel engines are notorious for the black smoke they can emit, and gas turbines because of soot emission. The ideal efficiency of an ordinary automobile engine is about 56%, but in practice the actual efficiency is about 26% (fig.4.6). Engines of higher operating temperatures (compared to sink temperatures) would be more efficient, but the melting point of the material the engines are made of suppresses the upper temperature limit at which they can operate. Higher efficiencies await engines made with new materials with higher melting points as already proved with ceramic engines. The ideal efficiency (according to the 2nd law of thermodynamics) given in [%]: |
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  η = 100· |
Thot - Tcold_ Thot |
Thot, combustion temperature in the engine [K] Tcold, exhaust gas temperature at engine [K] |
As can be see in the representative image, an average car (at sea level) uses only about 26% of the provided energy, the remaining 74% are lost. However, at altitudes of 2240 m with reduced partial oxygen pressure, the combustion process is even further restricted, and along with it, resulting in a reduced energetic output of the engine; in Mexico City it can be as low as 20%! | Furthermore, the use of a built-in air-conditioner, lowered tire pressure, speeds exceeding 90 km/h, and aggressive driving pushes efficiency even further down. In Mexico City the idling and coasting losses are probably excessively elevated, as countless "stop and go" intervals in clogged roads are chronically; an average poorly maintained Mexican car could run on an efficiency level of less than 15%! | ![]() Fig.4.6: Efficiency of the internal combustion engine (50kB) |
Until the traditional combustion engine will be replaced by newer cleaner alternatives, like the Hydrogen Fuel Cell, the design of automobile engines is now being guided by requirements to reduce emissions of these pollutants. |
5. Control Strategies
As in many other big cities of the world, Mexico City has made important efforts in order to reduce
air pollution. Recognizing that transportation has proved to be a major pollution source within the Mexico City
Metropolitan Area (ZMCM), any strategy that aims to reduce or control atmospheric pollution has to include a
transportation improvement program. The main programs to combat air pollution in the ZMCM are: |
5.b IMECA
To provide information related to air quality conditions in the metropolitan area, the Mexican authorities have
developed a pollution standard index called IMECA (from the Spanish 'Indice Metropolitano de la Calidad del Aire',
fig.5.4). The index of the quality of the air, is defined as a representative although relative value of the levels of
atmospheric contamination and its effects in the health within a certain region. Pollution levels exceeding 100 points
on the IMECA scale are considered a threat to human health -
see table. |
6. Conclusion
Prior to the 1940s, Mexico City was known for its clear air and spectacular views of snow-capped volcanoes.
Today, the city's mountains are only rarely visible due to some of the worst air pollution in the World. Many factors
have contributed to this situation national policies that have promoted industrial growth and a concentration of wealth
and employment in Mexico's capital; a population boom from 3 million in 1950 to roughly 20 million today; and heavy
reliance on motorized transportation. The city sits in a basin 2,240 meters above sea level, and is surrounded by
mountains that rise one kilometer or more above the basin (a former lake bed). High elevation and intense sunlight are
key factors in ozone formation. Air pollution is generally worse in the winter, when there is less rain and events of
thermal inversion are more common. |
7. Literature Brimblecombe P. (1986); Air composition and chemistry 2nd ed.; Cambridge Environmental Chemistry Series 6; Cambridge University Press, UK Brown, L. T., LeMay, Jr. H. E.; (1977); Chemistry, the Central Science, Prentice Hall, Inc. Englewood Cliffs, New Jersey, USA. Chang R. 1994; Chemistry 5th ed.; McGraw-Hill; New York, USA Edgerton S.A, et al. (1997); Particulate Air Pollution in Mexico City; North American Research Strategy for Tropospheric Ozone; CDN, MEX, USA Gloxhuber, C.; (1994); Toxicology; 5th ed.; Thieme Verlag; Stuttgart, FRG Haider, M.; (1999); Lecture Script - Air analysis; Wacker Chemie; FRG Hofmann, W.; (2001); Lecture Script - Aerosol Physics; University of Salzburg; AUT Kisser, W.; (2000); Lecture Script - Toxicology; University of Salzburg; AUT Lutgens, F., Tarbuck, E.; (1998); The Atmosphere; 7th Edition; Prentice Hall; NJ, USA Mage D. et al. (1996) Urban Air Pollution in Megacities of the World; Atmospheric Environment Vol.30, No.5, pp681-686; Elsevier Science, UK Meszaros E.; (1999); Fundamentals of Atmospheric Aerosol Chemistry; Akademiai Kiado, Budapest, HUN Morawska, L.; (1999); Lecturing Script: Introduction of Aerosols; QUT, Brisbane, AUS Reichl, F.; (1997); Taschenatlas der Toxikologie; Thieme Verlag; Stuttgart, FRG Wilbraham, A. C., Staley, D. D., Matta, M. S.; (1998), Chemistry, Teacher's Edition, 4th edition, Addision-Wesley Publishing Company, New York, USA
BBC-World TV, Earth Report - City Smog (1999), UK - www.tve.org Mexico City lifts pollution alert No relief for Mexico's air pollution crisis Americas No let-up for Mexico City pollution Americas Mexico City pollution emergency San Antonio blames Bret, evacuees for air pollution Sustainable Development Cases Studies NAEC releases air pollution report http://www.epa.gov/ INDOEX http://www.epa.gov/oms/t2models.htm http://www.epa.gov/epahome/faq.htm#jobs Air Pollution Technology Fact Sheets Information Center on Air Pollution Air Pollution in Mexico City Air Pollution & Lung Disease Mexico City Fights Smog by Electrifying Air PEMEX Case Studies of Urban Air Pollution life-threatening air pollution The Great Smog Of 1952 Initiative for Sustainable Energy Atmospheric Chemistry of Organic, Oxidants and Their Precursors |