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Chapter 7

Air Quality in Mexico City 1,2

1

J. Garfias and R. González

1Subsecretaría de Ecología, Secretaría de Desarrollo Urbano y Ecología, Río Elba 20, México, D.F., CP 06500 Facultad de Química, Universidad Nacional Autónoma de Ciudad Universitaria, México, D.F., CP 04510

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2

In Mexico City, several air quality parameters are measured continuously by an Automated Monitoring Network operated by the Under Secretariat of Ecology. Carbon monoxide, particulate matter, sulfur dioxide, nitrogen oxide, and ozone are the contaminants exceeding Air Quality Standards. Emissions produced by 2.7 million vehicles and 35,000 commercial and industrial outfits are not easily dispersed in a Valley located at 2240 m and surrounded by two mountain chains which hinder air circulation. An Integral Program, recently established to alleviate pollution, is briefly described. Illustrated writings in the Mendocino Codex indicate that the Aztec pilgrimage of 1327 ended with the founding of Mexico City on an island where an eagle was seen eating a snake atop a cactus tree. At that time, the surface area covered by water in the Valley of Mexico was as large as the land area. Old paintings belonging to the 16th and 17th century depict a valley with three large lakes encircling the city: Xochimilco, Texcoco and Chalco. Mexico City was usually flooded during the rainy season: the worst deluge took place in 1629, lasting five years and decimating the population. It is then not surprising to trace, as early as the 17th century, efforts directed to dry the lakes. To this day, dust storms, ensuing after Texcoco and Chalco Lakes were partially drained, sweep the city in February and March (7). Although particulate matter often exceeds the air quality standard, dust has now been reduced by seeding a grass variety resistant to saline soils. Air at 2240 m is 23% lighter than air at sea level; this fact led Humboldt to observe, at the beginning of the last century, that Mexico City's air was "the most transparent one." Intense industrialization and population growth in the last 40 years, plus adverse geographical and

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meteorological conditions, have transformed Mexico City to one of the most contaminated metropolises in the world.


Geographical and Meteorological Considerations Combustion efficiency at Mexico City's elevation is considerably reduced, compared to sea level, and carbon monoxide (CO) and hydrocarbon (HC) emissions enhanced, unless an extra 23% volume of air is fed. Contaminant dispersion is not easily achieved in Mexico City as she lies in a valley flanked by two cordilleras, averting strong wind formation. Average wind velocity is 2.4 m/s. The direction of prevailing winds is NESW during the day (see Figure 1). At night, a light mountain wind descends towards the Valley. Thermal inversions occur almost daily during the winter but less often in summer (see Figure 2a). Inversions break at around 9:30 A M in winter (Figure 2b). The average thickness of the inversion layer is less than 200 m in 64% of the days showing an inversion. Particularly adverse conditions may prevail in winter whenever a combined mechanism of high air pressure and thermal inversion occurs. Seasonal rains that fall from June to September help to clear the atmosphere. However, a quasi-static atmosphere associated with high pressures may lead to high ozone levels at any time. Pollution Inventory Eighteen and a half percent of Mexico's total population live in the Mexico City Metropolitan Area (MCMA). The M C M A comprises the Federal District and 17 Municipalities from the State of Mexico. The M C M A has an estimated total area of 2,000 square kilometers, of which 34% is urban, 28% forest, 27% agricultural, and 11% arid. Fifteen million people living in the M C M A produce 36% of the Gross National Product, and consume 15% of total fuel. Mobile, fixed and natural sources contribute 83,12, and 5% respectively to the 4.9 million metric tons of contaminants yearly emitted into the atmosphere (Table I). Table I. Sources of Atmospheric Pollution in 1989 (metric tons per year) Contaminant

Industrial

Vehicular Natural

Total

Particulates Sulfur Oxides Hydrocarbons Carbon Monoxide Nitrogen Oxides

128,000 184,000 137,500 53,000 68,000

41,000 7,300 310,000 3,573,000 111,300

251,000

420,000 191,300 447,500 3,626,000 179,300

Total

570,500

4,042,800

251,000

4,864,300

Dunnette and O'Brien; The Science of Global Change ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

7.

Air Quality in Mexico City

GARFIAS & G O N Z A L E Z

151

NE


STATE OB' MEXICO

10 Km.

0 OI4-T

p,

OI5-QL

Ù FEDERAL

SW

DISTRICT

SE

Figure 1. Map of the Mexico City Metropolitan Area showing the location of the following monitoring stations: 10-C Azcapotzalco; 11-F Tlanepantla; 12-L XalostoC.; 13-X Merced; 14-T Pedregal; 15-Q Cerro de la Estrella; 16-U Plateros; 17-Y Hangeras; 18-P UAM-Iztapalapa; M u Museo.




N U M B E R O F DAYS

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

•

1989

Nov

Dec

Nov

Dec

MONTH

(a)

CZ]1986

AVERAGE BREAKING

12

H

1987

•

1988

HOUR

ι

11

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

β

1989

MONTH

(b)

CZI1986

E l 1987

•

1988

Figure 2. Thermal Inversions, (a) Number of days occurring, (b) Average breaking hour.



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153


Mexico depends mostly on oil and gas (89.8%) as a source of energy. The relative contribution to pollution by different activities at National and M C M A levels may be assessed by comparing fuel consumptions (Table II). With the exception of kerosenes, each fuel has a distinctive usage in Mexico. Consequently, 51.6% of fuel (gasoline and diesel) consumed in the M C M A is used in transportation, 15.2% (L.P.G.) in residential water heating and cooking, and 27% (natural gas and fuel oil) in industrial and power generation enterprises. Furthermore, by remembering that almost one fifth of Mexico's population lives in the MCMA, it can be inferred from inspection of Table II, that there is a substantially lower per capita consumption of diesel, and in particular of fuel oil in the M C M A than in the rest of the country. The proportionally low diesel figure may be explained by the importance of metro transportation in Mexico City; the even lower fuel oil figure may be explained by the establishment of heavy industry in other regions, and by the fact that power generation in situ has been largely curtailed in Mexico City to reduce pollution. Table IL Fuel Consumption in Mexico and in Mexico City Metropolitan Area in 1989 (metric tons per day) Fuel Natural Gas L.P.G. Gasoline Kerosene Diesel Fuel Oil Total 3 b c

b

Mexico"

%

MCMA

%

27,452 16,213 46,821 6,078 26,273 65,510

14.6 8.6 24.9 3.2 13.9 34.8

4,840 4,224 10,947° 1,734 3,367 2,625

17.5 15.2 39.5 6.2 12.1 9.5

188,347

100.0

27,737

100.0

Reference 2 Equivalent to fuel oil 2.5% is unleaded gasoline

Fuel consumption is very sensitive to economic growth, price structure, and ecological policies. After seven years of economic stagnation, a recovery initiated in 1989 led to a 6.2% annual increase in gasoline demand in the MCMA. However, a rise in gasoline prices plus the permanent establishment of a program in the M C M A to keep vehicles out of circulation one day a week, have lowered the gasoline demand growth rate for 1990. On the other hand, unleaded gasoline is expected to reach 7% of total gasoline demand in 1990. It's increased availability is required by the appearance of 1991 models running exclusively on unleaded petrol. An additional daily substitution of 1,590 tons of fuel oil by 1.93 million cubic



154


meters of natural gas in power stations has also changed the consumption pattern for 1990. There are 35,000 commercial and industrial establishments and 2.5 million vehicles using fuels. Major industries, such as an oil refining (16,000 tons per day, operating until 18 March 1991), two power stations (1,000 total M W ) , foundries and several chemical and manufacturing plants are located upwind in the northern part of the M C M A . The average life of a car is 10 years. New car sales amounted to 368,000 units in the M C M A in 1989, compared to 136,000 in 1984. Vehicular transportation accounts for 22.4 million personal journeys a day: 51% are taken to go to work, 24% to school, 8% for shopping, 3% for entertainment, and 14% in other activities. Although 79.4% of personal journeys are made by public transport, those 19.0% related to the use of private cars are sufficient to create traffic jams, low transit speeds and 70.4% of vehicular emissions. Vehicular Emission Standards The latest revision of Emission Standards for New Vehicles was issued in September 1988. A summary of past and present emission standards is given in Table III.

Table III. Emission Standards for New Vehicles (g/km) Model Year

CO

HC

NO

33.0 27.0 22.0 18.0 7.0 2.11

3.0 2.8 2.0 1.8 0.7 0.25

2.3 2.3 2.0 1.4 0.62

35.0 22.0 8.75

3.0 2.0 0.63

3.5 2.3 1.44

x

Automobiles: 1976 to 1985 1986 to 1987 1988 to 1989 1990 1991* and 1992* 1993*

_

Commercial Trucks: 1990 and 1991 1992 and 1993 1994

T h e standard may be met by emission weighting of a company's car production. The Emission Standard for New Diesel Vehicles, issued in December 1988, established a maximum of 50 Hartridge Opacity Units. The Ecological Technical Standard of June, 1988, enforced the maximum allowable emissions for circulating cars (Table IV). The Diesel Regulation


7.

GARFIAS & GONZALEZ


155

of November 1988, requires biannual inspections. The maximum allowable opacity in diesel engines is related to nominal gas flow. There are 636 Car Inspection Centers in the Federal District and 250 in the State of Mexico, and 114 Diesel Inspection Centers operated by the Secretaria de Communicaciones y Transportes.


Table IV. Maximum Allowable Emissions for Cars in Circulation

Year Model

CO (%vol)HC(ppm)

Previous to 1979 1980 to 1986 Later than 1987

6.0 4.0 3.0

700 500 400

Fuel Quality. Petroleos Mexicanos markets two gasoline brands: leaded Nova Plus and unleaded Magna Sin. Nova Plus is specified to have tetraethyl lead (TEL) in the 0.5 to 1 milliliters per gallon range, a minimum Research Octane Number of 81, and a Reid Vapor Pressure (RVP) in the 7 to 9.5 psi range. Magna Sin is similar to American Regular, with a minimum 86 Road Octane Number, a maximum of 0.01 grams of lead per gallon, and the same R V P as Nova Plus. Diesel for combustion engines contains no more than 0.5% sulfur with a minimum 40 octane number. Industrial Fuel Oil is limited to 3% sulfur, but has from 0.2 to 0.7% of compounded nitrogen. To meet Emission Standards, new 1991 models are designed to run on unleaded petrol, and therefore, new engines working at a higher compression ratio will be from 8 to 10% more efficient than older ones. Air Quality Monitoring Network. The first systematic effort to measure air quality began in 1966 with the installation of 4 manned monitoring stations. The Mexican A i r Quality Standards (MAQS), given in Table V , were promulgated in November 1982. The air monitoring network has been expanded and transformed over the years. A t present, the Undersecretariat of Ecology operates an Automated Monitoring Network comprising 25 stations, of which 15 measure sulfur dioxide, 5 nitrogen oxides, 15 carbon monoxide, 3 non-methane hydrocarbons, 10 ozone and 2 hydrogen sulfide. Ten stations measure the meteorological parameters wind velocity, wind direction, relative humidity, and temperature. The information received by telephone in a computer at the Central Office is processed and relayed to officers in charge of emergencies and to the press. There is an additional Manual Monitoring Network, made up by 16 stations, committed to evaluate total particulate matter, PM10 (suspended particulate matter less than 10 μτη in diameter), sulfur dioxide and heavy


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metals. Two acoustic radars are used to measure the heights of inversion layers. Geopotentials are appraised at 300, 500 and 700 millibar.


Table V. Air Quality Standards* Contaminant

Averaging Time

Mexican Standard

USA. Standard

Carbon Monoxide

8h 1 h

13 ppm

9 ppm 35 ppm

Sulfur Dioxide

annual average 24 h

Hydrocarbons (corrected for metane) Nitrogen Dioxide

003 ppm 014 ppm

0.13 ppm

3 h (6-9 a.m.)

021 ppm 005 ppm

annual average 1 h

0.21 ppm

Ozone

1 h

0.11 ppm

Total Particulate Matter

annual geometric mean 275 /ig/m 24 h

Q12ppm 3

75/igta 3

* Standards, other than those based on annual average or annual geometric average, are not to be exceeded more than once a year. Thermal inversions make winter the most unfavorable season for clean air. Vast differences in air quality are found in the industrialized north, and the residential southwest regions. Particulate matter influences mainly the north, where industries, landfills, and the dried bed of Texcoco Lake are located. Sulfur oxides impinge primarily on the northeast and southwest. High carbon monoxide concentrations are found in heavy traffic areas such as the northwest. Ozone affects predominantly the southwest at any season. We have selected air quality records from data generated by stations registering the higher pollutant levels, as follows: Carbon Monoxide. The Standard is often exceeded at the Cuitlahuac Station (northwest) during winter,as displayed in Figure 3a. However, a diminishing trend on the number of hours reaching a 26 ppm level can be appreciated in Figure 3b. Sulfur Dioxide. High concentrations are found at Xalostoc (NE) and Santa Ursula (SW) stations during fall and winter, the former is located close to a power plant, and the latter near an asphalt factor; the Standard was exceeded 5 and 9 days respectively from October 1989 to February 1990.


7.


GARFIAS & GONZALEZ

NUMBER OF 8-H MOVING

157

PERIODS

50

40

30

20


10

Sep

Oct

Nov

Dec

Jan

Feb

MONTH YEAR

(a)

•

1989

Β

1990

N U M B E R O F H O U R S E X C E E D I N G 26 P P M 60

50

40

30

20

10

(b)

SPAN

•

O C T 86 - F E B 87

OCT

87 - F E B

88

Β

O C T 88 - F E B

OCT

89 - F E B

90

89

Figure 3. Carbon Monoxide at Cuitlahuac Station, (a) Number of 8-h moving periods in which the Standard is exceeded, (b) Number of hours in which 26 ppm is exceeded.


158


Nitrogen Dioxide. At Merced Station, located in the central region, the Standard was exceeded on 19 days from October 1988 to April 1989. In the same period, the Standard was exceeded 12 days at Tlalnepantla Station (northwest), and 3 days at Pedregal Station (southwest). Particulate Matter. Maximum daily average concentration of particulate matter, smaller than 10 microns, measured at Xalostoc (NW), from July 1989 to July 1990 is exhibited in Figure 4a. The American Standard is 150 μg m" as a daily average. Maximum PM10 concentrations measured in several stations in June 1990 are presented in Figure 4b.


3

Lead. Lead in petrol was decreased to 1.2 mL of T E L per gallon in 1983. T E L has been kept in leaded gasoline in the 0.5-1.0 range since 1986. The lead concentration trend in petrol is followed closely by that measured in the atmosphere in downtown Museo Station. Lead concentration at Museo is within International Standards (average 1.5 /ug/m in 3 months), however, Xalostoc Station placed in an industrial area shows twice as high lead concentrations as the international standards, pointing to the need for relocating some foundries outside the Valley, and controlling others. Replacement of T E L in gasoline byy 5% methyl-terbutyl ether (MTBE) and the increased use of unleaded petrol will gradually lower lead emissions. 3

Ozone. High ozone concentrations are confronted in the southwest region, where an air parcel accumulates photochemical contaminants collected and generated after sweeping the north and central regions. The number of days exceeding 0.11 and 0.22 ppm at Pedregal Station are illustrated in Figure 5. The maximum hourly average ozone concentration ever measured was 0.441 ppm at Pedregal Station in December 1986. The maximum hourly ozone concentrations determined in a year at Pedregal and Plateros Stations from 1986 to October 1990 are given in Table VI. Table VI. Maximum Ozone Concentrations in an hour at Pedregal and Plateros Stations (ppm) Year

Pedregal

Plateros

1986 1987 1988 1989 1990*

0.441 0.344 0.405 0.340 0.403

0.398 0.331 0.351 0.310 0.375

* Up to October 31st.


GARFIAS & GONZALEZ


MAXIMUM CONCENTRATION (yg/m3) 400

300

200


100

I Jul

Aug Sep

Oct

Nov

Dec

Jan

Feb

• 1 Mar

Apr

May

Jun

Jul

MONTH YEAR

(a)

!

I 1989

MAXIMUM CONCENTRATION

H

1990

(ug/m3)

300

250

200

150

100

50

0 JUNE

(b)

1990

STATION TLALNEPANTLA

L Î 1

XALOSTOC

PEDREGAL

ÎlDii

MERCED

CZH

C.ESTRELLA

Figure 4. Particulate matter less than 10 microns (PM10). (a) Maximum daily average concentration observed at Xalostoc Station (northwest), (b) Maximum daily average concentration observed at five stations in June 1990.



N U M B E R O F DAYS E X C E E D I N G


30

STANDARD

ι

J

F

M

A

M

J

J

A

S

O

N

D

MONTH

YEAR

•

1987

H

1988

•

1989

H

1990

N U M B E R O F DAYS

30

ι

25

20

J

F

M

A

M

J

J

A

S

O

N

D

MONTH

•

1987

Ell 1988

CZ3 1989

HI

1990

Figure 5. Ozone at Pedregal Station (southwest), (a) Number of days exceeding the Standard, (b) Number of days exceeding twice the Standard concentration.


7.

GARFIAS & GONZALEZ


161


Integral Programs to Control Atmospheric Pollution Some invaluable isolated actions were taken from time to time in the last twenty to twenty five years that prevented air quality from declining at an even faster rate. For example, energy demand was met (electricity and refined oil products) by expanding capacity of plants located outside of the MCMA; a metro was built; the vehicular traffic system was reordered; and new hydrotreating, reforming and catalytic cracking units reduced lead and sulfur in refined products. Four months after the September 1985 earthquake, a set of wider actions was implemented: 1600 and 320 tons per day of fuel oil were substituted respectivedly by natural gas at a power station and at Atzcapotzalco Refinery in 1986; a detergent additive was incorporated into gasoline to keep carburetors cleaner in 1986; a far reaching Federal Law on Ecological Ordering and Environmental Protection was enacted in March 1988; Emission Standards for cars in circulation and new cars were legislated in June and September 1988 respectively; and Emission Standards for Industrial Combustion Processes were issued in 1988, among other actions. In the last two years, the federal and city authorities, with the financial assistance and the expert advice of specialists from Japan, the United States of America, Germany, France, and Great Britain, devised an Integral Program (3) aimed at bettering air quality in the MCMA. The Integral Program includes fuel quality improvement and reduction of emissions in gasoline distribution; a public transport system more efficient and less contaminating; industrial assimilation of advanced process technology and pollution control systems; reforestation and ecological restoration of barren land and open dumping sites; and the strengthening of ecological research, education and communication activities. Several actions have already started: 5% MTBE is added to gasoline; two additional metro lines are under construction; the program "No Circulation Today" was commenced in November 1989; and loans have been approved for expanding the Automated Air Quality Network, for initiating air modeling in real time, and for building new plants outside the M A M C to produce MTBE and isomeric gasoline, to hydrotreat diesel (0.1% maximum sulfur) and fuel oil (0.8% maximum sulfur), and to increase reforming capacity. A long and difficult task will be confronted, in particular to bring ozone within the air quality standards, as the physics and chemistry of ozone formation in the MCMA are not sufficiently understood. Literature Cited 1. Jauregui, E., Int. J. Climatology 1989, 9, 169-180. 2. Memoria de Labores 1989; Petróleos Mexicanos: Mexico City, 1989. 3. Programa Integral Contra la Contaminación Atmosférica de la Zona Metropolitana de la Ciudad de México, Departamento del Distrito Federal: Mexico City, 1990. RECEIVED October 7, 1991


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