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Air Pollution

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 (1998). According to local census data, the main source of most pollutants are internal combustion engines (75%), followed by natural sources (12%), services (10%) and industries (3%). Some particles emissions in the city are due to natural sources (erosion), sulphur dioxide is related to industrial activity, while carbon monoxide, nitrogen dioxide and hydrocarbons arise mainly from transport emissions.

The main sources of sulphur dioxide (SO2) are industries (57%), followed by internal combustion engines (27%) and services (16%).

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.


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 Calidad del Aire'). The value of this index is equal to 100 at the air quality standard.

Historical Data. Percentage of days in which some air quality standard (IMECA 100) is exceeded at each zone of the ZMCM (data taken from http://www.ine.gob.mx/cal_aire/ingles/sinhis.html).




The graphic shows the percentage of days for IMECA values above 100, 150 and 200. Values greater or equal to 250 IMECA during those years were observed mainly in 1991 (15) and 1992 (11). Values above 300 IMECA were observed in small percentages (1 or 3%) before 1994.

Particulate matter (PST, from the Spanish 'Particulas Suspendidas Totales') is manually monitored in the ZMCM. Its value frequently exceeds the accepted standards.

Concentrations of particulate matter smaller than ten microns (PM10) has been monitored since 1995. Standards were exceeded on 98 days during that year, and on 182 days during 1996.

Air Quality Summary for 1996

  • In 1996, some of the standards for air quality were exceeded in the ZMCM on 91% of days (333), including 71 days (19%) measured at more than 200 IMECA points and 5 days up to 250 points.
  • The ozone standard was exceeded on 89% of days (327) and for 20% of them the value of standard was doubled. Levels were exceeded on average on 75% of the days in each month. The PM10 standard was exceeded on 182 days (50%).
  • Nitrogen dioxide went beyond the 100 IMECA point standard on 22% of days during the year. The highest value measured was approximately 160 IMECA points.
  • On eight days, carbon monoxide values went beyond the standard. The maximum value was almost 160 IMECA points. Due to an extraordinary situation caused by a natural gas shortage, fuel oil was burnt for two days in the ZMCM and concentrations above the standards set for sulphur dioxide were registered.
  • Levels of lead have been within air quality standards since 1995.

Northwest Zone. Percentage of days greater or equal to 100 and 150 IMECA points are shown. Levels of 200 IMECA were reached on 5.2% of days for O3 and none for PM10, CO, NO2 and SO2. This year the percentage of days greater or equal to 250 points was only reached for O3 (0.3% days).

Northeast Zone. As above, the graphic shows levels of 100 and 150 IMECA. The percentage of days greater or equal to 200 IMECA was only 0.3% for O3.

Centre Zone. 4.4% of days presented IMECA values greater or equal to 200, and there were no recorded levels which exceeded 250 IMECA.

Southwest Zone. The southern part of the ZMCM is commonly the most polluted area for ozone. The percentage of days with levels greater or equal to 200 IMECA was 13.4%, with 0.5% registering more than 250.

Southeast Zone. High concentrations of ozone are also more frequently seen in this area. The percentage of days greater or equal to 200 IMECA for ozone was 2.7%, including 0.5% greater or equal to 250.

Maximum IMECA. In 1996, maximum values of IMECA were reached in the southwest zone with 274 points for ozone. In the northeast area the maximums were 177 IMECA for PM10 and 122 IMECA for sulphur dioxide. In the northwest zone two other pollutants registered their maximum values for the year: carbon monoxide (142 IMECA) and nitrogen oxides (157).

Monthly Variation. During 1996, as in many other years, the highest pollutant concentrations were measured in winter time (mainly January). Ozone reached its peak in January, with 269 points, and its minimum in April, with 200. For small particulates (PM10) the maximum was 177, also in January, and a minimum of 87 IMECA.

Carbon monoxide concentration also exceeded the standard. Maximum levels reached 142 IMECA, with a minimum of 72 IMECA observed in June. NO2 levels ranged from 157 to 75 IMECAS, while SO2 had a maximum of 122 and a minimum of 38 IMECA. It should be noted that over the year concentrations of all major pollutants exceeded the standards set.

Source: http://www.ine.gob.mx/cal_aire/ingles/sica.html



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