Air Pollution in Mexico City

Mexico City Smog

Air Pollution in Mexico City is a severe environmental problem. Living in the city, if have expenienced some of the most insanse traffic jams and on a daily basis. There are so many people living here so consumption and the burning of fossil fuels are enormous. But environmental programs over the last 20 years have ensured that the air quality in Mexico City is improving. The  Sistema de Monitoreo Atmosférico de la Ciudad de México (SIMAT) has reported improvements in practicaly every contiminant, although it must be said that concentrations of ozone and suspended particles are above recommended levels given by the Norma Oficial Mexicana.

Air pollution in Mexico City is a mammoth topic and so I hope to update this section of the website regularly, as like many people of this era, the environment for me is a sensitive and important issue.

Explained below is how air pollution in Mexico City is not helped by the fact that it is a valley.

Air has a weight and therefore exerts a force over any object it encounters. This force is proportional to that of the atmosphere over any determined area, known as atmospheric pressure. In the case of Mexico City, air pressure is 0.8 kg/cm2.

A region of the atmosphere where air pressure can reach a maximum or minimum value is known as a pressure system. The region where air pressure reaches the highest is known as the high pressure system and the region of the lowest air pressure, the low pressure system. These variations in pressure are associated with changes in temperature and density and composition of the atmosphere.

On the earth’s surface, the wind moves towards the centre of the low pressure area in an anticlockwise manner. When the air converges (joins) towards the centre it rises slowly and, at a certain height, it diverges (separates). The rising air basically moves away from the centre to compensate for the converging air that is moving towards the centre on the earth’s surface. The humidity of the rising air condenses at the highest and coldest heights, forming clouds. This system is what leads to rain, storms and in extreme cases, hurricanes.

In systems of high pressure, the wind at the earth’s surface diverges or, in other words, it moves away from the centre of the area of high pressure in a clockwise motion. To compensate for this displacement, the air converges towards and above the centre of the high pressure region. The air then descends and heats up in the centre, avoiding condensation and resulting in a pleasant dry and warm climate.

The descending movement of air in high pressure systems is known as subsidence whilst the movement of air in low pressure systems is known as divergence. Subsidence impedes the formation of clouds and creates clear skies. Wind is generally weak.

SubsidenceDivergence1

The dry air from the upper layer moves downwards on the lower layer creating a condition known as thermal inversion. This acts like a stopper, trapping pollution and provoking a reduction in visibility. The thermal inversion layer impedes the entry of fresh air that dilutes the pollution below. Thermal inversion can also remain for hours, even days.

Inversion Polution

Air pollution trapped below the boundary limit is also dangerously exposed to intense solar radiation, because of the clear skies that high pressure systems create. The photochemical transformation of the pollution stimulates production of ozone and fine particles. The lack of dispersion creates the accumulation of air pollution in Mexico City, through which ozone can easily reach elevated levels and the fine particles produced start to diminish visibility.

 

Main Contaminants to Air Pollution in Mexico City

Below is a list and the levels of the main contaminants that cause air pollution in Mexico City.

Details are taken from an air pollution in Mexico City report

Sulphur Dioxide

Sulphur Dioxide is a gaseous compound formed by the combustion of sulphur, or compounds that contain sulphur, in the presence of air. In Mexico City, its principal origin is through the burning of fossil fuels, although it can be created in a natural manner. Furthermore, levels of sulphur dioxide provide an important precursor to levels of secondary aerosols and acid rain.

Acceptable levels of sulphur dioxide stated by the Norma Oficial Mexicana are 0.130 ppm as a maximum daily average and 0.030 ppm as a yearly average.

Current levels of sulphur dioxide are 0.076 ppm as a maximum daily average and 0.10 ppm as a yearly average. Both levels are acceptable according to the Norma Oficial Mexicana.

However, the standards set by the World Health Organisation are 20 µg/m3 as a daily average and 500 µg/m3 as a peak 10 minute average. Against these standards, Mexico City fails with 156 µg/m3 of sulphur dioxide daily and 967 µg/m3 maximum average over 10 minutes.

The daily averages between 1992 and 2010 are as follows:

1992 0.205 ppm

2001 0.297 ppm

2010 0.076 ppm

SulphurDioxide2

Nitrogen Dioxide

Nitrogen dioxide is a compound gas that is reddish-brown in colour and has a distinctly unpleasant smell.  It is produced in the majority of combustion processes that use air as an oxidant. It is the principal component in the formation of ozone and contributes to poor visibility. In Mexico City, the Norma Oficial Mexicana recommends acceptable levels of Nitrogen Dioxide at 0.210 ppm each hour.

According to the Norma Oficial Mexicana, Mexico City is within its limits, producing 0.171 ppm of Nitrogen Dioxide per hour.

However, according to the World Health Organisation Mexico City fails its hourly standard of 200 µg/m3 by producing 252 µg/m3.

The hourly averages of Nitrogen Dioxide between 1996 and 2010 are as follows:

1996 0.463 ppm

2000 0.304 ppm<

2010 0.171 ppm

NitrogenDioxide2
NitricOxides2

Carbon Monoxide

Carbon Monoxide is colourless and odourless and is the product of incomplete combustion of hydrocarbons or biomass in the presence of air as an oxidant. It is also produced when there is not sufficient oxygen to reach the product of complete oxidation of carbon dioxide. The Norma Oficial Mexicana puts acceptable levels of carbon monoxide at 11.0 ppm every 8 hours.

According to the Norma Oficial Mexicana, Mexico City is within its limits of carbon monoxide at 7.5 ppm every 8 hours.

The World Health Organisation also agrees that Mexico City is within its limits of 10 000 µg/m3, producing 6731 µg/m3 every 8 hours.

CarbonMonoxide2

Ozone

Ozone is a highly oxidant compound gas that is very reactive and when at surface level, it is considered a contaminant. In urban environments, it is the principal component of photochemical smog and is formed by reactions between hydrocarbons and nitrogen oxides in the presence of solar radiation. The Norma Oficial Mexicana puts safe levels of ozone at 0.080 ppm every 8 hours.

According to the Norma Oficial Mexicana, Mexico City is above limits of ozone with 0.123 ppm every 8 hours.

According to the World Health Organisation, who puts safe limits at 100 µg/m3, Mexico City is again above with 237 µg/m3.


Details on this page about air pollution in Mexico City are up to date to 2010.

All facts on this page were taken from the report ‘Calidad del aire en la Ciudad de México – Informe 2010′, published by the Secretaria del Medio Ambiente del Gobierno del Distrito Federal.

Click here for a link to the publication.

Or click here to found out more about air pollution in Mexico City by going to the SIMAT homepage.

Photo Credit: Smog by arndw on Flickr