Air Pollution In Mexico City - Traveller's Beware

Here's eight reasons why you should be concerned about the air pollution in Mexico City:

1. Mexico City is one of the largest cities in the world with a population close to twenty million people. The majority of the population is exposed daily to fumes produced by 3.6 million vehicles. This, of course, means that many roads throughout Mexico City are highly congested.
   
   In addition to this, many vehicle owners fill up their vehicles with poor quality fuel.
   These vehicle fumes coupled with other air pollution sources are responsible for many residents suffering from severe respiratory health problems.
   
   
2. It has been known for many years that the air pollution in Mexico City is much worse than that found in similar sized cities in other developing nations.
   
   
   
3. Mexico City is situated in a valley surrounded by mountains on three sides. The city is situated at 2,240 metres above sea level, and because there's less oxygen at this altitude, most of the air pollution is the result of incomplete combustion of hydrocarbons, mainly diesel emissions. Since the diesel fuels aren't burnt properly at this altitude, this leads to the formation of high amounts of soot particles, (also known as black carbon).
   
   
4. You can be sure that when you breathe in the air at Mexico City, you're in fact inhaling a sizeable proportion of these soot particles as well. Soot particles are also known to aggravate your eyes, throat and worse still can become dislodged in your lungs. Unfortunately, given the tiny size of the soot particles, no coughing or sneezing will expel this particulate matter from your respiratory system.
   
   
5. These black carbon aerosols are quite difficult to remove from the atmosphere since the soot is hydrophobic. That is, the soot is water repellent and not easily washed out of the atmosphere by precipitation unlike water attracting (hygroscopic) particulates like nitrate and sulfate. So, for this reason, the Mexican government has had more success with reducing most forms of air pollutants in Mexico City, except for the soot.
   
   
6. The air pollution in Mexico City is compounded by the fact that this city is located within the tropics. You can expect a lot of intense sunshine all year round, (more so in the winter months).
   
   
   

   For that reason, Mexico City receives high levels of ozone and various other types of photochemical smog. A large proportion of the ozone air pollution in Mexico City is formed when nitrogen oxide (NO_x), carbon monoxide (CO) and volatile organic compounds (VOCs) react with the intense sunlight. But as soon as the sunsets, surface ozone levels decrease rapidly.

   
   
   
7. Another contributor (though less important) to the daytime ozone problems in this region occurs due to the leakage of reactive olefins such as butenes and propene which are found in liquefied petroleum gas (LPG). Most of the LPG comes from household heating and cooking, which then leaks into the atmosphere. Much of this leaked LPG can't be burned properly due to the altitude of Mexico City and so when the intense sunlight reacts with the unburned component of the LPG, ozone is produced.
   
   
   
8. You need to be aware that the worst time of the year for air pollution in Mexico City is near the end of the winter months (particularly February and March). However, air pollution concentrations are always high from 6am to 9am, which of course corresponds with peak hour traffic. High concentrations of air pollution in Mexico City also occurs on days when the there's little wind to disperse the pollutants beyond the cities boundaries.

What months have the lowest air pollution levels?

During the summer months, Mexico City has frequent heavy rain and storms which wash many of the air pollutants from the atmosphere.

In fact, air pollution in Mexico City is much lower in the mid-afternoon than any other part of the day. The main reason is due to the fact that there's enough wind in the afternoon to blow the majority of the pollutants out of the area.

However, once the sun begins to set, the wind drops and many pollutants stagnate near the emission sources since a temperature inversion forms a few hundred metres above the ground.

In order for you avoid the majority of the smog in Mexico City, it's wise to travel outside the peak hour traffic times during weekdays.

You can also expect that the air pollution in Mexico City will be quite low during public holidays. Obviously, since there's much less vehicle traffic on the roads and many industries are closed during this period, you can expect that smog levels due to vehicle and industrial fumes will be markedly lower than on a weekday when there's no public holiday.

Factors that lead to the build up of air pollution in Mexico City:

• Unusually low rainfall (build up of black carbon particulates)
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• Very little cloud cover with hot weather (leading to the formation of ground-level ozone)
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• A strong high pressure system lying over Mexico. This leads to high atmospheric stability.
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• Cold clear nights
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• Lack of wind (very common in the winter months)
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What's been done to reduce air pollution in Mexico City?

The Mexican government has worked considerably hard to reduce key air pollutants over Mexico City. Firstly they have managed to completely remove reactive olefins from LPG. Secondly, new legislation brought out in 1991, ensures that new vehicles have to be fitted with catalytic converters. Both of these strategies have made an immediate effect by improving the air quality over Mexico City. The reduction of these two air pollutants has also decreased the formation of ozone pollution. Ozone air pollution is a powerful irritant to your eyes, nose and throat. So, by reducing this pollutant, everyone will be able to breath a little easier.

However, while these air quality management programmes have contributed to significant reductions in hazardous emissions to the atmosphere, the fact remains that there's still serious air pollution issues in Mexico City that have not been addressed. For example, there appears to be no reduction in the level of black carbon being emitted to the atmosphere, which is certainly a concern.

As an interesting sideline, a number of scientific studies have shown that the local residents have lost their sense of smell due to Mexico City's reoccurring air pollution problems.

References

1. Barfoot KM, Vargas-Aburto C, MacArthur JD, Jaidar A, Garcia-Santibanez F and Fuentes-Gea V. Multi-elemental measurements of air particulate pollution at a site in Mexico City. Atmospheric Environment. 1984;18(2):467-471.
   
   
2. Blake DR and Rowland SF. Urban leakage of liquefied petroleum gas and its impact on Mexico City air quality. Science. 1995;269(5226):953-956.
   
   
3. Jiang M, Marr LC, Dunlea EJ, Herndon EJ, Jayne JT, Kolb CE, Knighton WB, Rogers TM, Zavala M, Molina LT and Molina MJ. Vehicle fleet emissions of black carbon, polycyclic aromatic hydrocarbons, and other pollutants measured by a mobile laboratory in Mexico City. Atmospheric Chemistry and Physics. 2005;5:3377-3387.
   
   
4. Marley NA, Gaffney JS, Ramos-Villegas R, Gonzalez BC. Comparison of measurements of peroxyacyl nitrates and primary carbonaceous aerosol concentrations in Mexico City determined in 1997 and 2003. Atmospheric Chemistry and Physics. 2007;7:2277-2285.
   
   
5. Mollina LT, Kolb CE, de Foy B, Lamb BK, Brune WH, Jimenez JL, Ramos-Villegas R, Sarmiento J, Paramo-Figueroa VH, Cardenas B, Gutierrez-Avedoy V and Molina MJ. Air quality in North America's most populous city - overview of the MCMA-2003 campaign. Atmospheric Chemistry and Physics. 2007;7:2447-2473.