Smog

Claude Monets The Houses of Parliament, painted in 1904

depicts the filtering of sunlight, intercepted by the thick smog
of Edwardian London

London shrouded in smog, April 2011.

Smog is a type of ground-level ozone that is produced by complex photochemical reactions between organic compounds, nitrogen oxides and sunlight. These particles of pollutants become suspended in the atmosphere appear as a brown haze above a city. Certain weather conditions and geography affect the location and severity of smog.

Winter smog can form under an anticyclone, when a temperature inversion is coupled with high concentrations of sulphur dioxide and other gasses produced from the combustion of fossil fuels emitted by vehicles and industry. Calm conditions exacerbate the conditions; pollutants remain in situe as they cannot be flushed away by air currents. Exposure to air pollution is linked to the contraction of respiratory diseases and aggravates asthma.

Conditions for smog development:

Temperatures and sunlight determine the length of time it takes for smog to form – high temperatures and high sunlight intensity increase the rate of smog development when upper air is warm enough to inhibit vertical circulation. A temperature inversion can induce smog formation, where normal temperature decrease with height (lapse rate) switches to temperature increase with height. The inversion acts like a lid, preventing convective overturning air currents from breaking through the layer of warm air.

The topography of a city can determine the severity; cities that are most prone to smog lie in geological basins, enclosed by hills/mountains that trap the pollutants. Mexico City and Los Angeles are prime examples of smog exacerbated by the local topography.  

The population density of a city affects pollution levels – densely populated mega-cities like London, New York in the USA, Cairo in Egypt, Seoul in South Korea, New Deli in India, to name but a few.

Case Study – Mexico City

 

Situated in a basin surrounded by two major mountain chains, Mexico Cities geography contributes to its susceptibility to smog. Cold air sinks down the mountains and over the urban zone, trapping the heavy volumes of industrial and vehicular pollution underneath a layer of warm air - this is known as a temperature inversion. Due to the topography of the land, winds are unable to push the smog over the surrounding mountains. The highest concentrations of carbon monoxide in the air are usually during the weekday morning commuting period, from 7 – 9 am, when low temperatures, low atmospheric stability and high emissions occur simultaneously. Although winds normally circulate over the city by the evening, particles are not dispersed and transported away from the city completely and are easily blown back down the mountains the following day.



Smog in Mexico City - a temperature inversion caused by the natural topography.  



Strategies for improvement:

1. Imposing driving restrictions by banning cars from the road one day a week
2. Upgrading the bus network to encourage commuters to switch from private to public transport
3. The government has cooperated with industry to set attainable goals in reducing emissions.

Recommended Resource:

 

Out Reach World provides informative worksheets including an excellent one about pollution in Mexico City and other Mega Cities. A series of exercises with prepared questions or tasks such as ‘propose a pollution prevention plan’ could be used in the classroom or set as homework.

http://www.outreachworld.org/Files/u_texas/mexico_city_pollution.pdf