The Urban and Industrial Environment:
The Atmosphere

01/04/2006

Format for printing

We wish to learn:

  • What environmental changes have been produced by urbanization?
  • How do cities affect local and regional weather?
  • What is the heat island effect?
  • Which are the main atmospheric pollutants?
  • How are photochemical smog and acid rain produced?
  • How does air pollution impact weather?
  • How does weather affect pollutants? 
Jump to: [Introduction] [Mexico City] [Atmospheric Effects] [Atmospheric Pollutants] [Photochemical Smog] [Impact on Weather] [Concentration of Pollutants] [Greenhouse Effect]

1. Introduction

Environmental effects due to urbanization have been observed since early in the the 18th century. Until the middle of the 20th century, urbanization levels were too low and the number of large cities was too small for there to be any significant effects other than local climatic and hydrologic impacts. 

As late as 1900, there were barely 43 cities in the world exceeding 500,000 population, of which only 16 exceeded 1,000,000. Since 1950 the number of large cities has increased very rapidly (about 400 exceed 1,000,000)1 . Metropolitan areas have grown to form even larger agglomerations and some very large urban areas with populations in the tens of millions have emerged. For example, by the year 2,000, and Calcutta, Bombay, Cairo, Jakarta and Seoul are each expected to be in the 15-20 million people range. The environment in large urban areas has deteriorated and resulted in an increased incidence of diseases and ailments. 

The effects of urban development can be defined by many factors. The most important of these factors relate to air quality, the availability of safe water supplies, and provisions for sanitation and waste management. In this chapter we will consider the effects of urbanization on the atmosphere. 
 

2.  Problems associated with the urban environment: Mexico City

Mexico City today is one of the largest cities in the world. The environmental issues present in this megapolis are a good example of the problems that plague many cities around the world. 

The population of Mexico City is over 20 million people. The population has gone from approximately 3 million people in 1950 to near 20 million in 1997, with an average growth rate close to 5%. Much of the high growth rate of the City is due to the continuous arrival of migrants from the economically depressed rural areas. Rapid urbanization has outstripped society's capacity to meet basic human needs, leaving enormous portions of the city's population with inadequate incomes, diets, housing, services and health care. 

The land area of Mexico, D.F. 1,499 km2; the area of Mexico D.F. plus suburbs, is near 3,129 km2. The growth rate of the city in spatial extent, is close to that of the population (~5%). New developments are more dense and less planned, and generally include less open space. Many developments are now built on hill slopes, generating a considerable amount of soil erosion and a significant increase in flash floods after rainstorms. Overcrowding and inadequate housing has contributed to the urban health problems like respiratory disease and tuberculosis. 
 

Waste

Approximately 4-5 m3/s of sewage water are treated and used mainly for irrigation in parks and public spaces. The remainder is eliminated from the Basin by means of the deep drainage system that was built in the early 1970s and by the old surface drainage canals. This waste is mostly used for irrigation, constituting an important source of agricultural pollution. 

The health and environmental effects of inadequate solid waste disposal are felt far beyond the city in the form of water, land and air contamination over a wide area. The city produces approximately 10,000 tons of domestic garbage per day. In contrast to developed countries, which generate litter with a low proportion of organic residues, the garbage of Mexico City is rich in vegetable and fruit waste, and consequently could be used for making compost at a relatively low cost. Until 1987 most of the residues were disposed of in open fields. Recently, a more modern system of sanitary filling into dredged pits has become operational. Extending and improving waste collection, processing and disposal services is vital to control mounting health problems and prevent further damage to the environment. 
 

Water

Mexico City is built on an underground aquifer which significantly depleted. In general, much of the water in the Basin is heavily polluted; aquifers and rivers that supply its water are overextended and polluted; cholera recently have killed many people. 

Today, most of the food and water consumed within the Basin comes from outside its boundaries. Many parts of the city suffer from water shortage and industrial use of water is very inefficient. Water recycling uses only 7% of the sewage, and nearly 20% of the water supply is lost through deficient pipe systems. Pipe breakage represents a continuous health hazard, as microorganisms from the sewage system can contaminate the fractured pipes. Gastrointestinal diseases are among the most frequent health problems in the urban area and are a primary cause of infant mortality. In general, the Mexico City water resources should be protected through the introduction of a variety of techniques, including: sanitary waste disposal facilities, urban storm water runoff and drainage programs, the promotion of recycling and reuse of wastewaters and solid wastes, and the control of industrial pollution sources. 
 

Air

City streets clogged with traffic and air pollution is perhaps the worst problem the city faces. Photochemical smog is critical during the winter months, when a temperature inversion stabilizes the atmosphere above the Basin and the air pollutants accumulate in the stationary layer of air that covers the city. Encircling mountains trap the smog generated by cars and factories. 

Most of the air pollution originates from automobile exhaust, which are responsible for ~72% of all atmospheric pollutants in Mexico City. Mean ozone levels in Mexico City are ten time the normal atmospheric concentration, and are more than twice the maximum limit allowed in the United States. In order to alleviate this problem, the vehicles of Mexico City are under a permanent vehicular restriction program called "Hoy No Circula". All vehicles according to the last digit of their license plate are distributed by colors along the week, and are not allowed to be driven once in a week. 

Pollution monitoring is carried out by the Automatic Atmospheric Monitoring Network (R.A.M.A.), which operates over the México City Metropolitan Area. R.A.M.A.'s 33 automatic atmospheric monitoring stations are equipped with air pollution analyzers and meteorological sensors, in order to analyze air quality every minute of the year. Air quality reports (see Table 1 below) are published daily through the national media. Based on this reports vehicle use and industrial activity are restricted, in order to protect Mexican people's health. 

 

Zone

Ozone

Nitrogen Dioxide
NO2

Sulfur Dioxide
SO2

Carbon Monoxide
CO

Suspended Particles
PM-10

UV
Index

Northwest

083
13:00

057
10:00

010
7:00

050
10:00

067
8:77

009
13:00

Northeast

075
16:00

029
10:00

020
20:00

045
07:00

111
07:00

009
13:00

Downtown

066
14:00

037
09:00

020
07:00

069
10:00

092
09:00

009
13:00

Southwest

104
13:00

058
10:00

006
07:00

037
14:00

057
14:00

009
13:00

Southeast

074
15:00

023
09:00

012
09:00

033
13:00

091
07:00

009
13:00 

Statistics on population, environment, economics, and other factors can be found at the following web addresses: www.ddf.gob.mx and lib-www.lanl.gov/pubs/mexico.htm


3. Atmospheric Effects

In the remaining of this lecture we will try to understand the changes that take place in the atmosphere around cities. Major cities are covered by a dome of dust, smoke, and haze which are the product of a convective circulation or air that concentrates pollutants over a city. The convective circulation is produced by a temperature difference between the city and surrounding areas. 
 

Urban Heat Island

There is a marked difference in temperature between the city core and adjacent rural areas. This temperature difference, which increases with city size, is usually referred to as the urban heat island . In the larger cities over 10,000,000, the mean annual minimum temperature can be as much as 4 degrees F higher than that of the surrounding rural periphery. This difference is much greater in summer than in winter. 

The causes for the urban heat island are twofold:

    1. Compared to rural surfaces, city surfaces absorb and store significantly more solar radiation. Specifically during the summer months, the buildings, pavement, and concrete of the city absorb and store solar radiation. The concreted city surfaces have both great thermal capacity and conductivity, so that heat is stored during the day and released at night. Because of the high water runoff in the city, less of this energy is used for evaporation. In contrast, the countryside, the presence of evaporation and transpiration results in relatively lower temperatures during the day and night.
    2. Manmade energy produced by industrial, commercial and domestic users. 

The presence of pollutants over the city enhances the heat island effect. H2O, CO2, and other pollutants enhance the surface temperature through convection and radiation processes (atmospheric greenhouse effect*). 
 

Urban Wind

Wind speed is influenced not only by horizontal pressure gradients, but also by surface roughness. The buildings in a city provide a powerful frictional force on air moving over and around them. This creates turbulence, with characteristically rapid spatial and temporal changes in velocity. 

Winds have a cooling effect on the urban heat island. For example, wind speeds of 5 m/s can eliminate a heat island in a city of 250,000, but speeds of 10 m/s are needed when the size reaches 1,000,000. The frictional force of the city, however, can reduce wind speeds and inhibit this cooling: average wind speed can be reduced as much as 30% by a big city. 

4. Atmospheric Pollutants

Many gases and aerosols that can be air pollutants are normal constituents of the atmosphere. These substances become pollutants when their concentration increase to levels that can be harmful to humans, animals or plants. Table II describes the health effects, and the natural and anthropogenic sources of the main atmospheric pollutants. 

 

Carbon Dioxide: Contributes to Greenhouse warming

  • Natural sources and sinks:
    • Cellular respiration of organisms release carbon dioxide to the atmosphere, and through photosynthesis, plants take up carbon dioxide. Other natural sources are forest and brush fires and volcanic activity.
  • Anthropogenic sources:
    • Combustion of fossil fuels (coals, oil, and natural gas) for electric power generation, transportation, and heating.

Carbon Monoxide: Asphyxiating agent that constitutes a serious health hazard

  • Natural sources and sinks:
    • It is produced by combination of oxygen with methane and other volatile organic compounds; it is removed from the atmosphere by the activity of certain soil microorganisms and by chemical reactions that convert CO to CO2. 
  • Anthropogenic sources:
    • Incomplete combustion of fossil fuels, especially by motor vehicles; burning of forests and savannas to clear land. 

Volatile Organic Compounds: Produce photochemical smog; carcinogens

  • Natural sources and sinks
    • All vegetation emits various hydrocarbons. Terpenes are reactive and responsible for the aromas of pine, eucalyptus and sandalwood trees. 
  • Anthropogenic sources:
    • Incomplete combustion of gasoline by motor vehicles (hundreds of different hydrocarbons); escape during gasoline delivery and refueling (~15% of the total release into the atmosphere); solvents used in industrial and commercial processes (painting and cleaning); chemical manufacturing and petroleum refining.

Methane: Greenhouse gas; explosive at concentrations of 5% or bigger. 

  • Sources and sinks:
    • Methane is produced when organic material decays in the absence of oxygen (anaerobic decay), for example in rice paddies and marshes; biological activity in termites and the stomachs of cattle and sheep; non-reactive at normal background levels.
  • Anthropogenic sources:
    • Landfills

Oxides of Nitrogen (NO and NO2): NO2 contributes to heart, lung, liver, and kidney damage; linked to incidence of bronchitis and pneumonia; reduces visibility; precursor of photochemical smog and acid rain.

  • Natural sources and sinks:
    • NO is produced by soil bacteria; it combines readily with O2 to form NO2. 
  • Anthropogenic sources:
    • Power plants and motor vehicles are the main source. Oxides of nitrogen form when high combustion temperatures, such as those inside an automobile engine cause nitrogen and oxygen in the air to combine; oxidation of nitrogen compounds in coal and other fuels.

Compounds of Sulfur: Acid rain

  • Natural sources and sinks:
    • Sulfur dioxide (SO2) is produced by volcanic eruptions; sulfate particles are injected into the atmosphere by sea spray; and hydrogen sulfide (H2S) is produced in anaerobic decay. These sulfur compounds are removed from the atmosphere by precipitation and transfer to the soil, vegetation and surface water. In the atmosphere sulfur dioxide is converted to sulfur trioxide (SO3) and sulfate-containing aerosols. The latter in the presence of water form droplets of sulfuric acid. Sulfuric acid dissolves in precipitation and increases its acidity. In the absence of precipitation sulfuric acid droplets convert to acidic aerosols that reduce visibility and may cause human health problems when inhaled. Eventually, acidic aerosols settle to the ground as dry deposition. 
  • Anthropogenic sources:
    • Fossil fuel (coal and oil) contain sulfur as an impurity and emit sulfur dioxide when burned. Certain industrial activities, such as paper and pulp processing emit hydrogen sulfide and other sulfur-containing gases.


5. Photochemical Smog
Photochemical smog is present in the urban environment. It forms when oxides of nitrogen (present in motor vehicle exhaust) and hydrocarbons (from various anthropogenic and biogenic sources) react in the presence of sunlight to produce a mixture of aerosols and gases. By-products of this reaction include ozone (O3), formaldehyde (CH2O), ketones, and PAN (peroxyacetyl nitrates). These substances irritate the eyes and damage the respiratory system. Anthropogenic hydrocarbons result from the evaporation of solvents and fuels, as well as incomplete combustion of fossil fuels. Natural hydrocarbons are produced by vegetation. Recent studies suggest that such natural hydrocarbons may play an important role in the formation of photochemical smog. 

The normal levels of ozone at the earth's surface average about 0.02 ppm, while in in thick photochemical smog, ozone concentration may exceed 0.5 ppm . Chronic exposure to ozone may cause lung disease. It also degrades rubber and fabrics, and damages some crops. 

 

6. Air Pollution Impact on Weather

Air pollution can affect the amount of cloudiness and the quantity and quality of precipitation, especially downwind from large urban-industrial areas. Urban-industrial areas are sources of: 

  • Condensation nuclei that enhance cloud development;
  • Water vapor that increases the relative humidity; and
  • Heat (urban heat island) that favors convection.

Jet contrails

Jet contrails are composed of ice crystals originating from water vapor and condensation nuclei produced by jet engines as by-products of combustion. 

There is evidence of a linkage between cloud cover and jet contrails. Cloud cover: 

  • Enhance local precipitation;
  • Reduce the penetration of solar radiation; and
  • Absorb terrestrial infrared radiation at night.

The above implies lower daytime temperatures and higher nighttime temperatures. This temperature trend has been observed over the United States in the last few decades. 

Acid Rain

The combination of dry deposition and wet deposition of acids and acid-forming compounds onto the surface of the earth is known as acid rain. Oxides of sulfur and nitrogen in the atmosphere interact with moisture to form droplets of sulfuric acid and nitric acid. These acids dissolve in precipitation and increase its acidity nearly 200 times, bringing the pH of rainwater from above 5 (5.6 is considered normal) to values as low as 3. 

In the absence of precipitation sulfuric acid droplets convert to acidic aerosols that reduce visibility and may cause human health problems when inhaled. Eventually, acidic aerosols settle to the ground as dry deposition. 

Among the effects of acid deposition are: 

  • It lowers the pH of lakes and streams, affecting fish reproduction, aquatic life and microorganisms;
  • It removes metals from the soil, washing them into lakes and streams where they may be harmful to fish and plants;
  • It weakens or kills trees, especially conifers at high elevation, by leaching nutrients from soil and bottom sediments; and it accelerates weathering of building materials and metal corrosion

. 

 

Figure 1: (A) If a smokestack is too low, effluent may be trapped within the wake of nearby buildings or the chimney itself. 
(B) If a smokestack is constructed to the height of a good engineering practice (2.5 times the height of the nearest obstacle), effluents clear the wake, and downwash and trapping are avoided.
 

7. Atmospheric Factors that Affect the Concentration of Pollutants

Once pollutants enter the atmosphere their concentration decreases as they mix with clean air. The rate of dilution depends on atmospheric conditions. Wind speed and atmospheric stability are important factors that determine the rate of dilution. 
 

The Urban Wind
Wind speed is important in accelerating dilution of pollutants. When it is windy, turbulence is responsible for mixing polluted air with clean air, accelerating dilution. When the wind is not present, the much slower molecular diffusion processes control the rate of dilution. In the urban environment winds are affected by the surface, which produces friction and slows the wind down. Dilution of pollutants is thus hampered in urban localities. 

Another consequence of the frictional interaction of winds with the urban surface is the formation of zones of light and irregular winds that can trap pollutants. The horizontal wind becomes turbulent to the lee of obstacles, like buildings. A distance in front of the obstacle the irregular motion forms a closed circulation, known as a wake (see Figure 1). This is why, if a smokestack is too low, effluents may be trapped within the wake of nearby buildings. In general smokestacks must have a height 2.5 times the height of the nearest obstacle, so that effluents clear the wake and trapping is avoided.

Atmospheric Stability
 
 


Figure 2

Stability affects vertical motion within the atmosphere. In the presence of stable air, convection and turbulence are inhibited, while they are enhanced if the air is unstable. Consequently, when pollutants are emitted into stable air, they are not transported upward, and remain in a stable layer of air that acts as a lid in the troposphere (see Figure 2). 

Temperature Inversions

In the troposphere, temperature decreases with altitude. During the day, the sun warms the air near the surface. This heated air expands and rises, carrying near surface pollutants higher into the troposphere. Colder, denser air from surrounding high-pressure areas then sinks into the low-pressure area created when the hot air rises. This process produces a continuing mixing of warm and cold air which help dilute pollutants. Sometimes the reverse occurs, i.e., warmer, lighter air overlies cooler, denser air. This is called a temperature inversion. Temperature inversions produce stable atmospheric layers which inhibit mixing and dispersion of pollutants. In effect, a lid of warm air covers the region and prevents upward-flowing air currents from developing. In this case it is possible for air pollutants at ground level to build up to harmful levels. 

Next we describe three ways in which a temperature inversion can happen: 

Subsidence temperature inversion: A temperature inversion that develops aloft as a result of air gradually sinking over a broad area and warmed by adiabatic compression2.

Radiational temperature inversion: Cooling of a surface air layer by emission of infrared radiation so that the coldest air is adjacent to the Earth's surface and the air temperature increases with altitude. Radiational temperature inversions can be strengthen when a city is in a valley between mountains. An example of this situation is provided by Mexico City (see Figure 3) , where under the influence of gravity, cold dense air moves downslope the mountains thus strengthening the radiational temperature inversion in the basin. 

Advection of air : This sometimes happens in the Rocky Mountains, where air descending the leeward (located on the side toward the wind is blowing) slopes is warmed by compression and overlies cold air advected on northerly surface winds. An example of this situation occurs in the city of Denver.( See Figure 4)

Figure 3: Under the influence of gravity, cold, dense air drains downslope and strengthens the temperature inversion in the valleys. 

 

Figure 4: A temperature inversion develops aloft on the leeward slopes of the Rocky Mountains. Air descending the leeward slopes is warmed by compression and overlies colder air advected on northerly surface winds. This situation creates a high air pollution potential for Denver, which would lie in the valley to the right of the mountain.



8. The Atmospheric Greenhouse Effect*

The presence of the earth's atmosphere causes the earth's average surface temperature (near 290 K) to be higher than the corresponding value (256 K) without an atmosphere. In order to understand how this happens it is necessary to invoke two physical processes that take place in the surface-atmospheric system: radiation and convection*. It is the interplay of these two processes in the presence of the earth's atmosphere that causes the surface temperature to be higher, and that determines the temperature profile of the troposphere. This effect is referred to as the Atmospheric Greenhouse Effect. 

About half of the solar radiation incident on the top of the atmosphere reaches the surface of the earth . This energy is absorbed by the surface, making it hotter than the surrounding atmosphere. The surface cools by convection and by long-wave radiation. Atmospheric gases, on the other hand, are also heated by long-wave radiation, but mostly by heat transfer from the surface (convection); they cool by emitting long-wave radiation, which in turn is absorbed by the surface, thus increasing on the average the amount of radiant energy reaching the surface. It needs to be emphasized that the amount of radiation emitted downward from the sky depends on the temperature of the atmosphere, which is determined by convective processes, rather than directly by radiative processes. The two most important atmospheric gases which contribute to the atmospheric greenhouse effect are water vapor and carbon dioxide. 

It should be noted that the atmospheric greenhouse effect is different from the warming observed at greenhouses. The main difference is that In the greenhouse effect convection does not play a role, and energy is trapped within the greenhouse walls by the non-absorbing properties of the walls in the long wavelength range. 

* Convection - Vertical air circulation in which cool air sinks and forces warm air to raise
 
 

Suggested Readings

  • Turner II, B. L., W. C. Clark, R. W. Kates, J. F. Richards, J. T. Mathews, and W. B. Meyer, "The Earth as Transformed by Human Action", Cambridge University Press, 1990; Ch. 7 
  • Ahrens, Donald C., "Essentials of Meteorology" West Publishing Co., 1994; Ch: 12 


Goudie, Andrew, "The Human Impact on the Natural Environment", The MIT Press, 1994. 

 


All materials © 2001 by the University of Michigan.