The URBAN AND INDUSTRIAL ENVIRONMENT:

The Atmosphere

• 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 weather affect pollutants?

Absorption and Scattering of solar radiation

• Solar radiation is attenuated as it passes through the earth’s atmosphere. The atmosphere contains particles and gases, which affect the incoming radiation through the mechanisms of scattering and absorption. 

• Scattering causes changes in direction and intensity of radiation.

• Absorption causes molecules in the atmosphere to absorb energy at various wavelengths.  The net result of absorption is that energy is attenuated or lost.  Ozone, carbon dioxide, and water vapor are the three main atmospheric constituents that absorb radiation. 

Earth Radiation Budget

 

Earth’s surface

5. Clouds scatter and absorb light – cool the atmosphere during day and keep surface warm at night.

2. Ground radiates in the infrared, cooling the surface 

4.Particles scatter light – net cooling effect; absorb light- heating

3. Molecules absorb infrared light – warms atmosphere by trapping radiation 

1. Light that reaches the ground (mostly visible) is absorbed and reflected; absorbed light increases ground temperature 

Convection Mixes the Atmosphere and Helps Dilute

Pollutants

3. Water vapor in air condenses and forms clouds.

3. Water vapor in air condenses and forms clouds.

1.Solar radiation is reflected and absorbed. The absorbed radiation heats the surface.

2. The air near the surface absorbs solar radiation and radiation emitted by the surface. Warm air from near the surface moves upward and is replaced by cold air from above (convection).

hot air

cold air

"Urban Heat Islands."

What causes this to happen?

• There are fewer trees, shrubs, and other plants to shade buildings, intercept solar radiation, and cool the air by "evapotranspiration."

• Buildings and pavement made of dark materials absorb the sun's rays instead of reflecting them away, causing the temperature of the surfaces and the air around them to rise.

• Surface roughness serves to reduce the wind speed and reduce ventilation.

Evapotranspiration

• Evapotranspiration occurs when plants secrete or "transpire" water through pores in their leaves. 

• The water absorbs energy as it evaporates, cooling the air in the process.  

• A single mature, properly watered tree with a crown of 30 feet can "evapotranspire" up to 40 gallons of water in a day, which is like removing all the heat produced in four hours by a small electric space heater.

Heat Islands and Energy Use

• Higher temperatures in urban heat islands bring with them increased energy use, mostly due to a greater demand for air conditioning. As power plants burn more fossil fuels, they increase both the pollution level and energy costs.

Urban heat islands and smog

• Urban heat islands are not only uncomfortably hot, they are also smoggier.

• Smog is created by photochemical reactions of pollutants in the air. These reactions are more likely to occur and intensify at higher temperatures.

• In Los Angeles, for example, for every degree Fahrenheit the temperature rises above 70°F, the incidence of smog increases by 3%.

Cool Pavements Lower Temperatures

Dark materials absorb more heat from the sun. Black surfaces in the sun can become up to 70°F (40°C) hotter than the most reflective white surfaces. If those dark surfaces are roofs, some of the heat collected by the roof is transferred inside.

PRE-INDUSTRIAL TROPOSPHERE

The chemical composition of the troposphere in pre-industrial times was the result of sunlight acting on natural chemicals emitted by the biosphere, from volcanoes, and biomass burning (forest fires).

     biogenic hydrocarbons,      minor releases of nitrogen oxides from biogenic

emissions and lighting      sulfur compounds and particulates from

volcanoes and forest fires.  The most important biogenic hydrocarbon was

methane, but isoprene and other biogenic hydrocarbons released by trees were also important.

Atmospheric Pollution

• 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.

• Which are the main pollutants?

• What are their natural sources or sinks?

• What are the anthropogenic sources?

Atmospheric Aerosols

  Atmospheric aerosols are particles suspended in air, with very short lifetimes compared to greenhouse gases.

Their diameters range from a few nanometers to ten micrometers.

They are generated in two ways:

• by direct emission to the atmosphere, for example from automobile exhaust and sea-spray (primary aerosols),

• by gas-to-particle conversion of chemical species in the atmosphere (secondary aerosols).

Aerosol Sources

• Natural and Anthropogenic Sources:– Sulfates from volcanic eruptions, – Black carbon (soot) and sulfates from

combustion– Mineral dust

Aerosol Sources

• The aerosol loading in the atmosphere has increased significantly with human activity.

• Mineral dust is a major contributor to aerosol loading and optical thickness, especially in subtropical and tropical regions.

• Deserts, dry lake beds, semi-arid desert fringes, and drier regions where vegetation has been reduced or soil surfaces have been disturbed by human activities, are the main sources of dust in the lower atmosphere. It has been estimated that up to 50% of the current atmospheric dust load originates from disturbed soil surfaces

Dust Devil

Aerosols are of interest because:

• Aerosols are the primary cause of visibility degradation in polluted areas.

• Aerosols can either absorb or scatter light. In this way they directly influence the Earth’s radiation balance and contribute to climate change.

Aerosols are of interest because:

Aerosols are a major component of urban smog and several recent epidemiological studies have shown that aerosols in urban areas have a significant negative impact on human health.

Aerosols control the formation of clouds. When the relative humidity exceeds 100% they are able to take up water, and grow to become droplets. The result is formation of clouds or fog. Aerosols with this ability are called Cloud Condensation Nuclei (CCN).

Carbon Dioxide: Contributes to Greehouse 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. 

Mercury rain

•As coal burns, it releases traces of mercury that come out ofsmokestacks.•Much of the mercury stays airborne for up to two years and spreads around the globe. •But some is emitted as a water soluble compound formed when mercury reacts with chlorine, an element often found in coal from eastern states. •Precipitation quickly washes this form of mercury into lakes, rivers and oceans, where microorganisms take it up and convert, it into toxic methylmercury. •The mercury passes up the food chain into fish and eventually into people.

Mercury rain

• High levels can cause learning problems or retardation in children and neurological damage in developing fetuses.

• One recent study found fetus-harming levels of mercury in about 8% of U.S. women of childbearing age.

Methane: Greenhouse gas; explosive

at concentrations of 5% or bigger. 

Natural 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

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.

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 and London smog

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. 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.

Photochemical Smog

• Photochemical smog forms when oxides of nitrogen 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 (ozone (O3), formaldehyde (CH2O), ketones, and PAN (peroxyacetyl nitrates).

• Average ozone level at the earth’s surface: 0.02 ppm

• Ozone concentration may exceed 0.5 ppm in thick photochemical smog.

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

pH of familiar itemspH Item

7.8 to 8.4 Sea water 6.3 to 6.6 Cow’s milk 5.6 to 6.0 Potatoes 5.2 to 5.4 Cabbage 4.0 to 5.0 Beer 3.2 to 4.0 Cherries 2.8 to 3.8 Wine 2.4 to 3.4 Vinegar 3.0 to 3.3 Grapefruit 1.8 to 2.0 Limes

1. to 3. Human gastric juices 2. to 4. Soft drinks

ACID RAIN

Precipitation pH Comments pH 5.6 Virtually no impact

from pollution 5.0 pH 5.6 Within the range of

natural variation; little influence prom pollution

pH < 5.0 Reasonable to assume that pollution has lowered the pH.

Effects of Acid Rain

• Lowers the pH of lakes and streams, affecting the reproduction cycle of fish.

• Leach metals from the soil, washing them into lakes and streams where they may harm fish and aquatic plants.

• Responsible for the decline and dieback of coniferous forests.

• Accelerated weathering of building materials and metal corrosion.

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

• 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. 

The frictional interaction of winds with the urban surface forms zones of light and irregular winds that can trap pollutants.

(A) If a smokestack is too low, effluents 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.

Convection Mixes the Atmosphere and Helps Dilute

Pollutants

3. Water vapor in air condenses and forms clouds.

3. Water vapor in air condenses and forms clouds.

1.Solar radiation is reflected and absorbed. The absorbed radiation heats the surface.

2. The air near the surface absorbs solar radiation and radiation emitted by the surface. Warm air from near the surface moves upward and is replaced by cold air from above (convection).

hot air

cold air

Atmospheric Stability

temperature

heig

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Atmospheric TemperatureTemperature of air parcel

Unstable SituationMixing Occurs

Atmospheric Stability

temperature

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Atmospheric Temperature:Temperature inversion

Temperature of air parcel

Very stable situationNo mixing occurs

Atmospheric Stability

• 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.

Temperature Inversions

Temperature inversions

Stable Air

Traps pollutants

Radiation Inversion

 • At night the surface cools by emission of

infrared radiation, so that the coldest air is adjacent to the Earth’s surface and the air temperature increases with altitude.

 • In still air, this inversion generally

persists until the surface is warmed again the next morning by absorption of sunlight. What happens if it is foggy?

URBAN SMOG

London Smog Los Angeles Smog Characteristics (Sulfurous) (Photochemical)

First recognized Centuries ago Mid- 1940’s Source Burning high-sulfur coal Burning oil in power plants and gasoline in

automobiles Primary pollutants SO2 (sulfur dioxide),

sooty particles Organic Molecules, NOx

Secondary pollutants H2SO4, sulfates, aerosols, …

O3, PAN (peroxyacetyl nitrate), aldehydes, nitrates, sulfates, particulates,…

Temperature Cool (2o C [35o F]) Warm (24oC[75oF]) Relative humidity High; usually foggy Low; usually hot and dry Light levels Dark, foggy Bright sunlight Time of peak pollution Early morning Afternoon Type of temperature inversion

Radiation Subsidence

Subsidence (“Overhead”) Inversions

     Happens in places where there are

mountains.      These inversions often come from the

air being transported over the mountains and sinking into the valley.

   As the air sinks, the atmosphere below is compressed by the overlying sinking air mass and it is warmed, producing a temperature inversion.

Subsidence Inversion

Important Similarities between London and Los Angeles Types of

Smog

• They are caused by combustion products

• The pollutants are concentrated near the ground due to temperature inversions 

• The primary and secondary pollutants are harmful to human, animals and plants.