AOSC 200

LESSON 25

Air Pollution

William Shakespeare 1564-1616, from his play ‘Hamlet’

Air Pollution

Air Pollution

History of Air Pollution• Air pollution is not a new problem• In England, wood for burning became scarce,

and the populace resorted to burning coal which had a high sulfur content. The by-products were soot (carbon particles) and sulfur dioxide.

• John Evelyn in 1661 wrote about the notorious London pea-soup fog. These occur in the fall when the Thames is warm but the ground is cold. The natural fog this produces is enhanced by the extra soot particles, and the sulfur dioxide reacts in the water droplets to produce sulfuric acid.

London Killer Smog

SMOG

• Word coined by Dr. Harold Des Veaux, a London physician in 1903.

• SMOKE + FOG = SMOG

• He meant London smog – sulfurous fumes from coal burning + large water droplets formed around smoke particles (soot)

• 1952 – Killer smog – 4000 deaths. Another episode in 1956 led to 1000 deaths.

• Similar events have also occurred in the US.

• Large industrial cities such as St.Louis and Pittsburg also suffered from ‘London’ smog, as the use of coal increased.

PHOTOCHEMICAL SMOG

• In 1940 vegetable crop damage began to be seen in the Los Angeles basin. Pine trees began to lose their needles.

• Haagen-Smit and colleagues at the University of California, Riverside studied this effect using smog chambers - large plastic tents into which pollutants could be injected and their reactions investigated.

• They showed that the effect was due to ozone in the atmosphere.

• The ozone was produced by a series of reactions involving the oxides of nitrogen and organic compounds (e.g. gasoline), both of which are emitted by automobiles.

• It is this form of smog that gives the pollution seen in the Baltimore/Washington corridor.

Sources and Types of Air Pollutants

• can be grouped into two categories: primary and secondary.

• Primary pollutants are emitted directly from identifiable sources. They pollute the air immediately upon being emitted.

• Secondary pollutants are produced in the atmosphere when certain chemical reactions take place among primary pollutants.

• Sources. Two types of sources are identified fixed sources and mobile sources.

N2

O2

Ar

O3

Inert gasesCO2

H2

←SO2, NO2,CFC’s, etc

PM

COCH4

N2O

Composition of the Earth’s Troposphere

Denver, Colorado on a clear day

Denver, Colorado when particulate matter reduced visibility

Fine Particles or Particulate Matter (PM)

• PM is made up of suspended particles of either solid or liquid pollutants.

• PM is grouped by size: under 10 microns is called PM10, under 2.5 microns is called PM2.5.

• PM causes increased mortality and morbidity.

• Examples of PM include diesel soot, acids, dust, sulfates, nitrates, and organics.

Photochemical SMOG

Radical Hydroxyl theasknown is

2

:vapor

er with watreacts atomoxygen excited Then the

:oxygen of atom

excitedan give osunlight t with reacts Ozone

2*

2*

3

OH

OHOHO

OOhO

SMOG Chemistry

MOMOO

ONOhNO

NOOHCNOOHC

OHCOHC

HC

OHHCOHHC

32

2

252252

252252

52

252 62

:by followed

sunlightby apart broken then is dioxidenitrogen The

DIOXIDE; NITROGEN

form toOXIDE NITRIC with reactsen oxidant th This

:OXIDANTan produce

toOXYGEN MOLECULAR with combines then

(Ethane)

nshydrocarbo from ATOM HYDROGEN a stripcan OH

Schematic of ozone production from a Volatile Organic Compound (VOC)

SMOG

• NEEDS

• Hydrocarbons and nitrogen oxides

• Strong sunlight to start reactions

• Warm temperatures to maintain reactions – the higher the temperature the faster the rate.

• Peak ozone will be when temperature is highest – in the afternoon.

Daily Ozone Cycle

Sunrise Sunset

Time of day

Ozone Concentration

Ozone productionfollows a dailycycle with maximumconcentrations typically observedin the late afternoon.

This cycle is a signature of the dynamic processes of atmospheric air pollution

Highly Polluted Areas

In highly polluted areas the concentration of nitric

oxide can be large. In this case nitric oxide can react

with ozone :

NOO3 NO2 O2

This reaction removes ozone and short circuits the ozone

formation chain of reactions.

In highly polluted areas the ozone concentration

falls off as the level of nitric oxide increases.

Ozone vs NOx for Non Methane HydroCarbon level of =0.6 ppmc

High Pollution days• The figure illustrates one of the problems in the

abatement of pollution. The ozone concentration is used as the standard, and yet one can reduce the nitrogen oxides by a significant fraction and see no change, or even an increase in the ozone level.

• Most of the pollution is emitted in the cities, which typically puts the atmosphere at the right of the figure. As the pollutants move away from the city center their concentration gets smaller, and the atmosphere is moved toward the left, and the ozone increases.

• Hence the suburbs can see more ozone than the cities.

Overall scheme for air pollution

Haze

Haze is a subset of PM and is primarily composed of sulfur and nitrogen compounds.

Particles of a certain size can reflect or refract light, causing a reduction in visibility. This reduction in visibility is known as haze.

Hazy conditions occur frequently in conjunction with

severe O3 events.

An Example of the Effects of Hazein the Mid-Atlantic

The Great Smoky Mountains National Park

A Clear Day A Hazy Day

Photos from www.epa.gov

VOC Emissions

33%

7%

29%

13%

18% SolventUtilization

Storage &Transport

On-RoadVehicles

Non-RoadVehicles

All Other

NOx emission inventory

28%

13%

5%30%

19%

5%Fuel Combustion- Electric Utility

Fuel Combustion- Industrial

Fuel Combustion- Other

On-RoadVehicles

Non-RoadVehicles

All Other

Sulfur Dioxide emissions

67%

17%

4%3%

2%7%

Fuel Combustion- Electric Utility

Fuel Combustion- Industrial

Fuel Combustion- Other

MetalsProcessing

Non-RoadVehicles

All Other

Robert Angus Smith (1817-1884) was a 19th-century Scottish

chemist who investigated numerous environmental issues.

Smith did innovative studies of air and water pollution and

was one of the few at the time to realize the importance of finding

solutions to the environmental problems caused by urban growth.

He is most famous for his 1852 research on air pollution, in the

course of which he discovered acid rain.

Acid Precipitation

• Rain is naturally weakly acidic because CO2 from the atmosphere dissolves in water.

• Unperturbed rainwater has a pH of 5.6

• Precipitation near urban areas has a much lower pH. This rain or snow is called acid precipitation.

Acid Precipitation