AIR POLLUTION FUNDAMENTALS

History

Many of the worst air pollution episodes occurred in the last two centuries in London

key ingredients–calm winds –fog –smoke from coal burning

in 1873 - 700 deaths in 1911 - 1150 deaths in 1952 - over 4000 deaths;

the last event prompted the parliament to pass a Clean Air Act in 1956

Other Events

in 1930 - Meuse Valley; pollution became trapped in a narrow valley - 600 people became ill, 63 were killed

in the US, air quality degraded quickly shortly after the industrial revolution and again, the problem was coal burning in the central and mid western U.S.

in 1948 - Donora, PA in the Monongahela River Valley with a five-day episode - 1000's became ill, 20 were killed

in 1960s - NYC experienced several dangerous episodes

in 1960s and 70s - Los Angeles - increase in industry and automobile usage led to many pollution episodes

Grand Canyon

The above events led to passing the Clean Air Act of 1970 (updated in 1977 and again in 1990) empowered the federal government to set emission standards that each state would

have to meet Even the most remote areas are affected by air pollution like

the Grand Canyon

Grand Canyon on a good day

The Grand Canyon on a bad day

Pollutant SourcesAir pollutants – airborne substances that occur in concentrations high enough to threaten the health of people and animals, to harm vegetation and structures, or to toxify a given environment (from Ahrens).

Natural Sources:

wind picking up dust

suspended particles

volcanic eruptions

dust, ash, gases like SO2, CO2

forest firessmoke, ash, unburned

hydrocarbons, CO2

Vegetation VOCs, pollen

Ocean Waves salt particles

Anthroprogenic Sources

industry paper mills

power plants refineries

manufacturing

particulate matter SOx NOx Ash

personal: transportation

home furnaces fireplaces

open burning of refuse

CONOx

VOCs particulate matter

forest firessmoke, ash, unburned

hydrocarbons, CO2

Primary Pollutants

injected into the atmosphere directly

carbon monoxide (CO) – odorless, colorless, poisonous gas – created by incomplete combustion (especially bad with

older cars) – generates headaches, drowsiness, fatigue, can result in death

oxides of nitrogen (NOx, NO) – NO - nitric oxide – emitted directly by autos, industry

Primary Pollutants - 2

sulfur oxides (SOx) – SO2 - sulfur dioxide – produced largely through coal burning – responsible for acid rain problem

volatile organic compounds (VOCs) – highly reactive organic compounds – release through incomplete combustion and industrial

sources

particulate matter (dust, ash, salt particles) – bad for lungs

Secondary Pollutants

form in the atmosphere through chemical and photochemical reactions from the primary pollutants

sulfuric acid H2SO4 can cause respiratory problems

nitrogen dioxide NO2

gives air a brownish coloration

ozone O3 colorless gas has a sweet smell is an oxidizing agent - lung tissue to rubber products irritates the eyes

Pollutants

The primary and secondary pollutants are found in either of the following two types of smog:

• London-type smog • Photochemical smog (produced in L.A. and many other urban areas around the world)

How are the London and L.A.-type smogs produced ??????

Smog - smoke + fog

• London Smog - This type of smog comes from coal smoke combining with the water vapor and liquid water in cool, humid or foggy air.

• L.A. smog has been identified as coming from auto exhaust, primarily (there is a significant "stationary" source)

London vs. L.A. SmogLondon Smog: requires humid/foggy, stagnant air have lots of coal burning

SO2 + H20 -> H2SO4

L.A. Smog:requires clear, sunny skies (since L.A. photochemical smog requires sunlight for at one of the key chemical reactions)

NOx + ROG + sunlight --> O3 + NO2 ROG are reactive organic gases from unburned gasoline NOx are oxides of nitrogen

London vs. L.A. Smog

London smog: • temperature inversion – the lower the better • humid foggy, stagnant air • air will look sooty, dirty, foggy

LA smog:• temperature inversion - the lower the better • hot sunny, stagnant weather • air looks hazy, brownish in color

Primary pollutants in LA smog

CO - carbon monoxide NO - nitric oxide ROG - reactive organic gases (unburned gasoline)

These are mainly direct combustion products from gasoline- or diesel-burning internal combustion engines.

There is a significant source of ROGs from stationary industries and small businesses

Secondary pollutants in LA smog

O3 - ozone

NO2 - nitrogen dioxide

Particles

PAN - peroxyacetyl nitrate

These are products of reactions in the atmosphere, NOT directly emitted

The main secondary pollutant is ozone, near the surface, up in the stratosphere, it's a good thing....

South Coast AQMD web site: http://www.aqmd.gov/smog/index.html

Map of L.A. Basin area and air quality data: http://www.aqmd.gov/telemweb/areamap.aspx

Real time air quality reading in the LA Basin area: http://www.aqmd.gov/telemweb/Reading.html

Map archive of U.S. air quality: http://www.epa.gov/airnow/mapselect.html

Another map showing NO2 and O3 concentrations with

meteorological data as well can be found at: http://www.atmos.ucla.edu/aqmd/currntobs.html

Source of the Primary Pollutants - Photochemical Smog

• Mobile sources (such as automobiles) are the largest sources of CO to the atmosphere.

• Stationary industrial sources are the largest source of particulate matter (PM).

• ROGs seem to be shared between the stationary and

mobile sources.

LA smog photochemistry – the null cycle

A null cycle neither produces nor destroys

anything overall

so, how do ozone and nitrogen dioxide

concentrations build up during the

day ?????????

something else is

missing........

Ozone Production - Null Cycle

Observations of O3 concentration vs. time show that there is a

significant increase in O3 during the afternoon.

If the null cycle were the only process occurring in L.A. smog, then this observation would be anomalous.

There must be something else. . .

Reactive Organic Gasses

Reactive organic gases (ROG) undergo a series of reactions to form radicals

The alkylperoxy radical (RO2*) reacts with and oxidizes NO to

form NO2 faster than NO reacts with O3 to produce the same

result

Thus, when ROGs are present, it is likely that O3 will not be

destroyed to produce NO2, and the null cycle is broken

Note that each time an NO2 molecule is formed (by whatever

method), it very quickly results in the production of O3 (via

photo dissociation and a recombination)

NEW CYCLE WITH REACTIVE

HYDROCARBONS

THE NULL CYCLE WITH NO HYDROCARBONS

Factors affecting smog concentration

Smog concentrations vary over time and space according to environmental conditions and sunlight (the source of energy for photochemical reactions)

The higher the sunlight intensity, the greater production rate

of O3

The greater the wind speeds and mixing heights the lower the smog concentration

In addition, the direction of the wind will control the areas where smog is transported.

Geographical Factors

Mountains stop the horizontal transport of smog, or divert it in another direction,

unless the wind is strong enough to blow over the mountain (not likely to happen in L.A., due to the inversion that prevents

vertical mixing) Since secondary pollution forms after the emission of primary pollution, we are likely to find the higher secondary pollution

concentrations downwind of the source regions.

Diurnal Smog Variation

Due to the time it takes for ozone to build up in the afternoon, the highest peaks of ozone occur inland, because the sea breeze

transports pollutants inland during the afternoon

Without ROGs, the ozone concentration would remain low most of the day

At night, there is no sunlight to photo dissociate NO2, so O3 is not produced. The O3 from the daytime photochemistry dissipates

overnight

Time series of ozone can be found at: http://www.atmos.ucla.edu/aqmd/currntobs.html

for the LA Basin

Seasonal Variations

Variations in sunlight intensity cause variations in O3 production rate

When the sun is most intense (i.e., Summer), O3 should reach

highest levels, and primary pollutants should be at low concentrations

In the winter, when the sun is weak, there will be reduced

production of O3

Primary pollutants, such as CO, reach seasonal maxima during the winter, but more from the lower mixing heights during this

season than from the reduced sunlight intensity

 

The effect of low mixing heights would be to reduce the dispersion volume in which pollutants can mix, which

increases the concentration if the source rate is the same

Health Effects

Air Pollution Dispersion

Air pollution dispersion is often studied with simple models called box models.  How is the box defined for the Los Angles

area???? The ventilation factor gives us a way of relating the pollution

concentration to the parameters that control dispersion of the pollution in the local environment.

Basically, increasing either the mixing height or the wind speed increases the effective volume in which pollutants are

allowed to mix. The larger the volume, the lower the pollution concentration

Chimney Plume Dispersion

In the stable atmosphere case (producing a fanning plume), there is horizontal dispersion at a right angle to the wind

due to turbulence and diffusion.

In the vertical, dispersion is suppressed by the stability of the atmosphere, so pollution does not spread toward the

ground.

This results in very low pollution concentrations at the ground.

In unstable air, the plume will whip up and down as the atmosphere mixes around (whenever an air parcel goes up, there must be air going down someplace else to maintain

continuity, and the plume follows these air currents).

This gives the plume the appearance that it is looping around

An inversion aloft will trap pollutants underneath it, since the stable inversion prevents vertical dispersion.

Pollution released underneath the inversion layer will fumigate the mixed layer.

Note that if the smokestack was high enough to release the pollution within the inversion layer, the plume would fan

because the plume occurs within stable air

………. Questions ???