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 ???