Environmental Transport and Fate
Chapter 8
–
Ground-level ozone
Smog over Los Angeles
Benoit Cushman-Roisin
Thayer School of Engineering
Dartmouth College
Layer of haze lingering over Los Angeles
(http://www.politeo.net/smog/index.html)
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Another photograph
(http://en.wikipedia.org/wiki/Los_Angeles,_
_California)
Los Angeles viewed from the hills of Griffith Park
A layer of brown haze lingers over the city.
It used to be really, really bad…
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The situation – The problem
Note: “Smog” = Smoke + Fog
Two components:
1) meteorological trapping of the air mass (the kettle),
2) Heavy dose of emissions by transportation and industry (the brew).
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The outcome:
Southern California is not the only region in the United States in non-attainment of the
National Ambient Air Quality Standards (NAAQS), but it appears to be the worst.
Basic ground-level ozone chemistry
Ground-level ozone proceeds from a photochemical set of reactions, that is,
chemical reactions that depend on sunlight.
Preliminary reactions:
N2 + O2 → 2 NO
2 NO + O2 → 2 NO2
← during engine combustion
← in the air shortly afterwards
Then in the atmosphere:
NO2 + sunlight → NO + O
O + O2 → O3
O3 + NO → NO2 + O2
and cycle repeats
← excitation by sunlight causing dissociation
← ozone formation
← relaxation
Nomenclature:
NO = nitric oxide
NO2 = nitrogen dioxide
O3 = ozone
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Actually, it is quite a bit more complicated…
The loose and highly reactive oxygen also breaks up water molecules (if there is some humidity)
O + H2O → 2 OH●
The hydroxyl radical OH● is extremely reactive and causes a chain of fast reactions,
in which the excitation is being bounced from molecule to molecule:
radical–CH3 + OH● → radical–CH2● + H2O
radical–CH2● + O2 → radical–CH2O2●
radical–CH2O2● + NO → radical–CH2O● + NO2
radical–CH2O● + O2 → HO2● + radical–CHO
HO2● + NO → OH● + NO2
(NO loss, NO2 creation)
(again loss of NO and creation of NO2)
The hydroxyl radical OH● is regenerated at the end of this chain of reactions, ready for another
round as long as there are hydrocarbons with which it can react.
The return of NO to NO2 (twice per cycle) means that less NO is available
to react with ozone in the relaxation reaction
NO + O3 → NO2 + O2
then letting ozone accumulate in the air.
Some additional reactions…
Atmospheric chemists are now
juggling with a set of 300-some
chemical reactions!
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Amounts of chemicals at any given time depend on sunlight and people’s commuting schedule.
High ozone levels pose significant health risks to the elderly and young children,
but even healthy adults may be affected, especially those with asthma.
Proof that smog depends on sunlight
(http://www.ucar.edu/news//updates/airquality.shtml)
Sunny Mexico City under a cover of smog
(http://en.wikipedia.org/wiki/Smog)
Beijing air on a day after rain (left) and a sunny but smoggy day (right).
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The peak ozone level during a smog episode depends on the
maximum temperature during the day.
The formation of smog is highly
sensitive to temperature: the higher the
temperature, the greater the formation
of ozone and, hence, the intensity of
smog.
In Los Angeles at temperatures below
70°F, the concentration of smog
(measured as ozone) is below the
national standard. At temperatures of
about 95°F all days are smoggy.
Cooling the city by about 5°F
5 F would
have a dramatic impact on smog
concentration.
This permits a smog forecast:
Weather forecast → Temperature forecast → Ozone peak forecast
National Ozone Standards
1-hr average 120 ppb
(standard attained when the expected number of days per calendar year with maximum hourly average
concentration above 120 ppb is no more than 1)
8-hr average
75 ppb
(3-year average of the fourth-highest daily maximum 8-hour average ozone concentration measured at each
monitor within an area over each yyear not to exceed 75 ppb))
(http://www.epa.g
gov/airtrends/ozone.html)
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What has been done over time?
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How has southern California
done in more recent times?
(Source: http://www.epa.gov/region09/annualreport/03/pdf/fullreport.pdf)
… and this despite growing road traffic !
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Reminder: 8-hr average standard is 75 ppb = 0.075 ppm.
(http://www.epa.gov/region9/air/trends/o3.html)
To see ozone concentrations over the United States
States, go to
http://airnow.gov/index.cfm?action=airnow.main
Some trends can be seen at
http://www.epa.gov/airtrends/index.html
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