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Chapter 8

Environmental Transport and Fate

Chapter 8

–

Ground-level ozone

Smog over Los AngelesSmog over Los Angeles

Benoit Cushman-RoisinThayer 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/w

iki/Los_Angeles,__C

alifornia)

Los Angeles viewed from the hills of Griffith ParkA 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 ← during engine combustion2 NO + O2 → 2 NO2 ← in the air shortly afterwards

Then in the atmosphere:

NO2 + sunlight → NO + O ← excitation by sunlight causing dissociationO + O2 → O3 ← ozone formationO3 + NO → NO2 + O2 ← relaxationand cycle repeatsand cycle repeats

Nomenclature:NO = nitric oxideNO2 = nitrogen dioxideO3 = 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:in which the excitation is being bounced from molecule to molecule:

radical–CH3 + OH● → radical–CH2● + H2Oradical–CH2● + O2 → radical–CH2O2●

radical–CH2O2● + NO → radical–CH2O● + NO2 (NO loss, NO2 creation)radical–CH2O● + O2 → HO2● + radical–CHOHO2● + NO → OH● + NO2 (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://en.wikipedia.org/w

Beijing air on a day after rain (left) and a sunny but smoggy day (right).

iki/Sm

og)

(http://ww

w.ucar.edu/new

s /

Sunny Mexico City under a cover of smog

/updates/airquality.shtml)

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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 would

This permits a smog forecast:Weather forecast → Temperature forecast → Ozone peak forecast

Cooling the city by about 5 F would have a dramatic impact on smog concentration.

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 year not to exceed 75 ppb)y )

(http://ww

w.epa.ggov/airtrends/ozone.htm

l)

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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 go toTo see ozone concentrations over the United States, go to

http://airnow.gov/index.cfm?action=airnow.main

http://www.epa.gov/airtrends/index.html

Some trends can be seen at