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Lecture 21

Urban Chemistry: Smog

ATOC/CHEM 5151

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What is smog?

• Term was coined in England in early

1900s to refer to the dark “clouds”

observed over British towns

SMoke + fOG = smog

• Today, we recognize two types:

– London-type smog

– Los Angeles-type smog

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London smog

• Mixture of smoke, fog, ash, soot, SO2, and

H2SO4

• Requires a temperature inversion

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Temperature Inversion

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Temperature Inversions and Smog

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Consequences of Smog

• London, 1952

• 4000 people died

from respiratory and

heart ailments

attributed to breathing

smog

• Most pollution

components were

from coal burning

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London Smog is not exclusive to

London: Donora, PA (1948)

• Donora lies along the Monongahela River

• Industry: zinc smelters and steel mills

• 26-31 Oct 1948: 20 people died; 43% of

population became ill

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London smog chemistry

• Pretty simple – carbon and sulfur:

– Carbon burns to become CO2 or CO, depending upon extent of combustion

– Some carbon remains unburned → soot• Particles about 0.1 µm in diameter

– Sulfur in coal also burns• S + O2 → SO2

• SO2 + OH → HSO3

• HSO3 + O2 → HO2 + SO3

• SO3 + H2O(ℓ) → H2SO4 (ℓ)

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Los Angeles smog

• Meteorology is also important – inversions trap pollutants in stagnant air and give them time to “cook”

Marine inversion Subsidence inversion

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“Classic” LA smog

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Primary PollutantsThings that are directly emitted

Nitrogen Oxides (NOx)

Hydrocarbons (VOCs)

Carbon Monoxide (CO)

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Secondary Pollutants

• Formed by

photochemical reactions

of primary pollutants

• Key elements are:

– NO2

– O3

– Peroxyacetyl nitrate (PAN)

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Comparison of London and LA -

type smogs

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Ozone formation• Provided there is sufficient NOx (> 50-100 ppt),

hydrocarbon oxidation will make O3.

• Recall CH4 oxidation:

CH4 + 10 O2 → CO2 + H2O + 5 O3 + 2 OH

• “Branched chain” reaction

– Several pathways

– Produces radicals as net products (which can go on

to oxidize other hydrocarbons)

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Typical alkane oxidation scheme

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Photochemistry: Smog Chamber

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Diurnal behavior of Ozone

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Ozone

production and

transport

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Types of Hydrocarbons

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General Reaction Scheme

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Hydrocarbon Reactivity

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“Group Reactivity”

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Incremental Ozone production

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Maximum O3 production

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Biogenic Hydrocarbons

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Isoprene Chemistry Each HCHO

leads to 3 O3

and 2 OH!

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Particulate

Matter

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Sulfate and Nitrate: Dominant Components

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Soot Particles: Host for Organics

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Urban pollution to global scale