Photochemical Smog: How Exhaust and Sunlight Make Ozone

0:00Why does city air turn hazy and harmful on hot, sunny days?

0:05Picture a city full of traffic on a hot, still, sunny day.

0:11Hot car engines make nitrogen and oxygen from the air react, forming nitrogen oxides.

0:18Exhaust also carries unburned hydrocarbons: compounds of hydrogen and carbon from the fuel.

0:25They are volatile organic compounds, or VOCs: carbon compounds that evaporate easily.

0:32Sunlight splits nitrogen dioxide into nitrogen monoxide and a single oxygen atom.

0:39That oxygen atom joins an oxygen molecule, O2, to make ozone, O3.

0:46Reactions involving VOCs turn nitrogen monoxide back into nitrogen dioxide, so ozone keeps building up.

0:55Nitrogen dioxide is a brown gas, so it gives the air a brown haze.

1:00This mix is called photochemical smog, because light drives its reactions.

1:06Usually, warm air near the ground rises and carries pollution away.

1:12In a temperature inversion, a layer of warm air sits above cooler air like a lid.

1:18The cool air cannot rise through it, and with no wind, the smog stays trapped.

1:24Ground-level ozone irritates the lungs and can make asthma worse.

1:29But high up, the same gas forms the ozone layer, about 15 to 35 kilometres up, which absorbs harmful ultraviolet light and protects us.

1:40Exhaust plus sunlight makes ground-level ozone, and a warm-air lid traps it over the city.