The vapors are an important source of chemicals that form urban smog.

The dragon was thrashing and choking, as the blood and smoke mixed in his nostril to form smog.

Nitrogen oxides react with other gases in sunlight to form smog.

The data measures ground-level concentrations of ozone, a chemical that forms smog.

Once they escape into the atmosphere, however, the solvents in current use can mix with other compounds to form ground-level ozone and smog.

They are an important ingredient of acid rain and combine with hydrocarbons to form smog.

Acetaldehyde emissions increase, which subsequently react in the atmosphere to form smog.

Ethyl nitrate is found in the atmosphere, where it can react with other gases to form smog.

NOx is changed to ozone, an irritating gas, forming smog that can travel for hundreds of miles.

NO react to form smog and acid rain.