peroxyacetyl nitrate

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A toxic organic compound, C₂H₃NO₅, formed photochemically in smog, causing eye irritation and damage to vegetation. Due to the stability of this compound, it may be transported in air over long distances before breaking down into smaller reactive compounds.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An urban air pollutant formed photochemically in the air as part of photochemical smog.