It is proposed that peroxyacetyl nitrate and its homologues are formed in polluted air by a reaction between acylate (or aroylate) radicals and nitrogen trioxide. The proposed reaction accounts for the delayed formation of peroxyacetyl nitrate upon the irradiation of low concentrations of hydrocarbons and nitrogen oxides in air. It also explains why peroxyacetyl nitrate is not formed in photooxidations at high reactant concentrations. It accounts for the formation of peroxyacetyl nitrate in the dark reaction of acetaldehyde, oxygen, and N2O5; and, finally, it explains the inhibiting effect that excess NO2 has on that dark reaction. The proposed reaction could yield either the accepted peroxyacetyl nitrate molecular structure or a molecular structure called acetyl pernitrate that does not include an oxygen-oxygen bond. The chemical properties of the molecule can be reconciled with either formulation.