Peroxyacetyl Nitrate (PAN) in the Atmosphere: A Comprehensive Review of Chemistry, Measurements, and Chemical-Transport Implications

Abstract

Peroxyacetyl nitrate (PAN) is a secondary pollutant formed through the photochemical oxidation of volatile organic compounds (VOCs) in the presence of nitrogen dioxide (NO₂). Since its identification in 1956, PAN has been recognized as a major reservoir for nitrogen (NO_x) and a component of photochemical smog and NO_y. Its thermal instability makes PAN lifetimes highly temperature dependent, with lifetimes ranging from minutes in the warm boundary layer to months in the upper troposphere. This behavior allows PAN to transport NO_x efficiently over regional and intercontinental scales, influencing downwind ozone (O₃) formation, oxidant cycling, and nitrogen deposition far from emission sources. PAN is lost primarily through thermal decomposition, with slower removal via photolysis, heterogeneous uptake, and minor reactions with OH and Cl. It correlates strongly with O₃, responds nonlinearly to precursor changes, and its formation is often VOC-limited in urban regions. While PAN decomposition can enhance downwind O₃ formation, PAN is also toxic in its own right, functioning as a potent lachrymator and phytotoxin that directly impacts human health and vegetation. Biomass burning, haze chemistry, and free-tropospheric transport further modulate PAN abundance and seasonality. This review synthesizes the current understanding of PAN chemistry, sources, and loss pathways; evaluates historical and modern measurement techniques; and examines PAN's role in photochemical smog, air quality, vegetation injury, and regional background O₃.