The Impacts Of Peroxyacetyl Nitrate In The Atmosphere Of Megacities And Large Urban Areas: A Historical Perspective

This paper is an overview of the history, discovery, and measurement of peroxyacetyl nitrate (PAN) and its analogs, the peroxyacyl nitrates (PANs), in highly populated urban areas and their downwind environments. The PANs are known to be strong lachrymators and highly phytotoxic. The discovery and characterization of PAN and its analogs helped to confirm that photochemical smog chemistry involved the oxidation of organics by the OH radical to produce organoperoxy radicals, which rapidly convert nitric oxide to nitrogen dioxide and lead to enhanced ozone formation. Advances in the knowledge of the chemistry of PAN and the PANs are reviewed including synthetic procedures, measurement techniques, field and satellite measurements, and important properties (rates of reaction, ultraviolet photolysis, and aqueous solubilities). Ground based measurements obtained in urban centers are highlighted, and the role of these large population centers is examined as sources of the PANs that impact downwind concentrations of tropospheric ozone. The phytotoxicity of PAN is discussed briefly along with its connections to other key trace species including the organic peracids, peroxides, and nitrates. The future potential of increasing wildfires due to climate change that are likely leading to large scale formation of the PANs is discussed along with the need to address the reactions of the PANs with polycyclic hydrocarbons on the surfaces of oily carbonaceous aerosols emitted from these fires. The work summarized here has led to the determination that PAN and its analogs play key roles in urban, regional, and global tropospheric chemistry.