Introduction

Abstract

Although numerous secondary components occur in the photochemical smog complex (Part 1, this vol.), only the most important phytotoxic components, ozone (03) and peroxyacetyl nitrate (PAN) are discussed. Phytotoxic concentrations of ozone are more widely distributed and generally occur in higher ambient concentrations than do those for PAN. However, on volume (molecular) basis, PAN is more phytotoxic. PAN is the first compound of a homologous series whose phytotoxicity increases with molecular weight in the following sequence: peroxyacetyl nitrate (PAN)<peroxypropionyl nitrate (PPN)<peroxyisobutyryl nitrate (PisoBN)<peroxybutyryl nitrate (PBN)<peroxybenzoyl nitrate (PBzN). Based on current knowledge, none of these compounds, except PAN, occurs at phytotoxic oncentrations in the atmosphere (Mudd 1975, EPA 1978 a). The possible practical importance of the co-occurrence of PAN and ozone (O. C. Taylor 1969, Temple 1982) will be discussed, as will the influence of the photochemical smog complex.