Experimental and Theoretical Study of the Atmospheric Degradation of Aldehydes

Abstract

Aldehydes are ubiquitous key components in the chemistry of the troposphere. They are common primary pollutants from biogenic emissions and in residues of incomplete combustion (Ciccioli et al., 1993). Relevant natural sources are vegetation, forest fires and microbiological processes (Kotzias et al, 1997). Aldehydes are also nearly mandatory intermediates in the photo-oxidation processes of most organic compounds in the troposphere (Kerr and Sheppard, 1981; Carlier et al, 1986). Formaldehyde (HCHO) and acetaldehyde (CH₃CHO) are among the most abundant carbonyls in the atmosphere. Ambient levels are in the order of a few tens of pptv in clean background conditions (Zhou et al., 1996; Ayers et al., 1997) but may reach tens of ppbv in polluted urban areas as a consequence of the elevated anthropogenic emissions of aldehydes and their precursors from automobile traffic, industrial and domestic heating, and industrial activity (Carlier et al, 1986; Yokouchi et al, 1990). The atmospheric loss processes include photolysis, day-time reaction with OH radicals and with Cl and Br atoms in the marine boundary layer, and reaction with NO₃ radicals during the night-time. The photolytic cleavage of aldehydes constitute an important source of free radicals, particularly in the moderately and strongly polluted areas (Carlier et al, 1986; Yokouchi et al, 1990). Aldehydes are toxic compounds themselves, and some of their photo-oxidation products, the peroxyacylnitrates, are phytotoxic and strong eye-irritant compounds (Carlier et al, 1986; Carter et al, 1981). Further, peroxyacylnitrates, such as peroxyacetyl-nitrate (PAN), are long-lived species, which can act as a NO₂ reservoir in the troposphere.