Aldehydes and organic acids

Abstract

Organic acids and aldehydes are ubiquitous chemical constituents in the atmosphere which are partially emitted from vegetation. Numerous publications indicate that formic and acetic acid as well as formaldehyde and acetaldehyde are of particular quantitative and qualitative importance for tropospheric chemistry. Other aldehydes and organic acids in the atmosphere which are possibly derived from biogenic sources include oxalic, propionic and pyruvic acids (Lunde et al. 1977; Norton et al. 1983; Kawamura et al. 1985; Andreae et al. 1987, 1988; Hofmann et al. 1997) as well as propanal, butanal and isobutanal (Isidorov et al. 1985; König et al. 1995). In remote areas, some semi-volatile aldehydes (hexanal, heptanal, octanal, nonanal, decanal, undecanal) were present ubiquitously in the atmosphere (Yokouchi et al. 1990). Acetaldehyde and formaldehyde concentrations measured in the atmosphere amounted to 0.3 to 5.0 ppbv at rural and forested sites, but increased up to 176 ppbv in urban areas (Table 1). The concentrations of acetic and formic acid are in a similar range between approximately 0.1 and 10.5 ppbv at urban and 0.1 and 7.5 ppbv at semi-rural and rural sites (Table 2). Less information is available for the concentrations of higher molecular weight aldehydes and carboxylic acids in ambient air. Values measured for these compounds varied between 0.01 and 0.26 ppbv (organic acids, Table 2) and 0.02 and 81 ppbv (aldehydes, Table 3). Due to their high reactivity (and therefore short half-life), the ambient mixing ratios of these compounds depend directly on the rates of production and destruction and only to a minor extent on long-range transport. In addition to anthropogenic sources, vegetation is considered a significant source, as well as a sink of these compounds. This chapter summarizes the actual knowledge on exchange processes of organic acids and aldehydes between trees and the atmosphere and discusses factors controlling these processes.