Biosynthesis of aldehydes and organic acids

Abstract

Trees emit a wide range of VOCs including oxygenated species such as aldehydes and organic acids into the atmosphere. Depending on their reactivity the calculated lifetimes of these compounds in the troposphere vary in the range from only a few (e.g. formaldehyde 3.6 h) up to some hours (e.g. acetaldehyde 20 h) which is in the same range as isoprene (c. 8 h) or monoterpenes (0.5 to 7 h) (Kotzias et al. 1997). Photochemical destruction and oxidation of aldehydes and organic acids lead to the generation of free radicals which participate in numerous atmospheric reactions and control many atmospheric chemical processes (Kotzias et al. 1997). These reactions strongly affect HO_x-radical chemistry in the troposphere thereby leading to the production of H0₂-radicals. In the presence of NO and NO₂ in ambient air, these reactions cause a net generation of ozone. Moreover, the destruction of acetaldehyde, for example, may be connected to the production of peroxyacylnitrates (PANs) which are known for their adverse effects on human health and plant growth (Sakaki 1998; Kotzias et al. 1997). Formic and acetic acids in the atmosphere strongly participate in the acidification of rainwater. Especially in rural regions the contribution of these acids to rainwater acidity is significant and amounts to around 50 to 64 % (see Galloway et al. 1982; Andreae et al. 1988; Bode et al. 1997).