Heterogeneous and Aqueous-Phase Transformations of Isoprene

摘要

Isoprene is a conjugated diene (2-methyl-buta-1,3-diene), volatile and hardly soluble in water; under normal pressure it boils at 34°C (Merck, 1999) and dissolves up to 1.47x10~(-2) M at 21.5°C, with a Henry's constant of 0.027 mole kg~(-1) atm~(-1) at 25°C (NIST, 2001). Isoprene is a metabolite in plants, microbes, animals and humans, and a major biogenic trace compound emitted to the atmosphere. It is very reactive towards atmospheric gas-phase oxidants such as hydroxyl and nitrate radicals or ozone. At higher concentrations, 220 - 7000 ppm, it is carcinogenic to rodents and possibly carcinogenic to humans (Melnick and Sills, 2001). Terrestrial vegetation produces most of the emitted isoprene - 500 Tg C per year (Guenther et al., 1995). The emitting plants synthesise isoprene in plastids (chloroplasts) from pyruvate and glyceraldehyde-3-phosphate, most probably to better resist the thermal stress (Harley et al., 1999; Kesselmeier and Staudt, 1999; Logan et al., 2000; Rodriquez-Concepcion and Boronat, 2002). However, many plants do not synthesise isoprene at all. Other natural sources of isoprene include sea phytoplankton (1 Tg C per year, Matsunaga et al., 2002), microbes (Wagner et al., 1999; Fall and Copley, 2000), animals and humans (Fenske and Paulson, 1999; Diskin et al., 2003). In humans, the synthesis of isoprene takes a mevalonate path that is related to the synthesis of cholesterol (Karl et al., 2001). Isoprene exhaled by humans totals roughly 4 Tg C per year.