Monoterpenes, C10H16, are volatile organic compounds emitted by plants and are key to the formation of cloud-seeding organic aerosols above the boreal forests of the northern regions of North America and Eurasia. Such forests also emit the C5 compound isoprene (2-methyl-1,3-butadiene), which researchers have now demonstrated inhibits the formation of particulate aerosols (Nature 2009, 461, 381). In plant chambers containing tree species typical of boreal forests, a research group led by Astrid Kiendler-Scharr, Thomas F. Mentel, and Jürgen Wildt of the Jülich Research Center, in Germany, studied the influence of isoprene concentration on aerosols. They found that isoprene’s inhibiting effect depends on hydroxyl radical concentration. The finding suggests that isoprene scavenges the radicals and prevents them from reacting with monoterpenes to seed aerosols. The results explain why aerosol formation above boreal forests is higher in the spring and fall than in the summer, when isoprene emissions are highest, the researchers say. And because plant species that emit isoprene are expected to flourish as global temperatures increase, the consequent reduction in cloud formation could lead to a feedback loop that encourages even higher temperatures.