Trees take in carbon dioxide, and give out oxygen – but they also emit other, highly reactive, gases such as isoprene and monoterpenes into the air. These gases, known as biogenic volatile organic compounds (BVOCs), are important because they are emitted in large quantities and react with oxidising molecules, such as ozone (O3) and the hydroxyl radical (OH). This reactivity means that BVOCs make an important contribution to the composition of the atmosphere wherever they are emitted. The level of OH and O3 present in the atmosphere, or its “oxidising capacity”, is important as this controls the lifetime of other gases, such as methane (CH4).
In 2008, researchers from the School of Chemistry at the University of Leeds went to Borneo for the Oxidant and Particle Photochemical Processes (OP3) campaign, making measurements on the ground and from the air. Results from this campaign indicated that levels of the OH radical were much higher in this region than would have been predicted by computer simulations, and that a previously unidentified source of OH radicals was likely.
After reacting with oxidant compounds, BVOCs form larger compounds that can stick onto particles in the atmosphere, and potentially form new ones. The emission of BVOCs therefore influences the size and number of particles in the atmosphere, which in turn determines the number of cloud droplets forming in clouds. The number of cloud droplets controls the brightness of clouds, and how much incoming solar radiation they reflect back to space. Researchers from the Institute for Climate and Atmospheric Science quantified this effect using detailed global model simulations and found that the emission of BVOCs by vegetation is having a cooling effect on the climate.