Plant-derived Secondary Organic Material in the Air and Ecosystems

Biogenic secondary organic aerosol (SOA) and deposited secondary organic material (SOM) are formed by oxidation of volatile organic compounds (VOCs) emitted by plants. Many SOA compounds have much longer chemical lifetimes than the original VOC, and may accumulate on plant surfaces and in soil as SOM because of their low volatility. This suggests that they may have important and presently unrecognized roles in plant adaptation. Using reactive plant terpenoids as a model we propose a three-tier (atmosphere–vegetation–soil) framework to better understand the ecological and evolutionary functions of SOM. In this framework, SOA in the atmosphere is known to affect solar radiation, SOM on the plant surfaces influences the interactive organisms, and wet and dry deposition of SOM on soil affects soil organisms. Oxidation of biogenic and anthropogenic VOCs in the atmosphere leads to formation of SOA particles which further grow and participate in cloud formation, and affect solar irradiation, temperature, and precipitation. Part of the atmospheric SOA also accumulates as SOM on foliage of plants and deposits on soil surface. Biogenic VOCs are emitted at high rates from plant foliage, but SOM formation under conditions found in the leaf boundary layer is unexplored. The ecological and evolutionary effects of biogenic VOCs may persist even after the original VOCs are oxidized. SOM formed from plant VOCs are expected to have even stronger ecological effects than the parent VOCs as these oxidation products are decorated with a number of reactive functional groups (carbonyls, carboxyls, hydroxyls, peroxy, hemiacetal, etc.) which may affect, for example, sensory organs of herbivores.