Aerosol Emissions From Vegetation

 Vegetation has been found to be the source of over 80 percent of all volatile organic compounds (VOCs) in the earth’s atmosphere.  Little is known about how the VOCs behave after being oxidized by ozone or other free radicals in the earth's atmosphere.  The purpose of this study was to determine which species emit significant particles and how these particles interact with water vapor in the atmosphere to form cloud condensation nuclei.  Various species were placed in an oxidizing environment with ozone and analyzed by a Scanning Mobility Particle Sizer (SMPS) to determine particle diameter and concentration.  Species were considered to have significant particulation if emissions were greater than 10 thousand parts per cubic centimeter.  It was found that VOCs emitted from evergreen species formed the greatest concentrations and sizes of particles. 

A Cloud Condensation Nuclei Counter (CCNC) was used to measure the diameters of particles that activated over various supersaturation percentages of water.  Similar to particle concentrations and diameters, most of the species that activated as cloud condensation nuclei were evergreens, with the exception of the American Basswood (a flowering tree) and the Hickory tree (a fruit-bearing tree).  It was noted that each supersaturation percentage resulted in nearly equivalent activation diameters for each species.  This did not include the flowering American Basswood, which activated at significantly larger diameters for each supersaturation.  The presence of flowers seemed to alter the particle properties and, therefore, the diameter at which they activated.

Gas Chromatography-Mass Spectroscopy (GC-MS) was also used to identify the VOCs emitted by certain tree species.  Three evergreens, the Blue Spruce, White Pine, and Eastern Hemlock, and one hardwood, the Japanese Cherry Blossom, were analyzed with the GC-MS.  It was found that the primary VOC emitted by all evergreens was the organic compound alpha-Pinene.  The evergreens also emitted large amounts of beta-Pinene, d-Limonene, and Carene.  The White Pine tree, which formed the highest concentration of particles, produced the largest peaks on the GC-MS, indicating the largest presence of VOCs compared to other species tested.  The hardwood Japanese Cherry Blossom was found to only emit particle concentrations on the order to 10³ parts per cubic centimeter, and, therefore, exhibited the smallest peaks on the GC-MS.  Additionally, the GC-MS did not detect any alpha- or beta- pinenes for the hardwood species.

The results of this study give insight into the future environment by identifying the behavior of biogenic aerosols and their interactions with the atmosphere.  It has been found that plant species emit more particles as a defense mechanism against environmental stressors.  Consequently, the species that emit higher concentrations of VOCs are expected to be more widespread in future habitats, while those that emit less will be overcome by the stressors.  With the expected rises in temperature, it is important to understand the behavior of plant species in order to predict the earth's future environment and this environment's contribution to the atmosphere.