Plants and Their Role in the Atmosphere
Typically, when the atmosphere is mentioned, it’s usually in the context of air quality and/or climate change. The former tends to focus on the emissions and subsequent transformations of air pollution that may be hazardous to human health. The latter, on the other hand, focuses on how greenhouse gases and atmospheric aerosols affect the radiative forcing of our planet. However, regardless of which one is being referring to, both are influenced by a complex interplay of an assortment of reactive compounds found within our atmosphere.
Of these reactive atmospheric compounds, two chemical families are held to high importance: the oxides of nitrogen (NOx = NO + NO2) and volatile organic compounds (VOCs). Though these two groups of compounds perform a wide array of chemistry, they’re most commonly studied due to their influence in the production of tropospheric ozone, a greenhouse gas and an irritating pollutant that can damage our crops as well as our lung tissue. Emissions of NOx are largely caused by anthropogenic means, with fossil fuel combustion serving as one of its primary sources. On the other hand, while human production of VOCs is a significant source, more than half of their annual emissions are of natural origin, predominately caused by terrestrial vegetation.1
The emission of these biogenic VOCs, or BVOCs, are due to a variety of reasons ranging from pest protection to hormone signaling.1 However, of the hundreds of BVOCs emitted by Earth’s vegetation, isoprene is perhaps the most important due to its high emission rate (accounting for 50% of all biogenic emissions) and equally high reactivity (lasting only hours to days in the atmosphere).2 It is due to these characteristics that isoprene plays a key role in atmospheric chemistry. That is, it not only plays a part in the ozone producing reactions through the NOx/VOC chemistry but subsequent products from isoprene reactions can form the secondary organic aerosol (effecting climate) and control the concentrations of the hydroxyl (OH) radical, the “atmospheric detergent” that removes many trace gases in the atmosphere.1,3
However, despite its obvious importance, the full mechanistic scheme of isoprene is still unknown, which can serve as an issue when predicting the full effects this important BVOC has on the atmosphere. Providing more complication, the magnitude of isoprene emissions can strongly depend on meteorological parameters such as temperature and radiation and even vary drastically between different plant species.1,2 This means that future changes in climate, land use and biofuels can have a large impact on how much isoprene is released into the atmosphere. Though much of the isoprene chemistry has been uncovered in the last decade, there is still much more research to be done. After all, a full understanding of this chemistry is required in order to evaluate future climate and air quality scenarios and produce appropriate policy decisions.
- Pike, RC, Young, PJ (2009) How plants can influence tropospheric chemistry: The role of isoprene emissions from the biosphere. Weather 12: 332-336
- Pacifico, F, Harrison, SP, Jones, CD, Sitch, S (2009) Isoprene emissions and climate. Atmospheric Environment 43: 6121-6135
- Mao, JQ, Paulot, F, Jacob, DJ, Cohen, RC, Crounse, JD, Wennberg, PO, Keller, CA, Hudman, RC, Barkley, MP, Horowitz, LW (2013) Ozone and organic nitrates over the eastern United States: Sensitivity to isoprene chemistry. Journal of Geophysical Research 118: 11256-11268