Sou Matsunaga - Desert Plants Have a Sunscreen for Themselves.

Plants release oxygen and water vapor, but that is not all. They also emit numerous types of organic gases called as biogenic volatile organic compounds (BVOCs). Surprisingly, the total emission of BVOC on a global scale easily exceeds the entire emission of anthropogenic VOCs. For example, isoprene (2-methyl-1,3-butadiene) is one of the major BVOCs and has a factor of 10 larger emission rate than total VOC from human activity. Another important feature of the BVOCs is that most of them are extremely reactive. The general fate of reactive compounds released into the atmosphere is degradation by strong atmospheric oxidants such as OH radical, ozone and NO 3 radicals. The degradation process is known to generate tropospheric ozone under a NO x presence condition. As a result, BVOCs are regarded as significantly important atmospheric components by atmospheric scientists. However, we do not presently know all of the important BVOCs. Recent research shows that there are still unknown BVOCs with significant emissions. My research interest is to determine unknown BVOCs and investigate their importance as an atmospheric component by development of analytical procedure and by field measurements.

I have developed an analytical procedure that enables measurement of high molecular weight BVOCs such as sesquiterpene (SQT, a group of BVOCs contain 15 carbon atoms) based on solvent extraction and gas chromatographic analysis. Field experiments in Las Vegas NV and Tomakomai (northern Japan) have been conducted from July 1 to 6, 2006 in several different sites around Las Vegas and from August 22 to September 2, 2006 in an experimental forest of Hokkaido University, respectively. Samples were collected onto an adsorbent in a glass tube from a branch enclosure bag with a flow rate of 220-300 ml min -1 for 3-4 hours for one sample. Oxidants and VOC free clean air was supplied into the bag with a flow rate of 800-2000 ml min -1 during the sampling. Therefore the inside of the bag continuously has a positive pressure and is isolated from the outside air. BVOCs were extracted from the adsorbent tube with approximately 2 ml of dichloromethane at the site, and the extracts were concentrated and analyzed at NCAR. 16 samples (10 species) and 51 samples (6 species) were collected in Las Vegas and Tomakomai, respectively.

Thirteen SQTs (including oxygenated SQTs) and two salicylic esters were determined from the sample. Emission rates of those compounds at Las Vegas and at Tomakomai ranged from 0 to 4.1 m gC dwg -1 h -1 and from 0 to 2.4 m gC dwg -1 h -1 , respectively. The emission rates were calculated as weight of carbon atoms in the molecule ( m g) per unit dry weight of leaves in the enclosure per unit time. I found some unknown compounds from desert plants that had higher emission rates than any known compounds at Las Vegas, and I determined that they were Octylate (2-ethylhexenyl salicylate) and Homosalate (3,3,5-trimethylcyclohexenyl salicylate) using a gas chromatography/mass spectrometer analysis and authentic standard compounds for those salicylic esters. Those esters are used as a sunscreen ingredient. They did not have significant emission rate from boreal plants at Tomakomai (factor of 150 smaller than those of Las Vegas ). It seems that desert plants have sunscreen compounds to protect themselves from high radiation, and those compounds are vaporized due to a high temperature in desert. Reaction constants with OH of those sunscreen compounds were estimated to be 2-4×10 -11 cm 3 molecule -1 sec -1 . The emission rate and OH reaction constants of those sunscreen compounds were close to those of a -Pinene which is considered to be one of the most important reactive BVOC. Those compounds can be significantly important in desert regions and could be a discovery of new class of important reactive atmospheric constituents.

ASP Spotlight March 2007
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