Why vegetative emissions of organic compounds are important?
Vegetation emits substantial amounts of organic compounds into the atmosphere. Their reactions in the air influence the abundance of important chemical compounds like ozone and the hydroxyl radical. They have therefore an important impact on the so-called oxidizing capacity of the atmosphere, i.e. its ability to cleanse itself of the many pollutants released by human activities and other emissions.
Furthermore, the degradation of many biogenic hydrocarbons generates low-volatility compounds which might increase the abundance of aerosols in the atmosphere. Among these hydrocarbons, monoterpenes (C10H16) and sesquiterpenes (C15H24) are thought to be of particular importance, in view of their relatively large emission rates and the low volatility of many among their oxidation products.
The main objective of IBOOT is to better understand and quantify the role of biogenic organic compounds in the atmosphere. IBOOT will focus on some of the major areas of uncertainties related these compounds :
- The experimental product yields obtained in laboratory investigations on the degradation of monoterpenes are difficult to translate to real atmospheric conditions because of large differences in terms of photochemical conditions. Our past work on the degradation of α-pinene (a monoterpene) upon reaction with hydroxyl (Peeters et al., 2001; Capouet et al., 2004) has shown that the product yields are strongly dependent on photochemical conditions. Semi-explicit degradation mechanisms are still lacking for the ozonolysis of α-pinene and for the degradation of primary products. Formation pathways have been proposed for known ozonolysis products (e.g. pinic acid), but they suffer from a lack of objective grounds.
- Field studies have shown that very reactive organic compounds (like the sesquiterpenes) are often a significant ozone sink in the boundary layer, rivaling dry deposition, and a source of hydroxyl radicals. The oxidation of sesquiterpenes might also trigger the formation of new particles. Only few laboratory investigations on the degradation of sesquiterpenes have been conducted so far.
- The formation of organic peroxides in the ozonolysis of terpenes is significant. Hydroperoxides have low vapor pressures and might contribute to a significant fraction of Secondary Organic Aerosols (SOA). Laboratory studies on the formation of complex peroxides are still lacking.
- It has been argued that the common absorptive model of Pankow (2004), which relates that gas/aerosol partitioning of organic compounds to their saturation vapor pressure, is unable to explain the experimental aerosol yields, i.e., the terpene oxidation products are found to be too volatile to explain the observations (e.g. Jenkin, 2004; Griffin et al., 2005). This failure has been often attributed to the formation of polymers and oligomers in the aerosol phase, especially under acidic conditions. Particle phase reactions of carbonyls like glyoxal and pinonaldehyde are thought to occur and could be a significant source of SOA in the atmosphere. The real importance of these processes has not been thoroughly quantified so far.
- Oxygenated organic compounds (often by-products of biogenic precursors) are present in the upper troposphere (UT) where they could influence the chemistry of ozone and hydroxyl radicals. Their sink mechanisms at the low temperatures typical of this region are still very uncertain. Our past work (Hermans et al., 2005a,b) has shown that the reactions of formaldehyde and acetone with HO2 are potentially significant. Other carbonyls could also undergo such reactions. Furthermore, the reactions of OH with certain oxygenated organic compounds have been found to proceed via non-traditional mechanisms at the low temperatures of the UT, with consequences on the reaction rates in these conditions.