Volatile organic compounds (VOC) are among the trace gases that have a role in controlling the distribution of chemical species in the atmosphere. Global emissions of VOC are dominated by natural (e.g., biogenic) sources. Scientific investigations of biogenic VOC emissions were initiated in the Soviet Union in 1928 and a considerable VOC flux measurement database was produced by the 1950s [Isidorov, 1993]. Biogenic VOC emissions research in North America began in the 1950's [Went, 1960] and reached a period of peak activity in the mid 1970s to early 1980s due to speculation that these compounds had a role in urban ozone formation. These efforts included systematic enclosure surveys of North American vegetation species as well as above-canopy flux measurements of emissions from entire ecosystems [e.g., Rassmussen, 1972; Zimmerman, 1979; Winer et al., 1982; Arnts et al. 1982].

This period of activity in North America was curtailed when some modeling studies failed to demonstrate that biogenic emissions were an important component of urban ozone formation [Altshuller, 1983]. At the same time, researchers in Europe [e.g., Knoeppel et al., 1981; Hov et al., 1983; Isidorov et al., 1985], Asia [e.g., Yokouchi et al., 1983] and Australia [Ayers and Gillett, 1988] were increasing efforts in this research area and making significant contributions.

The implementation of IGAC-GEIA was a significant step towards creating an international community of scientists interested in biogenic VOC emissions. Over thirty scientists from five continents attended the first IGAC-GEIA natural VOC working group meeting, held in Boulder Colorado in 1992. One outcome of this meeting was the initiation of an effort to produce a global biogenic VOC emission inventory [Guenther et al., 1995]. An equally important result was the strengthening of interactions among these scientists. This resulted in collaborations both within and outside of the IGAC-GEIA framework. In addition, the increasing interest in this field led to special sessions on biogenic VOC at international meetings as well as meetings solely devoted to this topic including a biennial Gordon Research Conference that focused on biogenic hydrocarbons and the atmosphere.

Among the challenges faced in developing a global natural VOC emission inventory are the large number of compounds and sources. The first IGAC-GEIA model included only one compound (isoprene) and three VOC categories (monoterpenes, other reactive VOC, and other VOC). Emissions from different sources were discussed but were not identified separately in the inventory. The next generation of the IGAC-GEIA natural VOC emission model will predict emissions of about 40 individual VOC compounds from seven different sources. Each source is responsible for the emission of more than one chemical species and some compounds are emitted by more than one source. The results of the past decade of biogenic VOC emissions research is being incorporated into the next generation model. This includes improved landscape characterization, emission factors, emission activity algorithms, and other model components.

The initial product of the IGAC-GEIA natural VOC working group was an inventory providing monthly total emission on a 1° latitude by 1° longitude grid. It was immediately clear that this was not sufficient for many chemistry and transport modelers. Since many biogenic VOC have very short lifetimes (minutes or hours), emission estimates were needed at relatively high spatial and temporal resolution. In addition, biogenic VOC emissions are strongly influenced by environmental conditions (e.g., temperature) and predictions for a specific scenario could vary greatly from that provided in the static IGAC-GEIA inventory. Thus many modelers by-passed the static inventory and directly used the model procedures to generate emission estimates for their own specific scenarios. As future efforts focus on Earth system models that simulate chemistry, climate and emissions, as well as the feedback coupling between them, it will be even more important to provide dynamic emission models rather than static emission inventories. There is also a need for a consistent approach to modeling emissions of different compounds from the same source, such as NO and VOC emissions from soil microbes. Even more important is coordination of procedures for modeling emission and deposition of the same compound since some emission routines may already account for deposition of a trace gas as it is transported through a vegetation canopy. These are all issues that IGAC-GEIA is considering as it moves to its next phase.

References

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