Grantee Research Project Results
Final Report: Biogenic Ozone-Precursors: From Mechanism to Algorithm
EPA Grant Number: R825259Title: Biogenic Ozone-Precursors: From Mechanism to Algorithm
Investigators: Fall, Ray
Institution: University of Colorado at Boulder
EPA Project Officer: Hahn, Intaek
Project Period: November 15, 1996 through November 14, 1999
Project Amount: $379,889
RFA: Air Quality (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:
To model regional air quality and make decisions about ozone abatement, it is desirable to estimate natural isoprene emissions from U.S. forests. The primary goals of this work were to: (1) understand those biological factors that regulate isoprene emission from forest canopies to the troposphere, and (2) use this information to develop a mechanistic isoprene emission algorithm.Summary/Accomplishments (Outputs/Outcomes):
Isoprene (2-methyl-1,3-butadiene) is a volatile organic compound (VOC) released in surprisingly large amounts to the atmosphere in U.S. forests. Interest in this biogenic emission stems from the fact that isoprene is very reactive and is a major contributor to ozone formation in many areas of the United States. We have worked to understand the regulation of isoprene formation in leaves so that a mechanistic, predictive model for forest isoprene emission can be developed. Such a model would be very useful to atmospheric scientists attempting to model and understand the role of isoprene in ozone episodes in the United States and elsewhere. In the period of this project, the following work was completed:- Chloroplasts of isoprene-emitting willow leaves were shown to contain two isoforms of the enzyme isoprene synthase, the enzyme responsible for isoprene formation (Wildermuth and Fall, 1998). These isoforms include a soluble, stromal form, and a thylakoid-bound form. The two partially purified isoforms exhibited similar catalytic properties. We demonstrated that two major regulators of foliar isoprene emission, leaf age and light, do not alter the ratio of the isoforms in chloroplasts; however, the complete mechanism of light activation and dark inactivation of isoprene synthase(s) has not been determined. The finding of two different forms of isoprene synthase in chloroplasts complicates the development of a new mechanistic model for leaf isoprene emission, but does provide a clearer biochemical and cellular basis for understanding controls of leaf isoprene emission.
- Our new findings on chloroplast isoprene synthases have been evaluated in terms of the current isoprene emission algorithm (referred to here as the G93 algorithm), which is in widespread use in modeling isoprene releases in U.S. counties. The results of this evaluation are published in Fall and Wildermuth (1998), and have been presented at a national meeting of atmospheric scientists (Fall, 1997). Short-term changes in the activity of the isoprene synthases in response to temperature are consistent with the temperature correction term of the G93 algorithm. The light dependence term of the G93 algorithm, which predicts light saturation of leaf isoprene emission, is consistent with the chloroplast location of the isoprene synthases and light-driven processes responsible for activation of the enzymes. Long-term changes in the amount of active isoprene synthase(s) correlate with the variations in basal isoprene emission rate seen as leaves mature and adapt to differences in their growth environment. However, recent reports show that the G93 algorithm can be used to model light-dependent emission of other VOCs, such as formic acid, 2-methyl-3-buten-2-ol, and monoterpenes; this finding raises serious questions about the algorithm's specificity and underlying assumptions.
- I published a comprehensive review of biogenic emissions of VOCs from higher plants, including a section on the mechanism, regulation, and role of isoprene emission (Fall 1999). This work has helped educate atmospheric scientists and students about the biological principles and complexities underlying formation and emission of VOCs in vegetation. This understanding is essential to the future development of VOC emission models that will be used to predict the impacts of vegetation on atmospheric processes.
- I contributed to a state-of-the-art review of natural emissions of VOCs, carbon monoxide, and nitrogen oxides from North America (Guenther, et al., 2000). For the first time, the natural processes giving rise to precursors of tropospheric ozone are reviewed and related to one another in a single document. This review points out that we can now model isoprene emissions from the United States during summer ozone episodes with an uncertainty of less than 50 percent, demonstrating that although the G93 algorithm is not mechanistically based, it has a high degree of utility.
- While this work was in progress, we discovered that soil bacteria are capable of producing substantial amounts of isoprene, which has significant implications for modeling isoprene levels in forest ecosystems. We have completed a minireview to describe this phenomenon and also point out that temperate forest soils contain high levels of isoprene-degrading bacteria (Fall and Copley, 2000). Thus, isoprene-emitting forest ecosystems also contain soil sources and sinks for isoprene, which must be considered in future work on modeling isoprene levels in the atmospheric boundary layer.
This work has contributed to a clearer understanding of the biological controls on isoprene formation and helped forge confidence in the use of the G93 algorithm, which is now being used extensively worldwide (Guenther, et al., 2000). Because of the complexity of light activation of isoprene synthase in chloroplasts, we were not able to develop a new mechanistic algorithm; although, we have outlined the steps needed to do so (Fall and Wildermuth, 1998). The biological insights of this work will guide future efforts to model and mitigate the impacts of isoprene emission on regional ozone formation in the United States.
Journal Articles on this Report : 4 Displayed | Download in RIS Format
| Other project views: | All 7 publications | 6 publications in selected types | All 4 journal articles |
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Fall R, Wildermuth MC. Isoprene synthase: From biochemical mechanism to emission algorithm. Journal of Geophysical Research-Atmospheres 1998;103(D19):25599-25609. |
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Fall R, Copley SD. Bacterial sources and sinks of isoprene, a reactive atmospheric hydrocarbon. Environmental Microbiology 2000, Volume: 2, Number: 2 (APR), Page: 123-130. |
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Guenther A, Geron C, Pierce T, Lamb B, Harley P, Fall R. Natural emissions of non-methane volatile organic compounds; carbon monoxide, and oxides of nitrogen from North America. Atmospheric Environment 2000;34(12-14):2205-223.0. |
R825259 (Final) R825419 (1998) |
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Wildermuth MC, Fall R. Biochemical characterization of stromal and thylakoid-bound isoforms of isoprene synthase in willow leaves. Plant Physiology 1998;116(3):1111-1123. |
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Supplemental Keywords:
tropospheric, metabolism, effects, chemicals, oxidants, organics, environmental chemistry, biology., RFA, Air, Scientific Discipline, Geographic Area, EPA Region, State, Environmental Chemistry, tropospheric ozone, Atmospheric Sciences, Biochemistry, biogenic ozone precursors, biogenic modeling, Ponderosa pine, Region 8, hydrocarbon oxidation, field assessment, immunochemical methods, ozone abatement, VOCs, Colorado (CO), forest ecosystem, isoprene emission algorithmProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.