Atmospheric oxidation capacity sustained by a tropical forest


Terrestrial vegetation, especially tropical rain forest, releases vast quantities of volatile organic compounds (VOCs) to the atmosphere1,2,3, which are removed by oxidation reactions and deposition of reaction products4,5,6. The oxidation is mainly initiated by hydroxyl radicals (OH), primarily formed through the photodissociation of ozone4. Previously it was thought that, in unpolluted air, biogenic VOCs deplete OH and reduce the atmospheric oxidation capacity5,6,7,8,9,10. Conversely, in polluted air VOC oxidation leads to noxious oxidant build-up by the catalytic action of nitrogen oxides5,6,7,8,9,10 (NOx = NO + NO2). Here we report aircraft measurements of atmospheric trace gases performed over the pristine Amazon forest. Our data reveal unexpectedly high OH concentrations. We propose that natural VOC oxidation, notably of isoprene, recycles OH efficiently in low-NOx air through reactions of organic peroxy radicals. Computations with an atmospheric chemistry model and the results of laboratory experiments suggest that an OH recycling efficiency of 40–80 per cent in isoprene oxidation may be able to explain the high OH levels we observed in the field. Although further laboratory studies are necessary to explore the chemical mechanism responsible for OH recycling in more detail, our results demonstrate that the biosphere maintains a remarkable balance with the atmospheric environment.

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Figure 1: OH recycling.
Figure 2: Model calculations (background) and measurements (circles) of HO x radicals over Suriname (4° N, 56° W) in October 2005.
Figure 3: Difference in OH in the boundary layer, calculated by including enhanced OH recycling in the model.


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We are grateful to the GABRIEL campaign team: S. Bartenbach, C. Becker, H. Bozem, S. Engemann, H. Franke, S. Gebhardt, C. Gurk, H. Hoeseni, R. Hofmann, T. Klüpfel, R. Königstedt, D. Kubistin, R. Maser, D. Noorden, U. Parchatka, D. Rodrigues, M. Rudolf, B. Scheeren, C. Schiller, V. Sinha, A. Stickler, B. Tan, P. van Velthoven, T. Warsodikromo and G. Wesenhagen. We thank the Modular Earth Submodel System (MESSy) team for model support, in particular P. Jöckel and H. Tost, and P. J. Crutzen for comments on the manuscript.

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Correspondence to J. Lelieveld.

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The file contains Supplementary Methods and Results, Supplementary Figures S1-S4 with Legends, Supplementary Table S1 and additional references. (PDF 1461 kb)

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Lelieveld, J., Butler, T., Crowley, J. et al. Atmospheric oxidation capacity sustained by a tropical forest. Nature 452, 737–740 (2008).

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