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Published June 21, 2012 | Supplemental Material
Journal Article Open

Protonation and Oligomerization of Gaseous Isoprene on Mildly Acidic Surfaces: Implications for Atmospheric Chemistry


In a global process linking the Earth's climate with its ecosystems, massive photosynthetic isoprene (ISOP) emissions are converted to light-scattering haze. This phenomenon is imperfectly captured by atmospheric chemistry models: predicted ISOP emissions atop forest canopies would deplete the oxidizing capacity of the overhead atmosphere, at variance with field observations. Here we address this key issue in novel laboratory experiments where we apply electrospray mass spectrometry to detect online the products of the reactive uptake of gaseous ISOP on the surface of aqueous jets as a function of acidity. We found that ISOP is already protonated to ISOPHˆ+ and undergoes cationic oligomerization to (ISOP)_2Hˆ+ and (ISOP)_3Hˆ+ on the surface of pH < 4 water jets. We estimate uptake coefficients, γISOP = (0.5 – 2.0) × 10ˆ(–6) on pH = 3 water, which translate into the significant reuptake of leaf-level ISOP emissions in typical (surface-to-volume ~ 5 mˆ(–1)) forests during realistic (a few minutes) in-canopy residence times. Our findings may also account for the rapid decay of ISOP in forests after sunset and help bring the global budget of volatile organic compounds closer to balance.

Additional Information

© 2012 American Chemical Society. Special Issue: A. R. Ravishankara Festschrift. Received: November 15, 2011. Revised: January 26, 2012. Publication Date (Web): February 22, 2012. S.E. is grateful to the Japan Society for the Promotion of Sciences Postdoctoral Fellowships for Research Abroad. This work was supported by the US National Science Foundation, Grant AGS-0964853. We thank Professor William A. Goddard III and Dr. Robert J. Nielsen for productive discussions.

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