Article pubs.acs.org/JPCA
Protonation and Oligomerization of Gaseous Isoprene on Mildly Acidic Surfaces: Implications for Atmospheric Chemistry Shinichi Enami,†,§ Himanshu Mishra,†,‡ Michael R. Hoffmann,† and Agustín J. Colussi*,† †
Ronald and Maxine Linde Center for Global Environmental Science and ‡Materials Science Department, California Institute of Technology, Pasadena, California 91125, United States S Supporting Information *
ABSTRACT: 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 ISOPH+ and undergoes cationic oligomerization to (ISOP)2H+ and (ISOP)3H+ on the surface of pH < 4 water jets. We estimate uptake coefficients, γISOP = (0.5 − 2.0) × 10−6 on pH = 3 water, which translate into the significant reuptake of leaf-level ISOP emissions in typical (surfaceto-volume ∼5 m−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.
■
INTRODUCTION Modeling the Earth’s system is vulnerable to the omission of feedback loops.1 A dominant force, the net input of radiant energy is particularly sensitive to feedbacks between the atmosphere and the biosphere.2 One such feedback comprises the absorption and scattering of (shortwave and thermal) radiation by the atmospheric aerosol produced from temperature-dependent biogenic gas emissions.3 It is estimated that ∼0.6 petagrams (1015 g) of gaseous isoprene (ISOP, 2-methyl 1,3-butadiene) are emitted by the biosphere annually,4 a flux that represents half of volatile organic compound (VOC) emissions and up to 10% of photosynthetically fixed carbon.5 In spite of the magnitude of these numbers and the anticipated response of biogenic gas emissions to global warming and anthropogenic perturbations, it is not entirely clear why some plants emit ISOP3 or how and how much it is converted to aerosol.4,6 Numerous field measurements, laboratory studies, and modeling exercises have addressed these phenomena.7 At present, however, ISOP emissions atop tropical forests are systematically overestimated.4,8,9 Evaluated from leaf-level (primary) ISOP emission fluxes by assuming
