Constraining Novel Photochemistry at the Biogenic Tailpipe

Glenn M. Wolfe

The forest is a complex chemical reactor where atmospheric composition is shaped by the interplay of biogenic volatile organic compound (VOC) emissions, transported anthropogenic pollution, radical-driven photochemistry and surface deposition. Our grasp of the fundamental processes in this environment directly impacts our ability to assess regional and global trends in air quality and chemistry-climate interactions. In this seminar, I will present results from a combination of field, modeling and laboratory studies that afford unique insights on the central role of chemistry at the forest-atmosphere interface. First, I will demonstrate that direct vertical flux observations can provide quantitative constraints on the relative magnitudes of in-canopy processes and shed light on previously unrecognized chemistry, such as fast oxidation of unidentified highly-reactive VOC emissions. With the aid of an explicit 1-D chemical-transport model, I will illustrate that such chemistry can markedly alter radical cycling in the canopy environment. Next, I will focus on our evolving understanding of the photochemistry of isoprene, the most ubiquitous biogenic VOC. Current-generation oxidation mechanisms are incomplete even for this well-studied molecule, and kinetic models struggle to accurately predict concentrations of the key daytime oxidant, the hydroxyl radical (OH), in high-isoprene environments. I will present results from recent laboratory investigations into the OH production and recycling potential of a putative new class of isoprene oxidation products, which may be important for reconciling the “missing OH” in remote forested environments.