Tight Coupling of Surface and In-Plant Biochemistry and Convection Governs Key Fine Particulate Components over the Amazon Rainforest

MANISHKUMAR SHRIVASTAVA, Quazi Rasool, Bin Zhao, Mega Octaviani, Rahul Zaveri, Alla Zelenyuk, Brian Gaudet, John Shilling, Johannes Schneider, Christiane Schulz, Ying Liu, Scot T. Martin, Jianhuai Ye, Alex Guenther, Rodrigo Souza, Martin Zoeger, Martin Wendisch, Ulrich Pöschl, Pacific Northwest National Laboratory

     Abstract Number: 195
     Working Group: Aerosol Chemistry

Abstract
The Amazon rainforest plays important roles in the Earth’s radiative balance, clouds and water cycle. A key aspect of forest-atmosphere interactions, which is not well understood, relates to how organic gases emitted by the forest form secondary organic aerosols. Understanding how the forest produces these particles could help us understand how deforestation and changing climate will affect global warming and the water cycle.

Combining unique high-altitude aircraft measurements and detailed regional model simulations, we show that in-plant biochemistry plays a central but previously unidentified role in fine particulate-forming processes and atmosphere–biosphere–climate interactions over the Amazon rainforest. Isoprene epoxydiol secondary organic aerosols (IEPOX-SOA) are key components of sub-micrometer aerosol particle mass throughout the troposphere over the Amazon rainforest and are traditionally thought to form by multiphase chemical pathways. Here, we show that these pathways are strongly inhibited by the solid thermodynamic phase state of aerosol particles and lack of particle and cloud liquid water in the upper troposphere. Strong diffusion limitations within organic aerosol coatings prevailing at low temperatures and low relative humidity in the upper troposphere strongly inhibit the reactive uptake of IEPOX to inorganic aerosols. We find that direct emissions of 2-methyltetrol gases formed by in-plant biochemical oxidation and/or oxidation of deposited IEPOX gases on the surfaces of soils and leaves and their transport by cloud updrafts followed by their condensation at low temperatures could explain over 90% of the IEPOX-SOA mass concentrations in the upper troposphere. Our simulations indicate that even near the surface, direct emissions of 2-methyltetrol gases represent a ubiquitous, but previously unaccounted for, source of IEPOX-SOA. Our results provide compelling evidence for new pathways related to land surface–aerosol–cloud interactions that have not been considered previously.