Turbulent transport and chemistry of plant-emitted gases in the central Amazonia

Description

TThe Amazon rainforest emits sufficient biogenic volatile organic compounds (BVOCs) that, after their oxidation, can contribute to the formation secondary organic aerosols, which can activate to become cloud condensation nuclei. In this presentation, the seasonal patterns of individual hydrocarbons are discussed to infer emission rates in response to timing and amount of precipitation. Of the compounds identified, hydrocarbons and methanol made up over half of the average atmospheric composition above the forest canopy, followed by isoprene, acetone, and acetaldehyde and monoterpenes, methyl vinyl ketone, and methyl ethyl ketone. The BVOCs responded strongly to precipitation events, including when ozone enhancements occurred following mesoscale convective storms. The vertical distribution of both BVOCs and oxidants is investigated to ascertain rates of chemical destruction as a function atmospheric turbulence and oxidant levels. Photochemical model simulations involving full chemical mechanisms of individual hydrocarbon molecules are then employed to supplement the field studies. Formation rates and steady-state concentrations of hydroxyl radical resulting from the oxidation of individual and aggregated biogenic hydrocarbon species under the general influences of varying levels of nitrogen oxides are determined. Model calculations indicate that reactions of isoprene and monoterpenes with enhanced ozone levels produce hydroxyl radical formation rates that are similar to those experienced in photochemically reactive environments. The present study advances the general hypotheses that convective storms in the Amazon region modify dynamics and thermodynamics of the lower atmosphere, and transport sufficient ozone amounts to the surface to create suitable conditions to accelerate the oxidation cycles of plant-emitted hydrocarbons. Results from Large Eddy Simulation (LES) will be presented to demonstrate that substantial amounts of the locally emitted hydrocarbons are chemically reacted within the forest canopy. The LES is coupled with a photochemical mechanism to investigate the interactions between turbulent transport and air chemistry and estimate the export of both plant emitted hydrocarbons and their reaction products. Simulation results indicate that amounts of hydrocarbons are chemically destroyed in the forest canopy.