Impacts of Turbulent Transport and Chemistry on Oxidation Rates of Isoprene and Monoterpenes in the Tropical Convective Boundary Layer

Tropical forests, such as the Amazon rainforest, emit large quantities of biogenic volatile organic compounds (BVOC), including isoprene and monoterpenes, which react with atmospheric oxidants, including the hydroxyl radical, ozone and the nitrate radical. Some of the resulting reaction products can condense to form secondary organic aerosols, which due to the typically clean tropical air masses can make up a large portion of the total atmospheric aerosol loading and may thus affect cloud development and climate. In order to ascertain the impact of forest BVOC emissions on regional cloud development and climate, it is necessary to quantify the impacts of turbulent transport and atmospheric chemistry in the convective boundary layer. One important question is the influence of within-canopy processes, which are typically not resolved in regional models that treat the forest as a lower boundary condition, while isoprene and monoterpenes are emitted within the forest canopy. This is highlighted by the fact that median air parcel residence times in the dense Amazon rainforest, which govern the time available for in-canopy reactions, range from a few seconds near the canopy top to tens of minutes close to the surface. Such residence times are both comparable to the chemical lifetime of many reactive trace gases and the convective timescale (z_i/w_*). This work uses Large Eddy Simulation (LES), proven to adequately represent turbulence structure (including sweeps and ejections) observed during a field campaign from April 2014 to January 2015 to quantify the impact of within-canopy and boundary-layer scale processes on the transport and air chemistry of isoprene, monoterpenes and primary reaction products on their export at the top of the boundary layer. For this, the LES is coupled with a chemical mechanism including isoprene and aggregated monoterpenes to investigate the interplay between turbulent transport and air chemistry for the canopy export of plant emitted hydrocarbons and primary reaction products such as methyl vinyl ketone and methacrolein. Simulation results indicate that isoprene export from the forest is reduced by approximately 10% due to within-canopy oxidation and that coherent sweeps and ejections lead to considerable temporal and spatial variability in vertical isoprene transport.