Controls on the Canonical Shapes of Terpene Concentration Gradients within the Canopy Sublayer of an Amazon Rain Forest

This study reports on the diversity blend of hydrocarbons emitted by the Amazon rain forest during the 2014 wet season. Twenty-five monoterpene and sesquiterpene species, with aggregated ambient mixing ratios of 1.5 parts per billion on a volume basis (ppbv), exhibited strong gradients within and above the forest canopy in response to the vertical distribution of sources and sinks. Isoprene was the most abundant hydrocarbon measured above the rain forest, with maximum mixing ratios reaching about 20 ppbv. Concentrations of emitted hydrocarbons increased with canopy depth as the dense forest inhibited their dispersion, resulting in greater reaction rates for a given gas with ozone and hydroxyl radical. Reactions of ozone with monoterpenes and sesquiterpenes can generate considerable amounts of hydroxyl radicals. Mesoscale convective storms transport substantial amounts of ozone from the middle of the troposphere to the surface. Biomass burning contributes to elevated nitric oxide levels, which combine with hydrocarbon reaction products to produce oxidants. Hence, one objective here is to determine hydroxyl radical yields from oxidation of speciated monoterpenes and sesquiterpenes as function of ozone and nitric oxide levels. Another goal is to estimate the secondary organic aerosol formation potential as a function of ozone and nitric oxide levels. Photochemical model calculations involving both detailed- and aggregated-chemical mechanisms are employed to estimate hydroxyl yields under the conditions encountered in the rain forest. Model calculations indicate that reactions of isoprene, monoterpenes, and sesquiterpenes with enhanced ozone levels (> 20 ppbv) produce hydroxyl radical formation rates (10⁶ radicals cm^-3 s^-1) that are similar to those experienced in photochemically reactive environments. Hence, the present study advances the general hypotheses that, despite the reduced actinic irradiance ordinarily experienced within the forest canopy during the rainy season to drive photochemical processes, the forest-emitted hydrocarbons undergo sufficient oxidation to generate substantial amounts of hydroxyl radicals. Results from the present study unfolds the processes responsible for the formation of secondary organic aerosols from oxidation of plant-emitted hydrocarbons in tropical forests.