Oxidation of Isoprene and Monoterpenes as a Function of Nitrogen Oxides in the Amazon Rain Forest

The Amazon rainforest represents the most expansive region of the world with the largest and the most diverse emissions of phytogenic hydrocarbons. Due to the suitable environmental conditions characterized by high air temperature and solar irradiance, the rainforest releases terpenoid molecules year round, mostly isoprene and monoterpenes. At times, the Amazon rainforest also experiences occasionally elevated concentrations of nitrogen oxides due to regional biomass burning or long-range transport of urban emissions. In dense and tall canopies such as the Amazon rainforest, median air parcel residence times can be comparable to monoterpene chemical lifetimes. As a result, reactions of plant-emitted hydrocarbons with ozone, hydroxyl radical, and nitrate radical can occur in the forest canopy, generating free radicals whose additional reactions convert nitric oxide to nitrogen dioxide. This presentation will include results from numerical studies involving a coupled one-dimensional photochemical model to estimate the fractions of locally emitted isoprene and monoterpenes exported out of the rainforest canopy and the boundary layer due to surface deposition and chemical reactions occurring under the influences of prevailing levels of ozone and nitrogen oxides. We pay particular attention to the interplay, as a function of nitrogen oxide concentrations, between potential net hydroxyl radical formation from monoterpene oxidation and hydroxyl radical consumption by isoprene. We found that yields of hydroxyl radical from in-canopy ozonolysis of monoterpenes were similar to the amounts of hydroxyl radical needed to oxidize isoprene. In general, the formation of hydroxyl radical increased with nitric oxide levels, with enhancements exceeding 150 % when nitric oxide changed from 0.2 to 1.0 parts per billion.