Chemistry of Biogenic Hydrocarbons in the Amazon Rainforest

During the GoAmazon project in 2014, field measurements in the Amazon region of Brazil revealed that mesoscale convective storms transport ozone from the middle of the troposphere to the surface. Ozone enhancements in the regional atmospheric boundary layer ranged from 5 to 40 parts per billion volume (ppbv) and occurred mostly at 4:00 or 16:00 hour local time. Occasionally, air plumes burdened with nitrogen oxides at levels of 5-10 ppbv reached the interior of the rainforest, where the field measurements were conducted. The enhanced nitrogen oxides resulted from the long-range transport from cities such as Manaus and regional biomass burning. Meanwhile, the rainforest emitted diverse monoterpenes and sesquiterpenes, with nearly 25 measurable species whose aggregated mixing ratios reached 1.5 ppbv. Isoprene was the most abundant hydrocarbon measured above the rainforest, with maximum mixing ratios of about 20 ppbv. Reactions of ozone with monoterpenes and sesquiterpenes generate copious amounts of hydroxyl radicals, which dominate the air chemistry in forested environments. Reactions of hydroxyl radicals with hydrocarbons generate secondary organic aerosols that can serve as cloud condensation nuclei. This overview presentation will report on the conditions associated with ozone enhancements and ensuing chemistry of biogenic hydrocarbons following the occurrences of mesoscale convective storms in the central Amazon. Photochemical model simulations involving full chemical mechanisms 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 (>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 convective storms in the Amazon region: (i) modify dynamics and thermodynamics of the lower atmosphere, and (ii) transport sufficient ozone amounts to the surface to create suitable conditions to accelerate the oxidation cycles of plant-emitted hydrocarbons. Reaction products lead to the formation of aerosols, which in turn can be transported out of the atmospheric boundary layer to influence cloud formation thereby generating a positive feedback to rainfall.