Biomass Burning Emission Disturbances of Isoprene Oxidation in a Tropical Forest

We present a characterization of the chemical composition of the atmosphere of the Brazilian Amazon rainforest based on trace gases measurements carried out during the South American Biomass Burning Analysis (SAMBBA) airborne experiment in September 2012. We analyzed the observations of the primary biomass burning emission tracers (carbon monoxide (CO) and nitrogen oxides (NOx)), ozone (O3), isoprene, and its main oxidation products, methyl vinyl ketone (MVK), methacrolein (MACR), and hydroxyhydroperoxides (ISOPOOH). The focus of SAMBBA was primarily on biomass burning emissions, but there were also several flights in areas of the Amazon forest not directly affected by biomass burning, revealing a background with a signature of biomass burning in the chemical composition due to long-range transport of biomass burning tracers from both Africa and the eastern part of Amazonia. We used the [MVK+MACR+ ISOPOOH]/[Isoprene] ratio and the hydroxyl radical (OH) indirect calculation to assess the oxidative capacity of the Amazon forest atmosphere. We compared the background regions (CO less than 150 ppbv), fresh and aged smoke plumes classified according to their photochemical age ([O3]/[CO]), to evaluate the impact of biomass burning emissions in the oxidative capacity of the Amazon forest atmosphere. We observed that biomass burning emissions disturb the isoprene oxidation reactions, especially for fresh plumes ([MVK+MACR+ISOPOOH]/[isoprene] = 7). The oxidation of isoprene is higher in fresh smoke plumes at lower altitudes (approx. 500 m) than in aged smoke plumes, anticipating near the surface a complex chain of oxidation reactions, which may be related to the secondary organic aerosols (SOA) formation. We proposed a refinement of the OH calculation based on the sequential reaction model, which considers vertical and horizontal transport for both biomass burning regimes and background environment. Our approach for the [OH] estimation resulted in values of the same order of magnitude of a recent observation in the Amazon rainforest [OH] congruent to 10(exp 6) (molecules cm(exp -3). During a fresh plume regime, the vertical profile of [OH] and [MVK+MACR+ISOPOOH]/[isoprene] ratio showed an evidence of an increase of the oxidizing power in the transition from PBL to cloud layer (1,000-1,500 m). These high values for [OH] (1.5 x 10(exp 6) molecules cm(exp -3) and [MVK+MACR+ISOPOOH]/[isoprene] (7.5) indicate a significant change above and inside of the cloud decks due to cloud edge effects on photolysis rates, which have a major impact on OH production rates