Abstract. We present a characterization of the chemical composition of the atmosphere
of the Brazilian Amazon rainforest based on trace gas measurements carried
out during the South AMerican Biomass Burning Analysis (SAMBBA) airborne
experiment in September 2012. We analyzed the observations of primary biomass
burning emission tracers, i.e., carbon monoxide (CO), nitrogen oxides
(NOx), ozone (O3), isoprene, and its main oxidation
products, methyl vinyl ketone (MVK), methacrolein (MACR), and isoprene
hydroxy hydroperoxide (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 < 150 ppbv), fresh and aged smoke plumes classified according
to their photochemical age ([O3] ∕ [CO]), to evaluate the impact
of biomass burning emissions on 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) downwind. The
oxidation of isoprene is higher in fresh smoke plumes at lower altitudes
(∼ 500 m) than in aged smoke plumes, anticipating near the surface a
complex chain of oxidation reactions which may be related to secondary organic aerosol (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 on the same order of magnitude of a
recent observation in the Amazon rainforest [OH] ≅ 106
(molecules cm−3). During the fresh plume regime, the vertical profile
of [OH] and the [MVK + MACR + ISOPOOH] ∕ [isoprene] ratio showed
evidence of an increase in the oxidizing power in the transition from
planetary boundary layer to cloud layer (1000–1500 m). These high values of
[OH] (1.5 × 106 molecules cm−3) and
[MVK + MACR + ISOPOOH] ∕ [isoprene] (7.5) indicate a significant
change above and inside the cloud decks due to cloud edge effects on
photolysis rates, which have a major impact on OH production rates.
Biomass burning related ozone damage on vegetation over the Amazon forest: a model sensitivity study
