Browsing by Author "Fisch, Gilberto"

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Environmental and biological controls on seasonal patterns of isoprene above a rain forest in central Amazonia
(2018-06) Wei, Dandan; Fuentes, Jose D.; Gerken, Tobias; Chamecki, Marcelo; Trowbridge, Amy M.; Stoy, Paul C.; Katul, Gabriel G.; Fisch, Gilberto; Acevedo, Otavio; Manzi, Antonio O.; Randow, Celso von; Nascimento dos Santos, Rosa Maria
The Amazon rain forest is a major global isoprene source, but little is known about its seasonal ambient concentration patterns. To investigate the environmental and phenological controls over isoprene seasonality, we measured isoprene mixing ratios, concurrent meteorological data, and leaf area indices from April 2014 to January 2015 above a rain forest in the central Amazon, Brazil. Daytime median isoprene mixing ratios varied throughout the year by a factor of two. The isoprene seasonal pattern was not solely driven by sunlight and temperature. Leaf age and quantity also contributed to the seasonal variations of isoprene concentrations, suggesting leaf phenology was a crucial variable needed to correctly estimate isoprene emissions. A zero-dimensional model incorporating the estimated emissions, atmospheric boundary layer dynamics, and air chemistry was used to assess the contributions of each process on the variability of isoprene. Surface deposition was an important sink mechanism and accounted for 78% of the nighttime loss of isoprene. Also, chemical reactions destroyed isoprene and during 6:00 to 18:00 h local time 56, 77, 69, and 69% of the emitted isoprene was chemically consumed in June, September, December, and January, respectively. Entrainment fluxes from the residual layer contributed 34% to the early-morning above-canopy isoprene mixing ratios. Sensitivity analysis showed that hydroxyl radical (HO) recycling and segregation of isoprene–HO played relatively lesser roles (up to 16%) in regulating ambient isoprene levels. Nitric oxide (NO) levels dominated isoprene chemical reaction pathways associated with consumption and production of HO under low-NO and high volatile organic compound (VOC) conditions. While surface deposition and oxidative processes altered isoprene levels, the relative importance of these factors varied seasonally with leaf phenology playing a more important role.
Linking meteorology, turbulence, and air chemistry in the Amazon Rain Forest
(2016-12) Fuentes, Jose D.; Chamecki, Marcelo; Nascimento dos Santos, Rosa Maria; von Randow, Celso; Stoy, Paul C.; Katul, Gabriel; Fitzjarrald, David; Manzi, Antonio O.; Gerken, Tobias; Trowbridge, Amy M.; Freire, Livia Souza; Ruiz-Plancarte, Jesus; Furtunato Maia, Jair Max; Tota, Julio; Dias, Nelson; Fisch, Gilberto; Schumacher, Courtney; Acevedo, Otavio C.; Mercer, Juliane Rezende; Yanez-Serrano, Ana Maria
We describe the salient features of a field study whose goals are to quantify the vertical distribution of plant-emitted hydrocarbons and their contribution to aerosol and cloud condensation nuclei production above a central Amazonian rain forest. Using observing systems deployed on a 50-m meteorological tower, complemented with tethered balloon deployments, the vertical distribution of hydrocarbons and aerosols was determined under different boundary layer thermodynamic states. The rain forest emits sufficient reactive hydrocarbons, such as isoprene and monoterpenes, to provide precursors of secondary organic aerosols and cloud condensation nuclei. Mesoscale convective systems transport ozone from the middle troposphere, enriching the atmospheric boundary layer as well as the forest canopy and surface layer. Through multiple chemical transformations, the ozone-enriched atmospheric surface layer can oxidize rain forest-emitted hydrocarbons. One conclusion derived from the field studies is that the rain forest produces the necessary chemical species and in sufficient amounts to undergo oxidation and generate aerosols that subsequently activate into cloud condensation nuclei.