Turbulence and chemistry in the atmosphere of a central Amazonian forest
Monday, 15 December 2014: 11:30 AM
This presentation provides an overview of a field project in the rainforest of the Brazilian Amazon region conducted during April to July 2014. Objectives were to investigate turbulent transport and chemical processing of vegetation emitted gases to assess the role of the forest in aerosol production. Turbulence within and above the 35-m forest was investigated with 10 sonic anemometers deployed on a 50-m tower. Temporal and spatial distribution of plant-emitted hydrocarbons, and reactive chemical species such as ozone, carbon monoxide, sulfur dioxide, and nitrogen oxides were determined using gas analyzers and mass spectrometers to estimate aerosol yields from hydrocarbon photo-oxidation. Particle sizes and concentrations were investigated with aerosol spectrometers. The upper canopy experienced strong turbulence, with standard deviation of vertical velocity normalized by the friction velocity similar to values above the forest. The lower canopy maintained a quiescent state with turbulence being four times weaker than near the canopy top. Such weak turbulent state is conducive to the development of strong and pronounced hydrocarbon mixing ratio mean gradients within the canopy, with highest gas levels within the forest crown responding to strong emissions. The forest emitted monoterpenes, resulting in a total sum exceeding one parts per billion (ppb). Ozone levels ranged from 5 to 10 ppb. Such low ozone mixing ratios can be attributed to the frequent presence of clouds and precipitation. However, following the passage of mesoscale convective storms, ozone levels abruptly increased in response to downward transport from aloft and reached maximum levels of 30 ppb. Photochemical process remained suppressed and resulted in low particle concentrations (<104 particles cm-3), dominated by small diameter (< 400 nanometers) particles. Oxidants remained at sufficient levels to allow reactions of the locally produced hydrocarbons to generate particles. One conclusion is that the rainforest produces necessary and sufficient precursors to form particles.