Compostos Orgânicos Voláteis: variabilidade em escala de subtipos florestais na Amazônia Central

Abstract

Forests play a key role in the exchange of volatile organic compounds (VOCs) between the biosphere and the Earth's atmosphere. The Amazon Rainforest, the largest and most diverse rainforest ecosystem on Earth, is made up of intercalated plateaus and flooded valleys, separated by streams and rivers that end up being joined by sloping and valley regions at spatial scales of hundreds of meters. The wide variation in forest diversity, mainly due to different factors such as water availability, sunlight and soil types, leads to substantial variability in VOC emissions, further resulting in a heterogeneous distribution of VOC concentrations in the atmosphere in the horizontal and vertical, within these spatial scales of hundreds of meters. However, the existing sampling platforms so far are not able to provide data on the heterogeneous distribution of VOC concentrations. Therefore, sufficient datasets of VOCs at these spatial scales are lacking, making it difficult to better understand the interactions between forest, atmospheric chemistry and climate. Thus, the present study aimed to investigate the heterogeneity of the spatial scale of the horizontal and vertical gradients of VOC concentrations in the roughness layer of a tropical forest, using unmanned aerial vehicle (UAV) sampling, flying over forest subtypes of plateau, slope and valley in Central Amazon, during the morning and early afternoon periods of the rainy season and dry season of the year 2018. In the rainy season, the spatial scale of the horizontal gradient over the forest subtypes of plateau and slope was investigated, atmospheric isoprene concentrations near the canopy over the plateau forest subtype were 60% higher than those over the slope forest subtype. A gradient transport model showed that isoprene concentrations differ from 220% to 330% between forest subtypes, which contrasts with the 0% difference implemented in most models for the biosphere (ie, homogeneous emissions). In the dry season, the spatial scale of the vertical gradient over the forest subtypes of plateau and valley was investigated, the mean concentrations of isoprene at 50 meters were 2.1 and 2.3 ppbv in the plateau and valley respectively, and at 100 meters were of 1.8 ppbv at both locations. However, the isoprene concentration was 4.9% at 50 meters and 31% at 100 meters, higher in the valley forest subtype than in the plateau. When the vertical gradient was verified, this increase was of 16% and 40% in 50 and 100 meters respectively, therefore, an increase of 23% with the increase in altitude. These results point to the significance that heteroneity between forest subtypes is real and that quantifying VOC concentrations, emissions and other processes at intermediate levels of horizontal and vertical spatial scales of different forest subtypes are essential to understand ecological aspects, functions the terrestrial system of VOCs to be able to represent them more realistically in air quality and climate models that currently do not consider them.