Intermediate-Scale Heterogeneity in Volatile and Semivolatile Organic Compounds over the Near-Canopy Atmosphere in Central Amazonia

The emissions, deposition, and chemistry of volatile organic compounds (VOCs) are thought to be influenced by underlying landscape heterogeneity at intermediate horizontal scales of several hundred meters across different forest sub-types within a tropical forest. Quantitative observations and scientific understanding at these scales, however, remain lacking, in large part due to a historical absence of canopy access and atmospheric observational approaches. Herein, horizontal heterogeneity in the concentrations of VOCs (e.g., isoprene and monoterpenes) and their oxidation products (i.e., semivolatile organic compounds, or SVOCs) over the near-canopy atmosphere were examined by sampling from a copter unmanned aerial vehicle flown over different forest sub-types separated by < 1000 m, specifically plateau and slope forests in central Amazonia, during the wet season of 2018. Results show that isoprene concentrations in the near-canopy atmosphere over the plateau forest were significantly greater than those over the slope forest. By comparison, monoterpene concentrations show no statistical difference. A gradient transport model constrained by the data was used to infer emission differences from these forest sub-types, which is in contrast to a 0% difference implemented in most present-day biosphere emissions models (i.e., homogeneous emissions). Heterogeneity of 2-methyltetrols (including 2-methylthreitol and 2-methylerythritol), the isoprene oxidation products, were also observed over the two sampling locations. Model simulations indicate that the uneven isoprene distribution over the near-canopy atmosphere plays a minor role in the difference of SVOC concentrations observed, due to the long SVOC lifetimes in the atmosphere (e.g., a few days). And the observed SVOC heterogeneity may be attributed to other factors such as primary vegetation emissions.The dataset obtained from this study at previously unexplored intermediate scales, linking the diversity among VOC emissions and atmospheric SVOC production, is essential for better evaluating the ecological and Earth system roles of VOCs and representing them in climate and air quality models.