River Winds and Transport of Forest Volatiles in the Amazonian Riparian Ecoregion

Volatile organic compounds (VOCs) emitted from forests are important chemical components that affect ecosystem functioning, atmospheric chemistry, and regional climate. Temperature differences between a forest and an adjacent river can induce winds that influence VOC fate and transport. Quantitative observations and scientific understanding, however, remain lacking. Herein, daytime VOC datasets were collected from the surface up to 500 m over the "Rio Negro" river in Amazonia. During time periods of river winds, isoprene, α-pinene, and β-pinene concentrations increased by 50, 60, and 80% over the river, respectively. The concentrations at 500 m were up to 80% greater compared to those at 100 m because of the transport path of river winds. By comparison, the concentration of methacrolein, a VOC oxidation product, did not depend on river winds or height. The differing observations for primary emissions and oxidation products can be explained by the coupling of timescales among emission, reaction, and transport. This behavior was captured in large-eddy simulations with a coupled chemistry model. The observed and simulated roles of river winds in VOC fate and transport highlight the need for improved representation of these processes in regional models of air quality and chemistry-climate coupling.

Air quality and photochemical reactions: analysis of NOx and NO2 concentrations in the urban area of Turin, Italy

In this work, based on the existing studies on photochemical reactions in the lower atmosphere, an analysis of the historical series of NO_x, NO₂, and O₃ concentrations measured in the period 2015-2019 by two monitoring stations located in the urban area of Turin, Italy, was elaborated. The objective was to investigate the concentration trends of the contaminants and evaluate possible simplified relationships based on the observed values. Concentration trends of these pollutants were compared in different time bands (diurnal or seasonal cycles), highlighting some differences in the dispersion of the validated data. Calculated [NO₂]/[NO_x] ratios were in agreement with the values observed in other urban areas worldwide. The influence of temperature on the [NO₂]/[NO_x] ratio was investigated. An increase of [NO₂]/[NO_x] concentration ratio was found with increasing temperature. Finally, a set of empirical relationships for the preliminary determination of NO₂ concentration values as a function of the NO_x was elaborated and compared with existing formulations. Polynomial functions were adapted to the average concentration values returned by the division into classes of 10 μg/m³ of NO_x. The choice of an empirical function to estimate the trend of NO₂ concentrations is potentially useful for the preliminary data analysis, especially in case of data scarcity. The scatter plots showed differences between the two monitoring stations, which may be attributable to a different urban context in which the stations are located. The dissonance between a purely residential context (Rubino station) and another characterised by the co-presence of residential buildings and industries of various kinds (Lingotto station) leads to the need to consider a greater contribution to the calculation of the concentrations emitted in an industrial/residential context due to a greater presence of industrial chimneys but also to more intense motorised vehicle transport. The analysis of the ratio between nitrogen oxides and tropospheric ozone confirmed that, as O₃ concentration increases, there is a consequent reduction of NO_x concentration, due to the chemical reactions of the photo-stationary cycle that takes place between the two species. This work highlighted that the use of an empirical formulation for the estimation of [NO_x] to [NO₂] conversion rate could in principle be adopted. However, the application of empirical models for the preliminary estimation of [NO_x] conversion to [NO₂] cannot replace advanced models and should be, in principle, restricted to a limited area and a limited range of NO_x concentrations.

Amazonian biogenic volatile organic compounds under global change

Biogenic volatile organic compounds (BVOCs) play important roles at cellular, foliar, ecosystem and atmospheric levels. The Amazonian rainforest represents one of the major global sources of BVOCs, so its study is essential for understanding BVOC dynamics. It also provides insights into the role of such large and biodiverse forest ecosystem in regional and global atmospheric chemistry and climate. We review the current information on Amazonian BVOCs and identify future research priorities exploring biogenic emissions and drivers, ecological interactions, atmospheric impacts, depositional processes and modifications to BVOC dynamics due to changes in climate and land cover. A feedback loop between Amazonian BVOCs and the trends of climate and land-use changes in Amazonia is then constructed. Satellite observations and model simulation time series demonstrate the validity of the proposed loop showing a combined effect of climate change and deforestation on BVOC emission in Amazonia. A decreasing trend of isoprene during the wet season, most likely due to forest biomass loss, and an increasing trend of the sesquiterpene to isoprene ratio during the dry season suggest increasing temperature stress-induced emissions due to climate change.

Observations of sesquiterpenes and their oxidation products in central Amazonia during the wet and dry seasons

Biogenic volatile organic compounds (BVOCs) from the Amazon forest region represent the largest source of organic carbon emissions to the atmosphere globally. These BVOC emissions dominantly consist of volatile and intermediate-volatility terpenoid compounds that undergo chemical transformations in the atmosphere to form oxygenated condensable gases and secondary organic aerosol (SOA). We collected quartz filter samples with 12 h time resolution and performed hourly in situ measurements with a semi-volatile thermal desorption aerosol gas chromatograph (SV-TAG) at a rural site ("T3") located to the west of the urban center of Manaus, Brazil as part of the Green Ocean Amazon (GoAmazon2014/5) field campaign to measure intermediate-volatility and semi-volatile BVOCs and their oxidation products during the wet and dry seasons. We speciated and quantified 30 sesquiterpenes and 4 diterpenes with mean concentrations in the range 0.01-6.04 ngm^-3 (1-670ppq_v). We estimate that sesquiterpenes contribute approximately 14 and 12% to the total reactive loss of O₃ via reaction with isoprene or terpenes during the wet and dry seasons, respectively. This is reduced from ~ 50-70 % for within-canopy reactive O3 loss attributed to the ozonolysis of highly reactive sesquiterpenes (e.g., β-caryophyllene) that are reacted away before reaching our measurement site. We further identify a suite of their oxidation products in the gas and particle phases and explore their role in biogenic SOA formation in the central Amazon region. Synthesized authentic standards were also used to quantify gas- and particle-phase oxidation products derived from β-caryophyllene. Using tracer-based scaling methods for these products, we roughly estimate that sesquiterpene oxidation contributes at least 0.4-5 % (median 1 %) of total submicron OA mass. However, this is likely a low-end estimate, as evidence for additional unaccounted sesquiterpenes and their oxidation products clearly exists. By comparing our field data to laboratory-based sesquiterpene oxidation experiments we confirm that more than 40 additional observed compounds produced through sesquiterpene oxidation are present in Amazonian SOA, warranting further efforts towards more complete quantification.