Mirror, mirror in the forest…
Some organic compounds exist in two mirror-image forms. This is called chirality, and such chiral forms are identical in most physical and chemical properties. However, some properties such as melting point and water solubility can differ between chiral forms. Therefore, the abundance of one form over the other in the atmosphere can have far-reaching consequences and might even affect cloud formation processes.
That is because of the formation of so-called secondary organic aerosols (SOA) from chiral precursors. SOA formation describes the process when organic particles are oxidized in the atmosphere to create tiny particles, which then act as seeds for cloud formation.
A chiral source
However, there are very few studies that distinguish between chiral forms. Thus, researchers still know little about their relevance to atmospheric processes. A few years ago, Nora Zannoni and her colleagues performed a study at ATTO focusing on the chiral compound α-pinene. Among other things, they found that its two chiral forms are neither equally abundant nor is the ratio of the two forms constant over time, season or height above the forest floor.
In a new study, Denis Leppla, Thorsten Hoffmann and their colleagues looked at pinic acid and its chiral forms. Pinic acid forms in the atmosphere through SAO formation from α-pinenes. The team wanted to find out how the chemical reactions in the atmosphere affect the chirality of its product pinic acid.
A chiral product
They found that the abundance of the two chiral forms remains the same. Similar to α-pinenes, their analysis shows a gradient in the ratios of the two forms of pinic acid with increasing height. These results were consistent over three measurement campaigns at ATTO in 2018 and 2019. They conclude that the chiral information of the precursor molecule α-pinene is merely transferred to pinic acid. That means that large-scale emission processes of the two precursor chiral forms mainly determine their chiral ratio. Meteorological, chemical, or physicochemical processes, on the other hand, do not play a particular role.
Overall, the results show that the chiral relationship of the biogenic precursor compound α-pinene is preserved in the oxidation products. Thus, future studies can use it to interpret the biogenic emission sources. Because pinic acid exists in the atmosphere in particle form and because it has a longer lifetime before it reacts to something else again, it provides a larger-scale picture of precursor emissions, while also revealing local and regional influences. Thus, this study by Denis Leppla and his team offers a better alternative for studying chirality.
Denis Leppla et al. published the study “Varying chiral ratio of pinic acid enantiomers above the Amazon rainforest” Open Access in Atmos. Chem. Phys.
Direct measurements of OH radicals are rare and difficult to achieve. However, since they react with BVOCs, Ringsdorf et al. inferred them from isoprene measurements at ATTO. To do so, they applied a technique called ‘Dynamical Time Warping’ from the field of speech recognition. Akima Ringsdorf et al. published the study “Inferring the diurnal variability of OH radical concentrations over the Amazon from BVOC measurements” Open Access in Nature Scientific Reports.
Eliane Gomes Alves and her colleagues measured isoprene emissions at the ATTO 80-meter tower across three years to better understand how these emissions vary seasonally and under extreme climatic conditions like El Niño events. They also looked into which biological and environmental factors regulate the emission of isoprene to the atmosphere.
Bioaerosols influence the dynamics of the biosphere underneath. In a new study, Sylvia Mota de Oliveira and her colleagues used the ATTO site to collect air samples at 300 m above the forest. Then, they used DNA sequencing to analyze the biological components that were present and figure out what species of plant or fungi they belong to. One of the most striking new insights is the stark contrast between the species composition in the near-pristine Amazonian atmosphere compared to urban areas.
BVOC emissions in the Amazon have been studied for decades, but we still don’t fully understand when and under what conditions tree species or even individual trees emit more or fewer isoprenoids. To address this, Eliane Gomes Alves and her colleagues measured isoprenoid emission capacities of three Amazonian hyperdominant tree species.
Mosses and lichen appear to play a previously overlooked but important role in the atmospheric chemistry of tropical rainforests. A new study from Achim Edtbauer and colleagues shows that such cryptogams emit highly reactive and particle-forming compounds (BVOCs) that are important for air quality, climate, and ecosystem processes.
The Amazon rain forest plays a major role in global hydrological cycling. Biogenic aerosols, such as pollen, fungi, and spores likely influence the formation of clouds and precipitation. However, there are many different types of bioaerosols. The particles vary considerably in size, morphology, mixing state, as well as behavior like hygroscopicity (how much particles attract water) and metabolic activity. Therefore, it is likely that not only the amount of bioaerosols affects the hydrological cycle, but also the types of aerosols present.
Bioaerosols may act as cloud condensation nuclei and ice nuclei, thereby influencing the formation of clouds and precipitation. But so far there is less knowledge about the ice nucleation activity of each bioaerosol group and atmospheric models hitherto have not differentiated between them. Patade et al. created a new empirical parameterization for five groups of bioaerosols, based on analysis of the characteristics of bioaerosols at ATTO: fungal spores, bacteria, pollen, plant/animal/viral detritus, and algae. This makes it possible for any cloud model to access the role of an individual group of bioaerosols in altering cloud properties and precipitation formation.
Biogenic volatile organic compounds remove OH from the atmosphere through chemical reactions, which affects processes such as cloud formation. In a new study, Pfannerstill et al. reveal the important contributions of previously not-considered BVOCs species and underestimated OVOCs to the total OH reactivity.
Felipe Souza, Price Mathai and their co-authors published a new study analyzing the diverse bacterial population in the Amazonian atmosphere. The composition varied mainly with seasonal changes in temperature, relative humidity, and precipitation. On the other hand, they did not detect significant differences between the ground and canopy levels. They also identified bacterial species that participate in the nitrogen cycle.
Nora Zannoni and her colleagues measured BVOC emissions at the ATTO tall tower in several heights. Specifically, they looked at one particular BVOC called α-pinene. They found that chiral BOVs at ATTO are neither equally abundant nor is the ratio of the two forms constant over time, season, or height. Surprisingly, they also discovered that termites might be a previously unknown source for BVOCs.