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.
Total OH reactivity
The tropical forests are Earth’s largest source of biogenic volatile organic compounds (BVOCs). These compounds are highly chemically reactive, which makes them very relevant to atmospheric processes. Once released to the atmosphere, BVOCs undergo oxidation reactions within seconds to days. BVOCs react mainly with OH radicals, which form during the daytime primarily from the ozone (O3) and water. They thus remove part of the OH from the atmosphere, making tropical forests an OH-sink. The abundance of OH in the atmosphere in turn affects processes such as cloud formation and the residence of greenhouse gases such as methane.
Despite the impact of BVOCs on large-scale atmospheric processes, most studies in tropical forests to date have been limited to a small number of known, abundant BVOC species. But they cannot explain the total OH-sink, and substantial fractions of up to 80% remained unaccounted for.
Now Eva Pfannerstill and her colleagues wanted to solve this problem and close the budget. During several field campaigns at ATTO, they measured the loss of OH molecules per second, which is called “Total OH reactivity”. At the same time, they measured BVOC emissions and ozone concentrations.
Finding the missing link
Due to this comprehensive approach, they were able to largely attribute all the OH reactivity to specific compounds. This includes a total of 83 different VOC species and 3 inorganic trace gases. Previously unaccounted for OH reactivity in the rainforest was due to a combination of things.
On the one hand, Pfannerstill and her co-authors found a number of primary BVOCs that were not included in previous studies, but that do contribute significantly to OH reactivity.
On the other hand, they found that oxygenated compounds (OVOCs) appear to play an important role. The team’s analysis reveals a contribution of OVOCs to the seasonal total OH reactivity of roughly 1/3. This is up to 5 times more than what scientists found in previous studies. An explanation for the large contribution of OVOCs to the OH sink is likely the meteorological conditions in the tropics. Intense solar irradiation, high humidity leading to high OH levels, and elevated temperatures cause fast photochemical oxidation processes.
In addition, they analyzed daily, season, interannual cycles of OH-reactivity at various heights above the forest canopy. Among others, they found that rainfall causes short-term spikes in total OH reactivity, which are followed by below-normal OH reactivity for several hours.
Pfannerstill et al. published all their results Open Access in “Total OH reactivity over the Amazon rainforest: variability with temperature, wind, rain, altitude, time of day, season, and an overall budget closure” in Atmos. Chem. Phys.
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.
Fungal spore emissions are an important contributor to biogenic aerosols, but we have yet to understand under what conditions fungi release their spores. Nina Löbs and co-authors developed a new technique to measure emissions from single organisms and tested this out at ATTO and with controlled lab experiments. They published their results in the Open Access Journal Atmospheric Measurement Techniques.
Felipe Souza and co-authors now collected bioaerosols at our ATTO site. Then they extracted and analyzed the DNA to determine the communities present. This is the first study that described the community of microorganisms within aerosols in the Amazon. They found many different types of bacteria and fungi. Some were cosmopolitan taxa, but they also identified many that are specific to certain environments such as soil or water. This suggests that the atmosphere may act as an important gateway for bacteria to be exchanged between plants, soil, and water.
Pfannerstill et al. compared VOC emissions at ATTO between a normal year and one characterized by a strong El Nino with severe droughts in the Amazon. The did not find large differences, except in the time of day that the plants release the VOCs. They published their results in the journal Frontiers in Forest and Global Change.