Finding the missing components to total OH reactivity

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.

Air inlet tube at the top of the ATTO tall tower. © Eva Pfannerstill / MPI-C

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.

Total OH reactivity budget in a previous study with few compounds measured (left) and this study with many more studied compounds (right). The unattributed fraction is reduced drastically. Figure from Eva Pfannerstil / MPI-C
Total OH reactivity budget in a previous study with few compounds measured (left) and this study with many more studied compounds (right). The unattributed fraction is reduced drastically. Figure from Eva Pfannerstil / MPI-C

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.

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