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https://doi.org/10.5194/acp-2020-752
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-752
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  08 Oct 2020

08 Oct 2020

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This preprint is currently under review for the journal ACP.

Total OH reactivity over the Amazon rainforest: variability with temperature, wind, rain, altitude, time of day, season, and an overall budget closure

Eva Y. Pfannerstill1, Nina G. Reijrink1,2, Achim Edtbauer1, Akima Ringsdorf1, Nora Zannoni1, Alessandro Araújo3, Florian Ditas1,a, Bruna A. Holanda1, Marta O. Sá4, Anywhere Tsokanku1, David Walter1, Stefan Wolff1, Jošt V. Lavrič5, Christopher Pöhlker1, Matthias Sörgel1, and Jonathan Williams1,6 Eva Y. Pfannerstill et al.
  • 1Atmospheric Chemistry and Multiphase Chemistry Departments, Max Planck Institute for Chemistry, 55128 Mainz, Germany
  • 2Département Sciences de l'Atmosphère et Génie de l'Environnement (SAGE), IMT Lille Douai, 59508 Douai, France
  • 3Empresa Brasileira de Pesquisa Agropecuária (Embrapa) Amazonia Oriental, CEP 66095-100, Belém, Brazil
  • 4Instituto Nacional de Pesquisas da Amazônia (INPA), CEP 69067-375, Manaus, Brazil
  • 5Biogeochemical Processes Department, Max Planck Institute for Biogeochemistry, 07745 Jena, Germany
  • 6Energy, Environment and Water Research Center, The Cyprus Institute, 1645 Nicosia, Cyprus
  • anow at: Hessisches Landesamt für Naturschutz, Umwelt und Geologie, 65203 Wiesbaden, Germany

Abstract. The tropical forests are Earth’s largest source of biogenic volatile organic compounds (BVOCs) and thus also the largest atmospheric sink region for the hydroxyl radical (OH). However, the OH sink above tropical forests is poorly understood, as past studies revealed large unattributed fractions of total OH reactivity. We present the first total OH reactivity and VOC measurements made at the Amazon Tall Tower Observatory (ATTO) at 80, 150, and 320 m above ground level, covering two dry seasons, one wet and one transition season in 2018–2019. By considering a wide range of previously unaccounted for VOCs, which we identified by PTR-ToF-MS, the unattributed fraction was with an overall average of 19 % within the measurement uncertainty of ~ 35 %. In terms of seasonal average OH reactivity, isoprene accounted for 23–43 % of the total, oxygenated VOCs (OVOCs) for 22–40 %, while monoterpenes, sesquiterpenes, and green leaf volatiles combined were responsible for 9–14 %. These findings show that OVOCs were until now an underestimated contributor to the OH sink above the Amazon forest.

By day, total OH reactivity decreased towards higher altitudes with strongest vertical gradients observed around noon during the dry season (−0.026 s−1 m−1), while the gradient was inverted at night. Seasonal differences in total OH reactivity were observed, with the lowest daytime average and standard deviation of 19.9 ± 6.2 s−1 during a wet–dry transition season with frequent precipitation, 23.7 ± 6.5 s−1 during the wet season, and the highest average OH reactivities during two dry season observation periods with 28.1 ± 7.9 s−1 and 29.1 ± 10.8 s−1, respectively. The effects of different environmental parameters on the OH sink were investigated, and quantified, where possible. Precipitation caused short-term spikes in total OH reactivity, which were followed by below-normal OH reactivity for several hours. Biomass burning increased total OH reactivity by 2.7 s−1 to 9.5 s−1. We present a temperature-dependent parameterization of OH reactivity that could be applied in future models of the OH sink to further reduce our knowledge gaps in tropical forest OH chemistry.

Eva Y. Pfannerstill et al.

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Eva Y. Pfannerstill et al.

Data sets

ATTO total OH reactivity March 2018 Eva Y. Pfannerstill, A. Edtbauer, and J. Williams https://doi.org/10.17871/atto.158.8.646

ATTO total OH reactivity October 2018 Eva Y. Pfannerstill, N. Reijrink, A. Edtbauer, and J. Williams https://doi.org/10.17871/atto.159.10.647

ATTO total OH reactivity June 2019 Eva Y. Pfannerstill, A. Edtbauer, and J. Williams https://doi.org/10.17871/atto.160.7.648

ATTO total OH reactivity September 2020 Eva Y. Pfannerstill, A. Edtbauer, A. Ringsdorf, and J. Williams https://doi.org/10.17871/atto.161.14.649

Eva Y. Pfannerstill et al.

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Short summary
Tropical forests are globally significant for atmospheric chemistry. However, the mixture of reactive organic gases emitted by this ecosystem is poorly understood. By comprehensive observations at an Amazon forest site, we show that oxygenated species were previously underestimated in their contribution to the tropical forest reactant mix. Our results show rain and temperature effects, and have implications for models and the understanding of ozone and particle formation above tropical forests.
Tropical forests are globally significant for atmospheric chemistry. However, the mixture of...
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