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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 20 Feb 2019

Submitted as: research article | 20 Feb 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Long-term total OH reactivity measurements in a boreal forest

Arnaud P. Praplan1, Toni Tykkä1, Dean Chen2, Michael Boy2, Ditte Taipale2, Ville Vakkari1,3, Putian Zhou2, Tuukka Petäjä2, and Heidi Hellén1 Arnaud P. Praplan et al.
  • 1Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
  • 2Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, P.O. Box 64, 00014 University of Helsinki, Finland
  • 3Unit for Environmental Sciences and Management, North-West University, 2520 Potchefstroom, South Africa

Abstract. Total hydroxyl radical (OH) reactivity measurements were conducted at the second Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II), a boreal forest site located in Hyytiälä, Finland, from April to July 2016. The measured values were compared with OH reactivity calculated from a combination of data from the routine trace gas measurements (station mast) as well as online and offline analysis with gas chromatography coupled to mass spectrometry (GC-MS) and offline liquid chromatography. Up to 104 compounds, mostly Volatile Organic Compounds (VOCs) and oxidised VOCs, but also inorganic compounds, were included in the analysis, even though the data availability for each compound varied with time. The averaged experimental total OH reactivity increased from April to June (from 5.3 to 11.3 s−1) and decreased in July (8.8 s−1) due to different environmental conditions during the measurement days. In general, the total OH reactivity increased in late-afternoon and is high at night. It decreases in the morning and is low during the day, following the pattern of mixing ratios due to change of the boundary layer height. The missing reactivity fraction (defined as the different between measured and calculated OH reactivity) was found to be high. Several reasons that can explain the missing reactivity are discussed in detail such as (1) missing measurements due to technical issues, (2) not measuring oxidation compounds of detected biogenic VOCs, (3) missing important reactive compounds or classes of compounds with the available measurements. In order to test the second hypothesis, a one-dimensional chemical transport model (SOSAA) has been used to estimate the amount of unmeasured oxidation products and their expected contribution to the reactivity for three different short periods in April, May, and July. However, only a small fraction (< 9 %) of the missing reactivity can be explained by modelled secondary compounds (mostly oxidised VOCs). These findings indicate that compounds measured but not included in the model as well as unmeasured primary emissions contribute the missing reactivity. In the future, non-hydrocarbon compounds from other sources than trees (e.g. soil) should be included in OH reactivity studies.

Arnaud P. Praplan et al.
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Arnaud P. Praplan et al.
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Short summary
Our study shows that, despite our best efforts and recent progress, our knowledge of the chemical composition of the air under the canopy of a boreal forest cannot still be fully characterized. The discrepancy between the measured total reactivity or the air and the reactivity derived from the known chemical composition highlights the need to understand better the emissions from vegetation, but also other sources, such as the forest soil.
Our study shows that, despite our best efforts and recent progress, our knowledge of the...