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A large fraction of the reactive compounds in air forest remains unknown

A large fraction of the reactive compounds in air forest remains unknown

Despite the best efforts to measure as much of the compounds present in the forest air, researchers are still unable to explain the measured total hydroxyl radical (OH) reactivity with the measured chemical composition of the air.

This is due to the difficulty to quantify every possible compound present. Using a model to estimate the concentration of compounds that were not directly measured (such as oxidation products from reactive compounds) only a small fraction of the unexplained reactivity could be accounted for.

While compounds emitted from vegetation are important, especially in summer, there is some evidence that unknown reactive compounds emitted from the forest floor might play a significant role, in particular in early spring when the soil is thawing.

These conclusions are derived from total OH reactivity and chemical composition measurements of the forest air between April and July 2016 at the SMEAR II boreal forest research station in Hyytiälä, Finland. A detailed account of the findings have recently been published in Atmospheric Chemistry and Physics.

Total hydroxyl radical (OH) reactivity measurements are a tool to assess our degree of understanding of atmospheric chemistry because OH reacts with virtually any other compound in the atmosphere. By comparing the experimentally measured total OH reactivity with the expected OH reactivity calculated from the known chemical composition at a given location, it is possible to estimate the extend of unexplained (or missing) reactivity. While the missing fraction is small in urban environments, it has been found to be higher in forested environments, which is why researcher focus on this type of environments to identify these unknown reactive compounds.

Further information:

Researcher Arnaud P. Praplan, Finnish Meteorological Institute,

Air Quality Group Leader Heidi Hellén, Finnish Meteorological Institute, heidi.hellé

Praplan, A. P., Tykkä, T., Chen, D., Boy, M., Taipale, D., Vakkari, V., Zhou, P., Petäjä, T., and Hellén, H.: Long-term total OH reactivity measurements in a boreal forest, Atmos. Chem. Phys., 19, 14431–14453,, 2019.

Arnaud Praplan's blog post about the research (published November 29, 2019): Seasonality and knowledge gaps: Total hydroxyl radical (OH) reactivity measurements in the boreal forest


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