# Sesquiterpenes and oxygenated sesquiterpenes dominate the VOC (C 5–C 20) emissions of downy birches

Biogenic volatile organic compounds (BVOCs) emitted by the forests are known to have strong impacts in the atmosphere. However, lots of missing reactivity is found, especially in the forest air. Therefore better characterization of sources and identification/quantification of unknown reactive compounds is needed. While isoprene and monoterpene (MT) emissions of boreal needle trees have been studied quite intensively, there is much less knowledge on the emissions of boreal deciduous trees and emissions of larger terpenes and oxygenated volatile organic compounds (OVOCs). Here we quantified the downy birch (Betula pubescens) leaf emissions of terpenes, oxygenated terpenes and green leaf volatiles (GLVs) at the SMEAR II boreal forest site using in situ gas chromatographs with mass spectrometers.

Sesquiterpenes (SQTs) and oxygenated sesquiterpenes (OSQTs) were the main emitted compounds. Mean emission rates of SQTs and OSQTs were significantly higher in the early growing season (510 and 650 ng ginline-formula $M3inlinescrollmathml{}_{\mathrm{normal dw}}^{-normal 1}$ 12pt17ptsvg-formulamathimg1b1fb9ed5a671545b8be99b30ebbd8ef acp-21-8045-2021-ie00001.svg12pt17ptacp-21-8045-2021-ie00001.png  hinline-formula−1, respectively) compared to in the main (40 and 130 ng ginline-formula $M5inlinescrollmathml{}_{\mathrm{normal dw}}^{-normal 1}$ 12pt17ptsvg-formulamathimgb7944d07d04706985578b665e6313e30 acp-21-8045-2021-ie00002.svg12pt17ptacp-21-8045-2021-ie00002.png  hinline-formula−1, respectively) and late (14 and 46 ng ginline-formula $M7inlinescrollmathml{}_{\mathrm{normal dw}}^{-normal 1}$ 12pt17ptsvg-formulamathimg439672bdf887b11d77bb49dd05dad63b acp-21-8045-2021-ie00003.svg12pt17ptacp-21-8045-2021-ie00003.png  hinline-formula−1, respectively) periods, indicating that early leaf growth is a strong source of these compounds. The emissions had a very clear diurnal variation with afternoon maxima being on average 4 to 8 times higher than seasonal means for SQTs and OSQTs, respectively. inline-formulaβ-Caryophyllene and inline-formulaβ-farnesene were the main SQTs emitted. The main emitted OSQTs were tentatively identified as 14-hydroxy-inline-formulaβ-caryophyllene acetate (inline-formulaM=262 g molinline-formula−1) and 6-hydroxy-inline-formulaβ-caryophyllene (inline-formulaM=220 g molinline-formula−1). Over the whole growing season, the total MT emissions were only 24 % and 17 % of the total SQT and OSQT emissions, respectively. A stressed tree growing in a pot was also studied, and high emissions of inline-formulaα-farnesene and an unidentified SQT were detected together with high emissions of GLVs. Due to the relatively low volatility and the high reactivity of SQTs and OSQTs, downy birch emissions are expected to have strong impacts on atmospheric chemistry, especially on secondary organic aerosol (SOA) production.

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Hellén, Heidi / Praplan, Arnaud P. / Tykkä, Toni / et al: Sesquiterpenes and oxygenated sesquiterpenes dominate the VOC (C5–C20) emissions of downy birches. 2021. Copernicus Publications.

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