Abstract
Abstract. In forested area, a large fraction of total hydroxyl radical (OH) reactivity remains unaccounted for. Very few studies have looked at the variations in total OH reactivity from biogenic emissions. In the present study, we investigate the total OH reactivity from three common boreal tree species (Scots pine, Norway spruce, and downy birch) by comparing it with the calculated reactivity from the chemically identified emissions. Total OH reactivity was measured using the comparative reactivity method (CRM), and the chemical composition of the emissions was quantified with two gas chromatographs coupled with mass spectrometers (GC–MSs). Dynamic branch enclosures were used, and emissions from one branch of a tree at the time were measured by periodically rotating between them. Results show that birch had the highest values of total OH reactivity of the emissions (TOHRE), while pine had the lowest. The main drivers for the known reactivity of pine and spruce were monoterpenes and sesquiterpenes. Birch emissions were dominated by sesquiterpenes, but monoterpenes and green leaf volatiles (GLVs) were present as well. However, calculated reactivity values remained low, leading to the highest missing fraction of reactivity (>96 %), while pine and spruce had similar missing reactivity fractions between 56 % and 82 % (higher in the spring and decreasing as the summer proceeded). The high average values were driven by low-reactivity periods, and the fraction of missing reactivity got smaller for pine and spruce when the TOHRE values increased. Important exceptions were identified for periods when the emission profiles changed from terpenes to GLVs, a family of compounds containing a backbone of six carbon atoms with various functionalities (e.g. alcohols, aldehydes, esters) that indicate that the plant is suffering from stress. Then, very high TOHRE values were measured, and the missing fraction remained high. This study found a different trend in the missing OHRE fraction of the Norway spruce from spring to autumn compared to one previous study (Nölscher et al., 2013), which indicates that additional studies are required to fully understand the complexity of biogenic reactive emissions. Future studies of boreal trees in situ should be conducted to confirm the findings presented.
Highlights
The boreal forest is the largest continuous terrestrial biome and represents a third of forested areas (Keenan et al, 2015)
A few compounds per class of biogenic volatile organic compounds (VOCs) were identified as the main drivers of the reactivity, which will be discussed in the following subsections for each tree individually
For total OH reactivity of emissions (TOHRE), the correction for deviation from pseudo firstorder kinetics applied to comparative reactivity method (CRM) data is based on calibration with α-pinene as a surrogate for biogenic emissions, but monoterpenes do not always represent the largest fraction of the emissions, which result in some uncertainty in TOHRE
Summary
The boreal forest is the largest continuous terrestrial biome and represents a third of forested areas (Keenan et al, 2015). It is a large source of volatile organic compounds (VOCs), such as isoprene (C5H8), monoterpenes (C10H16), and sesquiterpenes (C15H24), as well as some oxidised compounds such as methanol, acetaldehyde, and acetone Lindfors and Laurila, 2000; Rinne et al, 2009) These compounds are emitted by vegetation and are referred to as biogenic VOCs (BVOCs). The oxidation of VOCs in the atmosphere can lead to the formation of secondary aerosol formation and may play a role in photochemical air pollution by affecting levels of oxidants and pollutants.
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