OH reactivity from the emissions of different tree species: investigating the missing reactivity in a boreal forest

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.

Cryptogamic organisms are a substantial source and sink for volatile organic compounds in the Amazon region

“…Average TOHRE of the samples was 1.82*10 −4 ± 1.79*10 −4 m 3 g À1 dry s À2 , and therefore in the same order of magnitude as the measured TOHRE of a boreal pine tree and one order of magnitude lower than of boreal spruce and birch trees 43 . In contrast to the only two other TOHRE studies, the bryophyte emissions did not exhibit any significant unattributed OH reactivity fraction 42,43 . Bryophyte TOHRE was approximately 2/3 due to monoterpene-attributed reactivity emissions and 1/3 to sesquiterpene-attributed reactivity emissions (Fig.…”

“…The relevance of BVOC emissions from bryophytes for atmospheric chemistry can be assessed by measuring the total OH sink of the emitted substances, named the total OH reactivity emission (TOHRE, see Methods) 42,43 . Average TOHRE of the samples was 1.82*10 −4 ± 1.79*10 −4 m 3 g À1 dry s À2 , and therefore in the same order of magnitude as the measured TOHRE of a boreal pine tree and one order of magnitude lower than of boreal spruce and birch trees 43 . In contrast to the only two other TOHRE studies, the bryophyte emissions did not exhibit any significant unattributed OH reactivity fraction 42,43 .…”

Sesquiterpenes and oxygenated sesquiterpenes dominate the VOC (C<sub>5</sub>–C<sub>20</sub>) emissions of downy birches

“…3). High SQT emissions have also been observed in earlier studies of downy birch (Hakola et al, 2001;Räsänen et al, 2017) and mountain birch (Betula pubescens ssp. czerepanovii), which is a subspecies of downy birch (Haapanala et al, 2009).…”

“…A recent study (Praplan et al, 2019) showed that currently known oxidation products are able to explain only a minor fraction (<4.5 %) of the missing reactivity in the air of boreal forest. A large fraction of missing reactivity has been found directly in the emissions of the main boreal tree species (Nölcher et al, 2013;Praplan et al, 2020). To explain the missing reactivity, we need to identify the unknown compounds causing it.…”

Sesquiterpenes and oxygenated sesquiterpenes dominate the emissions of downy birches

“…Missing reactivity has been suggested to originate from unknown emitted compounds or atmospheric oxidation products of VOCs. A recent study (Praplan et al 2019) showed that 15 currently known oxidation products are able to explain only a minor fraction (< 4.5%) of the missing reactivity in the air of boreal forest and large fractions of missing reactivity, which was not explained by isoprene, MTs or SQTs, have been found directly in the emissions of main boreal tree species (Nölcher et al 2013, Praplan et al, 2020. To explain this unknown reactivity, we need to search for new compounds.…”