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.
Sesquiterpenes and oxygenated sesquiterpenes dominate the emissions of
downy birches
