Abstract. The concentrations of terpenoids (isoprene; monoterpenes, MTs; and sesquiterpenes, SQTs) and oxygenated volatile organic compounds (OVOCs; i.e. aldehydes, alcohols, acetates and volatile organic acids, VOAs) were investigated during 2 years at a boreal forest site in Hyytiälä, Finland, using in situ gas chromatograph mass spectrometers (GC-MSs). Seasonal and diurnal variations of terpenoid and OVOC concentrations as well as their relationship with meteorological factors were studied. Of the VOCs examined, C2–C7 unbranched VOAs showed the highest concentrations, mainly due to their low reactivity. Of the terpenoids, MTs showed the highest concentrations at the site, but seven different highly reactive SQTs were also detected. The monthly and daily mean concentrations of most terpenoids, aldehydes and VOAs were highly dependent on the temperature. The highest exponential correlation with temperature was found for a SQT (β-caryophyllene) in summer. The diurnal variations in the concentrations could be explained by sources, sinks and vertical mixing. The diurnal variations in MT concentrations were strongly affected by vertical mixing. Based on the temperature correlations and mixing layer height (MLH), simple proxies were developed for estimating the MT and SQT concentrations. To estimate the importance of different compound groups and compounds in local atmospheric chemistry, reactivity with main oxidants (hydroxyl radical, OH; nitrate radical, NO3; and ozone, O3) and production rates of oxidation products (OxPRs) were calculated. The MTs dominated OH and NO3 radical chemistry, but the SQTs greatly impacted O3 chemistry, even though the concentrations of SQT were 30 times lower than the MT concentrations. SQTs were also the most important for the production of oxidation products. Since the SQTs show high secondary organic aerosol (SOA) yields, the results clearly indicate the importance of SQTs for local SOA production.

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Abstract. 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 gdw-1 h−1, respectively) compared to in the main (40 and 130 ng gdw-1 h−1, respectively) and late (14 and 46 ng gdw-1 h−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. β-Caryophyllene and β-farnesene were the main SQTs emitted. The main emitted OSQTs were tentatively identified as 14-hydroxy-β-caryophyllene acetate (M=262 g mol−1) and 6-hydroxy-β-caryophyllene (M=220 g mol−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 α-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.

Abstract. We have studied biogenic volatile organic compound (BVOC) emissions and their ambient concentrations at a sub-Arctic wetland (Lompolojänkkä, Finland), which is an open, nutrient-rich sedge fen and a part of the Pallas-Sodankylä Global Atmosphere Watch (GAW) station. Measurements were conducted during the growing season in 2018 using an in situ thermal-desorption–gas-chromatograph–mass-spectrometer (TD-GC-MS). Earlier studies have shown that isoprene is emitted from boreal wetlands, and it also turned out to be the most abundant compound in the current study. Monoterpene (MT) emissions were generally less than 10 % of the isoprene emissions (mean isoprene emission over the growing season, 44 µg m−2 h−1), but sesquiterpene (SQT) emissions were higher than MT emissions all the time. The main MTs emitted were α-pinene, 1,8-cineol, myrcene, limonene and 3Δ-carene. Of SQTs cadinene, β-cadinene and α-farnesene had the major contribution. During early growing season the SQT∕MT emission rate ratio was ∼10, but it became smaller as summer proceeded, being only ∼3 in July. Isoprene, MT and SQT emissions were exponentially dependent on temperature (correlation coefficients (R2) 0.75, 0.66 and 0.52, respectively). Isoprene emission rates were also found to be exponentially correlated with the gross primary production of CO2 (R2=0.85 in July). Even with the higher emissions from the wetland, ambient air concentrations of isoprene were on average > 100, > 10 and > 6 times lower than MT concentrations in May, June and July, respectively. This indicates that wetland was not the only source affecting atmospheric concentrations at the site, but surrounding coniferous forests, which are high MT emitters, contribute as well. Daily mean MT concentrations had high negative exponential correlation (R2=0.96) with daily mean ozone concentrations indicating that vegetation emissions can be a significant chemical sink of ozone in this sub-Arctic area.

Abstract. Total hydroxyl radical (OH) reactivity measurements were conducted at the second Station for Measuring Ecosystem–Atmosphere Relations (SMEAR II), a boreal forest site located in Hyytiälä, Finland, from April to July 2016. The measured values were compared with OH reactivity calculated from a combination of data from the routine trace gas measurements (station mast) as well as online and offline analysis with a gas chromatographer coupled to a mass spectrometer (GC–MS) and offline liquid chromatography. Up to 104 compounds, mostly volatile organic compounds (VOCs) and oxidized VOCs, but also inorganic compounds, were included in the analysis, even though the data availability for each compound varied with time. The monthly averaged experimental total OH reactivity was found to be higher in April and May (ca. 20 s−1) than in June and July (7.6 and 15.4 s−1, respectively). The measured values varied much more in spring with high reactivity peaks in late afternoon, with values higher than in the summer, in particular when the soil was thawing. Total OH reactivity values generally followed the pattern of mixing ratios due to change of the boundary layer height. The missing reactivity fraction (defined as the difference between measured and calculated OH reactivity) was found to be high. Several reasons that can explain the missing reactivity are discussed in detail such as (1) missing measurements due to technical issues, (2) not measuring oxidation compounds of detected biogenic VOCs, and (3) missing important reactive compounds or classes of compounds with the available measurements. In order to test the second hypothesis, a one-dimensional chemical transport model (SOSAA) has been used to estimate the amount of unmeasured oxidation products and their expected contribution to the reactivity for three different short periods in April, May, and July. However, only a small fraction (<4.5 %) of the missing reactivity can be explained by modelled secondary compounds (mostly oxidized VOCs). These findings indicate that compounds measured but not included in the model as well as unmeasured primary emissions contribute the missing reactivity. In the future, non-hydrocarbon compounds from sources other than vegetation (e.g. soil) should be included in OH reactivity studies.