Design and characterization of a semi-open dynamic chamber for measuring biogenic volatile organic compound (BVOC) emissions from plants

With the accumulation of data about biogenic volatile organic compound (BVOC) emissions from plants based on branch-scale enclosure measurements worldwide, it is vital to assure that measurements are conducted using well-characterized dynamic chambers with good transfer efficiencies and less disturbance on natural growing microenvironments. In this study, a self-made cylindrical semi-open dynamic chamber with a Teflon-coated inner surface was characterized both in the lab with standard BVOC mixtures and in the field with typical broadleaf and coniferous trees. The lab simulation with a constant flow of standard mixtures and online monitoring of BVOCs by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) revealed lower real-time mixing ratios and shorter equilibrium times than theoretically predicted due to wall loss in the chamber and that larger flow rates (shorter residence times) can reduce the adsorptive loss and improve the transfer efficiencies. However, even when flow rates were raised to secure residence times of less than 1 min, transfer efficiencies were still below 70 % for heavier BVOCs like α-pinene and β-caryophyllene. Relative humidity (RH) impacted the adsorptive loss of BVOCs less significantly when compared to flow rates, with compound-specific patterns related to the influence of RH on their adsorption behaviour. When the chamber was applied in the field to a branch of a Mangifera indica tree, the ambient–enclosure temperature differences decreased from 4.5±0.3 to 1.0±0.2 ∘C and the RH differences decreased from 9.8 ± 0.5 % to 1.2±0.1 % as flow rates increased from 3 L min−1 (residence time ∼4.5 min) to 15 L min−1 (residence time ∼0.9 min). At a medium flow rate of 9 L min−1 (residence time ∼1.5 min), field tests with the dynamic chamber for Mangifera indica and Pinus massoniana branches revealed enclosure temperature increase within +2 ∘C and CO2 depletion within −50 ppm when compared to their ambient counterparts. The results suggested that substantially higher air circulating rates would benefit by reducing equilibrium time, adsorptive loss, and the ambient–enclosure temperature and RH differences. However, even under higher air circulating rates and with inert Teflon-coated inner surfaces, the transfer efficiencies for monoterpene and sesquiterpene species are not so satisfactory, implying that emission factors for these species might be underestimated if they are obtained by dynamic chambers without certified transfer efficiencies and that further efforts are needed for field measurements to improve accuracies and narrow the uncertainties of the emission factors.

Leaf-Scale Study of Biogenic Volatile Organic Compound Emissions from Willow (Salix spp.) Short Rotation Coppices Covering Two Growing Seasons

In Europe, willow (Salix spp.) trees have been used commercially since the 1980s at a large scale to produce renewable energy. While reducing fossil fuel needs, growing short rotation coppices (SRCs), such as poplar or willow, may have a high impact on local air quality as these species are known to produce high amounts of isoprene, which can lead to the production of tropospheric ozone (O3). Here, we present a long-term leaf-scale study of biogenic volatile organic compound (BVOC) emissions from a Swedish managed willow site with the aim of providing information on the seasonal variability in BVOC emissions during two growing seasons, 2015–2016. Total BVOC emissions during these two seasons were dominated by isoprene (>96% by mass) and the monoterpene (MT) ocimene. The average standardized (STD, temperature of 30 °C and photosynthetically active radiation of 1000 µmol m−2 s−1) emission rate for isoprene was 45.2 (±42.9, standard deviation (SD)) μg gdw−1 h−1. Isoprene varied through the season, mainly depending on the prevailing temperature and light, where the measured emissions peaked in July 2015 and August 2016. The average STD emission for MTs was 0.301 (±0.201) μg gdw−1 h−1 and the MT emissions decreased from spring to autumn. The average STD emission for sesquiterpenes (SQTs) was 0.103 (±0.249) μg gdw−1 h−1, where caryophyllene was the most abundant SQT. The measured emissions of SQTs peaked in August both in 2015 and 2016. Non-terpenoid compounds were grouped as other VOCs (0.751 ± 0.159 μg gdw−1 h−1), containing alkanes, aldehydes, ketones, and other compounds. Emissions from all the BVOC groups decreased towards the end of the growing season. The more sun-adapted leaves in the upper part of the plantation canopy emitted higher rates of isoprene, MTs, and SQTs compared with more shade-adapted leaves in the lower canopy. On the other hand, emissions of other VOCs were lower from the upper part of the canopy compared with the lower part. Light response curves showed that ocimene and α-farnesene increased with light but only for the sun-adapted leaves, since the shade-adapted leaves did not emit ocimene and α-farnesene. An infestation with Melampsora spp. likely induced high emissions of, e.g., hexanal and nonanal in August 2015. The results from this study imply that upscaling BVOC emissions with model approaches should account for seasonality and also include the canopy position of leaves as a parameter to allow for better estimates for the regional and global budgets of ecosystem emissions.

Ozone Reactivity Measurement of Biogenic Volatile Organic Compound Emissions

Previous research on atmospheric chemistry in the forest environment has shown that the total reactivity by biogenic volatile organic compound (BVOC) emission is not well considered in for-est chemistry models. One possible explanation for this discrepancy is the unawareness and ne-glect of reactive biogenic emission that have eluded common monitoring methods. This ques-tion motivated the development of a total ozone reactivity monitor (TORM) for the direct de-termination of the reactivity of foliage emissions. Emissions samples drawn from a vegetation branch enclosure experiment are mixed with a known and controlled amount of ozone (e.g. re-sulting in 100 ppb of ozone) and directed through a temperature-controlled glass flow reactor to allow reactive biogenic emissions to react with ozone during the approximately 2-minute residence time in the reactor. The ozone reactivity is determined from the difference in the ozone mole fraction before and after the reaction vessel. An inherent challenge of the experi-ment is the influence of changing water vapor in the sample air on the ozone signal. A com-mercial UV absorption ozone monitor was modified to directly determine the ozone differential with one instrument and sample air was drawn through Nafion dryer membrane tubing. These two modifications significantly reduced errors associated with the determination of the reacted ozone compared to determining the difference from two individual measurements and errors from interferences from water vapor, resulting in a much improved and sensitive determina-tion of the ozone reactivity. This paper provides a detailed description of the measurement de-sign, the instrument apparatus, and its characterization. Examples and results from field de-ployments demonstrate the applicability and usefulness of the TORM.

Measurement report: Biogenic volatile organic compound emission profiles of rapeseed leaf litter and its secondary organic aerosol formation potential

We analysed the biogenic volatile organic compound (BVOC) emissions from rapeseed leaf litter and their potential to create secondary organic aerosols (SOAs) under three different conditions, i.e., (i) in the presence of UV light irradiation, (ii) in the presence of ozone, and (iii) with both ozone and UV light. These experiments were performed in a controlled atmospheric simulation chamber containing leaf litter samples, where BVOC and aerosol number concentrations were measured for 6 d. Our results show that BVOC emission profiles were affected by UV light irradiation which increased the summed BVOC emissions compared to the experiment with solely O3. Furthermore, the diversity of emitted VOCs from the rapeseed litter also increased in the presence of UV light irradiation. SOA formation was observed when leaf litter was exposed to both UV light and O3, indicating a potential contribution to particle formation or growth at local scales. To our knowledge, this study investigates, for the first time, the effect of UV irradiation and O3 exposure on both VOC emissions and SOA formation for leaf litter samples. A detailed discussion about the processes behind the biological production of the most important VOC is proposed.