The variability in Salix BVOC emissions and possible consequences for managed SRC plantations

Willow (Salix spp.) trees have been used commercially since the 1980s to produce renewable energy. Some benefits with these trees are that they can clean soil from heavy metals, reduce the risk of nutrient leakage and eutrophication. They could be an alternative to reduce carbon dioxide emissions from fossil fuel, but they are known to emit high rates of biogenic volatile organic compounds (BVOCs). Many thousands of different compounds are included in this group and they can be divided into terpenes, alcohols, alkanes and esters, to mention a few. These compounds are important. For instance, BVOCs help plants to attract pollinators. They serve as a protection to different kinds of stress, e.g., herbivores, heat and pollutions. Once they are released into the atmosphere, they will be involved in many chemical processes. Hydroxyl radicals, which are known to clean the air from pollutions, are depleted by BVOCs leading to increased lifetime of methane. Emissions of BVOCs could also act as precursors for aerosol formation, which in turn might lead to changes in cloud properties and radiative forcing. Photochemical smog like ozone (O3) is another result of the reactions in the atmosphere where BVOCs are involved, which impairs regional air quality. The reported range of emission rates from Salix is wide. Details about age, canopy position of leaves and how commercially used Salix varieties differ in their emission potentials are lacking, which lead to large errors if modelled emissions are based on too simple assumptions. Therefore, the aim of the work in this thesis was to investigate how the emissions vary with height, age, variety and during the growing season to get more reliable emission rates that can be used in models to better assess the impacts on the regional air quality. Measurements of aerosol formation were conducted to be able to determine the potential particle production near the Salix site. The outcome from the Salix measurements was then combined with BVOC measurements on spruce to assess how an expansion of Salix plantations could affect the regional air quality if a conversion of the land was shifted into these plantations, e.g., by using more arable and forest land.
The study between 2015 and 2016, and the measurements in 2017, showed that Salix mainly emitted isoprene, which peaked during summer. Emissions of monoterpenes (MTs) showed a decreasing trend through the growing season. The BVOC emission rates differed depending on if the leaves were acclimatized to sunlit or shaded conditions, where the sun-adapted leaves emitted twice as much isoprene as the shaded. There was also a significant difference among the studied varieties, where Wilhelm emitted approx. three times more isoprene than Tora. Age influenced the emission rates. The emissions of non-terpenes from younger trees were substantially higher than from the older trees. Emissions of MTs were higher from younger trees compared to older as well. When comparing isoprene emissions, the emission rates from the older trees were almost five times larger than from the younger. Induced emissions of stress-related compounds like hexanal was observed due to an outbreak of Melampsora in 2015. Together with some other compounds, these emissions increased several times. All these results highlight the complexity of BVOC emissions and existing models need to be improved by including parameters like season, age, microclimate adaption and stress to better predict and estimate BVOC emissions.
No particle enhancement could be observed from the Salix plantation near the site in 2015. The explanation for this result was the lack of precursors, e.g., MTs. However, spruce trees emitted higher rates of MTs and are probably more prone to generate particles compared to Salix trees. The high isoprene emissions from Salix is more likely to produce O3 if sources of anthropogenic NOx are sufficiently close. An expansion of Salix plantations where spruce forest and traditional agriculture are converted into willow fields would lead to considerably more regional isoprene in the atmosphere. Thus, plantations of Salix should be strategically located to avoid prerequisites to form O3, and preferably the variety Tora should be chosen since it emitted the lowest rates of isoprene.