Development of a twin-cuvette measurement system for trace gas flux measurements and its application to various gas species under laboratory conditions
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Abstract
Ozone (O3), Peroxyacetyl nitrate (PAN) and trimethylamine (TMA) are reactive gases in the atmosphere and play an important role in atmospheric chemistry as well as for particle formation. They have an impact on the air quality, human health and ecosystems. Therefore, understanding their sources and sinks is essential for the determination of global atmospheric budgets and for the improvement of the environmental quality.
It is well-known that O3 and PAN are taken up by terrestrial vegetation. But the mechanism determining their non-stomatal deposition process is not totally understood. Especially, the influence of humidity on the non-stomatal deposition pathway is mainly deduced from field measurements. But for the understanding of their underlying mechanisms more systematical investigations of the processes are required. The role of TMA in the atmosphere is a special topic, which becomes more important in view of its impact on new particle formation (NPF). The contribution of its emission from terrestrial vegetation is poorly understood and needs more investigations to estimate the contribution to the chemical and physical processes.
Within this thesis a twin-cuvette measurement system to analyze the exchange fluxes of reactive gases as O3 and PAN with plants under controlled environmental conditions has been developed.
In the first case study, the influence of liquid surface films on the O3 and PAN deposition was investigated. We observed a clear relationship between the O3 deposition on plants (Quercus ilex) and the relative humidity (RH). In the presence of light, the opening of leaf stomata were responsible for the increasing O3 deposition, while during the absence of light the liquid surface films were the reason for O3 deposition as shown by closing stomata by the addition of abscisic acid (ABA). Therefore, the liquid surface film significantly contributes to the total deposition of O3. Additionally, we demonstrated that the formation of the liquid surface film on leaves as well as the non-stomatal O3 deposition are depending on the chemical composition of the particles deposited on the leaf cuticles as has been previously proposed by Fuentes and Gillespie (1992). In the case of PAN, a similar correlation between the deposition flux and RH was found in the presence of light, indicating the dominance of stomatal uptake. In the dark period, no relationship with RH was found, which may be related to the lower rate of aqueous partitioning for PAN than O3. A contribution of non-stomatal PAN deposition was found to be ~ 20 %, which was not affected by liquid surface films. Therefore, the ratio of the O3 and PAN deposition velocities is not constant when relative humidity changes, as assumed in many models.
In the second case study, we investigated the emission of TMA by Chenopodium vulvaria. The analysis of the leaf anatomy showed that the emission of TMA can be related to the epidermal gland vesicles, which were well distributed on both sides of the plant leaf. Additionally, we could show that mechanical stimuli such as slight air streams were able to destroy these gland vesicles, which led to a high emission of TMA up to 8 nmol m-2 s-1 or a local mixing ratio of 100 ppb. Also, environmental factors as temperature and relative humidity seem to be a driving force for the emission of TMA from that plant, which is in agreement with the suggestion from Cromwell (1949). Additionally, a screening of different flowering plants showed that plant emissions of TMA might not be restricted to specialized plants. Furthermore, their potential impact on the atmospheric new particle formation could be demonstrated qualitatively by the ternary nucleation systems with sulfuric acid – TMA – H2O and formic acid – TMA – H2O. Sulfuric acid was used to test if enhanced formation can be reproduced according to recent publications (e.g. Almeida et al., 2013). A new approach was to study the nucleation with organic acids. For formic acid, the particle concentration was about 3 orders of magnitude less than for sulfuric acid, whereas for the other tested organic acids (acetic acid and tartaric acid) no particle formation was observed.