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Abstract:
Volatile organic compounds (VOCs) play a crucial role in atmospheric chemistry, contributing to the formation of ozone and secondary organic aerosols. Exchange processes at the atmosphere-soil interface can potentially affect the budget of VOCs in the atmosphere. Knowledge about VOC exchange at the atmosphere-soil interface, however, is still very limited. Thus, the main parts of the PhD project presented in this thesis are aimed at exploring the atmosphere-soil exchange properties and processes of different species of VOC.
The first two parts of this work cover laboratory experiments and field measurements. For both studies, a coated-wall flow tube technique is employed, which allows us to investigate the uptake and heterogeneous reaction kinetics of VOCs at soil surfaces. The lab experiment focuses on formaldehyde (HCHO), which is an important precursor of OH radical and a key intermediate of VOC oxidation reactions but has one or several missing sinks/sources to be identified. The observed uptake and re-emission of HCHO reveals a bi-directional exchange on soil surfaces, suggesting that soil could serve either as a source or a sink depending on ambient conditions and trace gas concentrations. Further kinetic analysis shows that HCHO uptake on soil is a partially reversible process involving both adsorption/desorption and chemical reactions. With co-existence of water molecules, water vapor can exert a competition effect for reactive sites on soil with HCHO molecules and therefore influence the exchange behavior of HCHO. The field measurement, which is performed at an urban background site in Beijing, is intended to investigate exchange properties of VOCs at the atmosphere-soil interface under ambient conditions. The derived uptake coefficients (γ) and corresponding deposition velocities (Vd) or surface resistances (Rc) exhibit different average values and temporal variabilities for different VOCs. Most of the VOCs show a long-term net deposition. However, formic acid is emitted from the soil. The correlation analysis suggests that the emitted formic acid is most probably arising from the heterogeneous oxidation of other deposited VOCs.
The last part is designed to optimize the usage of the coated-wall flow tube technique from a technical point of view. Coated-wall flow tube reactors are frequently used to investigate gas uptake and heterogeneous or multiphase reaction kinetics under laminar flow conditions. Coating surface roughness may potentially distort the laminar flow pattern, induce turbulence and introduce uncertainties in the calculated uptake coefficient based on molecular diffusion assumptions (e.g., Brown/CKD/KPS methods), which hasn't been sufficiently addressed in previous applications. Here, we suggest using a critical height δc to evaluate turbulence effects in the design and analysis of coated-wall flow tube experiments. When a geometric coating thickness δg is larger than δc, the roughness elements of the coating may cause local turbulence and result in overestimation of the real uptake coefficient (γ). We further develop modified CKD/KPS methods (i.e., CKD-LT/KPS-LT) to account for roughness-induced local turbulence effects. By combination of the original methods and their modified versions, the maximum error range can be quantified and finally γ can be constrained. Additionally, the critical height δc can also be adjusted by optimizing flow tube configurations and operation conditions, to ensure not only unaffected laminar flow patterns but also other specific requirements of an individual flow tube experiment.
