title: Atmospheric measurements including AirCore measurements
Creators: name: Mandrakis, Vasileios
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 530 Physics
subject: 550 Earth sciences &amp; geology

description: This thesis describes the AirCore sampling technique which is able to retain a continuous vertical profile of air that extends from the ground until the mid-stratosphere. The majority of the data that are presented in the following pages were recorded from various sensors that are attached on the AirCore. These data were acquired during two intensive AirCore sampling campaigns in Sodankylä, Finland during June of 2019, and in Traînou, France in 2020.
Chapter 1 describes some of the main techniques that we use today to measure the concentration of greenhouse gases in the atmosphere. Whether it is for ground-based observations, commercial flights or intensive scientific aircraft campaigns, onboard instruments on ships and unmanned vehicles, the information we derive about the vertical distribution of greenhouse gases is limited to a maximum altitude which is much lower compared to the altitude at which an AirCore is able to sample.
In Chapter 2, the stratification of the atmosphere is presented and the most important characteristics of each layer are described. The main focus is given on the two layers that a typical AirCore flight covers, namely the troposphere and the stratosphere. The typical pressure and temperature profiles with changing altitude are shown in this chapter.
Chapter 3 provides important information related to fluid dynamics. It starts with the description of the laminar and the turbulent flow, two regimes that can affect significantly the sampling conditions during an AirCore flight or the measurement of the sample during the analysis. We can decide about the type of flow regime by calculating the dimensionless Reynolds number, which is the next definition provided in this chapter. One of the most important variables, especially during the analysis of the sample, is how fast we measure the total sampled volume of the AirCore, which is given by the volumetric flow rate. Next in this chapter, the Hagen-Poiseuille law for fluid flow along a tube is described, which explains the parabolic velocity profile we get for a gas stream that flows through a cylindrical pipe under laminar conditions. The chapter continues with the description of molecular diffusion in gases, which is described by Fick’s first and second law, as well as the dependence of the diffusion coefficient of gases on temperature and pressure. This chapter ends with the description of the effects of molecular diffusion and Taylor-Aris dispersion of a finite pulse of gas that flows along a cylindrical tube and the ideal gas law.
In Chapter 4 the different phases that comprise an AirCore flight are described, namely the ascent and the descent phase. It explains the basic principle that lies behind the AirCore sampling, which is the passive outflow (evacuation) during the first half of the AirCore flight and the passive inflow (filling-sampling) during the second half. This is a result of the continuously changing pressure with changing altitude and the need of the AirCore for pressure equilibration with the surrounding atmosphere. This chapter ends with the detailed information and characteristics of the AirCore used in this study. In Chapter 5 and Chapter 6 data from the two sampling campaigns in Sodankylä and Traînou are presented. Our group participated in the first campaign with three different AirCores, while an improved version of the UniBern lightweight AirCore was the sampler that was used during the second campaign. The data were recorded from the various sensors that are attached to the AirCore(s), and are mainly related to meteorological data and important flight information such as the velocity and the acceleration of the AirCore during the sampling flights. Additional focus is given to the temperature profiles of the AirCore coil that were measured during the Traînou campaign and the offsets that were observed between the different temperature sensors.
Chapter 7 presents the results of two experiments that were conducted to check the behavior of the different temperature sensors that were attached on the AirCore during the Traînou campaign. The motivation behind these experiments was to investigate the behavior of the insulated sensors in more detail and explain the offsets observed between these sensors during the Traînou campaign. The first experiment was conducted prior to the participation in the Traînou campaign, under stable room conditions and a temperature of approximately 23.5°C, while the second experiment was performed in one of the ice core freezers of the Physics department, under a temperature of approximately -20°C. The temperature profiles that were acquired by the AirCore temperature sensors during the ice core freezer experiment are directly compared to the profiles acquired by a set of temperature sensors that were provided to us by the Applied Physics department.
Chapter 8 provides a detailed description of the laboratory-simulated AirCore flights that were performed before the field campaign at Traînou. Several simulations were conducted with the goal of simulating the ascent and the descent of an AirCore while at the same time logging different variables. In order to achieve this, we used as input the pressure profile that was acquired by the UniBern lightweight AirCore during one of the Sodankylä flights. During the various simulations, the performance of two newly introduced systems was investigated, namely the differential pressure sensor and the CO-spiking system. In addition, this chapter also provides detailed information about slag tests that were performed prior to the AirCore simulation flights and aimed to verify the robustness of the experimental set-up.
Finally, Chapter 9 presents the differential pressure profiles that were acquired from the three sampling flights during the Traînou measurement campaign. Additionally, the velocity and acceleration profiles from the Traînou flights are shown in this chapter. During the simulations, differential pressure profiles that have the same behavior like the ones seen during the real flights, have been successfully simulated under controlled laboratory conditions. The measured profiles show a very important aspect of the AirCore sampling technique, namely that the pressure disequilibrium between the AirCore and the ambient atmospheric pressure. The differential pressure profiles can be converted into differential altitude profiles, which could potentially be an effective way of attributing the correct ”AirCore altitude” compared to the GPS-measured altitude. An empirical linear relationship is found to exist between the maximum descent velocity and the minimum differential pressure that is recorded between the open inlet and the closed outlet of the AirCore. This relationship was also observed during the laboratory simulations.