Künstler, Christopher
(2020).
Measurements of Atmospheric OH and HO2 Radicals by Laser-Induced Fluorescence on the HALO Aircraft during the OMO-ASIA 2015 Campaign.
PhD thesis, Universität zu Köln.
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Abstract
The goal of this work was to investigate the chemistry of atmospheric OH and HO2 radicals in the upper troposphere during the Asian summer monsoon period 2015 within the Oxidation Mechanism Observation (OMO) campaign. Concentrations of OH and HO2 were measured by a laser-induced fluorescence instrument (AirLIF) on the German research aircraft HALO between the Mediterranean Sea and the Maldives in the Indian Ocean. The measured data are compared to theoretical model predictions in order to test the understanding of atmospheric oxidation processes. For this purpose the precedingly developed AirLIF instrument at Forschungszentrum Jülich was thoroughly characterized in the laboratory and different calibration concepts applied and compared. The radical measurements during OMO were then evaluated and a zero-dimensional chemical box-model calculation for the expected OH and HO2 radical concentrations was tested against the measurement results. For the radical measurements using the AirLIF instrument on HALO, the ambient air is first sampled and decelerated by a factor of 10 inside a shrouded inlet (designed and built at Forschungszentrum Jülich). The air is then expanded into a measurement cell at low pressure inside the aircraft, where OH is detected by laser excited fluorescence. The OH and HO2 channel of AirLIF needed to be characterized for the flight conditions during OMO. Different calibration concepts have been applied and combined to determine the OH and HO2 detection sensitivities as a function of flight altitude, ambient pressure and temperature. These include the well established ground-based calibrations between flights to track the absolute sensitivities. The relative dependence with altitude was measured in the laboratory using a newly designed photochemical radical source which allows calibration at reduced pressure to simulate ambient air pressure at flight conditions. For the OH-channel - as an additional option - an in-flight calibration unit inside the shrouded inlet was used. It is however limited to below 10 km, because the radical production by the artificial photolysis of ambient water vapour becomes too small. To simulate the in-flight conditions, other research groups have confided in using different nozzle sizes to change the mass-flow through the system instead of varying ambient pressure. As part of a consistency check, both methods have been compared in detail and it is confirmed that they essentially agree. However, discontinuities in the pressure dependence of the OH calibration curve are presumably related to a change in conditions of the gas expansion and are thereby unique to a specific nozzle. The correct detection of this jump in sensitivity is therefore limited to the newly developed radical source. During the laboratory characterization of the HO2 channel a fluid dynamical effect on the HO2 nozzle was discovered, which is due to the lacking shrouded inlet and led to an overestimated HO2 inlet pressure, originally assumed to be static ambient pressure. It was possible to correct this by calculating the true mass flow through the nozzle using the computational fluid dynamics (CFD) software ANSYS Fluent. The OMO campaign took place from 21 July until 27 August 2015 and was divided in three phases. Till 01 August 2015 HALO was stationed on the airport of Paphos (Cyprus) and mainly flew over Cyprus and the Mediterranean Sea. During the second phase HALO was stationed on Gan (Maldives) aiming for the flight targets Bahrain and Sri Lanka, west and east of India respectively. From 10 August till the end of the campaign, HALO was again stationed on Cyprus and covered the Arabian Peninsula, Egypt and Greece as primary flight targets. At the end of the campaign for two flights Mount Etna was visited. In total OMO Asia comprised 17 flights up to 15 km of which AirLIF measured 2/3 of the time. Other institutes involved in OMO were the Karlsruhe Institute of Technology (KIT), the German Aerospace Center (DLR), the Max-Planck Institute for Chemistry (MPIC Mainz) and the universities Bremen, Wuppertal, Heidelberg and Leipzig. The MPIC Mainz provided a second LIF instrument measuring OH and HO2 radicals contemporaneously for the first time. Both, AirLIF and HORUS OH and HO2 in-flight measurements are intercompared flight-wise showing a general good agreement of their calibrations. The vertical profile of OH and HO2 up to 15 km is discussed in particular with respect to important atmospheric controlling parameters such as CO and NO. The HO2/OH ratio is then analyzed by a simple analytic approach. It is found that the latter is well explained above 7 km, while below a gap of a factor up to 3 remains. The altitude profiles and oxidation processes are further studied by using a zero-dimensional chemical box-model which is constrained by parameters measured by other instruments during the OMO campaign. Good agreement of OH and HO2 is found between 7 km and 11.5 km, while below 7 km OH is overestimated by a factor of 2.5 and HO2 is predicted correctly within the combined model-measurement uncertainty. This result is consistent with the underestimation of the HO2/OH ratio by up to a factor of 3 which is in agreement by the analytical model. Above 11.5 km both, OH and HO2, are overestimated by a factor up to 2.5. In the HO2/OH ratio however, this overestimation cancels out which indicates that there is either a missing HOx termination reaction or an overestimated HOx primary source in the model. The discrepancies observed in the upper and lower troposphere are finally addressed by sensitivity studies. In the lower troposphere these are most likely due to missing VOC reactivity, which primarily acts as an OH sink. The addition of a small amount of OH reactivity (0.1 s-1) due to unmeasured VOCs during the OMO Asia campaign, closed the gap for OH, while HO2 stayed in agreement. Only below 2 km a discrepancy of the HO2/OH ratio by a factor up to 2.5 remained. In the upper troposphere there are indications, that formaldehyde from the EMAC MPIC model is overestimated, which results in a contemporaneous increase in OH and HO2. This work, in particular the correction necessary to the HO2 channel, hints to further technical improvements for prospective LIF-aircraft applications.
| Item Type: | Thesis (PhD thesis) | ||||||||
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| URN: | urn:nbn:de:hbz:38-117020 | ||||||||
| Series Name: | Schriften des Forschungszentrums Jülich . Energie & Umwelt | ||||||||
| Volume: | 495 | ||||||||
| Date: | 2020 | ||||||||
| ISSN: | 1866-1793 | ||||||||
| Language: | English | ||||||||
| Faculty: | Faculty of Mathematics and Natural Sciences | ||||||||
| Divisions: | Außeruniversitäre Forschungseinrichtungen > Forschungszentrum Jülich | ||||||||
| Subjects: | Generalities, Science Natural sciences and mathematics Physics Chemistry and allied sciences |
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| Date of oral exam: | 18 May 2020 | ||||||||
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| Refereed: | Yes | ||||||||
| URI: | http://kups.ub.uni-koeln.de/id/eprint/11702 |
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