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Characterization and optimization of a DUAL channel PERCA for the investigation of the chemistry of peroxy radicals in the upper troposphere
| Content Provider | Semantic Scholar |
|---|---|
| Author | Kartal, Deniz Irtem |
| Copyright Year | 2009 |
| Abstract | The research of peroxy radical chemistry is an important topic that provides essential knowledge about photo oxidant formation and night time chemistry. Peroxy radicals play an important role in the formation and depletion reactions of ozone in the troposphere. They also play a key role in the cleaning processes of the atmospheric pollution. At the Institute of Environmental Physics (IUP) of the University of Bremen, laboratory studies are performed to characterize and design a dual channel reactor system (DUALER: DUal channel Airborne peroxy radical chemicaL amplifiER) for the measurements of peroxy radicals on an airborne platform in the upper troposphere. The IUP DUALER system, based on the PERCA (Peroxy Radical Chemical Amplification) technique was deployed on the DLR-Falcon 20 of German Aerospace Centre (DLR) during the measurement campaign AMMA SOP2 (African Monsoon Multidisciplinary Analysis, Special Observation Period 2). The West African Monsoon (WAM) is associated with deep convective transport of air masses to the upper troposphere impacted by anthropogenic and natural emissions. The analysis of the measurement results of the DUALER has the aim of understanding the identification of different air masses with different photochemical activity and developing knowledge of source and sink processes of peroxy radicals, as well as the formation and destruction mechanisms of ozone in the upper levels of the troposphere during the WAM. At the experimental part of this work, the implementation of a pressure chamber for improving the characterization procedure of the IUP DUALER for different pressure levels is presented and the results are compared with the performance of the instrumentation during AMMA. INTRODUCTION Peroxy radicals, HO2 and RO2, where R stands for any organic chain, play an essential role in the chemistry of the troposphere, particularly in the formation and depletion mechanisms of ozone. In addition, they can be used as indicators for the photochemical activity of the air masses. Radical chemistry in the troposphere has been the subject of intensive research and reviews (Clemitshaw et al., 2004; Monks et al., 2005). The quantification of the impact of radicals in a particular environment is a complex issue. Radicals are the intermediates of many chemical reactions. Therefore their impact results from the balance between existing sources and sinks of NOx (NO+NO2), CO, volatile organic compounds (VOC) and O3. Consequently, there are still many unknowns concerning radical formation and effect both in clean and polluted atmospheres. Measurement data for high levels of the troposphere is very scarce; therefore the measurements performed at higher levels of the troposphere are important for understanding tropospheric chemistry. The West African Monsoon (WAM) is believed to be critical for global atmospheric chemistry. The anthropogenic and biogenic sources of the trace gases play an important role in the oxidizing cycles of the troposphere and are summarized in Figure 1. The natural sources depending on the vegetation type, soil moisture and temperature are expected to change with changing climate. The emissions of VOC, which are important precursors of peroxy radicals, depend on the vegetation type. The tropical forest canopy is, for instance, the main source of isoprene (Zimmerman et al., 1988, Guenther et al., 1995). Higher atmospheric carbondioxide concentrations and temperatures are expected to increase isoprene emissions. Lelieveld et al., 2008 have proposed that natural isoprene oxidation recycles OH efficiency in low NOx air through reactions of organic peroxy radicals. In addition, tropical soils are an important source of NO (Yienger and Levy et al., 1995), and forest soils are one of the main sources of atmospheric dinitrogenoxide which is an important greenhouse gas. Regarding the NOx budget, the production of NO by lightening in convective clouds is also an important source of NOx especially at West Africa which is the most electrically active region of the world. Another important issue in the study of atmospheric photochemistry over Africa is the biomass burning which remains as the most important anthropogenic source of trace gases in West Africa. Under ideal conditions when the oxygen supply is enough, combustion of the organic matter produces water vapour and carbon dioxide. As the oxygen supply is never sufficient consequently the combustion is incomplete and pyrolysis of vegetable matter lead to the formation of reduced compounds such as CO, CH4, VOC, NO, NH3, H2S, SOx and aerosols. The deep convection events associated with the monsoon can transport these precursors and their oxidation products to the upper troposphere and lower stratosphere where they can be transported on regional and global scales. (International Science Plan for AMMA May 2005). West Africa is a vast zone where several types of mesoscale convective systems (MCS) develop according to the latitude, surface conditions and topography. MCS is usually defined as a single cumuliform and well vertically developed cloud or a cluster of such clouds, of typical horizontal extent 100x100 km, the mesoscale, between the local and synoptic scale. In particular, the MCS enclosed into synoptic-scale African Easterly Waves during the West African Monsoon are considered to be the origin of about 40% of the Atlantic tropical cyclones and responsible for troposphere-stratosphere exchange (Augustí-Panareda, and Beljaars et al., 2008). The outflow of the boundary of a MCS is a suitable environment for lifting, leading to an effective transport of trace gases, aerosols, and water vapour from the boundary layer into the free atmosphere. West Africa is therefore a suitable environment for investigating the photochemical activity by the measurement of peroxy radicals in air masses impacted by MCS. The knowledge about the chemical composition of these air masses during intense convective episodes is scarce. The ozone formation is expected to be favoured by the vertical transport of hydrocarbons and peroxides as the peroxy radicals are produced by UV photolysis and react rapidly with NO which is also transported vertically and horizontally transported, or produced by lightning. The total yield of formation of ozone depends on UV radiation, potential losses of radicals (aerosols, clouds), and the vertical budget of radical precursors (Cantrell et al., 2003a, c). Figure 1 Overview of major chemical species in the WAM region, their fates and impacts. (International Science Plan for AMMA May 2005). The Institute of Environmental Physics of the University of Bremen (IUP-UB) participated in the international measurement campaign AMMA (African Monsoon Multidisciplinary Analysis) which took place during the wet monsoon season in August 2006 in West Africa. IUP-UB contributed with the measurements of the total sum of peroxy radicals using the PERCA (Peroxy Radical Chemical Amplification) technique on board of the German scientific aircraft DLR-Falcon (DLR: Deutsches Zentrum für Luft und Raumfahrt). PERCA is one of the most frequently used technique for the measurement of the total sum of peroxy radicals, has been gradually improved since it was proposed by Cantrell and Stedman et al., 1982, and there is abundant literature describing new developments (Reiner et al., 1997; Cantrell et al., 1996, 2003a-b; T.J. Green et al., 2003, Mihele and Hastie et al., 1998; Mihele et al., 1999; Reichert et al., 2003), and characterisation for the ambient measurement of peroxy radicals, as well as the deployment in diverse polluted and remote areas (e.g. Monks et al., 1996, Carslaw et al., 1999; Burkert et al., 2001a-b, 2003; M.D. Andrés Hernández et al, 2001; Cantrell et al., 1996a; Volz-Thomas et al., 2003; Zanis et al., 2003; Fleming et al., 2006a-b). Recent developments addresses the speciation of different peroxy radicals, in particular the separate detection of the organic peroxy radicals to HO2 (Edwards et al., 2003; Fuchs et al., 2008). In most of the cases, the measurement system consists of a single reactor and detector. However, for remote areas and airborne measurements, dual systems, comprising two identical reactors and one or two detectors, have recently been developed in order to increase sensitivity and accuracy in the case of rapid changing background concentrations which can interfere in the radical determination (Cantrell et al, 1996b; Green et al., 2003). The main points of the present work are the improvement of the PERCA technique by developing a DUALER instrument (DUal channel Airborne peroxy radical chemicaL amplifiER) for airborne measurements and the analysis of the peroxy radical measurements carried out during the AMMA measurement campaign on board of the research aircraft DLRFalcon. This doctoral thesis reports on the laboratory studies carried out for the characterization and the optimization of the DUALER instrument and on the peroxy radical airborne measurements performed with the same DUALER. The present study is therefore a contribution to the investigation of the PERCA technique applications on airborne platforms. The results obtained during the measurement campaign in West Africa provide important information for further understanding of photochemical processes and photochemical activity of air masses during the WAM. Theoretical background 1 1 Theoretical background Formel-Kapitel 1 Abschnitt 1 The investigation of the peroxy radical chemistry in the upper troposphere is a central objective of the present work. In this context, the chemistry of peroxy radicals in the troposphere and the related aspects of atmospheric physics are briefly introduced in this chapter. More detailed information can be found in consulting books (Seinfeld and Pandis, 1997; Egbert Boeker, 1994; Richard P. Wayne, 2000). In addition the experimental techniques for peroxy radicals are summarized. The last section of this chapter gives brief informati |
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| Alternate Webpage(s) | http://elib.suub.uni-bremen.de/diss/docs/00011703.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
