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Authors: Dienhart, Dirk
Title: Atmospheric oxidation precursors in the marine boundary layer around the Arabian Peninsula
Online publication date: 8-Mar-2023
Year of first publication: 2023
Language: english
Abstract: The Arabian Gulf and the Suez Canal are famous for their Oil and Gas industry which brought wealth but also intensive ship traffic and severe amounts of air pollution to the region. Precise specification and quantification of these trace gases and pollutants is necessary to derive not only the accompanied regional impact on air quality, but also the global effects with respect to climate change. The Air Quality and Climate Change around the Arabian Basin ship expedition AQABA characterized the air quality around the Arabian Peninsula during summer 2017 covering a variety of meteorological, particulate matter and trace gas measurements in the marine boundary layer (MBL). Formaldehyde (HCHO), hydrogen peroxide (H2O2) and organic hydroperoxides (ROOH) reflect the photochemical activity of an air mass, as they are reservoir species for the primary oxidizing reagent of the troposphere, the hydroxyl radical (OH). Furthermore, HCHO is the most important intermediate during the oxidation of volatile organic compounds (VOCs). Measurements of HCHO, H2O2 and ROOH were performed with three different instruments based on fluorescence spectroscopy during AQABA onboard the Kommandor Iona and are presented and discussed in this doctoral thesis. The first section gives an introduction on the topic of atmospheric chemistry, oxidation precursors and the main gas phase reactions in the MBL. Characterization of the involved instrumentation and analytical methods is presented in chapter two. HCHO measurements are based on the AL4021 (Aero-Laser GmbH) which utilizes the Hantzsch reaction to convert dissolved HCHO into a fluorescent dye. During AQABA, this technique achieved a detection limit of 128 pptv HCHO. H2O2 and ROOH were measured with a similar detection principle based on the dual-enzyme method with detection limits of 13 pptv H2O2 and 8 pptv ROOH (AL2021). Furthermore, hydroperoxide samples were analyzed with high-performance liquid chromatography (HPLC) which verified the abundance of methyl hydroperoxide (MHP), peracetic acid (PAA) and ethyl hydroperoxide (EHP) over the Arabian Gulf. Ship exhaust plumes were excluded from the final HCHO dataset with a filter based on nitrogen oxides (NOx = NO + NO2), carbon monoxide (CO), sulphur dioxide (SO2) and wind direction data, while no significant influences on the mixing ratios of H2O2 and ROOH were identified. Extreme anthropogenic air pollution events were encountered over the Arabian Gulf with a considerably increased mean (± 1σ) of tropospheric ozone (O3) with 74.1 ± 9.2 ppbv, reaching a maximum of 167 ppbv during a pollution event on the first leg. This event also contained the largest amounts of HCHO (12.4 ppbv) and ROOH (2.32 ppbv), but only slightly elevated H2O2 (0.421 ppbv). Highest mean values of HCHO (3.77 ± 2.44 ppbv) and ROOH (0.357 ± 0.283 ppbv) were determined over the Arabian Gulf, while H2O2 reached its highest mean of 0.273 ± 0.113 ppbv over the Suez Canal. Cleaner air masses were detected over the Arabian Sea, the Mediterranean Sea and the southern Red Sea, where we also encountered a dust storm. The in situ observations were used to evaluate results of the general circulation model EMAC (ECHAM/MESSy Atmospheric Chemistry). Both HCHO datasets agree mostly within a factor of 2, while H2O2 was overestimated drastically by at least a factor of 5. Calculation of the photochemical budget of H2O2 revealed the overestimation of OH and hydroperoxyl (HO2) radicals, and a slight underestimation of the dry deposition velocity. The high resolution model WRF-Chem did not improve results of HCHO over the polluted Arabian Gulf and the Suez Canal, but more accurate results were achieved for HCHO, H2O2 and O3 for less polluted MBL conditions like the Arabian Sea, even though both models overestimated HOx. HCHO production rates were calculated based on the photochemical steady-state assumption (PSS) and were used for a regression analysis with OH reactivity and OH radical observations to estimate the HCHO yield factor αeff, which represents the theoretical yield of HCHO per OH radical consumed. Largest αeff were derived for the Arabian Gulf (0.315), followed by the Suez Canal (0.115) and the Gulf of Oman (0.077). According to the speciated OH reactivity, largest αeff were accompanied by elevated reactivity towards alkanes, alkenes and OVOCs, while a decrease of αeff over the southern Red Sea (0.013) was accompanied by reduced NOx. The study thus successfully established a new method to estimate HCHO production efficiencies without the need for a calculation of the HCHO production budget and was published as a measurement report in ACP. Further analysis of the Arabian Gulf with HYSPLIT trajectories showed that air pollution was mainly transported from Iraq, Kuwait and the northwestern coastline of Iran. According to satellite-based flare emission data there are many gas flaring sites located in these areas, which are strong sources of HCHO and other VOCs. Analysis of the cleaner regions during AQABA revealed enhanced HCHO compared to measurements in pristine MBL conditions, with ship emissions as a major cause even though the Arabian Sea is known as a strong source of biogenic emissions. Furthermore, heterogeneous loss of H2O2 and to a lesser extent HCHO and ROOH was detected during a sand storm over the southern Red Sea. The AQABA expedition successfully analyzed the complex and highly variable air quality around the Arabian Peninsula and achieved the first measurements of HCHO, H2O2 and ROOH in the MBL over the Arabian Gulf, where enhanced levels of air pollution are a threat for human health and the environment. Atmospheric chemistry models were evaluated with the measurements and demonstrated missing sources of HCHO while they simultaneously overestimated HOx. Further studies over the Arabian Gulf are necessary to increase the accuracy of anthropogenic emission databases, a local trace gas measurement network in the region would thus be highly valuable.
DDC: 333.7 Natürliche Ressourcen
333.7 Natural resources
500 Naturwissenschaften
500 Natural sciences and mathematics
540 Chemie
540 Chemistry and allied sciences
Institution: Johannes Gutenberg-Universität Mainz
Department: FB 09 Chemie, Pharmazie u. Geowissensch.
Place: Mainz
URN: urn:nbn:de:hebis:77-openscience-a1a61c8e-3e6a-413f-a022-c83ad32574d74
Version: Original work
Publication type: Dissertation
License: In Copyright
Information on rights of use:
Extent: 162 Seiten ; Illustrationen, Diagramme
Appears in collections:JGU-Publikationen

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