Aircraft-based in situ measurements of CH4 and CO2 downstream of European and Asian urban centres at local to synoptic scales

Die zwei wichtigsten anthropogenen Treibhausgase (THG) sind Kohlenstoffdioxid (CO2) und Methan (CH4), mit globalen Konzentrationen von zurzeit ~414,5 ppm bzw. ~1897 ppb. Zur Begrenzung der globalen Erwärmung ist ein genaues Verständnis ihrer Quellen und Senken erforderlich. Städtische Gebiete sind relevante THG Emittenten, aber aufgrund ihrer vielen individuellen kleinen Quellen sind die gesamten städtischen CO2 und CH4 Emissionen, deren Aufteilung in einzelne Quellsektoren und deren räumliche Verteilung unzureichend bekannt. In dieser Arbeit wird die Hypothese evaluiert, dass flugzeuggestützte in-situ Messungen geeignet sind, um die Auswirkungen urbaner CO2 und CH4 Emissionen auf die lokale bis synoptische THG Verteilung zu identifizieren und zu quantifizieren. Eine hoch empfindliche laser-gestützten Absorptionstechnik (Cavity Ring-Down Spektroskopie) wurde bei drei wissenschaftlichen Messkampagnen eingesetzt: [UC]2 (Urban Climate Under Change), EMeRGe-Europa und EMeRGe-Asien (Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales). Aufgrund der umfangreichen Charakterisierung des Instruments sowie der Verwendung von Kalibrationsstandards, welche auf die WMO Skala rückführbar sind, beläuft sich die Gesamtunsicherheit der CO2 und CH4 Messungen auf 0,2 ppm bzw. 1,1 ppb (~1 % der atmosphärischen Mischungsverhältnisse). Anhand einer lokalen Fallstudie am 20. Juli während [UC]2 wurde die Berliner THG Fahne vom atmosphärischen Hintergrund abgegrenzt und Emissionsraten für CH4 (5,20 ± 1,70 kg s-1) und CO2 (1,39 ± 0,76 t s-1) mit Hilfe einer Massenbilanz-Methode abgeleitet. Während die extrapolierten jährlichen CO2 Emissionsraten innerhalb der Fehlergrenzen mit aktuellen Emissionskatastern übereinstimmen, liegen sie für CH4 zwei bis siebenmal höher. Der Grund für die Diskrepanz wurde mithilfe von Ergebnissen eines hochauflösenden regionalen Chemie-Klimamodells auf eine Unterschätzung der CH4 Emissionen innerhalb der Stadt, sowie auf fehlende Inventarquellen im Umland zurückgeführt. Für letzteres könnten zahlreiche Mülldeponien und/oder Kläranlagen verantwortlich sein. Diese Arbeit zeigt erfolgreich, dass unabhängige top-down Schätzungen wichtig sind um bottom-up Emissionsraten zu überprüfen. Signaturen von europäischen und asiatischen urbanen CO2 und CH4 Emissionen konnten während EMeRGe im Abwind von London (UK), Barcelona (Spanien) und Manila (Philippinen) detektiert werden. Da die Messentfernung zu den jeweiligen Städten bis zu 250 km betrug, und sich somit die Abluftfahnen bereits mit der Umgebungsluft vermischten, wurde ihre Herkunft mit numerischen Modellsimulationen und zeitgleichen Messungen von kurzlebigen Spurengasen verifiziert. Die Beobachtung von großräumigen CH4 und CO2 Erhöhungen in der freien Troposphäre deuten darauf hin, dass das regionale THG Budget im Frühjahr stark durch den Einfluss vermischter Emissionen von Clustern von Megastädten des chinesischen Festlandes bestimmt wird. Frühere Messkampagnen in Asien (TRACE-P und KORUS-AQ der NASA) weisen ähnliche Muster in der regionalen THG Verteilung auf. Wie erwartet wurden jedoch höhere mittlere Mischungsverhältnisse während EMeRGe-Asien aufgrund der globalen Zunahme atmosphärischer CO2 und CH4 Konzentrationen detektiert. Diese Arbeiten bestätigen, dass in-situ Messungen ebenso erfolgreich eingesetzt werden können, um städtische THG Emissionen auf der meso- bis synoptischen Skala zu untersuchen.The two most important anthropogenic greenhouse gases (GHG) are carbon dioxide (CO2) and methane (CH4) with current global mole fractions of ~414.5 ppm CO2 and ~1897 ppb CH4. In order to develop efficient mitigation strategies limiting global warming, an accurate understanding of their sources and sinks is necessary. Urban areas are recognised as significant GHG emitters but constitute of a large variety of individual smaller sources. Hence, there is a lack of information on the magnitude of total urban CO2 and CH4 emissions, on their division into different source sectors and on their spatial distribution. This thesis evaluates the hypothesis that aircraft-borne in situ measurements are a useful tool to identify and quantify the impact of urban CH4 and CO2 emissions on the local to synoptic scale GHG distribution. A sensitive laser-based absorption technique, cavity ring-down spectroscopy, was deployed within three scientific field campaigns: [UC]2 (Urban Climate Under Change), EMeRGe-Europe and EMeRGe-Asia (Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales). The extensive characterisation and calibration with gas standards traceable to the WMO scales allows for measuring CO2 and CH4 mole fractions with an overall uncertainty of 0.2 ppm and 1.1 ppb, respectively, representing less than 1 % of the current atmospheric background. Based on a local case study on July 20th during [UC]2 it was possible to clearly distinguish Berlin’s urban GHG plume from the atmospheric background and to derive emission rates for CH4 (5.20 ± 1.70 kg s-1) and CO2 (1.39 ± 0.76 t s-1) using a mass balance method. While extrapolated annual CO2 emission rates agree within error bars with current inventories, they are two to seven times higher for CH4. Combining aircraft measurements with results from a high-resolution regional chemistry climate model, it was shown that the discrepancy is due to an underestimation of urban CH4 emissions within the city, as well as due to missing inventorial sources in the surroundings, which may include numerous waste dumps and/or wastewater treatment plants. This study successfully demonstrates that such independent airborne top-down estimates are important to evaluate bottom-up emission inventories. Signatures of European and Asian urban CO2 and CH4 emissions were detected in the regional GHG budget during EMeRGe for London (United Kingdom), Barcelona (Spain) and Manila (the Philippines) even at downwind distances up to 250 km. Due to the large distances from the respective sources, emissions were already mixed with cleaner background air or other pollution plumes. Their identification therefore was verified by numerical model simulations and co-measured short-lived species. The frequent observation of large-scale GHG plumes in the free troposphere downstream of China, indicate that the regional GHG budget during springtime is heavily impacted by the outflow from mixed emissions from megacity clusters from mainland China. A comparison with previous aircraft campaigns conducted in Asia (TRACE-P and KORUS-AQ of NASA) shows that similar patterns were observed in the regional GHG distributions. However, as expected, larger mean mole fractions were detected during EMeRGe-Asia due to the increase in global atmospheric CO2 and CH4 concentrations. These studies show that in situ instruments can also be successfully used to study the impact of urban emissions on the meso- to synoptic scale GHG budget

Validation of XCO2 and XCH4 retrieved from a portable Fourier transform spectrometer with those from in situ profiles from aircraft-borne instruments

Column-averaged dry-air mole fractions of carbon dioxide (XCO2) and methane (XCH4) measured by a solar viewing portable Fourier transform spectrometer (FTS, EM27/SUN) have been characterized and validated by comparison using in situ profile measurements made during the transfer flights of two aircraft campaigns: Korea-United States Air Quality Study (KORUS-AQ) and Effect of Megacities on the Transport and Transformation of Pollutants at Regional and Global Scales (EMeRGe). The aircraft flew over two Total Carbon Column Observing Network (TCCON) sites: Rikubetsu, Japan (43.46∘ N, 143.77∘ E), for the KORUS-AQ campaign and Burgos, Philippines (18.53∘ N, 120.65∘ E), for the EMeRGe campaign. The EM27/SUN was deployed at the corresponding TCCON sites during the overflights. The mole fraction profiles obtained by the aircraft over Rikubetsu differed between the ascending and the descending flights above approximately 8 km for both CO2 and CH4. Because the spatial pattern of tropopause heights based on potential vorticity values from the ERA5 reanalysis shows that the tropopause height over the Rikubetsu site was consistent with the descending profile, we used only the descending profile to compare with the EM27/SUN data. Both the XCO2 and XCH4 derived from the descending profiles over Burgos were lower than those from the ascending profiles. Output from the Weather Research and Forecasting Model indicates that higher CO2 for the ascending profile originated in central Luzon, an industrialized and densely populated region about 400 km south of the Burgos TCCON site. Air masses observed with the EM27/SUN overlap better with those from the descending aircraft profiles than those from the ascending aircraft profiles with respect to their properties such as origin and atmospheric residence times. Consequently, the descending aircraft profiles were used for the comparison with the EM27/SUN data. The EM27/SUN XCO2 and XCH4 data were derived by using the GGG2014 software without applying air-mass-independent correction factors (AICFs). The comparison of the EM27/SUN observations with the aircraft data revealed that, on average, the EM27/SUN XCO2 data were biased low by 1.22 % and the EM27/SUN XCH4 data were biased low by 1.71 %. The resulting AICFs of 0.9878 for XCO2 and 0.9829 for XCH4 were obtained for the EM27/SUN. Applying AICFs being utilized for the TCCON data (0.9898 for XCO2 and 0.9765 for XCH4) to the EM27/SUN data induces an underestimate for XCO2 and an overestimate for XCH4

Validation of XCO2 and XCH4 retrieved from a portable Fourier transform spectrometer with those from in situ profiles from aircraft-borne instruments

Column-averaged dry-air mole fractions of CO2 and CH4 measured by a solar viewing portable Fourier transform spectrometer (EM27/SUN) were validated with in situ profile data obtained during the transfer flights of two aircraft campaigns. Atmospheric dynamical properties based on ERA5 and WRF-Chem were used as criteria for selecting the best aircraft profiles for the validation. The resulting air-mass-independent correction factors for the EM27/SUN data were 0.9878 for CO2 and 0.9829 for CH4