Zugl.: Hamburg, Univ., Diss.

Aerosols have a significant effect on the global radiation budget through their interactions with radiation and clouds. However, estimates of their effect are the dominant source of uncertainty in current estimates of total anthropogenic effect on climate. A major cause of this uncertainty is the high degree of variability of aerosol properties and processes that affect their lifetime. Prediction of the aerosol effect on climate depends on the ability of three-dimensional numerical models to accurately estimate aerosol properties. However, a limitation of traditional grid-based models is their inability to resolve variability on scales smaller than a grid box. Past research has shown that significant aerosol variability exists on scales smaller than these grid-boxes, which can lead to discrepancies between observations and aerosol models. This thesis uses a synthesis of aerosol observations, global climate model (GCM) data, and a new aerosol modelling technique implemented within a regional-scale model to quantify the important scales of aerosol variability and the extent to which different sub-grid scale processes contribute to discrepancies in aerosol modelling. Analysis of black carbon (BC) plumes from aircraft observations shows that BC plumes represent a large portion of total BC mass and typically exist on scales of 65{ 100 km. Comparison of observed plume scales to those simulated by GCMs at multiple resolutions show that GCMs overestimate the scales of along- ight-track variability by 64% at the highest resolution. Variability is shown to be greater near sources than in remote regions, indicating that models may benefit from higher resolutions in regions of high emissions. Additionally, GCMs at all resolutions show higher variability in the latitudinal direction than the longitudinal direction, suggesting that capturing latitudinal variability may result in greater improvements in aerosol modelling. This work additionally presents a novel technique to allow one to isolate the effect of aerosol variability from other sources of variability within the model. Processes most affected by neglecting aerosol sub-grid variability are gas-phase chemistry and aerosol uptake of water through the aerosol/gas equilibrium reactions. The inherent non-linearities in these processes result in large changes in aerosol parameters when aerosol and gaseous species are artificially mixed over large spatial scales. These changes in aerosol and gas concentrations are exaggerated by convective transport, which transports these altered concentrations to altitudes where their effect is more pronounced. Future aerosol model development should focus on accounting for the effect of sub-grid variability on these processes at global scales in order to improve model predictions of the aerosol effect on climate.

Die Wechselwirkung von Aerosol und Strahlung in der Atmosphäre beeinflusst stark die Energiebilanz der Erde. Durch die großräumige Erfassung der horizontalen und vertikalen Verteilung von Aerosoleigenschaften in der Atmosphäre liefern Fernerkundungstechniken einen wichtigen Beitrag zu unserem Verständnis des Klimasystems. Genaue Beobachtungen durch langfristige operationelle Satellitenmissionen und zuverlässige Referenzmessungen vom Boden aus sind auch für die Ableitung und Verbesserung satelliten- und modellgestützter Aerosoldatensätze unerlässlich. Dies gilt insbesondere über dem Ozean. Mittels Fernerkundungsmethoden werden in dieser Dissertation bestimmte optische Eigenschaften von Aerosol und dessen Strahlungseffekt untersucht. Ein Teil der Datengrundlage hierfür wurde auf fünf Fahrten mit dem Forschungsschiff Polarstern mittels eines multispektralen Schattenbandradiometers erhoben. Anhand dieser Daten werden die aus theoretischen Betrachtungen abgeleitete Unsicherheit der Irradianzmessung von etwa 2 % anhand eines Vergleichs mit Sonnenphotometerbeobachtungen an Land und auf dem Schiff bestätigt. Unter Verwendung Schiffs-gestützter Referenzdaten werden im Rahmen dieser Dissertation mehrere weitere Aerosoldatensätze evaluiert. Für zwei satellitengestützte Datensätze können die erwarteten Fehlergrenzen bestätigt und die vom Aerosoltyp abhängigen Einschränkungen aufgrund von Modellannahmen diskutiert werden. Darüber hinaus werden die optischen Eigenschaften von Aerosol in der CAMS-Reanalyse betrachtet. Dabei findet sich die größte Diskrepanz in der Aerosolabsorption, die von der CAMS-Reanalyse um etwa 30 % überschätzt wird. Schließlich wird der Strahlungseffekt von Aerosol für die Region Deutschland und das Jahr 2015 unter unbewölkten Bedingungen mit zwei komplementären Ansätzen untersucht. Hierbei werden Messungen der solaren Einstrahlung an 25 Stationen des Beobachtungsnetzes des Deutschen Wetterdienstes als Datengrundlage verwendet. Einerseits wird ein Ensemble von empirischen Modellen verwendet, um die direkte Strahlungswirkung von Aerosol am Boden mithilfe einer Fehlerminimierung zu bestimmen. Die zugrundeliegenden Annahmen über Aerosol- und atmosphärische Eigenschaften in diesen Modellen werden kritisch analysiert und diskutiert. Im zweiten Ansatz werden explizite Strahlungstransfersimulationen des Strahlungseffekts unter Verwendung der CAMS-Reanalyse genutzt. Weiterhin wird die Sensitivität der Simulationen auf Unsicherheiten in den Eingangsgrößen untersucht, und damit die resultierende Unsicherheit im Strahlungseffekt abgeschätzt. Nach Korrektur von systematischen Abweichungen in der CAMS-Reanalyse hat Aerosol im Jahre 2015 einen mittleren abkühlenden Strahlungseffekt von -10.6 Wm-2 am Boden in Deutschland. / The interaction of aerosol and radiation in the atmosphere exerts a strong influence on the Earth's energy balance. Remote sensing techniques provide an important contribution to our understanding of the climate system, by observing the horizontal and vertical distribution of aerosol properties in the atmosphere on a large scale. Accurate observations from long-term operational satellite missions and reliable ground-based reference measurements are essential for deriving and improving satellite- and model-based aerosol data sets. This is especially true over the ocean. In this dissertation, certain optical properties of aerosol particles and their radiation effect are investigated using remote sensing methods. Parts of the considered data basis were collected on five cruises with the research vessel Polarstern using a multispectral shadow-band radiometer. This unique data set contributes to the global available reference observations over the ocean by partially filling known gaps. On this database, an algorithm to evaluate shadow-band radiometer observations for the determination of spectral irradiance and optical properties of aerosol has been advanced. The basis algorithm was developed by the author as part of his master's thesis. The uncertainty of the irradiance measurement of about 2 % derived from theoretical considerations is validated by comparison with sun photometer observations on land and on ship. Using ship-borne reference data, several aerosol products are evaluated as part of this dissertation. For two satellite-based datasets, the expected error bounds has been confirmed and the aerosol-type dependent limitations due to model assumptions in the satellite retrievals are discussed. Furthermore, the optical properties of aerosol considered in the CAMS reanalysis are evaluated. The largest discrepancy is found in the aerosol absorption, which is overestimated by the CAMS reanalysis by about 30 %. Finally, the radiative effect of aerosol is investigated for the region of Germany and the year 2015 under cloud-free conditions using two complementary approaches. Here, measurements of solar irradiance at 25 stations of the observation network of the German Weather Service are used as a data basis. In the first approach, an ensemble of empirical models is used to determine the direct radiative effect of aerosols on the ground using error minimization. The underlying assumptions about aerosol and atmospheric properties in these models are critically analysed and discussed. The second approach quantifies the radiative effect by applying explicit radiative transfer simulations using CAMS reanalysis. The uncertainty in the radiative effect is estimated by studying the sensitivity of the simulations to uncertainties in the input variables. After correcting for systematic deviations in the CAMS reanalysis, aerosol has a cooling radiative effect of -10.6 Wm-2 on the ground in Germany in the annual mean of 2015.