Aerosol – remote sensing, characterization and aerosol-radiation interaction

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

Evaluation of satellite-based aerosol datasets and the CAMS reanalysis over the ocean utilizing shipborne reference observations

Reliable reference measurements over the ocean are essential for the evaluation and improvement of satelliteand model-based aerosol datasets. Within the framework of the Maritime Aerosol Network, shipborne reference datasets have been collected over the Atlantic Ocean since 2004 with Microtops Sun photometers. These were recently complemented by measurements with the multi-spectral GUVis- 3511 shadowband radiometer during five cruises with the research vessel Polarstern. The aerosol optical depth (AOD) uncertainty estimate of both shipborne instruments of ±0:02 can be confirmed if the GUVis instrument is cross calibrated to the Microtops instrument to account for differences in calibration, and if an empirical correction to account for the broad shadowband as well as the effects of forward scattering is introduced. Based on these two datasets, a comprehensive evaluation of aerosol products from the Moderate Resolution Imaging Spectroradiometer (MODIS) flown on NASA's Earth Observing System satellites, the Spinning Enhanced Visible and Infrared Imager (SEVIRI) aboard the geostationary Meteosat satellite, and the Copernicus Atmosphere Monitoring Service reanalysis (CAMS RA) is presented. For this purpose, focus is given to the accuracy of the AOD at 630 nm in combination with the Ångström exponent (AE), discussed in the context of the ambient aerosol type. In general, the evaluation of MODIS AOD from the official level-2 aerosol products of C6.1 against the Microtops AOD product confirms that 76% of data points fall into the expected error limits given by previous validation studies. The SEVIRI-based AOD product exhibits a 25% larger scatter than the MODIS AOD products at the instrument's native spectral channels. Further, the comparison of CAMS RA and MODIS AOD versus the shipborne reference shows similar performance for both datasets, with some differences arising from the assimilation and model assumptions. When considering aerosol conditions, an overestimation of AE is found for scenes dominated by desert dust for MODIS and SEVIRI products versus the shipborne reference dataset. As the composition of the mixture of aerosol in satellite products is constrained by model assumptions, this highlights the importance of considering the aerosol type in evaluation studies for identifying problematic aspects. © Author(s) 2020

Algorithms and uncertainties for the determination of multispectral irradiance components and aerosol optical depth from a shipborne rotating shadowband radiometer

The 19-channel rotating shadowband radiometer GUVis-3511 built by Biospherical Instruments provides automated shipborne measurements of the direct, diffuse and global spectral irradiance components without a requirement for platform stabilization. Several direct sun products, including spectral direct beam transmittance, aerosol optical depth, Ångström exponent and precipitable water, can be derived from these observations. The individual steps of the data analysis are described, and the different sources of uncertainty are discussed. The total uncertainty of the observed direct beam transmittances is estimated to be about 4% for most channels within a 95% confidence interval for shipborne operation. The calibration is identified as the dominating contribution to the total uncertainty. A comparison of direct beam transmittance with those obtained from a Cimel sunphotometer at a land site and a manually operated Microtops II sunphotometer on a ship is presented. Measurements deviate by less than 3 and 4% on land and on ship, respectively, for most channels and in agreement with our previous uncertainty estimate. These numbers demonstrate that the instrument is well suited for shipborne operation, and the applied methods for motion correction work accurately. Based on spectral direct beam transmittance, aerosol optical depth can be retrieved with an uncertainty of 0.02 for all channels within a 95% confidence interval. The different methods to account for Rayleigh scattering and gas absorption in our scheme and in the Aerosol Robotic Network processing for Cimel sunphotometers lead to minor deviations. Relying on the cross calibration of the 940 nm water vapor channel with the Cimel sunphotometer, the column amount of precipitable water can be estimated with an uncertainty of ±0.034 cm

Irradiance and cloud optical properties from solar photovoltaic systems

Solar photovoltaic power output is modulated by atmospheric aerosols and clouds and thus contains valuable information on the optical properties of the atmosphere. As a ground-based data source with high spatiotemporal resolution it has great potential to complement other ground-based solar irradiance measurements as well as those of weather models and satellites, thus leading to an improved characterisation of global horizontal irradiance. In this work several algorithms are presented that can retrieve global tilted and horizontal irradiance and atmospheric optical properties from solar photovoltaic data and/or pyranometer measurements. Specifically, the aerosol (cloud) optical depth is inferred during clear sky (completely overcast) conditions. The method is tested on data from two measurement campaigns that took place in Allgäu, Germany in autumn 2018 and summer 2019, and the results are compared with local pyranometer measurements as well as satellite and weather model data. Using power data measured at 1 Hz and averaged to 1 minute resolution, the hourly global horizontal irradiance is extracted with a mean bias error compared to concurrent pyranometer measurements of 11.45 W m−2, averaged over the two campaigns, whereas for the retrieval using coarser 15 minute power data the mean bias error is 16.39 W m−2. During completely overcast periods the cloud optical depth is extracted from photovoltaic power using a lookup table method based on a one-dimensional radiative transfer simulation, and the results are compared to both satellite retrievals as well as data from the COSMO weather model. Potential applications of this approach for extracting cloud optical properties are discussed, as well as certain limitations, such as the representation of 3D radiative effects that occur under broken cloud conditions. In principle this method could provide an unprecedented amount of ground-based data on both irradiance and optical properties of the atmosphere, as long as the required photovoltaic power data are available and are properly pre-screened to remove unwanted artefacts in the signal. Possible solutions to this problem are discussed in the context of future work

Effects of Aerosols and Clouds on the Levels of Surface Solar Radiation and Solar Energy in Cyprus

Cyprus plans to drastically increase the share of renewable energy sources from 13.9% in 2020 to 22.9% in 2030. Solar energy can play a key role in the effort to fulfil this goal. The potential for production of solar energy over the island is much higher than most of European territory because of the low latitude of the island and the nearly cloudless summers. In this study, high quality and fine resolution satellite retrievals of aerosols and dust, from the newly developed MIDAS climatology, and information for clouds from CM SAF are used in order to quantify the effects of aerosols, dust, and clouds on the levels of surface solar radiation for 2004–2017 and the corresponding financial loss for different types of installations for the production of solar energy. Surface solar radiation climatology has also been developed based on the above information. Ground-based measurements were also incorporated to study the contribution of different species to the aerosol mixture and the effects of day-to-day variability of aerosols on SSR. Aerosols attenuate 5–10% of the annual global horizontal irradiation and 15–35% of the annual direct normal irradiation, while clouds attenuate 25–30% and 35–50% respectively. Dust is responsible for 30–50% of the overall attenuation by aerosols and is the main regulator of the variability of total aerosol. All-sky annual global horizontal irradiation increased significantly in the period of study by 2%, which was mainly attributed to changes in cloudiness

Shipborne rotating shadowband radiometer data of spectral irradiance components and aerosol optical depth during Polarstern cruises PS83, PS95, PS98, PS102 and PS113

The dataset consists of continuous measurements of global, direct and diffuse spectral irradiance and aerosol optical thickness during Polarstern cruises PS83, PS95, PS98, PS102 and PS113 with the GUVis-3511 radiometer and the BioSHADE accessory build by Biospherical Instruments Inc.. To derive this dataset, the raw GUVis-3511 data is processed as described in Witthuhn et al. 2017, doi:10.5194/amt-10-709-2017 A first version of this data from cruise PS83 is already published at pangaea: doi:10.1594/PANGAEA.872377 Since then the processing algorithm receives major updates as described in Witthuhn et al. 2019 ( in review, doi:10.5194/amt-2019-321)