Planetary boundary layer height variability over Athens, Greece, based on the synergy of Raman lidar and radiosonde data: application of the Kalman filter and other techniques (2011-2016)

The temporal evolution of the Planetary Boundary Layer height over Athens, Greece for a 5-year period (2011-2016) is presented. Using the EOLE Raman lidar system, the range-corrected lidar signals were selected around 12:00 UTC and 00:00 UTC for a total of 332 cases (165 days and 167 nights). The Kalman filter and other techniques were used to determine PBL height. The mean PBL height was found to be around 1617±324 m (12:00 UTC) and 892±130 m (00:00 UTC).Peer ReviewedPostprint (published version

A tropospheric NO2 research product from TROPOMI for air quality applications in Europe

This study focuses on a tropospheric NO₂ research product from TROPOMI measurements over Europe based on an improved retrieval algorithm. We present an overview of the DLR NO₂ algorithm and validation with ground-based measurements. In addition, the use of TROPOMI tropospheric NO₂ columns for air quality purposes in Europe will be discussed. The DLR NO₂ retrieval algorithm for TROPOMI consists of mainly three steps: (1) the spectral fitting of the slant column based on the differential optical absorption spectroscopy (DOAS) method, (2) the separation of stratospheric and tropospheric contributions, and (3) the conversion of the slant column to a vertical column using an air mass factor (AMF) calculation. To calculate the NO₂ slant columns, a 405-465 nm fitting window is applied in the DOAS fit for consistency with other NO₂ retrievals from OMI and TROPOMI. Absorption cross-sections of interfering species and a linear intensity offset correction are applied. The stratospheric NO₂ columns are estimated using a directionally dependent STRatospheric Estimation Algorithm from Mainz (DSTREAM) method to correct for the dependency of the stratospheric NO₂ on the viewing geometry. For AMF computation, the climatological OMI surface albedo database is replaced by the geometry-dependent effective Lambertian equivalent reflectivity (GE_LER) and directionally dependent (DLER) data obtained from TROPOMI measurements with higher spatial resolution. As surface albedo is an important parameter for accurate retrieval of trace gas columns, the effect of surface albedo in TROPOMI NO₂ retrieval was investigated by comparing results applying different surface albedo datasets. Mesoscale-resolution a priori NO₂ profiles obtained from the regional chemistry transport model POLYPHEMUS/DLR and LOTOS-EUROS are used. The cloud correction in this TROPOMI NO₂ retrieval is improved using the Clouds-As-Layers (CAL) model from the ROCINN cloud algorithm which is more representative of the real situation than the Clouds-As-Reflecting-Boundaries (CRB) model. Validation of the TROPOMI tropospheric NO₂ columns is performed by comparisons with ground-based MAX-DOAS measurements at nine European stations with urban/suburban conditions. The improved DLR tropospheric NO₂ product shows a similar seasonal variation and good agreement with MAX-DOAS measurements. In particular, the retrievals applying a priori NO₂ profiles from the regional model with a high spatial resolution and recent emission inventory improve an underestimation in TROPOMI tropospheric NO₂ columns in polluted urban areas. Finally, we present the use of the TROPOMI tropospheric NO₂ research product in the regional POLYPHEMUS and LOTOS-EUROS chemistry transport models to analyse the effect of traffic emission on air quality in Germany with the framework of the S-VELD project

Intercomparison of Sentinel-5P TROPOMI cloud products for tropospheric trace gas retrievals

Clouds have a strong impact on satellite measurements of tropospheric trace gases in the ultraviolet, visible, and near-infrared spectral ranges from space. Therefore, trace gas retrievals rely on information on cloud fraction, cloud albedo, and cloud height from cloud products. In this study, the cloud parameters from different cloud retrieval algorithms for the Sentinel-5 Precursor (S5P) TROPOspheric Monitoring Instrument (TROPOMI) are compared: the Optical Cloud Recognition Algorithm (OCRA) a priori cloud fraction, the Retrieval Of Cloud Information using Neural Networks (ROCINN) CAL (Clouds-As-Layers) cloud fraction and cloud top and base height, the ROCINN CRB (Clouds-as-Reflecting-Boundaries) cloud fraction and cloud height, the Fast Retrieval Scheme for Clouds from the Oxygen A-band (FRESCO) cloud fraction, the interpolated FRESCO cloud height from the TROPOMI NO2 product, the cloud fraction from the NO2 fitting window, the O2–O2 cloud fraction and cloud height, the Mainz Iterative Cloud Retrieval Utilities (MICRU) cloud fraction, and the Visible Infrared Imaging Radiometer Suite (VIIRS) cloud fraction. Two different versions of the TROPOMI cloud products OCRA/ROCINN, FRESCO, and the TROPOMI NO2 product are included in the comparisons (processor version 1.x and 2.x). Overall, the cloud parameters retrieved by the different algorithms show qualitative consistency in version 1.x and good agreement in version 2.x with the exception of the VIIRS cloud fraction, which cannot be directly compared to the other data. Differences between the cloud retrievals are found especially for small cloud heights with a cloud fraction threshold of 0.2, i.e. clouds that are particularly relevant for tropospheric trace gas retrievals. The cloud fractions of the different version 2 cloud products primarily differ over snow- and ice-covered pixels and scenes with sun glint, for which only MICRU includes an explicit treatment. All cloud parameters show some systematic problems related to the across-track dependence, where larger values are found at the edges of the satellite view. The consistency between the cloud parameters from different algorithms depends strongly on how the data are filtered for the comparison, for example, what quality value is used or whether snow- and ice-covered pixels are excluded from the analysis. In summary, clear differences were found between the results of various algorithms, but these differences are reduced in the most recent versions of the cloud data

Μελέτη του ατμοσφαιρικού οριακού στρώματος και της επίδρασής του στις ιδιότητες των νεφών

The main objective of this Ph.D. Thesis was to study the impact of aerosols on cloud properties under the effect of turbulence in the PBL. The current work has been triggered by the uncertainties of cloud adjustments due to their interaction with aerosols within the PBL. The impact of turbulence and aerosols' properties on cloud development was studied in the urban environment of Athens from the database of the HygrA-CD campaign which took place in May-June 2014. This Thesis is structured in eight chapters. In Chapter 1, we present the structure and the diurnal variation of the PBL. Its turbulent characteristics are certainly important for the cloud development and are identified from the energy spectrum. The kinetic energy dissipation rate, defined from the turbulent kinetic energy budget is a variable which we can retrieve from a Doppler lidar in the PBL. Moreover, methods for tracking the PBL height with the use of lidar remote sensing techniques are discussed in the same chapter. In Chapter 2, we introduce the concept of the aerosol-cloud interactions and their impact to climate change. More specifically, the relations between aerosol properties and cloud droplet activation are briefly explained and, finally, the growth mechanisms of cloud droplets' radius in warm clouds are presented. Chapter 3 deals with the remote sensing of the atmosphere and especially of aerosols. The detailed description of the NTUA lidar instrumentation (EOLE and AIAS lidars) is part of this chapter. We provide, in an analytic method, the technical characteristics of the NTUA lidar systems. Furthermore, we describe the Klett and Raman methods for deriving aerosol characteristics regarding their optical properties, based on the received lidar signals. Calibration techniques and signal processing of raw lidar data, are also provided here. In Chapter 4, aiming to describe the interaction of light with matter, we make an introduction to the electromagnetic scattering theory and how we determine aerosols' optical properties from the scattering field. In the same chapter we also present the inversion techniques based on the aerosols optical properties, to extract information for aerosols' microphysical properties.One of these inversion algorithms has been used within this Ph.D. Thesis and it is presented in Chapter 5. The LIRIC algorithm combines active and passive remote sensing data and gives as output the profile of the aerosol volume concentrations for fine and coarse-mode particles. LIRIC was applied on several case studies over Athens. The results and sensitivity tests are included in Chapter 5. The subject of Chapter 6 is the description of the research objectives of the international experimental campaign HygrA-CD. The instrumentation network involved six participants (academic institutes and research centers). The prevailing meteorological conditions over the Athens basin were studied using a cluster analysis of the air mass backtrajectories arriving over Athens. The major findings of the campaign are included in this chapter. In particular, the vertical profiles of the aerosols' optical properties (retrieved from NTUA multi-wavelength Raman lidar data) are presented and an aerosol typing scheme has been performed. The thermodynamic state of the atmosphere was explored from radio soundings and the PBL dynamics from the wind lidar data.Moreover, in Chapter 7, two methodologies which were developed during this Ph.D. Thesis are presented. First, a methodology aimed to link the aerosol number concentrations to cloud droplet number was developed. The algorithm uses synergistic data obtained from a multi-wavelength Raman lidar and a Doppler lidar and provides estimation of the CCN spectra and the cloud droplet number. Second, a methodology aimed to quantify the vertical aerosol flux in the cloud-topped PBL, was developed. The collocated measurements from a Doppler lidar and the NTUA elastic aerosol lidar were used to estimate the aerosol turbulent flux through the eddy correlation technique. Both methodologies were applied to cases from the HygrA-CD campaign and the results are presented in Chapter 7. Finally, the conclusions and perspectives for future studies are provided in Chapter 8.Ο βασικός στόχος της διατριβής είναι η μελέτη των επιπτώσεων των αιωρούμενων σωματιδίων στις ιδιότητες των νεφών υπό την επίδραση της τύρβης στο Ατμοσφαιρικό Οριακό Στρώμα (ΑΟΣ). Η συγκεκριμένη μελέτη κρίθηκε σημαντική καθώς υπάρχουν μεγάλες αβεβαιότητες στις προσαρμογές των νεφών λόγω της αλληλεπίδρασής τους με τα αιωρούμενα σωματίδια μέσα στο ΑΟΣ. Η επίδραση της τύρβης και των φυσικο-χημικών ιδιοτήτων των σωματιδίων στην ανάπτυξη των νεφών μελετήθηκε στο αστικό περιβάλλον της Αθήνας με χρήση των μετρήσεων που συλλέχθηκαν στην Αθήνα την περίοδο Μαΐος-Ιούνιος 2014.Η διατριβή είναι δομημένη σε οκτώ κεφάλαια. Στο Κεφάλαιο 1 παρουσιάζονται τα χαρακτηριστικά του ΑΟΣ. Στο Κεφάλαιο 2, εισάγουμε στον αναγνώστη την έννοια της αλληλεπίδρασης σωματιδίων-νεφών και την επιδρασή τους στην κλιματική αλλαγή. Συγκεκριμένα, αναφερόμαστε στις οπτικές ιδιότητες των αιωρούμενων σηματιδίων και την ενεργοποίηση αυτών σε νεφοσταγονίδια καθώς επίσης παρουσιάζουμε τους μηχανισμούς ανάπτυξης των νεφοσταγονιδίων στα θερμά νέφη. Στο Κεφάλαιο 3 ασχολούμαστε με μεθόδους τηλεπισκόπησης της ατμόσφαιρας και ιδιαίτερα των αιωρούμενων σωματιδίων. Αναλυτικά περιγράφεται το σύστημα lidar του ΕΜΠ με τα διάφορα τεχνικά χαρακτηριστικά της διάταξής του. Επιπρόσθετα, περιγράφουμε τις μεθόδους (Klett και Raman) για την ανάκτηση των οπτικών ιδιοτήτων των αιωρούμενων σωματιδίων από σήματα lidar. Οι μέθοδοι βαθμονόμησης και επεξεργασίας σήματος lidar παρουσιάζονται, επίσης, σε αυτό το κεφάλαιο. Με στόχο την περιγραφή της αλληλεπίδρασης ακτινοβολίας--ύλης, στο Κεφάλαιο 4 εισάγουμε τον αναγνώστη στην κλασσική θεωρία σκέδασης και στον τρόπο ανάκτησης των οπτικών ιδιοτήτων των αιωρούμενων σωματιδίων από το πεδίο σκέδασης. Στο ίδιο κεφάλαιο παρουσιάζονται μαθηματικές μέθοδοι αντιστροφής των οπτικών ιδιοτήτων των σωματιδίων με σκοπό τον προσδιορισμό των μικροφυσικών τους ιδιοτήτων. Ένας από τους αλγορίθμους αντιστροφής που χρησιμοποιήθηκε σε αυτή τη διατριβή είναι ο αλγόριθμος LIRIC που παρουσιάζεται στο Κεφάλαιο 5. Συνδυάζει όργανα ενεργητικής και παθητικής τηλεπισκόπησης προκειμένου να εξάγει πληροφορία για την καθ' ύψος ογκομετρική συγκέντρωση λεπτόκοκκων και χονδρόκοκκων σωματιδίων. Ο αλγόριθμος LIRIC εφαρμόστηκε σε διάφορα επεισόδια σωματιδιακής ρύπανσης στην Αθήνα.Το αντικείμενο του Κεφαλαίου 6 είναι η περιγραφή των επιστημονικών στόχων του διεθνούς πειράματος HygrA-CD. Τα πιο σημαντικά ευρύματα από το πείραμα παρουσιάζονται σε αυτό το κεφάλαιο. Συγκεκριμένα, οι οπτικές ιδιότητες των αιωρούμενων σωματιδίων καθ' ύψος όπως ανακτήθηκαν από το σύστημα Raman lidar παρατίθενται σε αυτό το κεφάλαιο καθώς επίσης και ο χαρακτηρισμός του είδους των ανιχνευθέντων αιωρούμενων σωματιδίων. Η θερμοδυναμική κατάσταση της ατμόσφαιρας διερευνήθηκε με χρήση ραδιοβολίσεων. Τέλος, η δυναμική του ΑΟΣ μελετήθηκε με τη βοήθεια ενός συστήματος Doppler lidar, όργανο ενεργητικής τηλεπισκόπησης για την ανάκτηση των συνιστωσών της ταχύτητας του ανέμου.Στο Κεφάλαιο 7 παρουσιάζονται οι δύο μεθοδολογίες που αναπτύχθηκαν στο πλαίσιο της παρούσας διδακτορικής διατριβής. Η πρώτη μεθοδολογία έχει στόχο να συσχετίσει τις συγκεντρώσεις των αιωρούμενων σωματιδίων με τις συγκεντρώσεις νεφοσταγονιδίων. Ο αλγόριθμος που αναπτύχθηκε χρησιμοποιεί συνεργιστικές μετρήσεις από ένα πολυκαναλικό σύστημα Raman lidar και ένα Doppler lidar με στόχο να παρέχει εκτιμήσεις του φάσματος Πυρήνων Συμπύκνωσης Νέφωσης (ΠΣΝ) και του αριθμού των νεφοσταγονιδίων. Η δεύτερη μεθοδολογία έχει ως στόχο να παρέχει την ποσοτικοποίηση της καθ' ύψος μεταφοράς μάζας αιωρούμενων σωματιδίων μέσα στο ΑΟΣ με χρήση μεθόδων τηλεπισκόπησης. Οι ταυτόχρονες μετρήσεις από το σύστημα Doppler lidar και το ελαστικό lidar σωματιδίων είναι απαραίτητες για την εκτίμηση της κατακόρυφης μεταφοράς αερολύματων μέσω της τεχνικής συσχέτισης στροβιλισμών. Και οι δύο μεθοδολογίες εφαρμόσθηκαν σε πραγματικές ατμοσφαιρικές συνθήκες κάνοντας χρήση της βάσης δεδομένων από το πείραμα HygrA-CD και παρουσιάζονται στο Κεφάλαιο 7. Τέλος, στο Κεφάλαιο 8 συνοψίζονται τα συμπεράσματα της παρούσας διδακτορικής διατριβής και διερευνώνται οι προοπτικές για μελλοντικές εργασίες

Operational cloud products from TROPOMI on Sentinel-5 Precursor and prospects for Sentinel-4 on MTG-S

Accurate trace gas and aerosol retrieval for satellite missions focused on atmospheric composition requires a precise knowledge of cloud properties in the UV/VIS/NIR spectral region. Such missions dedicated to atmospheric composition and air quality monitoring include the operational GOME-2 instruments on-board MetOp-A/B, the recently launched TROPOMI instrument on Sentinel-5P and the upcoming UVN/Sentinel-4 instruments on MTG-S and the UVNS/Sentinel-5 instruments on EPS-SG. Furthermore, cloud information from UV/VIS/NIR spectrometers, such as S5P, S4, and S5, is complementary to the information retrieved using sensors operating in the thermal IR. In this work we present the operational cloud products from the Sentinel-5 Precursor mission and prospects for Sentinel-4. These operational cloud products are generated using the latest version of the OCRA (Optical Cloud Recognition Algorithm) and ROCINN (Retrieval of Cloud Information using Neural Networks) algorithms which have been successfully applied to the operational processing of GOME/ERS-2 and GOME-2 MetOp-A/B data. The ROCINN algorithm retrieves cloud height, cloud optical thickness and cloud albedo from NIR measurements in and around the oxygen A-band (~760nm) taking as input the cloud fraction computed with the OCRA algorithm based on a broad-band UV/VIS color space approach. Two approaches which treat the clouds differently are implemented to the ROCINN algorithm. The first approach called Clouds-as-Reflecting-Boundaries (CRB) assume that the cloud is a reflecting surface whereas the second and more realistic model called Clouds-as-Layers (CAL) represents the cloud as a homogeneous cluster of scattering spherical particles