"Contemporary satellite instruments, providing accurate long-term observations at global scale, consist a fundamental tool in aerosol research. Relied on different remote sensing techniques, spaceborne sensors provide information about suspended particles’ load and nature, which are both highly variable in spatial and temporal terms. Our team has vast experience on the implementation of active and passive aerosol retrievals, acquired from lidars (e.g., CALIOP, CATS) and spectroradiometers (e.g., MODIS), in a wide range of scientific tasks. The in-house developed ESA-LIVAS database, in which a global 4D depiction of aerosols and clouds has been realized through the utilization of CALIOP vertically resolved observations, consists the flagship of the satellite component of the ReACT group. Advanced techniques, applied and tested at the ground, have been adopted in spaceborne observations making feasible a global view of fine/coarse dust aerosols and dust-related CCN/IN concentrations. Moreover, a global fine resolution dust optical depth dataset has been developed via the exploitation of MODIS and CALIOP data, whereas any possible multi-sensor synergy is considered aiming at optimizing aerosol loads’ description and characterization. One of the priorities of our team is the participation in the Cal/Val activities of ongoing (AEOLUS) and forthcoming (EarthCARE) satellite missions of the European Space Agency (ESA). Satellite observations are also analyzed for the investigation of outstanding cases (e.g., dust outbreaks, smoke transport, volcanic eruptions) as well as for the evaluation of numerical simulation aerosol outputs. Finally, in collaboration with modelling groups, satellite data are ingested in data assimilation schemes towards improving dust forecasting and monitoring. "

-Dr. Antonis Gkikas

Novel Instruments

The remote sensing activities of ReACT involve the development of prototype instruments. More specifically:

High Power UV Lidar

The High Power UV lidar is proposed by the National Observatory of Athens (NOA) and will be developed by Raymetrics S.A.. It will be installed in the PANhellenic GEophysical observatory of Antikythera (PANGEA) of the National Observatory of Athens within 2021. Its main parts include a high power linear polarized laser at 355nm and two telescopes, one for near field and one for far field (main) operation. The instrument will be optimized for daytime and nighttime operation. It will perform temperature and water vapor profiling measurements from N2 and H2O vibrational Raman scattering. It will also allow the retrieval of the aerosol extinction coefficient profiles at 365nm and 387nm from rotation Raman and N2 vibrational Raman, respectively. The elastic co and crossed polarized backscattered signals will be collected and combined to the total aerosol backscatter coefficient and the volume and particle depolarization ratio profiles will also be obtained. The overall aim for this system is to perform operational measurements and deliver all aforementioned products (temperature, water vapor, aerosol extinction coefficient at 365 and 387nm, aerosol backscatter coefficient at 355nm, volume linear and particle linear depolarization ratio at 355nm) with high spatio-temporal resolution down to a few hundred meters above ground.

This lidar will also include a Multispectral Detector from Licel enabling it to perform spectral measurements in 32 channels. A flexible dedicated spectrometer is applied with tunable central wavelength in the ultraviolet and visible parts of the spectrum and offers up to 3 different spectral resolutions. Aerosol laser induced fluorescence at 355nm excitation will be mainly targeted during night. Lidar profiles of fluorescence spectra are a fresh topic in the remote sensing community and can be applied for detection of biogenic particles such as pollen, viruses, fungi, and bacteria. Furthermore, it is possible to target specific vibrational Raman peaks with the spectrometer that correspond to gases such as O2, and CH4 (current state-of-the-art) taking into advantage the high emitted power. Detection of additional gases that scatter less due to their lower concentration levels will also be explored with the novel instrument.

Enhancement and Validation of ESA products (eVe)

EVE is a depolarization lidar system developed for the European Space Agency (ESA) by Raymetrics S.A. in collaboration with National Observatory of Athens (NOA) and Ludwig-Maximilians University of Munich (LMU). It is an innovative, mobile system which consists of a dual-laser/dual-telescope configuration. . It enables the simultaneous emission of linearly and circularly polarized radiation at 355 nm and the detection of the elastically backscattered radiation with polarization sensitive channels, as well as the inelastic (Raman) backscattered radiation at 387 nm. EVE aims to provide the ESA-Aeolus mission with a flexible, mobile reference ground-based lidar system capable of providing well-characterized fiducial reference measurements of aerosol optical properties. Moreover, it can be upgraded to multi-wavelength system to support calibration/validation activities of the ATLID lidar on-board the upcoming EarthCARE satellite ReACT team has actively contributed in the calibration and first QA/QC tests of the instrument and will operate the instrument in the upcoming ASKOS campaign. Two relevant publications are under preparation concerning the first tests and obtained products.

New active/passive remote sensing synergies

Aerosol Species Separation Algorithm (ASSA)

Multi-spectral depolarization Raman lidar measurement are the basis of the forward model ASSA. The concept is described in Siomos et al. 2020b and the technique in Siomos et al. 2021. It is based on a look up table (LUT) of aerosol mixtures constructed by externally mixing raw aerosol species from the Optical Properties of Aerosols and Clouds (OPAC) database (Koepke et al 2015, Hess et al. 1998). Currently mixtures of water soluble, soot, coarse sea salt, coarse dust are tested. Hygroscopic growth is taken into account according to OPAC parameterizations. The algorithm operates in two phases. Initially, it simulates the attenuated backscatter coefficients and volume depolarization ratios per vertical level for each mixture in the LUT. It selects the mixture profiles (potential solutions) that best describe the actual measured ones with the lidar. Then, through a RTM (currently libRadtran, Emde et al. 2016) it reproduces measured-like radiance spectra (direct, diffuse, global) per mixture profile in parts of the solar spectrum that are practically unaffected by absorbent gases. These are compared with actual measured spectra from radiometers and the mixture profile with the best general agreement given instrumental uncertainties is selected as the final solution. Being based on an RTM, the overall approach is rather flexible and any lidar and radiometric system that operates in the UV/VIS/IR can be included.

-Dr. Nikos Siomos

References

• Emde, C. et al., The libRadtran software package for radiative transfer calculations (version 2.0.1), Geoscientific Model Development, 9, 1647-1672, 2016
• Hess, M. et al, Optical Properties of Aerosols and Clouds: The Software Package OPAC, Bulletin of the American Meteorological Society, 1998, 79, 831-844, 1998
• Koepke, P. et al., Technical Note: Optical properties of desert aerosol with non-spherical mineral particles: data incorporated to OPAC, Atmospheric Chemistry and Physics, 15, 5947-5956 , 2015
• Siomos, N. et al.,  Towards an Algorithm for Near Real Time Profiling of Aerosol Species, Trace Gases, and Clouds Based on the Synergy of Remote Sensing Instruments, EPJ Web Conf., 237, 08023, 2020b
• Siomos N. et al, A technique to retrieve vertical concentration profiles of individual aerosol species based on the synergy of lidar and spectrophotometer measurements, submitted in the 15th International Conference on Meteorology Climatology and Atmospheric Physics (COMECAP) ,  2021

"In ReACT we work on numerical model research and development, focusing on the representation of atmospheric dynamics and processes at various scales. Areas of research include:

• Aerosol interactions and feedbacks in the atmosphere (direct, semi-direct and indirect effects).
• Severe atmospheric phenomena (e.g. hurricanes, floods, duststorms, volcanic eruptions)
• Air pollution, dispersion modeling (e.g. dust, smoke, volcanic ash)
• Climate change (e.g. climatic indexes, climate scenarios)
• Data assimilation of in-situ and remote sensing data (e.g. weather radar, satellite data, radiosondes) 
• Nowcasting
• Stratosphere-troposphere interactions and modelling natural climate variability

Operational numerical support is also provided in the framework of several European and National projects (ASKOS, CalVal, NEWTON, PANACEA e.t.c.) and modeling services are available in both the public and private sector (companies, prefectures e.t.c). Atmospheric modeling research and applications in ReACT is facilitated through a number of numerical tools such as WRF (ARW, Chem, NMM, Hydro), DREAM, FLEXPART, HYSPLIT and CR-SIM".

- Dr. Christos Spyrou

In ReACT we are using active and passive remote sensing to investigate atmospheric processes of importance to climate research, weather forecast, aviation and air quality.

Our experimental and theoretical work is dedicated to studies of aerosols, clouds, water vapour and their interactions using measurements from our ground-based long-term stations (Athens, PANGEA-Antikythera, Finokalia), dedicated experimental campaigns, and long-lived space borne sensors.

Key topics of our research are:

• The investigation of the processes that determine the life cycle of dust and its interactions with clouds and radiation.
• The development of new lidar instruments for dedicated studies (e.g. new polarimetric observations, evaluation of space borne lidar missions).
• The investigation of key atmospheric parameters (e.g. optical and physical properties of aerosols, clouds, water vapor, temperature, wind) above the Mediterranean.
• The 24/7 monitoring of the atmosphere, from the ground up to the stratosphere, supporting governmental and European infrastructures regulating aviation and air quality hazards (e.g. volcano eruptions, fires)."