JRA1: Lidar and sunphotometer

Improved instruments, integrated observations and combined algorithms

WP Leader: Ulla Wandinger


This research activity focuses on the integration of two successfully operating aerosol observation networks, the European Aerosol Research Lidar Network EARLINET and the European part of the Aerosol Robotic NetworkAERONET with the objectives:  

  • To improve daytime capabilities of lidar instruments with emphasis on easy-to-implement solutions and continuous operation for an improved characterization of the four-dimensional distribution of aerosols over Europe and a better understanding of climate-relevant aerosol properties.
  • To develop and apply integrated observation strategies for lidar and sunphotometer for the best use of complementary information on atmospheric aerosols gained from active and passive remote-sensing instruments.
  • To retrieve advanced information on aerosol microphysics from multi-spectral, multi-angle columnar sunphotometer and height-resolved multi-wavelength lidar observations under consideration of polarization information.

Description of work

EARLINET relies to a large extent on high-sophisticated, multi-wavelength Raman lidar instruments to study the four-dimensional distribution of aerosols over Europe, to characterize the different types of aerosols and to gain knowledge on the particles’ optical and microphysical properties. The simultaneous measurement of particle extinction and backscatter coefficients with Raman lidar at several wavelengths is the key technique for the inversion of particle microphysical properties. Inversion algorithms to calculate vertical profiles of particle effective radius, volume concentration, refractive index, and single-scattering albedo from multi-wavelength lidar extinction and backscatter data have been developed and implemented in the EARLINET data evaluation scheme.
Eighteen EARLINET stations are currently equipped with AERONET sunphotometers. Microphysical aerosol properties of the atmospheric column retrieved with inversion algorithms from spectral optical depth and multi-angle photometer observations are standard output products of AERONET. In order to improve the observational capabilities of integrated lidar and sunphotometer remote-sensing stations the existing instruments, observation strategies, and analysis methods will be refined and synergistic effects will be explored within three tasks described in the following.



 Task 20.1  Improved daytime capabilities of lidar instruments (CNR, IFT, MPG, UPC,FORTH)

So far, most of the EARLINET multi-wavelength Raman lidar observations are performed at night. Raman signals are weak compared to daylight background and thus daytime capabilities of Raman lidars are still limited. Different techniques have been developed to improve Raman lidar daytime capabilities in the past years. The techniques are based on receiver systems with small bandwidth and small receiver field of view to suppress the daylight background. They have been successfully tested and implemented in a few systems and single instruments are in operational use within EARLINET (IFT, MPG). This task aims at an in-depth investigation of the optical and mechanical design of Raman lidar instruments for optimum performance under daylight conditions with focus on easy-to-implement and robust solutions. Recommendations for the respective improvement of existing instruments within the network will be given.

a) Evaluation of existing techniques
In the first year, existing techniques for Raman lidar daytime measurements will be reviewed. The performance of different instruments will be compared and the candidate techniques for further developments will be identified. This evaluation will be based on experience within the network (IFT, MPG) as well as on the review of solutions applied elsewhere. The investigations will consider the use of pure rotational vs. vibration-rotation Raman scattering, interference-filter vs. grating technique, near-range and far-range telescopes, analog and photon-counting detection, and required receiver field of view and bandwidth. (IFT, CNR, MPG)

b) Optical and mechanical design studies
Based on the evaluation of existing techniques optimized solutions for the optical and mechanical design of daytime Raman channels shall be developed. Optical ray-tracing simulations will be performed to maximize optical system efficiency, daylight suppression, and vertical measurement range. Compactness and stability of the optical configuration as well as easy-to-implement solutions will be considered in these studies. (CNR, IFT, MPG, FORTH)

c) Implementation and test of optimized solutions
Optimized optical and mechanical setups will be implemented in selected systems and test measurements will be performed. The daytime capabilities of the new setups will be characterized under different atmospheric conditions. Conclusions will be drawn with respect to the applicability of the developed techniques in other systems of the network. (CNR, IFT, MPG, UPC, FORTH)
d) Recommendations for the implementation of daytime Raman lidar channels in the network
The experience gained within this task will be summarized and recommendations of the implementation of daytime Raman channels in the network will be given. Common solutions will be considered. (CNR, IFT, MPG)



 Task 20.2

 Integrated observation strategies (IFT, CNR, IPNASB, UPC, FORTH)

This task aims at experimental approaches of combining AERONET sunphotometers and EARLINET
multi-wavelength lidars in different regions of Europe in order to make best use of complementary information under a broad variety of observational situations. Datasets gained in this way will be used for the development of respective inversion algorithms in Task 20.3.

a) Observations in different European core regions
A broad variety of aerosol types is observed over Europe. Besides locally produced anthropogenic aerosols from traffic, industrial and agricultural activities, several remote and natural sources contribute to the complex aerosol situation over Europe. Saharan dust is frequently observed especially in the Mediterranean region. Smoke and eroded soil particles are transported towards Europe from remote areas. In order to cover the variety of aerosol types and their different mixing states, we will perform combined lidar and sunphotometer observations in five European core regions: central Europe (Leipzig, IFT), eastern Europe (Minsk, IPNASB), Spain (Granada, UPC), Italy (Potenza, CNR), and Greece (Thessaloniki, Athens, FORTH). The stations are selected such that next to the regional representativeness of the observations, the best available instrumentation is applied. Each selected station is at least equipped with a multiwavelength lidar and a standard AERONET sunphotometer. The lidars have a sufficient degree of daytime capability (which will be improved during the project) and also measure the particle depolarization ratio. The latter quantity is needed to distinguish non-spherical dust particles from other aerosols and is an important prerequisite for the application of appropriate scattering models in the inversion algorithms developed in Task 20.3. (IFT, IPNASB, UPC, CNR, FORTH)

b) Harmonized datasets for algorithm development
Common principles for the evaluation of observations performed with lidar and sunphotometer at different locations are required to produce harmonized datasets which can be used for algorithm development (see Task 20.3). From the lidar profiles the atmospheric layering as well as the vertical distribution of characteristic aerosol properties (spectral extinction and backscattering, depolarization) is derived. Lidar profiles usually do not cover the lowermost part of the boundary layer. Sunphotometers help closing this gap, because they measure the optical depth of the entire atmospheric column. Furthermore, they provide complementary information on the atmospheric aerosol in terms of spectral optical depth and scattering phase function.
The contents and structure of combined lidar-sunphotometer datasets will be defined and a database of observations performed at five European core regions will be established. Respective algorithms that use combined datasets at different level of complexity will be developed within Task 20.3. (IFT, IPNASB, UPC, CNR, FORTH)


 Task 20.3

 Integrated retrieval schemes for aerosol microphysical properties (CNRS, IPNASB, IFT, CNR, UPC, FORTH)

This task aims at the development and implementation of integrated lidar and sunphotometer inversion algorithms to obtain advanced information on aerosol microphysical properties.
a) Combined sunphotometer and backscatter lidar algorithms
In a first step, an inversion algorithm based on sunphotometer data and data from single-wavelength or multiple-wavelength backscatter lidars will be made available for common use in the network. Backscatter lidar data derived from elastic-backscatter signals do not suffer from daylight background, and they are available at all EARLINET stations. Work on this task has already been performed in a collaboration of CNRS (LOA Lille) and IPNASB. It has been shown that improved information on columnar particle size distribution, spectral single-scattering albedo, and vertically resolved aerosol mode concentrations can be derived from the integrated dataset. The algorithm will be further improved and tested with data from the database established within Task 20.2. (IPNASB, CNRS)
b) Combined sunphotometer and multi-wavelength Raman lidar algorithms
In the second step, multi-wavelength extinction and backscatter Raman lidar data are considered. The improved daytime extinction observation capabilities will be used here. Preliminary studies reveal that the information content of this advanced dataset will allow us to retrieve vertical profiles of parameters of the aerosol fine and coarse modes as well as spectrally resolved single-scattering albedo. Algorithm tests will be performed by users at selected experimental sites (Leipzig, Potenza, Granada, Thessaloniki, Athens). By the end of the project a user-friendly algorithm shall be developed and implemented. (CNRS, IFT, CNR, UPC, FORTH)
c) Algorithms for desert dust and ash
Because of the non-sphericity and comparably large size of dust and ash particles, specific scattering models for non-spherical particles are required for the inversion of dust optical data. Scattering data for spheroidal particles calculated with the T-Matrix method have been implemented in AERONET retrievals. The few closure studies performed with lidar, sunphotometer and airborne in situ instrumentation revealed discrepancies in microphysical parameters derived from the inversion of optical data and in situ observations. Improved modelling of light scattering by non-spherical particles is planned. Observations performed in the framework of Task 20.2 will serve to test improved algorithms with respect to an adequate reproduction of lidar backscatter and depolarization values. Afterwards, the improved scattering models shall be integrated in the combined inversion schemes in order to use lidar and sunphotometer data in the retrieval of dust and ash microphysical properties. (CNRS, IFT)

The work progress of the JRA will be discussed at annual workshops which will be collocated with the technical workshops of NA2 allowing the distribution of new ideas and outcomes to all existing and new EARLINET-AERONET stations.

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