Lidar remote sensing for the characterization of the atmospheric aerosol on local and large spatial scale

Atmospheric aerosol What are THEY and why are THEY so important? Minute particles suspended in the atmosphere Aerosols interact both directly and indirectly with the Earth’s radiation budget and climate Aerosols reflect or absorb sunlight Aerosols modify the size of cloud particles, changing how the clouds reflect and absorb sunlight WHAT ABOUT THE ESTIMATION OF THEIR EFFECTS? MOTIVATION

from Intergovernmental Panel Climate Change

INTERACTION LIGHT - ATMOSPHERE Elastic scattering Anelastic scattering   Mie scattering Rayleigh scattering x << 1 molecules Rayleigh scatteringMie scattering Mie scattering, larger particles Direction of incident light  A    E Raman scattering Information on the species concentration

LIDAR remote sensing

THE REMOTE SENSING LIDAR TECHNIQUE Sorgente laser Nd-Yag Laser Receiver LIght Detection And Ranging Signal processing

ELASTIC LIDAR EQUATION (SINGLE SCATTERING) z: altitude : wavelength 1 equation 2 unknown parameters + a priori hypothesis Lidar Ratio (LR) P L : laser power Standard Atmosphere vertical resolution  : efficiency β = β m + β a backscatter coefficient  m  a  extinction coefficient acceptance angle

RAMAN LIDAR EQUATION (SINGLE SCATTERING) No a priori hypothesis 1 Elastic lidar equation + 1 Raman lidar equation 2 unknown parameters

RCS - RANGE CORRECTED SIGNAL = P(z)*z 2 PBL height Planetary Boundary Layer Directly influenced by the presence of the Earth's surface Aerosol as tracers Time (UT) 18:00 20:00 22:00 24:00 02:00 04:00 06:00 Height above lidar station (m) nm (a.u.) Naples, 9-10 May 2005

EARLINET (European Aerosol Research LIdar NETwork) Since May 2000 ARPAC Naples station (40.833°N, °E, 118 m. asl) regular measurements twice a week special measurements (Saharan dust, forest fires, volcanic eruption, etc…) intercomparison both for hardware and software 25 stations

THE NAPLES LIDAR SYSTEM Diaphragm Collimating Lens High 387 Low High 355 Low 355 > High 532 Low 607

CLOUD SCREENINGSharp variation cloud RCS (a.u.) Height (m) PRE - PROCESSING DATA

PILE UP CORRECTION Measure the same signal: - D 1 at low acquisition rate (< 500kHz) - D 2 at working condition Polinomial fit Rate D 2 (MHz) Rate D 1 (MHz)

PRE - PROCESSING DATA MERGE Height (m) Analog – low height Photocounting – high height RCS (a.u.)

CALIBRATION Height (m) RCS (a.u.) PRE - PROCESSING DATA Molecular signal “Clean” air

Depolarization measurement Why? Function of the particles’ morphology Identification of solid and liquid phases of the particles

How do we perform linear depolarization measurements? 1.Use a linearly polarized laser source 2.Align a detecting channel (P channel) in the same direction of the initial polarization of the laser 3.Align another detecting channel (S channel) orthogonal with respect to the laser initial direction of polarization 4.Calibration of the system

Total Depolarization coefficient Defined as: Is the backscattering coefficient S(z) and P(z) are the ortoghonal and parallel signals H is the calibration constant k takes into account the instrumental effects

Aerosol Depolarization coefficient Molecular depolarization ( ) R Backscatter ratio Total depolarization coefficient

How do we calibrate depolarization channels? The calibration constant measures the relative efficiency of the polarization channels. There were studied and evaluated 4 techniques: 1.Rayleigh method 2.90° rotation of the polarization of the laser 3.45° rotation 4.Depolarization

Eyjafjallajökull

Depolarization by ETNA volcanic particles