Measurements of Aerosol Optical Parameters with Backscatter and Side-scatter Lidars

Studies in Surveying and Mapping Science (SSMS) Volume 4, 2016

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Measurements of Aerosol Optical Parameters with Backscatter and Side-scatter Lidars Huihui Shan1, Junjian Liu2, Hui Zhang1, Xiaomin Ma1, Zongming Tao1* Department of Basic Sciences, Army Officer Academy, Hefei, 230031, China

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Postgraduate Company, Army Officer Academy, Hefei, 230031, China

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*zmtao@aiofm.ac.cn Abstract The extinction coefficient, backscattering coefficient and phase function are essential optical parameters of aerosol. General backscatter lidar is an important tool for aerosol detection, but it has the blind area in near-range. Combining backscatter with side-scatter lidars, the above three optical parameters can be measured. Our experimental system is presented, three optical parameters retrieval methods are introduced, and three cases are studied. The results of case studies show that our lidars system and the retrieval methods work well.

OCIS code: 010.1110, 280.3640, 290.5820 Key words Atmospheric Optics; Aerosol; Lidar; Aerosol Optical Parameters

Introduction Liquid and solid particles suspended in the atmosphere are entitled atmospheric aerosol. Their aerodynamic diameters are between 0.001μm and 100μm. Atmospheric aerosol is one of the current hot researches in atmospheric science, because it has a certain impact on human health and the global climate. Aerosol affects Earth’s radiation budget directly, and affects cloud-radiation interactions indirectly [1]. At the same time, aerosol is a part of the pollutants in the air. Air pollution often occurs on the layer of a few kilometers from ground which generally is referred to as the planetary boundary layer. Aerosol chemistry, concentrations and type change with altitude. In order to understand aerosol’s impact on the global climate change and air pollution control, aerosol vertical distribution monitoring is very important, so we need to get information of aerosol altitude profile [2].

So far, there are many methods to detect aerosol, such as sun photometer and remote sensing techniques. Using sunlight, sun photometer provides the total aerosol properties integrated through the entire atmosphere.

Backscatter lidar provides aerosol backscattering altitude profile, so it becomes a powerful tool for detection of atmospheric aerosol [3]. Due to backscatter lidar transmitters and receiver’s field of-view (FOV) in the same place, backscattering light is not received or received incompletely in near-range. This is the so-called geometric factor influence. So the backscatter lidar is not suit to the near-range measurement.

Side-scatter lidar which is based on CCD detector is a developing tool for detection of atmospheric aerosol. The CCD camera is set to record the side-scattering light of laser beam [4]. The transmitting device and the receiving device are arranged in two places, and have no geometric factor. The vertical altitude resolution of side-scatter lidar is variable by adjusting the distance between laser beam and CCD detector, and has a good vertical altitude resolution in near-range generally. So side-scatter lidar is especially suitable to measure aerosol in near-range [5-6].

In this paper, backscatter lidar and side-scatter lidar are united in a joint to retrieve aerosol phase function, backscattering coefficient and aerosol extinction coefficient.

Experimental System Experimental system contains backscatter lidar and side-scatter lidar. Backscatter and side-scatter lidars are united in a joint. The diagram of experimental system is shown in Fig. 1. Backscatter lidar consists of telescope，transient recorder, PMT and laser on the left. Side-scatter lidar system includes laser, CCD camera and geometric calibration on the right. The process of the experimental system can be described as a laser beam emitting into atmosphere,

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due to the interaction between the laser and the atmosphere; the scattered light contains the information of the scattering particles. The telescope receives the backscattering signals profile of laser beam. The profile of side-scattering signal is detected by CCD camera, and then aerosol optical parameters are obtained from backscatter lidar data and side-scatter lidar data.

FIG. 1 THE DIAGRAM OF THE COMBINED LIDAR SYSTEM

The key specifications of the combined lidar system are summarized in table 1; Reference 7 described the detailed description.

TABLE 1 KEY SPECIFICATIONS OF THE EXPERIMENTAL SYSTEM

Laser

(Quantel Brilliant)Nd:YAG

Pulse energy

200 mJ

Wavelength

532 nm

Divergence

0.5 mrad

Repetition rate

10 Hz

Computer

(Lenovo ×200)

CCD Camera

(SBIG)

ST-8300M

Quantum efficiency

~55% at 532nm

A/D convecter (bits)

16

Pixel array

3352×2532

Pixel size (μm)

5.4×5.4

Angle per pixel(deg/pix)

~0.02

Wide-angle lens

Walmexpro f/2.8

Lens focal length (mm)

14

Telescope

(Meade) Cassegrain LX400-ACF-14”

Diameter Focal length

14 inch 2854 mm

PMT R7400U-02 Cathode radiant sensitivity(mA/W) 80 at 532 nm Transient Recorder Sampling rate

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Licel TR-20-160 20 MHz

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For backscatter lidar, the lidar equation is familiar to readers, so we omitted it here.

For side-scatter lidar, the lidar equation can be described as [5]

P( ) 

KAP0 TzTr  ( ) D

(1)

P( ) in equation (1) is the received photon number at scattering angle  and altitude z by a pixel. K is a CCD camera system constant; A is the area of the CCD camera lens; P0 is the photon number emitted by laser and D is the distance between CCD camera and laser beam; Tz is molecular and aerosol atmospheric transmittance from the laser to altitude z; Tr is molecular and aerosol atmospheric transmittance from altitude z to the CCD camera along the slant path;  ( ) is the total side-scattering coefficient including aerosol and molecule;  is the FOV of a pixel.

For the side-scatter lidar configuration, lidar signals are recorded by different pixels of the CCD camera. The spatial resolution Δz from the geometry is expressed as

z 

R2  D

(2)

For the side-scatter lidar system, we measured the FOV of a pixel  by calibration experiment;  is an approximate constant in the experiment, the spatial resolution z increases with the slant distance R. By comparing the equations (1) and (2) with backscatter lidar equations, there are two main difference aspects between side-scatter lidar and backscatter lidar. One is that the spatial resolution is not a constant in side-scatter lidar, the other is that received power is not 1/ R2 dependence in side-scatter lidar. Because backscatter lidar system has a disadvantage in near-range, it is mainly responsible for the detection at high altitude. Side-scatter lidar has lower resolution at high altitude, so it is mainly responsible for the detection in near-range. Aerosol optical parameters can be measured accurately from ground to the tropopause by combining backscatter lidar with side-scatter lidar.

Method For backscatter lidar, the retrieval method is familiar to readers, we omitted it here too.

The total scattering coefficient  S and the side-scattering coefficient  ( ) have a relationship as

 ( )  pf ( ) s

(3)

pf ( ) in equation (3) is the aerosol phase function. Based on aerosol backscattering coefficient, the side-scattering coefficient  ( ) can be described as

 ( ) (4) pf ( )  f ( ) ( ) pf ( ) f ( ) is defined as the relative aerosol phase function. The side-scatter lidar equation in scattering angle from 900 to 1800 can be rewritten as  ( ) 

KAP0 z z /cos(  ) (5) exp{0 [ m ( z )   a ( z )]dz   0 [ m ( z )   a ( z )]dz }  [  m ( ) f m ( )  a ( ) f a ( )]   D where  m ( z ) and  a ( z ) are molecular and aerosol extinction coefficients respectively;  m ( z ) and  a ( z ) are P( ) 

molecular and aerosol backscattering coefficients respectively; f m ( ) and f a ( ) are relative molecular and aerosol phase function respectively.

In equation (5), extinction coefficients, backscattering coefficients and relative phase functions of molecule and aerosol are six unknown variables. Three variables of molecule can be measured by molecular model and Rayleigh scatter theory. Assuming that extinction-to-backscatter ratio of the aerosol is a constant, an unknown variable is reduced, and so only two unknown variables are left. Known to anyone, the other can be obtained according to

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equation (5). The specific methods are shown as follows.

Aerosol Phase Function Side-scatter lidar and backscatter lidar work at horizontal direction simultaneously, and then a method to retrieve the aerosol phase function can be described as follows. In this case, we can assume that the aerosol extinction coefficient is a constant due to the homogeneity. Selecting a reference point, then phase function, backscattering coefficient and extinction coefficient for molecule can be obtained by a standard atmosphere. Backscattering coefficient and extinction coefficient for aerosol can be determined by backscatter lidar in the simultaneous experiment. The relative phase function for aerosol within the scatter angles can be retrieved by the numerical inversion method [6].

Backscattering Coefficient Aerosol phase function is associated with aerosol component, size distribution and refractive index, and does not rely on the total concentration. The aerosol phase function is assumed as uniform in the planetary boundary layer, and then the backscattering coefficient can be retrieved by side-scatter lidar as the following way. Keeping side-scatter lidar point vertically, the same polarization angle as horizontal measurements, the vertical aerosol backscattering coefficient can be retrieved using the aerosol phase function which is measured by side-scatter lidar in horizontal pointing. Choosing a suitable reference point, the aerosol backscattering coefficient value at this reference point can be obtained by simultaneous backscatter lidar. The molecular phase function, backscattering coefficient and extinction coefficient can be calculated by a standard atmosphere. As the extinction-to-backscatter ratio of aerosol is a constant, backscattering coefficient for aerosol can be retrieved from equation (5) using numerical inversion method.

Aerosol Extinction Coefficient Still using the above method, selecting a suitable reference point, aerosol extinction coefficient value at this reference point also can be measured by simultaneous backscatter lidar, and then aerosol extinction coefficient can be retrieved from equation (5) using numerical inversion method, as the extinction-to-backscatter ratio of aerosol is a constant.

Case Study The aerosol phase function was obtained from side-scatter lidar which pointed horizontally in the evening on Nov.29, 2013. The distance from the laser beam to the CCD was 10.34 m. The sky was clear at that night. The aerosol extinction coefficient was 0.35km-1 near the surface. As the whole view angle of the CCD camera is about 95°, in order to get scattering signals from 0° to 180°scattering angle, the center axis of the CCD camera was pointed to two directions in turn. At first, the center axis of the CCD camera was put on 45° scattering angle, and the exposure time is about 150s. Then the center axis of the CCD camera was put on 135° scattering angle, and the exposure time is 300s. Figure 2 shows that the aerosol phase function was measured from 15° to 180°. The aerosol phase function changes with θ in Figure 2. The maximum of aerosol phase function is located at 15° and the minimum of aerosol phase function is located about at 110°.

FIG. 2 AEROSOL PHASE FUNCTION

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The evening on Nov.13.2014, the shy condition was clear, and the lowest temperature is about 4 0C near the ground, no more than 6 m/s southeast wind. The backscatter lidar and the side-scatter lidar were pointed vertically. The distance between laser beam and CCD was 19.33 m. The center axis of the CCD camera was pointed to 41° tilting angle, and the exposure time was 240s. Figure 3 shows the retrieved aerosol backscattering coefficient profile at 22:30. The aerosol backscattering coefficient is retrieved by side-scatter lidar within 1km, and the aerosol backscattering coefficient is retrieved by backscatter lidar above 1km. From Fig. 3, we know that the aerosol backscattering coefficient changes with altitude with multi-layers structure. The aerosol backscattering coefficient decreases with altitude within 0.5km. The aerosol backscattering coefficient increases with altitude from 0.5km to 1.2km, then the aerosol backscattering coefficient decreases with altitude.

FIG. 3 AEROSOL BACKSCATTERING COEFFICIENTS PROFILE

Figure 4 shows that the aerosol extinction coefficient profile was retrieved on Nov.13, 2014 at 22:30. The aerosol extinction coefficient also displayed multi-layers structure.

FIG. 4 AEROSOL EXTINCTION COEFFICIENT PROFILE

Discussion and Conclusions In this paper, the lidars system and the retrieval methods of aerosol optical parameters are introduced briefly.

Aerosol phase function, backscattering coefficient and extinction coefficient can be measured by combining backscatter lidar with side-scatter lidar. The validation of our system is not described because of paper length. The results of case study show that the retrieval methods of aerosol optical parameters by the combined lidar system are credible, and the retrieved aerosol optical parameters are reasonable.

This work was supported by the National Natural Science Foundation of China under Grant No.41475025 .

REFERENCES

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Studies in Surveying and Mapping Science (SSMS) Volume 4, 2016

A. Ansmann, D. Althausen, U. Wandinger, K. Franke, D. Miiller, F. Wagner, and J. Heintzenberg, “Vertical profiling of the Indian aerosol plume with six-wavelength lidar during INDOEX: a first case study, ” Geophys. Res. Lett. 27, 963–966, 2000.

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C. Weitkamp, “Lidar:Range-Resolved Optical Remote Sensing of the Atmosphere, ” Springer, New York, 2005.

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[6]

Z. Tao, D. Liu, Z. Wang, X. Ma, Q. Zhang, C. Xie, S. Hu, Y. Wang: Opt. Express. 22(1), DOI: 10.1364/OE.22.001127 (2014) Huihui Shan was born on June 17, 1984 at Huaibei city, Anhui province, China. She received her master's degree from Anhui University in 2009 at Hefei city, Anhui province, China. Her major field of study is material physics and chemistry.

She is a Lecturer in department of Basic Sciences, Army Officer Academy at Hefei city, Anhui province, China. Her current research interests focus on lidar measurement, lidar data analysis and side-scattering lidar development. She published some articles about side-scattering lidar, e.g. Zongmig. Tao, Dong Liu, Xiaomin Ma, Bo Shi, Huihui Shan et.al., “Vertical distribution of near-ground aerosol backscattering coefficient measured by a CCD side-scattering lidar.”Applied Physics B lasers and Optics (2015)120:631-635; Xiaomin Ma, Bo Shi, Huihui Shan et.al., “Geometric Calibration Method of Side-Scatter Lidar Based on Charge-Coupled Device.” ACTA PHOTONICA SINICA(2015):0201002; Bo Shi, Xiaomin Ma, Zongmig Tao, Huihui Shan et.al., “Measurements of Near-Ground Aerosol Backscattering Coefficient Profile with Side-Scatter Technique .” ACTA PHOTONICA SINICA(2015):0501006.

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