Aerosol phase function represents the angular scattering property of
aerosols, which is crucial for understanding the climate effects of aerosols
that have been identified as one of the largest uncertainties in the
evaluation of radiative forcing. So far, there is a lack of instruments with
which to measure the aerosol phase function directly and accurately in
laboratory studies and in situ measurements. A portable instrument with high
angular range and resolution has been developed for the measurement of the
phase function of ambient aerosols in this study. The charge-coupled
device-laser aerosol detective system (CCD-LADS) measures the aerosol phase
function both across a relatively wide angular range of 10–170
The climate effect of aerosol optical properties is one of the greatest
uncertainties in our understanding of climate change (Pachauri et al., 2014).
Instruments such as the integrating nephelometer have often been used to
measure the aerosol scattering coefficient in laboratory studies and field
campaigns (Anderson et al., 1996; Heintzenberg and Charlson, 1996; Ma et al.,
2011; Müller et al., 2011; Tao et al., 2014). However, besides the total
scattering coefficient, the distribution of aerosol scattering at different
directions also has a significant impact on the direct climate effect of
aerosols (Kuang et al., 2015, 2016b). The aerosol phase function (
In past years, different research groups have developed several versions of
polar nephelometers to measure how the scattering intensities of aerosol
particles, cloud droplets and ice crystals change with scattering angle.
Muñoz et al. (2001, 2010, 2011) mounted a photomultiplier tube (PMT) on a
mechanical arm which can rotate around a point on the laser light path in the
same plane with the laser beam to change the scattering angle of the signal
captured by the PMT. Castagner and Bigio (2006, 2007) focused the light
scattered at a single spot with different scattering angles to another single
spot by using two parabolic reflectors next to the light path. A plane mirror
was placed at that point to reflect the scattering signals with different
angles to a PMT by rotation. These two styles of instruments measured the
angular distribution of scattering signals by using the rotational mechanism.
This design will lead to an obvious uncertainty because the signals were not
measured simultaneously. Barkey et al. (2002, 2007) made the sample flow
perpendicular and intersected it with the light path. Then many PMTs were mounted
around the point of intersection in the same plane with the laser beam to
capture the scattering signal from different scattering angles. The signals
with different scattering angles were measured at the same time with this
design. However, the angular resolution which is limited to larger than
8
Recently, McCrowey et al. (2013) developed a miniaturised polar nephelometer,
which can be used in the in situ measurement based on the techniques of
Curtis et al. (2007) and can then be calibrated in the laboratory using
polystyrene latex (PSL) standard particles. A comparison between the results
measured from this instrument and calculated from a Mie model showed a good
agreement. The detection range of this instrument is from 20 to 155
In this paper, a novel instrument named “charge-coupled device-laser aerosol detective system” (CCD-LADS) based on the CCD imaging principle and the optical structure of the fisheye lens is developed to measure the ambient aerosol phase function in the field measurement at a wider range of detection angles and a higher accuracy. The validation in both laboratory and field measurement shows the ability of the CCD-LADS to measure the aerosol phase function.
The CCD-LADS includes several main components: a high-power continuous laser emitter, two CCD cameras, optical filters and fisheye lenses. The laser and CCD cameras are mounted on tripods and controlled by a laptop. Each component is portable and on a scale of a few cubic decimetres.
The emitting system of the CCD-LADS is mainly built with a solid continuous laser emitter. Nd : YAG is used as the solid laser material as the wavelength of the emitter is 532 nm. The transverse mode is near TEM00. The M2 factor is less than 2.0 while the divergence of the beam is less than 2.0 mrad. The diameter at the aperture is 3.0 mm. The power of the laser is 1 W. To change the polarisation state of the laser from linear to circular, a quarter-wave plate was mounted in front of the laser emitter. During the exposure time (a few minutes) of the image, the circular-polarisation light can be assumed to be unpolarised.
The receiving system of CCD-LADS has three main parts, the CCD cameras, the
optical filters and the fisheye lenses. The SBIG model STF-8300 CCD imaging
camera, which has the KAF-8300 CCD sensor (ON Semiconductor, Phoenix, AZ,
USA) is used. The area array (17.96
The fisheye lens (Sigma Corp., Japan) has a 10 mm focus length and a F2.8
aperture. When this lens is used with a Nikon camera, the field of view can
be 180
To filter out the background noise from the sky radiation, an optical filter
(Thorlabs, Newton, NJ, USA) is mounted between the CCD camera and the lens.
The filter has a 532
Figure 1a is the sketch map of the geometric relationship of CCD-LADS. The
laser is emitted horizontally, while a beam trap is used to receive the laser
beam on the other side. Besides the laser beam, two CCD cameras with fisheye
lenses are installed at the same altitude with the laser to capture the
scattering signal from the laser beam, while the directions of the cameras
are forward and backward. With the mounted lens, there is a one-to-one
correspondence between the image of the laser beam captured by CCDs and the
laser beam object according to the principle of image formation by lenses.
When two CCD cameras are used in this system, the detective angle can be
expanded to 10–170
Sketch map of the geometric relationship and the sampling image of CCD-LADS.
To decrease the total area of the instrument, the distance between the CCD cameras and the laser beam should be less than 1 m. Therefore the CCD-LADS system covers an area 12 m long and 1 m wide. When the instrument is set up, the first step is to measure the relative position of the CCD cameras, the laser beam and the laser emitter. From the geometric relationship shown in Fig. 1, we can know that the light scattered at different position on the laser beam will be collected by different pixels on the CCD, so that the scattering light at different angles can be retrieved from the image captured by CCD. Due to the open path structure of the CCD-LADS, the background noise is much higher in the daytime than at night-time. Currently, the CCD-LADS system can only estimate the nocturnal aerosol scattering phase function.
The data acquisition of CCD-LADS involves obtaining the angle-resolved
scattering signals from images captured by two independent CCD systems and
then merging the signals. Firstly, the CCD-LADS is set up as shown in
Fig. 1a. The geometric relationships between the CCDs, laser emitter and
light trap are measured by tape. Then the scattering angle of laser in the
image should be calibrated. The direction of the CCD cameras are adjusted to
make sure that the image from the laser goes through the centre of the pixel
arrays of CCD. By using a beam block, the backscattering light is blocked
from going into the CCD and the pixel related to the 90
At the beginning of the measurement, the CCDs are cooled down to
Noise subtraction of CCD-LADS:
After the image has been captured, the scattering light of the laser beam is separated
from the background noise in the image as in the following steps. Firstly, the
central axis of the scattering signals of laser beam is fitted in the programme
(the red line shown in Fig. 2b). Then the intensities of image on the
perpendicular of this central axis (the blue line shown in Fig. 2b) are
fitted with a normal distribution:
When the angle-resolved signals from two CCDs are obtained, the change of
signals with angles can be merged by following the steps below. Firstly, the
minimum angle
Figure 4 shows the flow chart of the retrieval algorithm used to determine
Signal merging of two CCD cameras. Besides the signals captured by the first CCD (blue dotted line) and the second CCD (red solid line), the lifted signal from the second CCD (red dashed line) is also shown in the left drawing. The merged signal is shown in the right drawing.
Scattering phase function
Flow chart of the retrieval algorithm used to determine aerosol phase function from CCD-LADS measurements (the processes in the dashed box are used to subtract the scattering signal of air molecules from the total scattering signal).
As the first step, the scattering coefficient of air molecules at near-surface
level
To resolve the ratio between the air molecules and the total hemispheric
scattering
To solve the intensity of the total hemispheric backscattering scattering
signals
Two types of uncertainties determine the error of the retrieved aerosol phase function: the measurement errors caused by the processes of obtaining the angle-resolved signals and an error introduced by the retrieval algorithm.
There are two sources of measurement errors in the data acquisition processes
introduced in Sect. 2.2.1. Firstly, the measurement error of CCD used in the
CCD-LADS is 10 % according to the related manual. The relative difference
between the fitted normal distribution introduced in Eq. (1) and the measured
signal in the laboratory study is 8.8 %
The relative errors on the merged angle-resolved signals
Uncertainties of the merged angle-resolved signal from CCD-LADS measurement.
The uncertainties of the retrieval algorithm are introduced by the
uncertainties of the input parameters. There are three groups of input
parameters in the retrieval algorithm: merged angle-resolved signals, aerosol
hemi-backscattering coefficient and temperature/pressure. The errors of the
temperature and pressure are about 0.1 K and 0.1 hPa (Box and Steffen,
2001), which will lead to a 0.02 % uncertainty on
To validate the ability of the CCD-LADS to measure the aerosol phase
function, an indoor experiment was held in the laboratory in the Physics
Building at Peking University during 7–8 November 2015. The time
resolution of CCD-LADS was set to 60 s during the experiment, while the
angular detection ranged from 10 to 170
Figure 6 shows the time series of several quantities during the laboratory experiment. The scattering/hemispheric backscattering coefficient of aerosols at 525 nm wavelength shown in Fig. 6b and the mass concentration of black carbon particles shown in Fig. 6c reveal the same pattern that first declines and climbs up afterwards. The same pattern can be discovered in the time series of particle number size distributions shown in Fig. 6d. The variation reflects the slow exchange between the indoor and outdoor air. The peak diameter of aerosol number size distribution was still around 100 nm, but it had a slight shift during the experiment. Therefore, the fine particles are dominant in the laboratory. The single scattering albedo (SSA) shown in Fig. 6c was around 0.85 which means that the black carbon aerosol took up a relatively large proportion of the aerosol species, resulting in strong particle light absorption ability.
Time series of
Having combined the particle number size distributions measured with SMPS/APS and
the mass concentration of black carbon aerosols measured with AE51 (Fig. 6)
into a modified Mie-scattering model, the aerosol optical properties
including the aerosol phase function could be modelled (Ma et al., 2011). In
this study (both laboratory and field study), the refractive index used for
black carbon component is 1.95–0.79
Comparison between aerosol phase function obtained from CCD-LADS measurements (red solid line shows the result estimated with the retrieval algorithm, brown dashed line shows that estimated directly with the measurements), modelled with modified Mie model (blue dashed line) and offered by previous studies with CALIPSO (different colours of dotted lines represent different aerosol types).
To further validate the quality of the retrieved result from the CCD-LADS
measurement, a comparison was also carried out among the
Comparison between aerosol phase function at 42
During January 2016, a comprehensive field campaign focused on air pollution
in winter was conducted on the roof of a school building at Yanqi campus of
the University of Chinese Academy of sciences (UCAS) in the Huairou district,
Beijing (40
During the field measurement, the scattering phase function of dry aerosols
could be resolved in two ways, with Aurora 4000 polar nephelometer measurements
or with the modified Mie-scattering model with the related aerosol measurements.
Under high relative humidity condition, aerosol particles will absorb
moisture in the atmosphere and exhibit hygroscopic growth significantly
(Bian et al., 2014; Chen et al., 2014; Kuang et al., 2016a), and hence the
scattering properties of ambient and dry aerosols are totally different.
Therefore, the data collected at a relative humidity above 70 % were
eliminated from the comparison between the scattering phase functions of dry
and ambient aerosols obtained by different methods. Figure 9 shows the
result of the comparison mentioned above. The results from the three methods are
consistent with one another in the overlap of the detectable scattering
angular range. Compared with the other results, the retrieval of CCD-LADS
measurement enhances the backward scattering fraction of aerosol. This might
be caused by the angular range (30–160
Comparison between aerosol phase function retrieved from CCD-LADS measurements (red line shows the average value, the error bar shows the standard deviation), measured from Aurora 4000 polar nephelometer (blue triangle) and modelled with modified Mie model (grayscale map).
A novel instrument named charge-coupled device-laser aerosol detective
system (CCD-LADS) was developed to measure the nocturnal ambient aerosol
phase function in the ambient atmosphere at a wider range of detection
angles and a higher accuracy. The validation in both laboratory and field
measurement shows the ability of CCD-LADS to measure the aerosol phase
function. A laser is emitted horizontally, while two CCD cameras with
fisheye lenses are installed besides the laser beam at the same altitude to
capture the scattering signal from the laser beam with the cameras facing
forward and backward. Then the signal captured by the two
cameras are merged into one signal curve. The detectable angular range is
from 10 to 170
To validate the ability of CCD-LADS to measure the aerosol phase function, an
indoor experiment was held in the laboratory of the Physics Building at
Peking University during 7–8 November 2015. During the experiment, the
angular detection range was from 10 to 170
During January 2016, a comprehensive field campaign focused on air pollution
in winter was organised at the roof of a school building in Yanqi campus of
UCAS. Depending on the limitation of the ambient condition, the angular
detection range of the CCD-LADS was 30–160
Both the laboratory experiment and the field measurement have demonstrated that the CCD-LADS is a robust instrument, fully capable of measuring the ambient aerosol phase function under different conditions. Overall, compared with the laboratory-scale instruments, the CCD-LADS measured aerosol phase functions with a wider angular range and at a higher angular resolution.
The averaged retrieved aerosol phase function used to create Fig. 7 is attached in Supplement. The CALIPSO aerosol classification data are listed in the reference. The entire data set can be accessed by request to the corresponding author at zcs@pku.edu.cn.
The authors declare that they have no conflict of interest.
This work is supported by the National Natural Science Foundation of China (41590872, 41375134). Edited by: Thomas Wagner Reviewed by: two anonymous referees