Diurnal Behavior of Aerosol Extinction Coefficient
Based on Lidar and Ground Sampling Instruments
Prane Mariel ONG1,2, Tatsuo SHIINA2, Nofel LAGROSAS1 and Hiroaki KUZE1,2
1

Center for Environmental Remote Sensing (CEReS), Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
2Graduate School of Science and Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

Abstract: One of the important factors that contribute to air visibility is the aerosol extinction coefficient. In this paper, we derive thediurnal behaviour of aerosol extinction coefficients at 349 nm from the data of the plan-position indicator (PPI) lidar and groundsampling instruments, namely a nephelometer and an aethalometer. For both instruments, the diurnal behavior of the aerosol
extinction coefficients is shown to appear in-phase in every change of the ambient relative humidity (RH), which tends to be higher
at night and lower at the mid-day.
Key Words: Lidar, Ground Sampling Instruments, Aerosol Extinction Coefficient
1. Introduction
An extensive long-term monitoring of the visual air quality
is of importance to be able to establish the current visibilityconditions, track changes over time and determine thecontributing factors of visibility changes1-3). One parameterthat controls visual air quality is the aerosol extinction
coefficient. In this report, we examine the diurnal behavior of
the aerosol extinction coefficient with respect to theatmospheric conditions, especially the ambient relative
humidity (RH).

The measured scattering coefficients by the nephelometerand the derived absorption coefficients by the aethalometer areused to compute the extinction coefficient, and then categorized
as the result from the ground sampling instruments.

2. Instruments
All the instruments used to measure the aerosol extinction(Plan-Position Indicator (PPI) lidar), scattering coefficients(nephelometer), and equivalent black carbon (EBC)(aethalometer) are located on the rooftop of the 8-story CEReS
building in Chiba University (35.63N, 140.10E), about 30 m
above ground. The site is also equipped with a weather monitor
that measures the ambient RH, rain, wind speed and direction
every 5 minutes routinely.
2.1. PPI Lidar
The PPI lidar is based on a diode-laser pumped Nd:YLF
with 349 nm wavelength and 60 J/pulse at 300 Hz pulse
repetition rate. Figure 1 illustrates the schematic configurationof the PPI lidar system. A photomultiplier tube (PMT,Hamamatsu H10304-00) is used as the receiver sensor. The
PMT is connected to the Licel transient recorder (TR20-160)
for data recording of the backscattered signal every 5 min.
These values are then used to calculate the aerosol extinction
coefficient. In addition, the PPI lidar was set to measure in
fixed direction facing the north direction.
2.2. Nephelometer and Aethalometer
The three-wavelength (450, 550 and 700 nm) integrating
nephelometer (TSI 3563) is used to measure the aerosolscattering. It can measure the scattered light with scattering
angles between 7 and 170. In CEReS, the nephelometer
makes a scattering measurement every minute.
The seven-wavelength (370, 470, 520, 590, 660, 880, and950 nm) aethalometer (Magee AE31) is used to measure theequivalent black carbon (EBC), which is then used to derive theabsorption coefficient of the aerosols given by the following
equation:14625
𝛼𝑎𝑏𝑠 () = EBC ∗ (),(1)-1where 𝛼𝑎𝑏𝑠 () [m ] is the absorption coefficient, EBC ismeasured in units of ngm-3, and (14625) is the mass extinction

efficiency factor [m2g-1] 4). The aethalometer is set to measure
the average value over 5 min.

Fig. 1. PPI lidar system: schematic configuration
3. Methodology
The PPI lidar was operated continuously from July 14 to 17,
2017 to investigate the diurnal behaviour of the aerosol
extinction at 349 nm wavelength. The aerosol extinction
coefficients were derived using the Klett method5) given theassumption of atmospheric homogeneity in the lower
atmosphere in addition to the small (10) elevation angle of thePPI laser beam.
To obtain the extinction coefficient from the groundsampling instruments at the wavelength similar to the PPI lidar,a linear extrapolation was necessary from the logarithmic
relation of the scattering coefficient (nephelometer) andabsorption coefficient (aethalometer) with wavelength. These
optical data (taken every 5 min) are then plotted against the RH
from the meteorological data with respect to the hour of day forthe entire 3-day period.
4. Results
During the observation period from July 14 to 16(afternoon) the atmosphere was relatively clear with slightcloud cover, especially on the 16th. We experienced small
rainfall at 20:00 JST of July 16 and at 08:00 JST of July 17. A
summary of weather data for this rain period is presented inTable 1. The RH for the 3-day duration was consistent to havea mid-day minimum with a slight abrupt increase during theshort rain period, as can be seen in Fig. 2. As expected due totemperature dependence, the RH is higher at night and lower
during mid-day, and its difference is about 30-40% per day.
Table 1. Weather data of the period with rain.

Fig. 2. Extinction coefficients at 349 nm derived from the dataof ground sampling instruments (nephelometer and aethalometer) plotted against the ambient RH over the 3-day period.

Figure 2 also presents the extinction coefficients from theground sampling instruments plotted over time. From Fig. 2, it
is apparent that the temporal change of aerosol extinction is inphase with the ambient RH. Also, during the onset of the 1strain (July 16) there is a noticeable, rapid increase in theextinction coefficient. Despite the fact that the rain lasted forjust a few minutes, the number of aerosol particles after the rainremained to be high for the whole night. This might be due tothe increase in the number of hygroscopic aerosol particles
during this time and the RH in nighttime is at its maximum, i.e.
70% or higher3). Interestingly, the usual drop of the RH in themorning due to the temperature change, drastically drop theaerosol extinction to a minimum even for just a very shortperiod of time. The onset of the 2nd rain (July 17) did not alter
the aerosol extinction right away. This might be due to the levelof the RH, which is below 70%, just before the precipitation. Inaddition, as the RH increased to more than 80%, the aerosol
extinction started to increase rapidly (inset red dotted line).
A similar analysis was carried out on the PPI lidar data (Fig.
3). The in-phase relation with RH is found also for theextinction coefficients from the lidar data throughout the 3-day
period. However, the minimum aerosol attenuation is in theorder of about 2.0x10-4 m-1 or 20% light attenuation per km,slightly higher than the ground sampling instruments. It is alsointeresting to see the sensitivity of the lidar system inmeasuring the aerosol extinction during the onset of rain,
especially the 2nd rain (July 17). As seen in Fig. 3, there isapparent spike of aerosol extinction in every rain occurrence
before it gradually neutralizes back close to its original value(see inset red dotted lines). This is not the case for thenephelometer and aethalometer data.
Figure 4 is the time-distance profile of the aerosol
extinction coefficients derived from the PPI lidar measurements.
The range covered here is up to 1.8 km away from the lidar. As
observed from the previous figures, there is a goodcorrespondence of aerosol increase/decrease in every
increase/decrease of the ambient RH. The only difference isthat, this figure added the spread of the aerosol extinction overcertain range along the field of view of the lidar receiver
telescope.
5. Conclusion

Fig. 3. Extinction coefficients 500 m away from the PPI lidar
plotted against the ambient RH over the 3-day period.

We have derived the aerosol extinction coefficients at 349
nm from the data of the PPI lidar and ground sampling
instruments, a nephelometer and an aethalometer. For bothschemes, the diurnal behavior of the aerosol extinction
coefficient was found to be in-phase with the change in the RH
in the 3-day period. Moreover, the observed RH exhibits apattern of a mid-day minimum and nighttime maximum due to
its temperature dependence.
For future work, it is planned to make an extensive seasonal
analysis of the aerosol extinction behavior.
ReferencesFig. 4. Time-distance profile of the extinction coefficients fromthe PPI lidar data at 349 nm wavelength.

1) IPCC: In: Climate change 2007: the physical science basis.
(Cambridge, United Kingdom: Cambridge UniversityPress; 2007).
2) Manago, N., et al.: Journal of Quantitative Spectroscopy
and Radiative Transfer 112 (2011) 285-291.
3) Gebhari, K.A., et al.: Atmospheric Environment 35 (2001)
5177-5191.
4) Hansen, A.D.A: Magee Scientific: The Aethalometer(Berkeley, CA, 2005) p. 22.
5) Klett, J. D.: Applied Optics 20, 2 (1981) 211-220.