Aerosol characterization in the lower atmospheric boundary layer from lidar and DOAS measurements

Nofel LAGROSAS, Prane Mariel ONG, Jamrud AMINUDDIN, Shinchiro OKUDE and Hiroaki KUZE
Center for Environmental Remote Sensing (CEReS), Chiba University（〒263-8522 千葉市稲毛区弥生町 1-33）

Abstract: A plan-position indicator (PPI) lidar and LED-DOAS system are operated simultaneously to measure and characterizetemporal changes of aerosol parameters in the lower atmosphere during clear and cloudy conditions in summer over Chiba University.
In this work, aerosol extinction coefficient values at 349 nm are derived using Fernald’s method. The temporal trends of lidar andDOAS derived optical depth values are compared and shows similar temporal behavior for cases where the visibility is less than 35km. For high visibility, extinction coefficients do not follow the trend of the extinction coefficient values from LED DOAS. Theresults also show extinction coefficients from PPI are more sensitive to relative humidity changes.
Key Words: LIDAR, aerosols, DOAS, lower atmosphere, extinction coefficient

1. Introduction
Optical parameters (e.g., extinction coefficient,
Angstrom exponent) of aerosols in the atmospherecan be quantified using lidar and differential opticalabsorption spectrometer (DOAS). In an ideal
situation, these parameters are best quantified if anumber of instruments are operated simultaneously.
In this work, a PPI lidar (349nm) and LED-DOAS(590 nm) are operated simultaneously to explore thepossibility of charactering aerosols and to observe
the temporal change of aerosol extinction
coefficients and consequently aerosol optical depths
during summer at Center for Environmental RemoteSensing (CEReS), Chiba University. This observation
also intends to look at the performance of each
instrument during a 24 hour operation.

Wavelength (nm)Frequency (Hz)Wavelength (nm)Wavelength (nm)Output Wavelength(nm)Optical particle
counter (OPC)
Particle sizes (  m)
Measurements

2. Description of the systems
The PPI lidar (  =349 nm, f=1 kH) and LED DOAS
are placed on the 9 t h floor of CEReS building andwas continuously operated 1) . A retroreflector isplaced inside the 9 t h floor of the Science building,
320m north of CEReS building to reflect the lightfrom the DOAS transmitter. During the operation, thePPI lidar is angled at 10 o above the horizontal and ispointed north so that both instrument observe thesame spatial location. The LED DOAS is operatedusing 2 colors (590 nm and 625 nm). PPI and LED
DOAS data are recorded every 5 min and 30 s,respectively. In addition to the lidar and LED-DOAS
system, additional ground instruments are also
simultaneously operated: nephelometer (TSI 3563),
visibility meter (Vaisala PWD52), optical particle
counter (Rion KC-22B & KC-01E) and weathermonitor. The results of simultaneous operation ofthese instruments provide a better description theeffect of weather parameters on the optical properties
of aerosols and even on the performance of theinstruments. Table 1 shows the summary of thespecifications of the instruments used in this work.
.
Table 1. Specification of the instruments
PPI3491000
LED DOAS (Thorlabs
LED4D067)* 590 and 650
Nephelometer (TSI3563)
450, * 550, 700
Visibility meter (Vaisala
PWD52)
550 (measured at 875nm)*KC-22B and KC-01E(Rion)
0.08, 0.1, 0.2, 0.3, 0.5Weather monitor
RH, temperature, windspeed, wind direction,rainfall* employed here

3. Methodology
The extinction coefficient from PPI are obtainedby using Fernald’s method 2) . The extinction tobackscatter coefficient, S 1 , is set between 40-70 sr -1 3) .
Since the measurements are near the ground, theextinction coefficient (at 550nm) obtained fromvisibility meter is used as a boundary value for theinversion of the lidar data and converted t o 349nm
using the factor (550/349).
In the case of the LED DOAS data, the aerosoloptical depth is obtained by first implementing alinear fit on the logarithm of nighttime LED DOAS
data plotted against the nephelometer data. When theaerosol scattering coefficient is zero, the y-interceptof the linear fit represents the contribution from both
molecular optical depth and system constant.
4. Results
The operation of the PPI lidar and LED DOAS
started at around 19:00 JST on 27 July 2017 until 30
July 2017. During the three-day continuous operation,observed rainfall is zero. However, relative humidity

(RH) values range from 56% at daytime to 96% atnighttime. The RH usually peaks after midnight andstarts to decrease after sunrise. Wind speed range
from 0 to 5 m/s while temperature range from 22 o C to
34 o C. Atmospheric conditions range from clear tocloudy. Nighttime is usually cloudy as reported fromtheJapan
MeteorologicalAgency(http://www.data.jma.go.jp).
Figure 1 shows the temporal change of extinction
coefficient from both PPI lidar (349 nm) and LED
DOAS data (590nm). The extinction coefficientsfrom lidar are taken at a distance 450m north of thelidar location. The increasing extinction coefficientvalues from PPI and LED DOAS at nighttime can be
attributed to increasing RH. The results also showthat the deterioration of extinction coefficients fromPPI is likely correlated to increasing RH compared asto LED DOAS. When RH values reach more than
90%, the PPI signals deteriorate producing unreliable
extinction coefficients and are not shown in thegraphs (Figs. 1 and 2). This occurred on the night of
27-28 and 28-29 of July when the RH reached above
91%. A minor deterioration of lidar signal occurredon the night of 29-30 July when RH values are justbelow 90%.
During daytime, the extinction coefficient s fromboth instruments are comparatively lower than that ofnighttime. The RH values during daytime range from56-77% that result to the evaporation of water
adhered to the surface of the aerosol droplets andconsequently decreasing aerosol size.
Figure 2 shows the temporal development ofAngstrom exponent (AE) and RH from the night of
27 to the night of 29 July 2017. The Angstrom
exponent is quotient of the logarithm of the ratio ofextinction coefficients from PPI and LED to thelogarithm of the ratio of wavelengths from LED andPPI. The increasing Angstrom exponent on the nightof 27-28 July can be attributed to the influx of fresh
fine aerosols that contribute to high and increasing

Fig. 1. Extinction coefficient values from PPI lidar and
LED DOAS measurements (27-30 July 2017).
scattering coefficient from nephelometer data(~0.10/km at 450nm) even though the RH during thistime increases from 90% to 94% before midnight.
The OPC at CEReS registered increasing counts in
the 0.2, 0.3 and 0.5 size range. After midnight, whenRH increases up to 96%, aerosol growth could have
happened as seen from the lower Angstrom exponent

values during the early part of the day on 28 July.
The increasing Angstrom exponent at daytime on the28 t h can be attributed to the decreasing RH (94% to57%) from 07:00 to 14:00. After this time, RH
increases until past midnight contributing to aerosol
growth as seen in the decrease of Angstrom exponent
values. The increase of Angstrom exponent fromearly evening to midnight can be observed and iscommon to all three nights. This can be attributed tothe influx of fresh aerosols in the area, possibly fromevening traffic/rush. This diurnal pattern is observed
each day during the measurement. From the data, it
can be inferred that the effect of RH on aerosol
growth happens after midnight when RH is at its
maximum value (~94%-96%) and when there isminimum influx of aerosol in the atmosphere . High
Angstrom exponent occur during daytime when RH isat its lowest (~59%). Evaporation of water may havecompletely occurred during this time effectively
drying-up of aerosols even though clouds are present.

Fig. 2. Extinction coefficient values from PPI lidar
and LED DOAS measurements and ambient RH(27-30 July 2017).
5. Conclusion
This work has shown the capability of an LED
DOAS and lidar systems to characterize the aerosolsin the atmosphere over Chiba during summer. The
diurnal trend shows extinction coefficient values
from PPI lidar are more sensitive to changes inambient relative humidity compared to the extinction
coefficients from the LED source. When ambient RH
is more than 90%, PPI signals deteriorate and cannot
provide reasonable extinction coefficients. The
results of this work also show the diurnal pattern ofaerosol growth in the atmosphere. The combined
operation of these two instruments shows thataerosols in the lower troposphere can becharacterized to effectively understand the changesof aerosol optical properties in the atmosphere.
References
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