Aerosol Optical/Radiative Forcing Properties over East Asia Determined fromSKYNET Radiation Measurements
B.J. Sohn1, Do-Hyeong Kim1, Teruyuki Nakajima2, and Tamio Takamura3
School of Earth and Environmental Sciences, Seoul National University, Seoul, Korea1
Center for Climate System Research, University of Tokyo, Tokyo, Japan2
Center for Environmental Remote Sensing, Chiba University, Chiba, Japan3
E-mail: Sohn@snu.ac.kr
Abstract
We developed a method of retrieving aerosol optical properties and their associated radiative forcing fromsimultaneously measured sky radiation and surface solar flux data. The method is then applied to data sets collected atMandalgovi, Dunhuang, Yinchuan, and Sri-Samrong sites of the Skyradiometer Network (SKYNET), and at Anmyon,Gosan, and Amami-Oshima, to examine the aerosol characteristics of East Asia. From the analysis for the SKYNET
sites, it was found that aerosols in East Asia have smaller single scattering albedos (i.e., 0.89 for Asian dusts inDunhuang, 0.9 for urban type aerosols in Yinchuan, and 0.88 for biomass burning aerosols in Sri-Samrong), comparedto the single scattering albedo for the same type of aerosols found in other areas. Lower single scattering albedo
suggests that the aerosols over East Asia absorb comparatively more solar radiation. The measurements taken during
April at the latter three sites over the Korean peninsula, and the East China Sea showed that the single scattering albedo
of Asian dust becomes smaller during the course of its movement from the source region to East Asian sea waters (i.e.,
0.86 at Anmyon, 0.84 at Gosan, and 0.80 in Amami-Oshima), compared with 0.89 found in the source region (i.e.,Dunhuang). These findings strongly suggest that Asian dusts become blackened during the movement because ofmixing with soot particles produced over the industrial/urban area of China. The overall atmospheric forcing
efficiency (radiation flux per unit aerosol optical thickness at 0.5 µm) of Asian dusts ranges from 65 W m-2 to 94 W m-2
near the East Asian seaboard area, indicating that atmospheric heating by Asian dusts can be significantly enhanced by
the mixing with soot particles.
Keyword: aerosol, East Asia, solar radiation, aerosol radiative forcing .

1. IntroductionIn East Asia, in order to determine the physical,chemical, and radiative properties of aerosols, as well as
to quantify aerosols-radiation interactions, theAsian-Pacific Regional Aerosol Characterization
Experiment (ACE-Asia) was conducted over the East
China Sea (Huebert et al., 2003). The physical andchemical processes controlling the evolution of the major
aerosols were also studied. The results of ACE-Asia
have shown that dusts in Asia can be transported half
way around the globe, and air pollution can significantly
change dust aerosols by adding black carbon, toxic
materials, and acidic gases to the mineral particles(Huebert et al., 2003). It was also found that a 24
hour-averaged clear-sky shortwave net radiative forcing
ranges from -26 ~ -30 W m-2 over East Asian oceanic andcoastal region (Markowicz et al., 2003; Bush and Valero,2003)
Despite the importance of the Asian continent as anaerosol source region there have been few measurements,
especially in the desert area of north China. Moreover,due to the large spatiotemporal variations of aerosols,
long-term monitoring near the source regions or theirdownwind areas is needed. Focusing on long-termmonitoring of aerosols and an assessment of aerosol
impact on the climate system over East Asia, aground-based aerosol/radiation observation network,
named the Skyradiometer Network (SKYNET), has been
operating since 1997. Instruments used for observing
surface solar radiation and aerosol characteristics include
skyradiometer, pyranometer, and pyrheliometer.
Inaddition, due to the interest in dust impact on the local

and global climate, field experiments were conducted
over East Asian seawaters, i.e., the Asian AtmosphericParticle Environmental Change Studies (APEX)(Nakajima et al., 2003).
In this study, we will investigate the magnitude andvarying degrees of aerosol influence on the solar
radiation budget at the surface and TOA over East Asia
using surface solar radiation measurements taken at
SKYNET sites. Effort will also be made to interpret therelationship of these measurements to retrieved aerosol
optical parameters. To this end, we developed a methodof estimating ARF using retrieved aerosol opticalparameters from skyradiometer measurements. In doing
so, adjusted imaginary parts of refractive indices (andthen single scattering albedos) estimated from thediffuse/direct method, originally introduced by King andHerman (1979) and extended by Nakajima et al. (1996),
were used as inputs into the exact radiative transfer (RT)model instead of using direct retrievals fromskyradiometer measurements.
In addition, special
measurements taken during April in Korea and Japan
were also analyzed in order to investigate the influencesof Asian dust on radiative forcing downwind of East
Asia.
2. Observation and analysis
Direct and diffuse solar radiation were measured
using a sky radiometer (POM-01L, Prede Co. Ltd.) indaytime at seven wavelengths of 315, 400, 500, 675, 870,940, and 1020 nm. Aerosol optical thickness,single-scattering albedo at five wavelengths (400, 500,675, 870, and 1020 nm), Ångström exponent, andvolume size distribution [dV/dlnr (cm3cm-2)] wereretrieved using an inversion software (i.e.:SKYRAD.pack version 3) developed by Nakajima et al.
(1996). A detailed explanation about quality control,retrieval methods, and sensitivity analysis can be foundin Kim (2003), and Kim et al. (2004).
For the radiative forcing calculation, surface solar
radiation measurements in the wavelength band between
0.3 and 4.0 µm were used in conjunction with aerosol
optical properties obtained from the skyradiometermeasurements. Direct solar fluxes were measured by thepyrheliometer, which measures the intensity of a radiantbeam at normal incidence coming only from the solar
disk with about 5 degrees of the full opening view angle.
A shaded pyranometer was used to measure diffuse solar
radiation, as both the pyranometer and the shading disk
were mounted on an automated solar tracker to ensurethat the pyranometer was continuously shaded. Then
global downwelling solar fluxes were determined fromdirect solar radiation measured by pyrheliometer
multiplied by the cosine of the solar zenith angle plus
diffuse radiation by the shaded pyranometer. Although
global downward solar fluxes were also measured by thepyranometer, the pyranometer tends to be more uncertain
when the solar zenith angle is high because the detector
responds differently to the solar incident angle(Michalsky et al., 1995; Satheesh et al., 1999). Thus,
the global fluxes determined from both pyrheliometerand shaded pyranometer were preferable to thosemeasured by the pyranometer.
Measurements were taken at seven sites, given inFigure 1. Among them, four observation sites, i.e.,
Mandalgovi, Dunhuang, Yinchuan, and Sri-Samrong,form the SKYNET, while the Anmyon, Gosan sites ofKorea, and the Amami-Oshima site of Japan are for thespecial radiation measurements made during the spring.
50Nin Figure 2. As illustrated at the top of Figure 2,
aerosol optical parameters and atmospheric profiles areused as inputs into a RT model for simulating surface
solar fluxes. For the flux calculation, we use the real partof refractive index retrieved from sky radiancemeasurements. Thus values vary with space and time,
ranging from 1.45 to 1.70. During the retrieval, thecolumnar water vapor amount and the imaginary part of
the refractive index (hereafter absorption index) areobtained by minimizing the difference between thecalculated and measured fluxes, as suggested in King andHerman (1979), and Nakajima et al. (1996). In doing so,
the absolute accuracies of the pyrheliometer andpyranometer are used as the convergence criteria (ε) inFigure 2. The adjusted absorption index and columnar
water vapor amount are then used for the forcing
calculation.

Mandalgovi(45.59N, 106.19E)

Dunhuang(40.16N, 94.80E)

Anmyon

40N

Yinchuan(36.52N, 126.32E)

Latitude(38.48N, 106.22E)

Gosan(33.29N, 126.17E)

30N

Amami-Oshima(28.44N, 129.70E)

Figure 2: A schematic diagram for the procedures usedin aerosol radiative forcing calculation fromskyradiometer and surface solar flux measurements.
Kdir and Kdif represent direct and diffuse fluxes, c and min parentheses represent calculated and measured values,
respectively.

20N

Sri-Samrong(17.17N, 99.87E)

10N

EQ
90E

100E

110E

120E

130E

140E

150E

Longitude

Figure 1: Geographical locations of observation sites.
3. Methodology for radiative forcing calculation
For the radiative forcing calculation, we have developed
an algorithm, adjusting aerosol parameters to bring in
calculated values of diffuse and direct solar radiation
close to observed values. Retrieval procedures used forthe calculation of aerosol radiative forcing fromground-based solar radiation measurements are provided

4. Aerosol Radiative Forcing over East AsiaFor
anARF
calculation,
simultaneous
measurements of sky radiation and diffuse/direct solar
radiation flux at the surface are required. Because oflimited match-ups of sky radiation vs. solar flux data dueto the inhomogeneous data coverage, the ARF coverageis also limited. Therefore, ARF is given with thenumber of match-ups used for the forcing calculation ineach month—see the left panels of Figure 3. The

middle panels of Figure 3 display the 24-hour averaged
surface ARFs (W m-2) given with AOT at 0.5 µm at
Dunhuang, Yinchuan, and Sri-Samrong. The match-ups
available for Mandalgovi were not enough to represent
aerosol forcing characteristics in that area, so that results
obtained from measurements for Mandalgovi are notpresented.
The relatively small number of available data foreach month was largely due to the cloud contamination.
However, it was also contributed by the employed
assumption that the SSA from sky radiation measurents
should be consistent with the adjusted SSA, allowing
only a small adjustment to satisfy the measured surface
radiation fluxes. The retrieved SSA from the flux
method was compared against the one from sky radiation
measurements only, and then the retrieval was discarded
if the difference was greater than 0.05. In Figure 3,
calculated ARFs (represented by cross hairs) are given as
a function of optical thickness at 0.5 µm. Also
provided are theoretically calculated SSAs (representedby curves) at 0.5 µm, which range from 1.0 (top curve)
to 0.6 (bottom curve) with a 0.05 interval. For thesimpler demonstration using the reduced number ofparameters, theoretical SSAs at each site were calculatedusing mean values of aerosol volume size distribution
and surface albedo, but with varied optical thickness.
Because of this, some points appear to have unrealistic
SSAs greater than unity. Also given in the middle
panels of Figure 3 are the mean SSA and g. In addition,mean β at the TOA, atmosphere, and surface areprovided in the right panels of Figure 3.
As expected Dunhuang is influenced mainly bydust particles all year around. In Yinchuan, the generalaerosol characteristics are related to the accumulation
processes of urban aerosols throughout the year, anddusts during the springtime. Sri-Samrong showedpatterns for biomass burning aerosols during
mid-October to mid-February. The results obtained inthis study over those three sites will be interpreted withthe aid of typical aerosol types found at thoseobservation sites. Details of aerosol characterizationand their seasonal variations for those sites are found in
Kim et al. (2004).
Because Dunhuang is largely affected by frequent
Asian dust episodes during the spring to early summer
period, ARFs for Dunhuang found in this study arelargely due to Asian dust aerosols. The mean SSA andg are 0.89 and 0.71, respectively. The SSA obtained inthis study at 0.5 µm is slightly smaller than 0.92∼0.93 at
0.44 µm found over the Saharan desert or Saudi Arabia(Dubovik et al., 2002a), implying that Asian dusts are
more absorbing aerosol. On the other hand, theobtained g (≈0.71) is close to the values found in theSaharan desert (≈0.72) or Saudi Arabia desert (≈0.68)(Dubovik et al., 2002a).
The resultant forcing efficiency in the atmosphereis about 67 W m-2, suggesting that Asian dusts over theDunhuang area absorb a significant amount of solar
radiation.
Combining with the relatively smaller

forcing efficiency at the TOA (≈-9 W m-2), the overallsurface forcing efficiency is estimated to be -76 W m-2,
indicating that the surface is deprived of a substantial
amount of solar energy in the presence of Asian dusts.
The smaller TOA forcing efficiency seems to beassociated with high surface reflectivity over theDunhuang area, since the negative TOA forcing can be
reduced by the aerosol absorption of surface-reflected
upwelling radiation, in particular under high surface
albedo conditions, as noted in Sahara dust studies(Haywood et al., 2001; Haywood et al., 2003; and Myhreet al., 2003). More discussion on the influences ofAsian dusts on the ARF is found in Kim et al. (2004).
Yinchuan represents the urban type aerosol
because the match-up data only cover theSeptember-December period. The obtained SSA ofaround 0.9 is similar to the SSA (0.87 to 0.90) foundover the Indian Ocean during the INDOEX period(Satheesh and Ramanathan, 2000). However, 0.9 of
SSA in Yinchuan is much smaller than 0.98 at 0.44 µm
for urban type aerosols observed at the NASA Goddard
Space Flight Center (GSFC) (Dubovik et al., 2002a).
This variability of SSA for urban type aerosols may be
due to differences in fuel types and emission conditions.
Automobile traffic should be the main local source of thepollution around the GSFC. On the other hand, thehighly absorbing optical properties in the INDOEX aremainly due to the presence of soot type aerosols frominefficient fossil fuel combustion for heating and cooking,and biomass burning (Satheesh et al., 1999).
Considering that aerosols from inefficient fossil fuelcombustion are also abundant over China (Chameides etal., 1999), we suspect that the small SSA value inYinchuan is also due to the contributions of soot typeaerosols to SSA.
The period from December to March corresponds
to the dry winter season over the Sri-Samrong area, andthus the prevailing aerosol type is the biomass burning(Kim et al., 2004).
The retrieved SSA (≈0.88)
representing biomass burning aerosols over theSri-Samrong area is slightly larger than the 0.85 for
African Savanna smoke but smaller than the 0.93 for thesmoke over Amazonian forested regions (Eck et al.,
1998; Eck et al., 2001; Dubovik et al., 2002). There are
two bundles of points separated by about 0.9 line ofSSA; one showing the mean SSA around 0.91 during
December 1998 and the other showing a smaller mean
SSA around 0.86 during January to March in 1998. The
different SSAs suggest that aerosol origin andcomposition during December are different from thosefor the January-March period, i.e., there are moreabsorbing aerosols during the latter period.
Acknowledgements: This research has been supported
by ADD/KAIST of Korea, and also by the APEX Projectof the Japan Science and Technology Corporation.

24 hour average ARF (W m-2)

Number of cases-2030(a) Dunhuang

25-20

20

TOA-9

Atmosphere

67

Surface-76

TOA-16

Atmosphere

39

Surface-55

TOA-10Atmosphere

53

Surface-63

1.0-4015-60

10ω = 0.89
g = 0.71-80501234567860

9 10 11 12-100
0.0

0.1

0.2

0.6
0.3

0.4

0.5

0.6

0.7

0(b) Yinchuan

50-2040-4030-60

20-80

10

ω = 0.90
g = 0.670123456787060

β (W m /τ0.5)-100
9 10 11 12
0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0(c) Sri-Samrong

-20

50
40-40

30-60

20-80

1001234567Month8-100
9 10 11 120.0

ω = 0.88
g = 0.62
0.1

0.2

0.3

0.4

0.5

0.6

0.7

AOT at 0.5 µm

Figure 3: Number of cases used in each month (left), 24-hour averaged surface radiative forcing (W m-2) with AOT at
0.5 µm (middle), and forcing efficiency (β) given in W m-2 (right) at Dunhuang (top), Yinchuan (middle), andSri-Samrong (bottom). Curves represent the theoretical radiative forcing calculated with SSAs of 1.0 (top) to 0.6(bottom), with a 0.05 interval. Cross hairs represent observed forcing. ω and g represent mean single scattering
albedo and asymmetry factor at 0.5 µm, respectively.
Nakajima, T., T. Hayasaka, A. Higurashi, G. Hashida, N.
Moharram-Nejad, Y. Najafi, and H. Valavi, Aerosol
References
optical properties in the Iranian region obtained byground-based solar radiation measurements in theDubovik, O., A. Smirnov, B. N. Holben, M. D. King, Y. J.
summer of 1991, J. Appl. Meteor., 35, 1265-1278,
Kaufman, T. F. Eck, and I. Slutsker, Accuracy
1996b.
assessment of aerosol optical properties retrieved
Ramanathan, V., and Co-authors, Indian Ocean
from Aerosol Robostic Network (AERONET) sunExperiment: An integrated analysis of the climate
and sky radiance measurements, J. Geophys. Res.,
forcing and effects of the great Indo-Asian haze, J.
105, 9791-9806, 2000.
Geophys. Res., 106, 28,371-28,398, 2001b.
Eck, T. F., B. N. Holben, I. Slutsker, and A. Setzer,Sugimoto, N., A. Shimizu, and I. Matsui, Observations ofMeasurements of irradiance attenuation andaerosols and clouds with lidars, Fifth APEX
estimation of aerosol single scattering albedo forInternational Workshop, Miyazaki, Japan, 3-5 Junebiomass burning aerosols in Amazonia, J. Geophys.
2002, 2002.
Res., 103, 31,865-31,878, 1998.
Tanre,D., Y. J. Kaufman, B. N. Holben, B. Chatenet, A.
Kim, D. H., B. J. Sohn, T. Nakajima, T. Takamura, T.
Karnieli,F. Lavenu, L. Blarel, O. Dubovik, L. A.
Takemura, B. C. Choi, and S. C. Yoon, Aerosol
Remer,andA. Sminov, Climatology of dust aerosol
optical properties over East Asia determined fromsize
distribution
and optical properties derived fromground-based sky radiation measurements, J.
remotely sensed data in the solar spectrum, J.
Geophys.
Res.
109,D02209,Geophys. Res., 106, 18,205-18,217, 2001.
doi:10.1029/2003JD003387, 2004.
King, M. D., and B. M. Herman, Determination of theground albedo and the index of absorption ofatmospheric particulates by remote sensing. Part I:
Theory, J. Atmos. Sci., 36, 163-173, 1979.
Nakajima, T., and M. Tanaka, Matrix formulations forthe transfer of solar radiation in a plane-parallel
scattering atmosphere, J. Quant. Spectrosc. Radiat.
Transfer, 35, 13-21, 1986.