Fraction of DARF and Intercomparison with Previous Study

1
Chemical Apportionment of Direct Aerosol
Radiative Forcing in Korea
Jiyoung Kim and Baek-Jo Kim National Institute
of Meteorological Research, Seoul 156-720,
Republic of Korea

• Introduction
• During the recent several decades, rapid
  economic
• growth and industrialization with population
  growth
• in East Asia have resulted in an increase of
• anthropogenic pollutant emissions, which can
• cause regional air quality impairment and
  anthro-
• pogenic climate change. In addition, dust
  storms
• originating from deserts and loess plateaus
  of the
• northern China, and Inner Mongolia frequently
  sweep
• over East Asia. The right figure shows the
  MODIS-derived AOD during the recent 6-year (2000-
• 2005) over East Asia, showing enhanced aerosol
  loading over the region compared to other
• regions in the the world.
• In order to quantitatively estimate the climatic
  impacts of aerosols, understanding of various
• aerosol properties as well as their spatial
  and temporal distribution are needed. Based on
  those
• information, aerosol climate forcing could be
  quantified.
• In this study, we intended to establish new
  approach for estimating direct aerosol radiative
  
• forcing estimation. Based on the integrated
  aerosol and radiation measurements at Gosan
• during the ACE-Asia 2001 Field Campaign,
  chemical apportionment of shortwave direct
• aerosol radiative forcing (DARF) will be
  achieved and the results will be also validated
  by the

Instantaneous Aerosol Radiative Forcing at the
Surface
Left figures show measured and modeled radiative
fluxes (total, direct, and diffuse) at Gosan on
April 15 and 22, 2001 (Most clear days during the
ACE-Asia IOP). In general, there are good
agreements between two, confirming that the
approaches used in this study are reasonable. The
difference in fluxes in the early morning on
April 22 is due to the fact that we use
bulk-sampled aerosol chemical composition.
Therefore, it is assumed that there is no
temporal (diurnal) change of aerosol
concentration and composition. Although it might
be not realistic, it can be easily solved if we
use temporally-resolved chemical composition
data. Diurnal variations of shortwave direct
aerosol radiative forcing at the surface (Gosan)
are presented at the bottom of each figure. Large
fraction of the forcing can be accounted for the
coarse-mode dust. Water-soluble and EC also
largely contributed to the forcing. However,
contribution due to sea salt is not so large. It
is also noteworthy that large forcing
efficiencies of fine aerosols (e.g., EC and
water-soluble components) should be considered in
determining DARF. Although their mass
concentrations are small, they may occupy a large
fraction of the forcing. The W-type of the
forcing is due to diurnal variation of solar
zenith angle and scattering phase function of
each aerosol component.
Fraction of DARF and Intercomparison with
Previous Study
According to Kim et al. (AE2006), the IOP can be
classified into the four episodic periods (dust
April 11-13, pollution from the Chinese sector
April 14-19, pollution from the Korean sector
April 20-24, and smoke from Russian boreal
biomass burning April 25-27). Figures in the
left show the fractional distribution of DARF
classified into several aerosol types. Large
fraction of DARF is composed of mineral dust
component, especially in coarse
mode. Water-soluble and EC components also
contributed to the forcing though their mass
concentration and chemically-apportioned AOD were
not so large. The result implies that accurate
measurements of fine aerosol components as well
as coarse mode aerosol may be more important to
determine DARF and reduce the forcing
uncertainty because of their high forcing
efficiency (for example, the forcing efficiency
of soot (or EC) is 10 times higher than that of
coarse mode sea salt). The lower figure exhibits
an intercomparison result of DARF at the surface
of this study with Bush and Valero (JGR2003). In
general, DARF of this study overestimates the
forcing compared to the Bush and Valero (2003).
The reason would be caused by the fact that we
assume vertically-uniform chemical composition of
all aerosol component. However, in fact fine
pollution aerosols are more enriched near
surface. In spite of some difference between two
approaches this intercomparison confirms that
this study may be widely applicable to the field
of aerosol and climate study.
A New Approach for Estimating DARF
In order to estimate chemically apportioned
DARF, size-resolved chemical composition
(water-soluble, insoluble, EC, OC, and dust) of
aerosols are used. The aerosol optical properties
calculated by OPAC model (Hess et al., BAMS 1998)
are compared with measurements by nephelometer
and PSAP (so-called optical closure
studies). Results from optical closure studies at
the surface are extended to the whole atmospheric
column under the assumption of vertically uniform
composition of aerosol. Therefore, thickness of
aerosol layer can be obtained by using surface
extinction and optical depth data.
Chemically-apportioned AOD at 550 nm was used to
obtain the apportioned DARF as input of RTM. This
is a new approach to estimate the DARF.
Optical Closure Studies and Chemical
Apportionment of sext

• Summary and Discussion
• Chemical apportionment of shortwave direct
  aerosol radiative forcing at the surface during
  the ACE-Asia 2001 campaign was achieved by a new
  approach. In situ measurements of the physical,
  chemical, optical, and hygroscopic properties of
  East Asian aerosols were integrated to study
  extinction closure of aerosols. Results of
  extinction closure study extended into the entire
  atmospheric column to obtain the
  chemically-apportioned aerosol optical depth
  (AOD) at 550 nm. The apportioned AOD was used to
  input for estimating DARD contributed by each
  aerosol component (e.g., mineral dust,
  water-soluble, elemental carbon, and sea salt).
  Validation results of modeled radiative flux
  (total, direct, and diffuse) with measured fluxes
  by radiometers show that there are good
  agreements between two, confirming the new
  approach used in this study is reasonable.
• Major contributors of the forcing are found to be
  dust, water-soluble, and EC aerosols. Sea salt is
  a minor forcing fraction in spite of the
  geographical environment of the sampling site
  (Jeju island). This new approach for estimating
  chemically-apportioned aerosol radiative forcing
  would be readily applicable to monitor long-term
  trends of the forcing agents and their forcing
  magnitude in regional or global baseline stations
  (e.g., GAW).

Figures in the left are exhibiting the
aerosol-light scattering and absorption
coefficients in both sub-?m and super-?m size
range. We divided into two size ranges because of
different extinction efficiency with particle
size. In light scattering due to Asian aerosols,
super-?m dust component is dominant.
Water-soluble component (mainly composed of fine
particles such as sulfate, nitrate, and organic
compounds) also greatly contributes to particle
scattering while the fraction contributed by sea
salt was minor. In the figure, open circle and
vertical bar indicate daily mean and its standard
deviation, respectively. In general, the temporal
variation of calculated scattering coefficients
are well coincided with the measured scattering
coefficients (R2 0.93). In light absorption due
to particles, elemental carbon (EC) was a major
contributor of the absorption. Dust and
water-soluble also contributed to the absorption.
The temporal variation of measured and calculated
values was well coincided too (R2 0.92). In
this closure study, we assumed each aerosol type
is externally mixed in the atmosphere. And
non-spherical effect of dust particles in not
considered. However, these agreement between two
are those effect may not be large (Tang et al.,
JGR 1996).

• References
• Kim, J., S.-C. Yoon, S.-W. Kim, F. Brechtel,
  A. Jefferson, E.G. Dutton, K.N. Bower, S. Cliff,
  and J.J. Schauer,
• 2006 Chemical
  apportionment of shortwave direct aerosol
  radiative forcing at the Gosan super-site,
• Korea during ACE-Asia.
  Atmos. Environ., 40, 6718-6729.
• Kim, J., S.C. Yoon, A. Jefferson, and S.W.
  Kim, 2006 Aerosol hygroscopic properties during
  Asian dust,
• pollution, and biomass
  burning episodes at Gosan, Korea in April 2001.
  Atmos. Environ., 40 (8),
• 1550-1560.
• Kim, J., S.-C. Yoon, A. Jefferson, W.
  Zahorowski, and C.-H. Kang, 2005 Air mass
  characterization and
• source region analysis for
  the Gosan super-site, Korea during ACE-Asia 2001
  field campaign.
• Atmos. Environ., 39,
  6513-6523.
• Yoon, S.-C. and J. Kim, 2006 Influences of
  relative humidity on aerosol optical properties
  and aerosol
• radiative forcing during
  ACE-Asia. Atmos. Environ., 40, 4328-4338.
• For more details about this poster, please
  refer to the above references
• If you have any questions or comments, please
  send e-mail to Jiyoung Kim (jykim_at_metri.re.kr)