Direct radiative forcing due to anthropogenic aerosols in East Asia during April 2001

ARTICLE IN PRESS AE International – Asia Atmospheric Environment 38 (2004) 4467–4482

Direct radiative forcing due to anthropogenic aerosols in East Asia during April 2001 Lim-Seok Chang, Soon-Ung Park* School of Earth and Environmental Sciences, Seoul National University, San 56-1, Shilim-Dong, Kwanak-Ku, Seoul 151-747, Republic of Korea Received 24 October 2003; received in revised form 30 March 2004; accepted 26 May 2004

Abstract An aerosol dynamic model including such processes of nucleation, condensation/evaporation, coagulation, sedimentation, hygroscopic growth and dry and wet deposition coupled with the gas-phase chemistry of the California Institute of Technology model and the aqueous-phase chemistry of the Regional Acid Deposition Model together with meteorological outputs of the MM5 model in a grid of 60
60 km2 has been used to estimate anthropogenic aerosols in East Asia (95–145E, 20–50N) for the period of 2–30 April 2001 in the ACE-Asia experimental period. During this period an Asian dust event has been observed from 10 to 13 and 24–26 April in the Korean peninsula. The estimated anthropogenic aerosols excluding the Asian dust are implemented to estimate radiative forcing at the surface, at the top of atmosphere (TOA) and atmospheric aerosol absorption in East Asia using the National Center for Atmospheric Research column radiation model of the community climate model. The results indicate that the area averaged column integrated anthropogenic aerosol concentration in East Asia is estimated to be about 20 mg m2, of which 46%, 29%, 20%, 4% and 1% are contributed by mixed type, inorganic (IOC), sea salt, organic carbon and black carbon aerosol, respectively. The daytime area mean direct shortwave radiative forcing at the surface is found to be about 5.9 W m2, of which IOC and the mixed type aerosol contribute about 95% whereas that at TOA is about 4.1 W m2, of which the IOC and the mixed type aerosol contribute more than 90%. Consequently the area mean atmospheric absorption due to anthropogenic aerosol layer in East Asia is about 1.8 W m2. The result clearly confirms the existence of a cooling effect (negative forcing) due to the direct effect of anthropogenic aerosols at the surface and TOA in East Asia. However, the atmosphere of the troposphere above the ground is slightly heated due to absorbing aerosol layers that are composed of black carbon and the mixed type aerosol. r 2004 Elsevier Ltd. All rights reserved. Keywords: ACE-Asia; Aerosol process; Aerosol type; Column radiation model; East Asia

1. Introduction Atmospheric aerosols play a major role in the global climate system. Many researchers have conducted studies on the radiative forcing of aerosols for recent years. Aerosol particles are known to cool or warm the atmosphere directly by absorption, scattering and *Corresponding author. Tel./fax: +82-2-880-6715. E-mail address: [email protected] (S.-U. Park).

emission of solar and terrestrial radiation and indirectly by changing the albedo and the life time of clouds by acting as cloud condensation nuclei (Charlson et al., 1992). Current estimates suggest that anthropogenic aerosols and biomass burning have climate forcing enough to offset warming caused by greenhouse gases such as carbon dioxide (Kiehl and Briegle, 1993). For example the present day global mean radiative forcing due to anthropogenic aerosols is estimated to be between 0.3 and 3.5 W m2 which is comparable to

1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.05.006

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the present day greenhouse gases forcing of between 2.0 and 2.8 W m2 (IPCC, 1996). In spite of great radiative impacts of aerosols, a large uncertainty of key variables makes it impossible to correctly quantify the magnitude of radiative forcing. The uncertainties are due in part to the limited data on aerosol climatology, and in part to the lack of our understanding on the processes responsible for the production, transport, physical and chemical evolution and the removal of aerosols at various spatial and time scales. East Asia, particularly in China, is a major source of natural and anthropogenic aerosols over the Northern Hemisphere due to rapid economic expansion in many Asian countries. Tropospheric aerosols that originate in China are the complex mixture of various aerosols such as soil dust and anthropogenic particles from a variety of sources. Dust particles raised in the source regions of inland China will experience acidic pollutants and anthropogenic aerosols emitted from the industrialized regions that are heavily concentrated in the eastern parts of China during long-range transport of dust. Consequently, a significant transformation of chemical composition of the dust is expected as indicated by observations (Saxena and Seigneur, 1983; Gao et al., 1991; Kang and Sang, 1991). To understand radiative forcing of anthropogenic aerosols, we need the spatial distribution of concentrations of these pollutants. Therefore, the objective of this study is to examine the spatial distribution of concentrations of acidic pollutants and anthropogenic aerosols in East Asia using the gas-phase chemistry of the California Institute of Technology (CIT) airshed model and the aqueous-phase chemistry of the Regional Acid Deposition Model (RADM) and to investigate radiative forcing of anthropogenic aerosols only with the MM5 meteorological model for a period from 2 to 30 April 2001. During this time period Asian dust (Hwangsa in Korean) events have been observed from 10 to 13 and 24–26 April in Korea. The interaction of Asian dust with anthropogenic aerosols and their radiative forcing will not be treated here.

2. Model description The mass conservation of aerosol can be described as the general dynamic equation.
 @Ci @Ci þ rðuCi Þ ¼ r ðKrCi Þ þ con=evap @t @t

 @Ci @Ci reac þ coag þ @t @t
 @Ci source=sink; ð1Þ þ @t

Table 1 Particle size divisions (mm) Bin No.

Rmin

Rmax

R

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

0.005 0.015 0.01536 0.031 0.039 0.067 0.094 0.151 0.221 0.343 0.514 0.788 1.189 1.812 2.744 4.174 6.330 9.619 14.596 22.169

0.015 0.01536 0.031 0.039 0.067 0.094 0.151 0.221 0.343 0.514 0.788 1.189 1.812 2.744 4.174 6.330 9.619 14.596 22.169 33.652

0.010 0.015 0.023 0.035 0.053 0.081 0.123 0.186 0.282 0.429 0.651 0.988 1.501 2.278 3.459 5.252 7.974 12.107 18.383 27.910

Rmin and Rmax represent the starting and ending radius of each bin and R is the mid-point.

where Ci is the mass concentration of species i of particles, K the eddy diffusivity and ð@Ci =@tÞ con=evap; ð@Ci =@tÞ reac; ð@Ci =@tÞ coag; ð@Ci =@tÞ source=sink are, respectively, the changes of a composition due to condensation/evaporation, heterogeneous reaction on the surface of aerosol, coagulation and source/sink. The operator-splitting technique (Chang et al., 1987) is applied to solve the Eq. (1) according to following order, TxTyTz,cTaTzTyTx, where Tx, Ty, Tz,c, Ta represent transport in x, y, z direction, gas chemistry and aerosol dynamics, respectively. Individual terms in Eq. (1) are independently computed in parallel for a short time period. Aerosol process includes emission, homogeneous nucleation, condensation, coagulation, transport, sedimentation, dry and wet deposition. Five aerosol types including black carbon (BC), organic carbon (OC), inorganic (IOC), IOC–BC–OC and sea salt (SS) and 20 bins for each aerosol type are treated (Table 1) in the model. The chemical component in each aerosol type (Table 2) is assumed to be constant in a size bin and internally mixed. 2.1. Gaseous chemistry The gas chemistry of CIT is used to simulate the gaseous species concentrations. The chemical mechanism of CIT (Russell et al., 1988) treats 29 species and 52 reactions including 8 photolytic reactions. This model is expanded to include the troe reaction of sulfur dioxide

ARTICLE IN PRESS L.-S. Chang, S.-U. Park / Atmospheric Environment 38 (2004) 4467–4482 Table 2 Chemical components of each type of aerosol

applied through all Knudsen number is Hi ¼

Name of aerosol type

Chemical components

IOC BC OC IOC–BC–OC Sea salt (SS)

 þ SO2 4 ; NO3 ; NH4 ; H2 O BC OC  þ SO2 4 ; NO3 ; NH4 ; BC; OC; H2 O Na+, Cl, H2O

with OH that is not negligible at dry air. The aqueousphase chemistry is adapted from the original RADM model (Chang et al., 1987; Walcek et al., 1986). The dissociation of absorbed compound into ions, oxidation of S(IV) to S(VI) and wet deposition are calculated. The numerical integration of the stiff systems of ordinary differential equations has been performed with the implicit solver of the so-called Livermore Solver for Ordinary Differential Equations (LSODE, Hindmarsh, 1983). 2.2. Aerosol process 2.2.1. Nucleation Nucleation process is an important mechanism for the new particle production. Homogeneous nucleation is assumed to occur only with H2SO4/H2O when the critical gas-phase sulfuric acid concentration exceeds the critical value (Ccrit), which can be evaluated empirically (Wexler, 1994). Ccrit ¼ 0:16 expð0:1T  3:5RH  27:7Þ;

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ð2Þ

where T is the temperature in Kelvin , RH the relative humidity on a scale of 0–1 and Ccrit in mg m3. 2.2.2. Condensation/evaporation The condensation/evaporation is the gas to particle transfer process with changing of mass and size of particles. Growth and shrinkage are driven by the difference between the concentration just above the particle surface and that in the bulk gas. If a normalized particle diameter is independent variable, the condensation/evaporation term is given by
 @Ci ðZj ; tÞ con=evap ¼ Hi ðZj ; tÞCðZj ; tÞ @t 1 @HCi ðZj ; tÞ  ; ð3Þ @Zj 3
 Dpj ; Dpj is the particle diameter in the j where Zj ¼ ln Dpo bin, Dpo the smallest particle diameter, C the sum of mass concentrations of all species in an aerosol type, Hi the normalized condensation/evaporation rate of species P i, H ¼ Hi and t the time. The condensation/evaporation rate by Wexler and Seinfeld (1991) which can be

1 dmji 2pDpj Di Cai  Cei ¼ ; mi dt mi Bþ1

ð4Þ

where B ¼ 2l=aDpj ; a the accommodation coefficient, l the mean free path of air, mji the mass of i species in the j bin, mi the sum of the masses of all components in the j bin, Di the molecular diffusivity of condensing species i in air, Cai and Cei the concentrations of the bulk gas phase and that at the particle surface. The accommodation coefficient has been estimated to range from near unity to 104 (Huntzicker et al., 1980; Li et al., 1993; Dingenen and Raes, 1991). Hybrid method was taken to describe gas to particle mass transfer. For small particle, the aerosol composition is determined by the bulk aerosol thermodynamics using the model for aerosol reacting system (MARS) which is based on fundamental thermodynamic concepts and computationally efficient while maintaining reasonable agreement with EQUIL and KEQUIL (Saxena et al., 1986). Even if the aerosol is in equilibrium with the gas phase, the size distribution of aerosol components often cannot be uniquely determined from equilibrium. The fraction of condensate in each size bin is proportional to aerosol surface area (Pandis et al., 1993). If vapor pressure is assumed to be constant, the fraction of condensate i in the j bin is given by fji ¼ R N 0

2Ci =mi pDPj Di ðCai  Cei ÞðB þ 1Þ1 : 2Ci =mi pDPj Di ðCai  Cei ÞðB þ 1Þ1 dDpj

ð5Þ

For large particles, the vapor concentration for the volatile inorganic compounds is estimated by the thermodynamic model and then condensation/evaporation Eq. (3) is solved numerically by Bott’s method (Dhaniyala and Wexler, 1996). 2.2.3. Coagulation Coagulation occurs when Brownian motion, differential sedimentation, or turbulent flows force two particles or drops into contact and the particles stick to form an aggregate or a larger drop. The coagulation rate depends on coagulation kernel and the number concentration of two colliding particles. If a particle volume is the independent variable, the coagulation is given by
 Z @Ci ðvÞ 1 v Ci ðv  v0 Þ 0 coag ¼ bðv  v0 ; v0 ÞCi ðv0 Þ dv @t 2 0 v  v0 Z N Ci ðv0 Þ  Ci ðvÞ bðv; v0 Þ 0 dv0 ; ð6Þ v 0 where bðv; v0 Þ is the coagulation kernel between particles of volumes v and v0 : The coagulation kernel is estimated according to the following colliding mechanisms: (a) Brownian: For the Brownian coagulation process, the collision kernel is classified into three (or four) size regions: the free-molecule region, the transition region,

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and the continuum region and near-continuum region (Leed et al., 1980). Pratsinis (1988) expressed the coagulation coefficient in terms of the harmonic mean between the free-molecular and near-continuum expressions. For the free-molecule region, 1=3

bfree ¼

2kb Tðvi

1=3

þ vj Þ2

ðvi vj Þ1=3

ð7Þ

:

For the continuum region

AðDp Þ ¼ 1:5

!

bcon ¼

2kb T 1=Df 1 1 1=D ðvi þ vj f Þ 1=D þ 1=D : f 3m vi vj f

ð8Þ

For the free-molecular and near-continuum bbrowin ¼

bfree bcon ; bfree þ bcon

ð9Þ

where vi, vj is the volume of aerosol in the i and j bin, respectively, kb the Boltzman constant, m the air dynamic viscosity, Df the mass fractal dimension which is 3 for a sphere and less than 2 for an agglomerated BC. (b) Turbulent shear: In turbulent flow, particles can collide with each other by the shear-induced velocity difference. So the collision kernel is determined by relative velocity and sum of radius of two colliding particles. The analysis of Safman and Turner (1956) gives the coagulation coefficient as
 pek 1=2 bts ¼ ðDp1 þ Dp2 Þ; ð10Þ 120Da where ek is dissipation of kinetic energy per unit mass and Da the kinematic viscosity of the fluid, and Dp1 and Dp2 are the diameter of colliding particles. (c) Sedimentation: When heavy particles move fast, they catch up with lighter particles and collide with them. Under conditions of a quasi-steady relative motion between particles, the collision frequency between particles of differing velocities can be given by (Leed et al., 1980)
 3 2pgrp 2 2 bgrav ¼ yc jDp1  D2p2 j; ð11Þ 32 9m yc ¼ Dp1 yc ¼ Dp2

if if

2.2.4. Dry and wet deposition The dry deposition process of aerosol uses the inferential method (Wesely et al., 1985; Park, 1989) with taking into account the gravitational settling velocity. Wet deposition occurs when aerosol particles enter clouds acting as a nuclei and washout by colliding with falling rain droplets. The scavenging coefficient A, depends on the rainfall intensity and particle size (Seinfeld, 1986). EðDw ; Dp Þpo ; Dw

ð12Þ

where Dw is the representative raindrop diameter, E(Dw,Dp) the collision efficiency and po the rainfall intensity. The collision efficiency follows the parameterization of Slinn and Slinn (1980) which includes the Brownian diffusion, interception and inertial impaction. The mass scavenging rate is given by integrating scavenging coefficient times particle size distribution in all particle size interval. 2.2.5. Hygroscopic growth Hygroscopic growth of the mixture aerosol differs significantly from that of pure ionic species. ZSR relationship (Zdanovskii, 1948; Stokes and Robinson, 1966) is used to calculate the water content of mixture aerosols for inorganic aerosol such as IOC type (Wexler and Seinfeld, 1991). But for the mixture of organic and inorganic aerosol such as IOC–BC–OC type the semiempirically determined growth model seems to be more appropriate. The semi-empirical growth model (Hanel, 1976) is given by
  Dp 3 RH 1=3 ¼ 1 þ Es rs EH
; ð13Þ Dpoo 1  RH

Dp1 pDp2 ; Dp2 pDp1 ;

where rp is the particle density and g the gravity. The volume conserving semi-implicit solution is derived from the coagulation equation (6) (Jacobson et al., 1994). It is assumed that when the external type particles hetero-coagulate with any particle except for the same type of particle, the resulting particle enters a multi-component mixture particle. In this study the coagulation among IOC, BC and OC is considered only.

Fig. 1. The model domain and monitoring sites of the aerosol chemical composition (d) and optical properties of aerosols (+).

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Fig. 2. Spatial distributions of (a) SO2, (b) NOx, (c) NMVOC, (d) NH3, (e) BC and (f) OC emissions in a grid 60
60 km2 (Gg gridcell1 yr1) over the whole model domain.

where Dp is the particle diameter at a given relative humidity RH, Dpoo the diameter of a dry particle, rs the density of the dry particle, Es the soluble fraction of dry mass and EH the composite function defined as (William

and Kreidensweis, 1997) EH ¼ Iv e

Mw ; /Ms S

ð14Þ

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SO2

10

40

20

0

0

0

(a)

NO2

60

MODEL (ppb)

MODEL (ppb)

20

10

0

20

20

60

OBS. (ppb)

(b)

OBS. (ppb)

40

O3

100

MODEL (ppb)

80 60 40 20 0

0 (c)

50

100

OBS. (ppb)

Fig. 3. Scatter plots, along with the 1:1 line, of modeled and observed concentrations (ppb) of (a) SO2, (b) NO2, (c) O3 averaged 8 sites over south Korea during the non-Asian dust period.

where e is the dissolved fraction of the aerosol mass, Iv the van’t Hoff factor, Mw the molecular weight of water and /MsS the average molecular weight of solute. The empirically determined EH value for continental urban aerosols (Sloane and Wollff, 1985) was used for the IOC–BC–OC type aerosol. When a modeled particle of volume takes water, the intermediate particle is partitioned between two bins by volume fractions of the intermediate particle (Jacobson et al., 1994).

chemical components (Appendix A) obtained from highresolution transmission molecular absorption database (HITRAN) except for OC and used as input data for the CRM. The refractive index of OC is assumed to be 1.5 for the real part and 0.001 for the imaginary part over all spectrum range. For the mixture type aerosols the two effective medium theories such as Burggeman and Maxwell Garnet (Bohren and Huffman, 1983; Cylek et al.,1988) are used to estimate the refractive index following the structure of the aerosol.

2.3. Radiation 3. A case study The Column Radiation Model (CRM) of Community Climate Model 3 (CCM3) (Kiehl et al., 1998; Briegleb, 1992) is used to estimate aerosol direct radiative forcing (ADRF) at the top of atmosphere (TOA) and the surface. The CCM3 CRM uses an d-Eddington approximation with 18 spectral intervals (7 for O3, 1 for the visible, 7 for H2O, and 3 for CO2) spanning the solar spectrum from 0.2 to 5.0 mm. The optical properties of aerosols at 11 spectral interval wavelengths are calculated by a Mie code with the refractive indices of several

The Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) took place during the spring of 2001 over the Asian-Pacific region. It was designed to investigate the aerosol properties and radiative forcing in eastern Asia and the northwestern Pacific. The period of 2–30 April 2001 in this campaign period was selected to make comparisons of model results with observations. ACE-Asia special observation sites that are used in this study are given in Fig. 1. During this chosen

ARTICLE IN PRESS L.-S. Chang, S.-U. Park / Atmospheric Environment 38 (2004) 4467–4482 Sulfate

20

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OC

3

MODEL

MODEL

15 10

2

1

5 0

0 0

5

(a)

10 OBS.

15

20 (d)

0

Nitrate

1

OBS.

2

3

BC

20

1

MODEL

MODEL

15 10

0.5

5

0

0

0

5

(b)

10

15

20

OBS.

0 (e)

0.5

1

OBS.

Ammonium

MODEL

10

5

0 0 (c)

5

10

OBS.

Fig. 4. The same as in Fig. 3 except for modeled and observed aerosol concentrations (mg m3) of (a) sulfate from Kwangju, Kosan and Wan-Li, (b) nitrate from Kwanju and Kosan, (c) ammonium from Kwanju, Kosan and Wan-Li, (d) OC and (e) BC from Rhshiri, Sado, Hachijo and Chichijima.

period weak Asian dust (Hwangsa) events are observed on 10–13 and 24–26 April in Korea. However, Asian dust aerosol is not taken into account in this study to examine the capability of the presently developed aerosol dynamic model for anthropogenic aerosols only.

4. Gaseous pollutants and carbon emissions Asian Pacific Regional Aerosol Characterization Experiment (Ace-Asia) Web site (http://www.cgrer. uiowa.edu/people/carmichael/ACESSEmission-data main.

html) emission data are used for SO2, NOx, NH3, CO, non-methane volatile organic carbon (NMVOC), BC and OC. The Global Emission Inventory Activity (GEIA) are used for biogenic organic compound (Isoprene), respectively. Emission estimations of SO2, NOx and NH3 by Park et al. (1997) are used in South Korea. The emission from biomass burning is not considered here. The emissions of NO and NO2 are, respectively, given by 90% and 10% of the total NOx emission in the whole domain. The emissions of 6 lumped hydrocarbon species (HCHO, RCHO, OLE, ALK, ARO, C2H4) are calculated from total

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Fig. 5. Spatial distributions of monthly mean column integrated concentrations (mg m2) of (a) IOC, (b) BC, (c) OC, (d) Sea Salt, (e) IOC–BC–OC and (f) Total during non-dust period in April 2001. The daily averaged mean wind vectors at 850 hPa are shown.

anthropogenic NMVOC divided by hydrocarbon splitting factor including isoprene that is classified as olefin. Fig. 2 shows spatial distributions of SO2 , NOx, NMVOC, NH3, BC and OC emissions in a grid of 60
60 km2. The emission amounts of SO2, NOx,

NMVOC, NH3, BC and OC in the whole domain are 2.57
107, 1.43
107, 1.00
108, 1.75
107, 9.5
105, 2.69
106 t yr1, respectively. The dominant source regions of most of pollutants are eastern China, southeastern part of South Korea and near Osaka and Tokyo in Japan.

ARTICLE IN PRESS L.-S. Chang, S.-U. Park / Atmospheric Environment 38 (2004) 4467–4482

Urban area (Seoul) ∂C/ ∂log Dp

4 3 2 1 0 0.01 (a)

0.1 1 Diameter (µm)

10

Remote area (Kosan) ∂C/ ∂log Dp

4 3 2 1 0 0.01 (b)

0.1

1

10

Diameter (µm)

Fig. 6. Time averaged size-resolved surface aerosol concentrations of BC ( ), sulfate(–), nitrate ( ), ammonium ( ) and OC ( ) at the (a) urban area and (b) remote area during non-dust period.

The number of sea salt aerosol emission F (particle m2 s1) is estimated as (Gong et al., 1997) dF 3 1:19 expðG 2 Þ ¼ 10:98 u3:41 ; 10 Dp ð1 þ 0:104Dp Þ10 dDp

ð15Þ

where G=(1.073-log Dp)/0.65, u10 (m s1) is the wind speed at 10 m. The spectral BC and OC emissions are assumed to be constant with the diameter less than 1 mm.

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with 1:1 line. The observed aerosol data are not sufficient enough to get any statistical significance of the model results but the model simulates aerosol concentrations of sulfate, nitrate, ammonium, OC and BC within the error of 73 mg m3 with slightly more scatter in organic carbon aerosol. This may be due to not taking into account secondary organic aerosols. Fig. 5 shows spatial distributions of the column integrated monthly mean aerosol concentrations during the non-dust period in April. All types of aerosols except for the sea salt aerosol that is emitted from the sea have similar spatial distribution patterns with maxima over eastern central China. The most abundant aerosol type in East Asia is mixed type aerosol (IOC–BC–OC) and  þ follows by IOC ðSO2 4 þ NO3 þ NH4 Þ: The BC concentration is relatively low. The sea salt aerosol is relatively large and mainly confined over the sea. Fig. 6 shows mean size-resolved mass concentration near the surface averaged for the non-dust period at the urban site (Seoul in Korea) and the remote site (Kosan in Korea). In the urban area where emissions are large, the size growth of hydrophobic aerosols such as BC and OC is largely controlled by the coagulation process, thereby resulting in fine mode aerosols, whereas that of hygroscopic aerosols is controlled by both the coagulation and hygroscopic process thereby resulting in larger particle sizes than that of hydrophobic aerosol in the accumulation mode (Fig. 6a). However, in the remote area where emissions are small most of aerosols are contributed by long-range transport, suggesting more chances for the hygroscopic aerosol to grow bigger through the hygroscopic process. Consequently the size of the hygroscopic aerosol in the remote area (Fig. 6b) is larger than that in the urban area (Fig. 6a) in the accumulation mode.

5. Results 5.2. Estimation of aerosol direct radiative forcing 5.1. Simulated model concentrations The comparisons of modeled and observed hourly surface concentrations of SO2, NO2 and O3 averaged over South Korea for the period from 2 to 30 April (Fig. 3) excluding the Asian dust period (10–13 and 24– 26 April) show that the model underestimates SO2 and NO2 concentrations (Figs. 3a,b) but simulates quite well O3 concentration (Fig. 3c). This is probably due to uncertainties in emission estimations. The diurnal variation of these concentrations are also well simulated (not shown here). The scatter plots of the daily mean concentrations of the modeled sulfate, nitrate, ammonium, BC and OC aerosols against those observed at Kwangju and Kosan in Korea, Sado, Rishiri, Hachijo and Chichijima in Japan and Wan-Li in Taiwan (Fig. 1) for the period of non-Asian dust period in April are given in Fig. 4 along

Vertical profiles of temperature, pressure, H2O mixing ratio obtained from the MM5 simulation and those of ozone and CO2 mixing ratios from the CRM model are used for the initial profiles in the CRM model. The aerosol direct radiative forcing is obtained as the difference in shortwave net radiative fluxes at TOA and the surface between CRM simulations with and without aerosol mass loading simulated by the present model. Fig. 7 shows the time series of the modeled and observed daily mean aerosol optical depths (AOD) at the ACE-Asia experimental sites (Fig. 1). The observed AOD (http://aeronet.gsfc.nasa.gov) is averaged one at 0.44 and 0.67 mm wavelengths whereas the model is taken at 0.5 mm wavelength. The model simulates slightly low values of AOD at all sites compared with observations. Relatively large difference in the periods

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OBS.

MODEL

Xianghe 1.5

AOD

1

0.5

0

(a)

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Anmyon

AOD

1

0.5

0 2

(b)

3

4

5

6

7

8

9

10 11 12 13 14 15

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Shirahama

AOD

0.6 0.4 0.2 0 2

(c)

3

4

5

6

7

8

9

10

11 12 13

14 15

16 17 18

19 20 21

22 23

24 25 26

27 28 29

30

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Noto

AOD

0.4

0.2

0 2

(d)

3

2001 APRIL

Fig. 7. Daily averaged observed (’) and modeled (—) aerosol optical depths at 0.5 mm wavelength at (a) Xianghe in China, (b) Anmyondo in Korea and (c) Shirahama and (d) Noto in Japan.

of 9–11 and 25–26 might be associated with Asian dust events (Fig. 7). Fig. 8 shows the spatial distribution of daytime mean shortwave direct radiative forcing at the surface (SRF) due to each type of aerosol. The spatial distribution pattern of SRF quite resemble that of the column integrated aerosol concentration (Fig. 5). The direct radiative forcing at the surface due to IOC is more than 5 W m2 in eastern central China (Fig. 8a). However, the direct radiative forcing at the surface contributed by BC, OC and sea salt aerosols (Figs. 8b–d) is relatively small due to their low concentrations. The radiative forcing at the surface due to all types of anthropogenic

aerosols (Fig. 8f) is larger than 10 W m2 over eastern China, Korea and northern Japan due to mainly the mixed type aerosol and IOC. This value is about 20% of the value estimated by Redemann et al. (2000) at the east coast of the US over the Atlantic Ocean. Fig. 9 shows the spatial distributions of the daytime mean SRF at the TOA. The spatial distribution patterns of the direct radiative forcing at TOA due to IOC (Fig. 9a), OC (Fig. 9c) and sea salt (Fig. 9d) aerosols quite resemble those at the surface (Fig. 8) with almost the same magnitude, suggesting negligible amounts of atmospheric absorption of shortwave radiation due to these types of aerosols. However, the spatial distribution

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Fig. 8. Daytime mean direct radiative forcing (W m2) at the surface due to (a) IOC, (b) BC, (c) OC, (d) Sea Salt, (e) IOC–BC–OC and (f) total aerosol during the non-dust period in April 2001.

of the radiative forcing at TOA attributed by BC (Fig. 9b) and the mixed type aerosol (Fig. 9e) is quite similar to that at the surface (Fig. 8) but their magnitudes are much reduced, implying large amounts of atmospheric absorption of shortwave radiation caused by BC and the mixed type aerosol. The total aerosol radiative forcing at TOA (Fig. 9f) has a similar distribution pattern to that at the surface (Fig. 8f) but its value is much reduced. This is attributed to the highly absorbing aerosols including BC and the mixed type aerosol (IOC–BC–OC). The maximum radiative forcing

at TOA is about 10 W m2 that occurs in eastern China and central Korea. This is largely contributed by the mixed type aerosol and IOC aerosol which has the single scattering albedo of 0.98. Fig. 10 shows the vertical profiles of daily mean aerosol concentrations and their daytime net radiative fluxes averaged over South Korea for the non-dust in April. The vertical distribution of aerosol concentrations varies quite significantly in association with meteorological conditions. Consequently, radiative forcing at all levels also varies significantly with time. The average

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Fig. 9. The same as in Fig. 8 except for TOA radiative forcing.

radiative forcing at the surface over South Korea (Fig. 10b) is largely contributed by the mixed type aerosol and IOC due to their high concentration near the surface (Fig. 10a) and high value of single scattering albedo. It is worthwhile to note that radiative forcing due to non-absorbing aerosols including OC and IOC aerosol does not change with height, whereas that of absorbing aerosols including BC and the mixed type aerosol (IOC–BC–OC) decreases with height. This is due to the fact that the downwelling radiation flux decreases with decreasing height due to aerosol scatterings. However, the upwelling radiation flux increases with height due to aerosol. The downwelling radiation flux decrease is compensated by

the upwelling radiation flux increase due to aerosol. This results in constant radiative forcing with height for OC and IOC aerosols. However, for the absorbing aerosols such as BC and mixed type aerosol both upwelling and downwelling radiation flux are absorbed in the aerosol layer that has a maximum concentration in the lower troposphere, thereby resulting in large negative radiative forcing in the lower layer with a decreasing trend with height. The vertical profiles of mean total anthropogenic aerosol concentration and daytime aerosol radiative forcing averaged over South Korea for the non-dust period in April in Fig. 11 clearly indicate that high aerosol concentration occurs near the surface and

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14 BC OC IOC SEA SALT IOC-BC-OC

10

10

8 6

8 6

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4

2

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0 0.01

0.1

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0 -6.00 -5.00 -4.00 -3.00 -2.00-0.08 -0.06 -0.04 -0.02 0.00 0.02

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CON. (µg m -3)

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HEIGHT (km)

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HEIGHT (km)

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W m-2

(b)

14

14

12

12

10

10 HEIGHT (km)

HEIGHT (km)

Fig. 10. Vertical profiles of (a) aerosol concentration (mg m3) and (b) daytime net radiative forcing (W m2) due to: BC (), OC(’), IOC (J), Sea salt (+) and IOC–BC–OC(&) averaged over South Korea during the non-dust period.

8 6

6

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0 5

(a)

8

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0 -10.00

30

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CON. (µg m )

(b)

-8.00

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-4.00

-2.00

0.00

W m-2

Fig. 11. Vertical distributions of (a) the mean total aerosol concentration (mg m3) and (b) the mean total daytime aerosol radiative forcing (W m2) averaged for the non-dust period.

decreases rapidly with height. The averaged short wave aerosol radiative forcings are about 7 W m2 at the surface and about 5 W m2 at the top of atmosphere over South Korea, which implies that anthropogenic aerosols heat the atmosphere over South Korea by about 2 W m2 in this period. The fractional contributions of each type of aerosol to the time-area averaged column integrated total mass, daytime radiative forcing at the surface and at TOA averaged in the whole analysis domain (East Asia) for the non-dust period in April are shown in Fig. 12. Averaged total aerosol mass in East Asia is about 20 mg m2, of which 46% and 29% are, respectively, contributed by the mixed type aerosol and IOC (Fig. 12a). The contribution of BC to the total aerosol mass is

the least (about 1%). Mean aerosol radiative forcing at the surface is about 6 W m2, of which 64% and 31% are, respectively, contributed by the mixed type and IOC aerosol. The contribution of BC and OC to the radiative forcing at the surface is small. On the other hand, mean aerosol radiative forcing at TOA is about 4.0 W m2. Most of this is contributed by IOC–BC–OC (50%) and IOC (43%).The contribution of the other types of aerosols to radiative forcing at TOA is small (Fig. 12c), suggesting the dependency of radiative forcing on different types of aerosols. The difference between radiative forcing at TOA and at the surface leads to estimate the aerosol absorption in the atmosphere. The presently estimated atmospheric aerosol absorption in East Asia is about 1.5 W m2. This implies that aerosols

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Aerosol mass IOC 20% IOC-BC-OC 46%

BC 1%

-2

20 mg m

OC 4% sea salt 29%

(a) Radiative forcing at the surface

IOC 31% OC 1%

-5.9W m-2

BC 0.5%

IOC-BC-OC 64% sea salt 3.5%

(b) Raditive forcing at TOA IOC-BC-OC 50%

IOC 44%

-4.1W m-2

BC 0.01%

(c)

sea salt 7%

OC 0.01%

Fig. 12. The fractional contributions of various aerosols on the (a) total mass (mg m2), (b) daytime direct radiative forcing (W m2) at the surface and (c) at TOA averaged for the nondust period in April 2001 in East Asia.

slightly heat the atmosphere over East Asia due to the absorbing aerosol.

6. Conclusions An aerosol dynamic model including aerosol processes of nucleation, condensation/evaporation, coagulation, sedimentation, dry and wet deposition and hygroscopic growth coupled with the gas-phase chemistry of the CIT airshed model and the aqueous-phase chemistry of the RADM with the MM5 meteorological model in a grid of 60
60 km2 has been used to estimate anthropogenic aerosols in East Asia (90–145E, 20–50N) for the non-dust period in April 2001 in the ACE-Asia

experimental period. The modeled anthropogenic aerosols are implemented to estimate radiative forcing at the surface and TOA and atmospheric aerosol absorption in East Asia using the NCAR CCM3 CRM model. The results show that the area averaged column integrated anthropogenic aerosol concentration is found to be about 20 mg m2, of which 46%, 29%, 20%, 4% and 1% are, respectively, contributed by the mixed type (IOC–BC–OC), IOC, sea salt, OC and BC aerosols, implying more than half of the anthropogenic aerosol being contributed by the transformed aerosol in East Asia. The area mean daytime radiative forcing at the surface is found to be about 6.0 W m2, of which more than 90% are contributed by mixed type aerosol and IOC. The sea salt and OC aerosols occupy more than 33% of the total anthropogenic aerosol mass in East Asia but their contribution to direct short wave radiative forcing at the surface are less than 5%, suggesting the importance of aerosol types for the estimation of radiative forcing at the surface. The area mean daytime direct radiative forcing at TOA in East Asia is found to be about 4.0 W m2, of which more than 90% is contributed by the mixed type aerosol and IOC aerosol, suggesting the importance of the transformed aerosols on radiative forcing at the top of atmosphere. The area mean daytime atmospheric absorption due to aerosol layer in East Asia is estimated to be about 2 W m2. This value is quite smaller than that value estimated by Yu et al. (2001) in the southeastern US. This study confirms the existence of a cooling effect (negative forcing) due to the direct effect of aerosol at the surface and TOA in East Asia. However, the atmosphere of the troposphere above the ground is still heated with the area averaged rate of 2.0 W m2 by the absorbing aerosol layer that consists of BC and the mixed type aerosol. This study mainly pertains to estimate direct radiative forcing due to anthropogenic aerosols transformed from various precursors emissions in East Asia. However, in this region especially in spring mineral dusts originated from desert areas of inland China are frequently dominated. In fact, a weak Asian dust events have been observed twice in Korea during the analysis period. The radiative forcing due to anthropogenic aerosol together with dust aerosols is prerequisite to understand the role of aerosols in the climate change in East Asia. Nevertheless, this also requires dust emission, transformation and transport processes that are now at hand.

Acknowledgements This research was partially supported by the Climate Environment System Research Center in Seoul National University and the Ministry of Education under the BK

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3.35E08 1.00E07 0.00E+00 4.50E01 5.00E06

3.35E08 1.00E07 0.00E+00 4.50E01 5.00E06

3.35E08 1.00E07 0.00E+00 4.50E01 5.00E06

1.60E08 1.00E07 0.00E+00 4.70E01 2.00E06

1.60E08 1.00E07 0.00E+00 4.70E01 2.00E06

1.18E09 1.00E07 0.00E+00 4.50E01 1.00E08

1.90E02 2.00E-03 2.36E01 5.20E01 7.00E03

1.35 1.55 1.34 1.92 1.48 1.36 1.55 1.33 1.62 1.51 1.40 1.55 1.33 1.50 1.51

1.36 1.55 1.33 1.62 1.51

1.36 1.55 1.33 1.62 1.51

1.36 1.55 1.33 1.62 1.51

1.35 1.54 1.33 1.74 1.51

1.35 1.54 1.33 1.74 1.51

1.33 1.53 1.33 1.75 1.50

1.19 1.41 1.26 1.83 1.40

References

1.19 1.41 1.14 1.83 1.40

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21 program. An anonymous reviewer’s comments are appreciated.

(1) Real part of refractive index Water Ammonium sulfate Nitric acid Black carbon Sea salt (2) Imaginary part of refractive index Water Ammonium sulfate Nitric acid Black carbon Sea salt

0.2–0.245 0.245–0.265 0.265–0.275 0.275–0.285 0.285–0.295 0.295–0.305 0.305–0.350 0.350–0.7 0.7–5.0 Wavelength (mm)

Appendix A. Refractive indices used in the present model

2.63–2.86 4.16–4.5

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