Environ Monit Assess (2012) 184:4763–4775 DOI 10.1007/s10661-011-2300-7

Columnar aerosol optical and radiative properties according to season and air mass transport pattern over East Asia Young M. Noh & Detlef Müller & Hanlim Lee & Kwonho Lee & Young Joon Kim

Received: 21 October 2010 / Accepted: 15 August 2011 / Published online: 6 September 2011 # Springer Science+Business Media B.V. 2011

Abstract The column-integrated optical and radiative properties of aerosols in the downwind area of East Asia were investigated based on sun/sky radiometer measurements performed from February 2004 to June 2005 at Gwangju (35.23° N, 126.84° E) and Anmyeon (36.54° N, 126.33° E), Korea. The observed aerosol data were analyzed for differences among three seasons: spring (March–May), summer (June–August),

Y. M. Noh : D. Müller : Y. J. Kim (*) School of Environmental Science & Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, South Korea e-mail: [email protected] Y. M. Noh e-mail: [email protected] D. Müller e-mail: [email protected] H. Lee Department of Atmospheric Sciences, Yonsei University, Seoul 120-749, South Korea e-mail: [email protected] K. Lee Department of Satellite Geoinformatics Engineering, Kyungil University, Buhori 33, Hayangeub, Gyeongsan, Gyeongsangbuk-do, South Korea e-mail: [email protected]

and autumn/winter (September–February). The data were also categorized into five types depending on the air mass origin in arriving in the measurement sites: (a) from a northerly direction in spring (SN), (b) from a westerly direction in spring (SW), (c) cases with a low Ångström exponent (<0.8) in spring (dust), (d) from a northerly direction in autumn/winter (AWN), and (e) from a westerly direction during other seasons (AWW). The highest Ångström exponents (α) at Gwangju and Anmyeon were 1.43±0.30 and 1.49±0.20, respectively, observed in summer. The lowest column-mean singlescattering albedo (ω) at 440 nm observed at Gwangju and Anmyeon were 0.89±0.02 and 0.88±0.02, respectively, during a period marked by the advection of dust from the Asian continent. The highest ω values at Gwangju and Anmyeon were 0.95±0.02 and 0.96± 0.02, respectively, observed in summer. Variations in the aerosol radiative-forcing efficiency (β) were related to the conditions of the air mass origin. The forcing efficiency in summer was −131.7 and −125.6 Wm–2 at the surface in Gwangju and Anmyeon, respectively. These values are lower than those under the atmospheric conditions of spring and autumn/ winter. The highest forcing efficiencies in autumn/ winter were −214.3 and −255.9 Wm–2 at the surface in Gwangju and Anmyeon, respectively, when the air mass was transported from westerly directions. Keywords Aerosol . Sun/sky radiometer . Ångström exponent . Single-scattering albedo . Radiative forcing

4764

Introduction Atmospheric aerosol concentrations and their optical and microphysical properties are one of the main sources of uncertainty in current assessments and predictions of regional and global climatic change [Intergovernmental Panel on Climate Change (IPCC) 2007]. Aerosol interactions result in both direct radiative forcing by scattering and absorption of sunlight and indirect effects that influence cloud formation and lifetime (Ackerman et al. 2000; Hansen et al. 1997; Jacobson 2001; Yu et al. 2007). Direct radiative forcing by aerosols is primarily a function of aerosol concentration in the total atmospheric column (e.g., aerosol optical depth), aerosol optical properties (e.g., extinction coefficient and single-scattering albedo), and temporal and spatial variability in aerosol concentrations (Eck et al. 2005; Yoon et al. 2005; Liu et al. 2008). In particular, the absorption of solar radiation by atmospheric aerosol is important in understanding the effects of aerosol on climate (Bergstrom et al. 2002). The light-absorption characteristics of aerosol are critical for investigations of the aerosol semi-direct effect (Hansen et al. 1997), whereby absorbing aerosols modify the heating rates of the surface and aerosol layer, thereby changing the stability of the atmosphere and potentially influencing the lifetime of clouds (Koren et al. 2004). In addition, accurate knowledge of the absorption properties of atmospheric aerosols is required for assessing the magnitude of direct radiative forcing at the top of the atmosphere and at the Earth’s surface (Ramanathan and Carmichael 2008). The aerosol single-scattering albedo (ω), defined as the ratio of aerosol light-scattering to total extinction, is a key parameter in determining the influence of aerosols on global and regional climate change (Novakov et al. 2003). A change in ω from 0.9 to 0.8 may change the sign of radiative forcing from negative to positive, depending on the reflectance of the underlying surface and the altitude of the aerosols (Hansen et al. 1997). The combined influences of arid dust production, large regional populations, and increasing fossil-fuel use mean that the East Asian region often experiences very high concentrations of tropospheric aerosols. This aerosol burden may increase in the future as both the population and economic activity (and associated combustion of fossil fuel) continue to grow (Lee et al. 2006). With the increase in concern regarding the properties of aerosols in East Asia, many studies have

Environ Monit Assess (2012) 184:4763–4775

sought to understand the optical characteristics of atmospheric aerosols in the region using various instruments. For example, the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) field campaign was conducted primarily in South Korea, Japan, China, and adjacent oceanic regions from late March through May 2001 to identify the complex aerosol mixtures (pollution and dust) in the region (Huebert et al. 2003). Lee et al. (2007) derived ω across China for a period of 1 year, providing insight into spatial and temporal variations in ω based on simultaneous measurements of direct transmittance (using hand-held Sun photometers on the ground) and reflected radiance at the top of the atmosphere (using spaceborne MODIS data; Kaufman et al. 2005). Kim et al. (2004), Eck et al. (2005), and Kim et al. (2010) examined the optical properties of aerosol over East Asia based on long-term measurements of the Sky Radiometer Network (SKYNET) and Aerosol Robotic Network (AERONET), respectively. While China has been the focus of many investigations that sought to gain a better understanding of optical properties of aerosols, there is a lack of continuous measurement data in downwind areas of East Asia. Aerosol studies in downwind areas are important in terms of understanding regional climate change, air quality, and temporal changes in optical properties during aerosol transport. To gain a better understanding of seasonal variations of aerosol optical properties based on air mass patterns in Korea, we analyzed continuous AERONET sun/sky radiometer measurements carried out in Gwangju and Anmyeon, Korea. The remainder of this paper is organized as follows. Brief descriptions of the AERONET sun/sky radiometers, the measured data, and methods of data analysis are provided in “Measurements and analysis”. In “Results and discussion,” we present the columnar aerosol optical, microphysical, and radiative properties at the two sites, according to season and patterns of air mass movement. Finally, the conclusions are provided in “Conclusions.”

Measurements and analysis Column-integrated spectral aerosol measurements were made with CIMEL 318 automatic tracking sun and sky-scanning radiometers (Holben et al. 1998) at two AERONET sites in Korea: Gwangju (35.23° N, 126.84 °E) and Anmyeon (36.54° N, 126.33° E). At

Environ Monit Assess (2012) 184:4763–4775

Anmyeon, direct sun measurements were made every 15 min at 340, 380, 440, 500, 675, 870, 940, and 1020 nm. At Gwangju, the polarized version of the CIMEL 318-1 sun/sky radiometer was used to take measurements at 440, 675, 870, 940, and 1,020 nm. The aerosol optical depth (AOD, τ), single-scattering albedo (ω) at four wavelengths (440, 675, 870, and 1,020 nm), the Ångström exponent (α), the fine-mode fraction, and the total volume size distribution were retrieved using the AERONET algorithm (Dubovik and King 2000). Detailed information on the cloud-screening and data-retrieval processes can be found in Dubovik and King (2000) and Smirnov et al. (2000). In this study, quality assured Level 2.0 AERONET data were used. Figure 1 shows the locations of AERONET sun/ sky radiometers utilized in this study. We analyzed radiometer data from February 2004 to June 2005. For comparison, we also consider data for Beijing (39.98° N, 116.38° E) measured during the same period. Anmyeon Island is located in the Yellow Sea off the west coast of Korea. The island is less affected by local pollution sources than is Gwangju, which is located southwest of the Korean Peninsula. Air masses move mainly from the west, northwest, and north to the Korean Peninsula during spring, autumn, and winter. In summer, air masses are mainly transported Fig. 1 Observation sites and study area showing the regional air mass patterns during the different seasons

4765

from southerly directions. A total of 136 and 110 clear days of data were used for Gwangju and Anmyeon, respectively. Single-scattering albedo values were retrieved for 73 and 58 days at Gwangju and Anmyeon, respectively. The collected data were categorized according to season (spring, March–May; summer, June–August; and autumn/winter, September– February), and the dominant air mass directions are shown in Table 1. Autumn and winter were combined as one seasonal period because of similarity in air mass origins and AOD values during these two seasonal periods compared to those in spring and summer. The limited number of observation days during those seasons was another reason for the combining. The NOAA/ARL Hybrid Single-Particle Lagrangian Integrated Trajectory model (Draxler and Rolph 2003) was simulated to find the origin of air mass that reached the observation site. The 5-day (120 h) backward trajectories of air masses at height of 800 m above sea level were derived. In order to associate the AODs with the air mass origins, which represent each season, backward trajectories were calculated at 12:00 AM local time on sun/sky radiometer observation days. Noon was selected as the starting time of backward trajectories since it is thought to represent sun/sky radiometer measurement time.

4766

Environ Monit Assess (2012) 184:4763–4775

Table 1 Number of measurement days for each category and values of aerosol optical depth (τ) at 440 nm, Ångström exponent (α, 440–870 nm), fine-mode fraction (F), and single-scattering albedo (ω) at 440 nm, at Gwangju and Anmyeon Site Gwangju

Classification Total Spring

Number of observed days (single-scattering albedo) τ

Α

F

ω

136 (73)

0.44±0.30 1.27±0.28 0.84±0.13 0.93±0.03

54 (28)

00.45±0.24 1.11±0.30 0.76±0.12 0.92±0.02

North (SN)

8 (5)

0.55±0.33 1.29±0.16 0.85±0.06 0.93±0.02

West (SW)

29 (16)

0.47±0.24 1.27±0.20 0.82±0.07 0.92±0.02

Dust

17 (7)

0.39±0.19 0.74±0.15 0.60±0.08 0.89±0.02

Summer

28 (23)

0.71±0.44 1.43±0.22 0.94±0.06 0.95±0.02

Autumn/winter

54 (17)

0.31±0.19 1.31±0.21 0.86±0.10 0.92±0.03

North (AWN)

38 (8)

0.28±0.18 1.36±0.18 0.88±0.09 0.94±0.02

West (AWW)

16 (9)

0.39±0.22 1.19±0.25 0.82±0.13 0.90±0.02

110 (58)

0.43±0.31 1.14±0.32 0.82±0.15 0.93±0.03

Anmyeon Total Spring

47 (25)

0.45±0.28 0.99±0.29 0.73±0.15 0.91±0.03

North (SN)

14 (7)

0.48±0.25 1.12±0.18 0.82±0.14 0.93±0.01

West (SW)

19 (13)

0.49±0.23 1.13±0.23 0.80±0.14 0.92±0.02

Dust

14 (5)

0.36±0.21 0.65±0.19 0.55±0.13 0.88±0.02

Summer

29 (19)

0.58±0.35 1.49±0.20 0.95±0.05 0.96±0.02

Autumn/winter

34 (14)

0.28±0.20 1.09±0.24 0.83±0.13 0.93±0.03

North (AWN)

16 (6)

0.19±0.12 1.06±0.24 0.80±0.12 0.94±0.01

West (AWW)

18 (8)

0.35±0.27 1.13±0.24 0.85±0.10 0.91±0.03

Air mass patterns in spring and autumn/winter were classified into the following categories according to air mass origin: atmospheric conditions affected by an air mass from the north in spring (SN), from the west in the spring (SW), cases with a low Ångström exponent (<0.8) in spring (Dust), an air mass that moved slowly from northern parts of the Korean Peninsula in autumn/winter (AWN), and transported from China in autumn/ winter (AWW). However, air mass origins were mainly from the south in summer. Figure 2 shows the classified pathways of air masses arriving in Gwangju. In the dust case, the air mass mostly originated from source region of Asian dust.

Results and discussion Aerosol optical depth (τ) and the Ångström exponent (α) Table 1 lists the aerosol optical depth (τ) at 440 nm, Ångström exponents (α, 440–870 nm), the finemode fraction of the particle volume size distribution (F), and single-scattering albedo (ω) at 440 nm,

observed at the Gwangju and Anmyeon sites. The total average value of τ at 440 nm was 0.44± 0.30 and 0.43± 0.31 at Gwangju and Anmyeon, respectively. The highest τ values of 0.71± 0.44 and 0.58± 0.35 were observed in summer at the Gwangju and Anmyeon sites, respectively. The total average value of α was 1.27 ±0.28 and 1.14 ±0.31 at Gwangju and Anmyeon, respectively. At both sites, the α value was lower in spring than during other seasons. The lowest α values of 0.73± 0.11 and 0.66 ±0.19 were observed under dusty conditions at Gwangju and Anmyeon, respectively. The highest α value coincided with the highest F value during summer. Figure 3a shows temporal variations in the monthly mean aerosol optical depth at 440 nm observed at Gwangju, Anmyeon, and Beijing. Monthly τ values at the two Korean sites are comparable, whereas a higher value was observed in Beijing. The average value for τ at the two Korean sites was generally higher in spring and summer than in autumn and winter. Figure 3b shows the monthly mean α values computed from τ measurements at 440–870 nm. At all sites, the lowest α value was measured in spring

Environ Monit Assess (2012) 184:4763–4775

4767

Fig. 2 Air mass transport patterns in Gwangju. a Air mass originated from the north in spring, b from west in spring, c dust case, d from north in autumn/winter, e from west in autumn/winter, and f air mass origin in summer

(March–April) due to the influence of desert-derived dust particles (Eck et al. 2005; Takemura et al. 2003). These observed monthly variations in τ and α are in a good agreement with those obtained from long-term measurements (1999–2002; Eck et al. 2005). The time series of τ and α reveal that the maximum τ occurs in June, and the minimum of α occurs in spring at Anmyeon. Figure 3c shows monthly variations in water vapor amount, revealing similar temporal variations to α, with a maximum in summer (July–August), when the

amount of water vapor is much higher in Gwangju and Anmyeon than in Beijing. Size distribution of aerosols Figure 4 show the seasonally averaged volume size distributions of aerosols at Gwangju and Anmyeon, revealing a bimodal logarithmic-normal distribution. The size distribution shows seasonal differences, with distinct two modes. The total volume of the fine-

4768

Environ Monit Assess (2012) 184:4763–4775

As shown in Fig. 4 and Table 1, the total volume of the fine mode and the value of τ observed in summer are higher than the equivalent values in spring and autumn/winter. In addition, the fine-mode peak radius of 0.2 μm observed in summer is larger than that for spring and autumn/winter (0.15 μm). This tendency toward larger particles in the fine-mode fraction at higher τ values in summer may reflect an increase in the rate of particle coagulation with increasing τ and the hygroscopic growth of aerosols under conditions of high relative humidity (RH). Single-scattering albedo (ω)

Fig. 3 a Monthly mean aerosol optical depth at 440 nm, b Ångström exponent (440–870 nm), and c water vapor at Gwangju (GJ, squares), Anmyeon (AM, circles), and Beijing (BJ, triangles)

mode fraction is clearly larger than that of the coarsemode fraction in summer and autumn/winter. However, the coarse mode dominates in spring due to the influence of long-range transported dust particles from Asia.

Figure 5 shows monthly averages of ω observed at 440 nm at Anmyeon, Gwangju, and Beijing. The overall average value of ω observed at the Beijing site was 0.88±0.04. The ω values measured at Beijing show a clear seasonal trend, with high values in summer and low values in winter. The highest and lowest monthly average ω values in Beijing were measured in July 2004 (0.93) and November 2004 (0.84), respectively. Values of ω observed at Gwangju and Anmyeon are higher than those at Beijing, although a similar seasonal trend is seen at all three locations. The average ω values at Anmyeon and Gwangju were 0.93±0.03 and 0.93±0.03, respectively. Low ω values observed in winter at all three sites are mainly due to the dominance of absorbing urban aerosols emitted from heating activities. A possible reason for differences in ω between Beijing and the two Korean sites is that the fine-mode aerosols produced in Beijing

Fig. 4 Average aerosol size distributions for spring (diamonds), summer (triangles), and autumn/winter (circles)

Environ Monit Assess (2012) 184:4763–4775

4769

Fig. 5 Monthly mean singlescattering albedo at 440 nm for Gwangju (squares), Anmyeon (circles), and Beijing (triangles)

possess a higher light-absorption capacity due to differences in fuel types, combustion methods, and pollution control technology (Eck et al. 2005; Noh et al. 2009). Another possible reason is the effect of aerosol aging during transport over the ocean from China to Korea. The minimum ω values at Gwangju and Anmyeon were observed in April 2004 and April 2005, respectively. It appears that long-range transported aerosols, such as Asian dust and anthropogenic particles from China, are dominant in spring because of the westerly wind direction: The air mass picks up aerosols while passing over desert and industrial areas in China before arriving at the Korean Peninsula. Table 1 shows seasonally averaged ω values. The highest ω values were observed in summer: 0.95± 0.02 and 0.96±0.02 at Gwangju and Anmyeon, respectively. The ω values in autumn/winter were 0.92±0.03 and 0.93±0.03 at the two sites, respectively. Similar ω values (0.92±0.02 and 0.91±0.03, respectively) were observed in spring. The relatively low absorption of the aerosols in summer is associated with the lower concentration of absorbing aerosol in the air mass from the south, in combination with the occurrence of water-soluble aerosols under conditions of high RH, which results in fine-mode particle growth and an increase in the light-scattering coefficient. The ω value of Asian anthropogenic aerosols measured during ACE-Asia increased from 0.91 to 0.96 due to an increase in RH from 40% to 85% (Carrico et al. 2003). Other reasons for the observed

seasonal variation are a low production rate of lightabsorbing aerosols in summer and the weaker effects of long-range transported aerosols from China in summer compared with other seasons. The seasonal data were categorized further, based on air mass patterns, into “north,” “west,” and “dust,” as shown in Table 1. The lowest ω values of 0.89±0.02 and 0.88±0.02 observed at the Gwangju and Anmyeon sites, respectively, were recorded during the dust period. These values are lower than that for Asian dust particles observed in the source region, Dunhauang in China (Eck et al. 2005). The lowest fine-mode fractions at Gwangju and Anmyeon (0.60 and 0.55, respectively) were measured during the dust period, indicating that fine particles were also detected along with dust particles. The low ω value observed at Gwangju and Anmyeon during the dust period suggests that dust interacts with anthropogenic pollutants during the transportation process, thereby increasing the absorbing characteristics of the aerosol (Huebert et al. 2003). A low ω value of 0.92±0.02 was observed at both Korean sites when air masses were transported from the west in spring. These values are lower than those observed at the time when the air mass moved from the north in spring, reflecting the high concentration of longrange transported light-absorbing particles that originate from China when the air mass moves from the west. Figure 6 shows the spectral dependence of ω in terms of the different categories of atmospheric conditions, revealing that the slope of the relation depends on the aerosol type. We observed a decrease

4770

Environ Monit Assess (2012) 184:4763–4775

Fig. 6 Spectral dependence of single-scattering albedo under different air-mass transport conditions at a Gwangju and b Anmyeon

in the slope of ω as a function of wavelength, except for the dust case. The spectral dependence of ω is such that ω decreases with increasing wavelength. This pattern is due in part to the relatively wavelength-independent imaginary part of the index of refraction of black carbon, which is the main absorbing component in the fine-mode fraction of the particle size distribution (Bergstrom et al. 2002; Eck et al. 2003, 2005). The high spectral dependence for the SN case must be induced from a large contribution of fine mode particles. The α and F averaged over five measurement days for ω measurement out of 8 days for SN observations were 1.39±0.15 and 0.90±0.02, respectively, which is supported by the previous investigation (Smirnov et al. 2002). Smirnov et al. (2002) reported that the slope of ω decreased with wavelength as the ratio of fine mode particle increased. The value of ω increases between 440 and 675 nm and decreases between 870 and 1020 nm, when the air mass was transported under the AWW condition at both sites, and under the AWN condition at Anmyeon. The variable spectral behavior is strongly related to the properties of long-range transported particles from China. This trend of increasing spectral behavior between 440 and 675 nm was observed at Beijing in all seasons, based on 6-year sunphotometer data (Yu et al. 2009).

The ω value of dust particles increases with increasing wavelength (Dubovik et al. 2002; Eck et al. 2005). Figure 6 shows that for both Gwangju and Anmyeon, ω increases with wavelength in the case of Asian dust. The ω values at Gwangju and Anmyeon show a similar spectral dependency to that observed in the source regions of Asian dust (Eck et al. 2005); however, the dependence on wavelength is less pronounced in the present study, suggesting that the differences in ω values of dust in the source region and in downwind areas in East Asia may reflect the fact that dust mixes with anthropogenic pollution during transport, thereby increasing the absorbing properties of aerosol (Huebert et al. 2003). Aerosol-direct radiative forcing Based on the analysis of column-integrated aerosol optical and microphysical properties, we investigated the aerosol-direct radiative forcing (ΔF, defined as the difference between the net radiative flux calculated with and without aerosols; W m –2 ) and the radiative forcing efficiency (β, defined as ΔF per unit AOD; W m–2/τ, λ=550 nm in this study) at both the surface and top of the atmosphere (TOA) under cloud-free conditions as shown in Table 2. AERONET provides clear-sky ΔF and β values in the spectral range of 0.2–4.0 μm, based on sunphotom-

Environ Monit Assess (2012) 184:4763–4775

4771

Table 2 Aerosol-direct radiative forcing (ΔF) and aerosol forcing efficiency (β) at the surface, top of the atmosphere, and in the atmosphere for each category Site

Classification

ΔF Surface

Gwangju

Anmyeon

β TOA

Atmosphere

Surface

TOA

Atmosphere

Spring

−59.9±23.5

−21.9±9.7

38.0±16.0

−187.0±37.2

−65.7±14.0

121.2±38.9

North (SN)

−64.8±27.4

−24.2±11.2

40.6±17.3

−173.2±34.5

−64.6±10.4

108.5±27.7

West (SW)

−60.1±23.4

−21.6±10.2

38.5±15.2

−189.0±38.6

−63.3±10.4

125.7±40.2

Dust

−57.3±22.9

−21.4±8.5

35.9±17.5

−190.0±36.8

−70.3±17.9

119.7±41.6

Summer

−62.3±27.9

−29.2±15.2

33.2±17.5

−131.7±35.7

−57.7±8.9

74.0±33.2

Autumn/winter

−45.2±21.6

−17.0±9.2

28.3±14.0

−213.5±41.1

−74.9±13.7

138.7±44.7

North (AWN)

−39.0±14.1

−15.1±7.6

23.9±8.0

−213.2±42.8

−75.2±13.5

138.0 ±45.5 140.4±44.1

West (AWW)

−60.1±28.7

−21.5±11.1

38.6±19.4

−214.3±38.2

−73.9±14.6

Spring

−62.7±31.4

−25.1±12.2

37.5±22.2

−188.9±38.9

−76.4±16.9

112.6±43.5

North (SN)

−63.5±40.8

−24.6±12.9

39.0±30.0

−183.7±41.6

−77.2±22.5

106.5±46.6

West (SW)

−64.3±27.9

−27.9±13.1

36.5±18.3

−183.1±38.2

−76.6±9.7

106.5±38.7

Dust

−59.1±27.6

−21.4±10.0

37.7±19.8

−205.1±34.5

−75.6±19.4

129.5±45.1

Summer

−52.7±31.4

−29.9±18.7

22.8±14.5

−125.6±20.7

−73.5±13.9

52.1±19.1

Autumn/winter

−41.2±21.3

−15.9±13.7

25.3±9.6

−230.7±55.5

−78.6±15.1

152.0±60.3

North (AWN)

−35.5±14.9

−11.3±6.8

24.2±9.7

−208.2±47.6

−81.7±14.4

126.5±41.4

West (AWW)

−46.3±25.1

−20.0±15.0

26.2±10.2

−255.9±53.9

−75.2±15.7

180.7±66.3

The measurement days are the same as in Table 1.

eter measurements. Detailed information on the retrieval method is given on the AERONET Web site (http://aeronet.gsfc.nasa.gov). Figure 7 shows that ΔF is proportional to τ at the Gwangju and Anmyeon sites. The correlation between ΔF and τ at Anmyeon is stronger (R2 =0.94 for TOA and R2 =0.79 for the surface) than at

Fig. 7 Relation between aerosol-direct radiative forcing (ΔF) and aerosol optical depth (τ) at 670 nm at the surface (squares) and top of the atmosphere (TOA; open circles) at a Gwangju and b Anmyeon

Gwangju (R2 =0.90 for TOA and R2 =0.77 for the surface). Figure 8 shows the average ΔF at the surface and at the TOA for the six categories. The largest ΔF values at the surface were obtained for the SN and SW conditions: –64.8 and −64.3 Wm–2 for τ=0.55 and τ=0.49 at 440 nm at Gwangju and Anmyeon, respectively.

4772

Environ Monit Assess (2012) 184:4763–4775

Fig. 8 Aerosol-direct radiative forcing (ΔF) at the surface, top of the atmosphere (TOA), and in the atmosphere under different air mass conditions at a Gwangju and b Anmyeon

ΔF values at the surface in summer were −62.3 and −52.7 Wm–2 at Gwangju and Anmyeon, respectively, which are the second and third highest values. In contrast to the surface values, the largest ΔF values at the TOA were observed in summer, for both Gwangju and Anmyeon (−29.2 and −29.9 Wm–2, respectively). These differences are probably induced by contrasting aerosol properties, except for AOD. Aerosol forcing efficiency (β) depends on the aerosol up-scatter fraction, as well as on ω (Kim et al. 2008).

Figure 9 shows the correlation between β observed at the atmosphere and each of single-scattering albedo, asymmetry factor, and water vapor, for Gwangju and Anmyeon. The asymmetry factor and water vapor show a weaker correlation with β than with ω, indicating that ω, which describes the aerosol light-absorbing property, has a greater effect on β in the atmosphere. Figure 10 shows values of β for the six categories. Because of weak absorption during summer, the lowest values of β at the surface, TOA, and in the atmosphere were found during this season, yielding values of −131.7, –57.7, and 74.0 Wm–2 at Gwangju,

Fig. 9 Relation between aerosol radiative forcing efficiency (β) and a single-scattering albedo at 670 nm, b asymmetry factor at 670 nm, and c water vapor at Gwangju (squares) and Anmyeon (open circles)

Environ Monit Assess (2012) 184:4763–4775

4773

Fig. 10 Aerosol radiative forcing efficiency (β) at the surface, top of the atmosphere (TOA), and in the atmosphere under different air mass conditions at a Gwangju and b Anmyeon

and −125.6, –73.5, and 52.1 Wm–2 at Anmyeon, respectively. Another reason for the difference is the high water vapor concentration in summer. Yoon and Kim (2006) reported that the radiative forcing efficiency decreases with increasing RH. The decrease in β with increasing RH results from the fact that the increasing rate of aerosol optical depth with RH is greater than the increasing rate of aerosol radiative forcing with RH (Yoon and Kim 2006); in addition, the light-absorbing property decreases with increasing RH (Carrico et al. 2003). Although β shows only minor differences among the other five cases, it varies with the season and air mass origin, being higher in autumn/winter than in spring, and higher when the air mass is transported from the west. Variations in β show a strong correlation with ω, indicating that the differences in β are associated with differences in light-absorbing properties, which vary with the season and air mass origin.

Conclusions Columnar aerosol optical properties were measured with sun/sky radiometers at two Korean sites (Gwangju and Anmyeon) and in Beijing from February 2004 to June 2005. The data were analyzed to evaluate seasonal variations in aerosol optical properties under various patterns of air mass transport in the downwind area of East Asia.

Aerosol optical depth and single-scattering albedo show distinct seasonal variations, with higher values in spring and summer than in autumn and winter. Different values are seen between Beijing, an upwind site, and the two Korean sites, which are downwind of China. A lower average ω value was observed in Beijing than at the Korean sites. The aerosol optical and radiative properties over Korea are strongly dependent on the air mass origin. Lower values of single-scattering albedo were generally measured when the air mass moved from the west. At both Gwangju and Anmyeon, the lowest ω values were observed during dust periods. This finding indicates that the aerosol optical properties in Korea are strongly affected by long-range transported aerosols at times of strong westerly winds. The aerosols measured in summer show the highest single-scattering albedo and the lowest aerosol radiative forcing efficiency, at both Gwangju and Anmyeon. This result means that summer aerosols influenced by the hygroscopic growth of fine particles under conditions of high relative humidity are clearly distinguished from those observed during other seasons. Changes in the aerosol light-absorbing and radiative properties are attributed mainly to the presence of anthropogenic pollutants transported from the Asian continent. Further investigations are required to understand in detail the aerosol transport patterns, including vertical profiles of aerosol layers in East Asia.

4774 Acknowledgments This work was funded by the Korea Meteorological Administration Research and Development Program under Grant CATER 2007-4108. The authors would like to thank GSFC/NASA for the use of AERONET sunphotometer data.

References Ackerman, A. S., Toon, O. B., Stevens, D. E., Heymsfield, A. J., Ramanathan, V., & Welton, E. J. (2000). Reduction of tropical cloudiness by soot. Science, 288, 1042–1047. Bergstrom, R. W., Russell, P. B., & Hignett, P. (2002). Wavelength dependence of the absorption of black carbon particles: Predictions and results from the TARFOX experiment and implications for the aerosol single scattering albedo. Journal of the Atmospheric Sciences, 59, 567– 577. Carrico, C. M., Kus, P., Rood, M. J., Quinn, P. K., & Bates, T. S. (2003). Mixtures of pollution, dust, sea salt, and volcanic aerosol during ACE-Asia: Radiative properties as a function of relative humidity. Journal of Geophysical Research, 108(D23), 8650. doi:10.1029/ 2003JD003405. Draxler, R.R., Rolph, G.D. (2003). HYSPLIT (HYbrid SingleParticle Lagrangian Integrated Trajectory) Model access via NOAA ARL READY Website /http://www.arl.noaa. gov/ready/hysplit4.htmlS. Silver Spring: MDNOAA Air Resources Laboratory. Dubovik, O., King, M.D. (2000). A flexible inversion algorithm for retrieval of aerosol optical properties from sun and sky radiance measurements. J Geophys Res 105, (D16), 20673–20696. Dubovik, O., Holben, B. N., Eck, T. F., Smirnov, A., Kaufman, Y. J., King, M. D., et al. (2002). Variability of absorption and optical properties of key aerosol types observed in worldwide locations. Journal of the Atmospheric Sciences, 59, 590–608. Eck, T. F., Holben, B. N., Reid, J. S., O’Neill, N. T., Schafer, J. S., Dubovik, O., et al. (2003). High aerosol optical depth biomass burning events: A comparison of optical properties for different source regions. Geophysical Research Letters, 30(20), 2035. doi:10.1029/2003GL017861. Eck, T. F., Holben, B. N., Dubovik, O., Smirnov, A., Goloub, P., Chen, H. B., et al. (2005). Columnar aerosol optical properties at AERONET sites in central eastern Asia and aerosol transport to the tropical mid-Pacific. Journal of Geophysical Research, 110, D06202. doi:10.1029/2004JD005274. Hansen, J., Sato, M., & Ruedy, R. (1997). Radiative forcing and climate response. Journal of Geophysical Research, 102, 6831–6864. Holben, B. N., Eck, T. F., Slutsker, I., Tanré, D., Buis, J. P., Setzer, A., et al. (1998). AERONET—A federated instrument network and data archive for aerosol characterization. Remote Sensing of Environment, 66, 1–16. Huebert, B. J., Bates, T., Russell, P. B., Shi, G., Kim, Y. J., Kawamura, K., et al. (2003). An overview of ACE-Asia: Strategies for quantifying the relationships between Asian aerosols and their climatic impacts. Journal of Geophysical Research, 108(D23), 8633.

Environ Monit Assess (2012) 184:4763–4775 Intergovernmental Panel on Climate Change (IPCC). (2007). Climate Change 2007: The scientific basis. In S. Solomon (Ed.), Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press. Jacobson, M. Z. (2001). Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols. Nature, 409, 695–697. Kaufman, Y. J., Koren, I., Remer, L. A., Tanré, D., Ginoux, P., & Fan, S. (2005). Dust transport and deposition observed from the terra-moderate resolution imaging spectroradiometer (MODIS) spacecraft over the Atlantic ocean. Journal of Geophysical Research, 110, D10S12. Kim, D. H., Sohn, B. J., Nakajima, T., Takamura, T., Takemura, T., Choi, B. C., et al. (2004). Aerosol optical properties over east Asia determined from ground-based sky radiation measurements. Journal of Geophysical Research, 109, D02209. doi:10.1029/2003JD003387. Kim, S. W., Yoon, S. C., & Kim, J. Y. (2008). Columnar Asian dust particle properties observed by sun/sky radiometers from 2000 to 2006 in Korea. Atmospheric Environment, 42, 492–504. Kim, S. W., Choi, I. J., & Yoon, S. C. (2010). A multi-year analysis of clear-sky aerosol optical properties and direct radiative forcing at Gosan, Korea (2001–2008). Atmospheric Research, 95, 279–287. Koren, I., Kaufman, Y. J., Remer, L. A., & Martins, J. V. (2004). Measurement of the effect of Amazon smoke on inhibition of cloud formation. Science, 303(5662), 1342–1345. Lee, D. H., Lee, K. H., Kim, J. E., & Kim, Y. J. (2006). Characteristics of atmospheric aerosol optical thickness over the northeast asia using TERRA/MODIS data during the year 2000–2005. Atmosphere, 16(2), 85–96. Lee, K. H., Li, Z., Wong, M. S., Xin, J., Wang, Y., Hao, W.-M., et al. (2007). Aerosol single scattering albedo estimated across China from a combination of ground and satellite measurements. Journal of Geophysical Research, 112, D22S15. doi:10.1029/2007JD009077. Liu, J., Zheng, Y., Li, Z., & Wu, R. (2008). Ground-based remote sensing of aerosol optical properties in one city in Northwest China. Atmospheric Research, 89, 194– 205. Noh, Y. M., Müller, D., Shin, D. H., Lee, H. L., Jung, J. S., Lee, K. H., et al. (2009). Optical and microphysical properties of severe haze and smoke aerosol measured by integrated remote sensing techniques in Gwangju, Korea. Atmospheric Environment, 43, 879–888. Novakov, T., Ramanathan, V., Hansen, J. E., Kirchstetter, T. W., Sato, M., Sinton, J. E., et al. (2003). Large historical changes of fossil-fuel black carbon aerosols. Geophysical R e s e a rc h L e t t e r s , 3 0 ( 6 ) , 1 3 2 4 . d o i : 1 0 . 1 0 2 9 / 2002GL016345, 57-1 to 57-4. Ramanathan, V., & Carmichael, G. (2008). Global and regional climate changes due to black carbon. Nature Geoscience, 1, 221–227. Smirnov, A., Holben, B. N., Eck, T. F., Dubovik, O., & Slutsker, I. (2000). Cloud screening and quality control algorithms for the AERONET database. Remote Sensing of Environment, 73, 337–349.

Environ Monit Assess (2012) 184:4763–4775 Smirnov, A., Holben, B. N., Eck, T. F., Slutsker, I., Chatenet, B., & Pinker, R. T. (2002). Diurnal variability of aerosol optical depth observed at AERONET (Aerosol Robotic Network) sites. Geophysical Research Letters, 29(23). doi:10.1029/2002GL016305. Takemura, T., Nakajima, T., Higurashi, A., Ohta, S., & Sugimoto, N. (2003). Aerosol distributions and radiative forcing over the Asian Pacific region simulated by Spectral Radiation-Transport Model for Aerosol Species (SPRINTARS). Journal of Geophysical Research, 108 (D23), 8659. doi:10.1029/2002JD003210. Yoon, S. C., & Kim, J. Y. (2006). Influence of relative humidity on aerosol optical properties and aerosol radiative forcing

4775 during ACE-Asia. Atmospheric Environment, 40, 4328– 4338. Yoon, S.-C., Won, J.-G., Omar, A. H., Kim, S.-W., & Sohn, B.-J. (2005). Estimation of the radiative forcing by key aerosol types in worldwide locations using a column model and the AERONET data. Atmospheric Environment, 39(35), 6620– 6630. Yu, X., Cheng, T., Chen, J., & Liu, Y. (2007). Climatology of aerosol radiative properties in northern China. Atmospheric Research, 84, 132–141. Yu, X., Zhu, B., & Zhang, M. (2009). Seasonal variability of aerosol optical properties over Beijing. Atmospheric Environment, 43, 4095–4101.