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Spectral aerosol optical depth variation with different types of aerosol at Gwangju, Korea
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Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 1609–1621 www.elsevier.com/locate/jastp
Spectral aerosol optical depth variation with different types of aerosol at Gwangju, Korea Jeong E. Kim, Seong Y. Ryu, Zhuanshi He, Young J. Kim Advanced Environmental Monitoring Research Center (ADEMRC), Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Republic of Korea Received 24 December 2004; received in revised form 18 March 2006; accepted 5 May 2006 Available online 21 July 2006
Abstract Simultaneous ground-based measurements of aerosol chemical composition and atmospheric aerosol optical depth (AOD) in the UV and visible regions were carried out at Gwangju (35.131N, 126.501E), Korea during a biomass burning period, October 4–November 12, 2002. An Asian dust event and biomass burning events were observed during the study period. The correlation coefficients (R) between AOD and PM2.5 and PM10 mass concentration during the study period are 0.5870.15 and 0.5270.14, respectively. Aerosol optical properties (AOD and A˚ngstro¨m exponent) and chemical characteristics were presented for selected days and their impacts on aerosol loading were examined. Both fine and coarse mode particle mass concentrations increased on biomass burning day resulting in high AOD values. The simultaneous occurrence of Asian dust and biomass burning resulted in the highest AOD of 1.20 at 311 nm and the lowest A˚ngstro¨m exponent of 0.42 in the visible range. When both transported urban pollutants and land dust aerosols affected the air quality over the study area, AOD and surface PM mass concentration increased 18–60% and 230–890%, respectively. r 2006 Elsevier Ltd. All rights reserved. Keywords: UV irradiance; Aerosol optical depth; MFRSR; Biomass burning aerosol; Asian dust
1. Introduction Studies on depletion of the stratospheric ozone have heightened the interest in the study of surface UV-B radiation because of associated anticorrelation aspects (Bais et al., 1997; Bernhard et al., 1997; Dickerson et al., 1997; Stolarski et al., 1998; Cappellani and Kochler, 1999). Positive UV trends Corresponding author. Tel.: +82 62 970 3401; fax: +82 62 970 3404. E-mail addresses: [email protected] (J.E. Kim), [email protected] (S.Y. Ryu), [email protected] (Z. He), [email protected] (Y.J. Kim).
1364-6826/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2006.05.008
can harm the ecosystem, such as terrestrial and aquatic biogeochemical cycles, and may also have adverse effects on human health (Tang and Madronich, 1995; Zepp et al., 1995; Caldwell et al., 1995; Longstreth et al., 1995). Total column ozone is the main factor affecting the surface UV-B level, but other factors such as cloud and aerosol can also affect the surface UV irradiance. The anticorrelation between ozone and the UV-B level has been well documented through the use of RAF (radiation amplification factor) for various spectral response functions (Madronich, 1993; Blumthaler et al., 1995). The aerosol effect on surface UV radiation has been actively studied during the last decade (Kylling et al.,
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1998; Meleti and Cappellani, 2000; Wenny et al., 2001) di Sarra et al. (2002) obtained RAF with higher accuracy by taking into account the aerosol effect with a sensitivity of 0.07–0.27. Krzyscin (2004) calculated RAF of aerosol to be 0.10–0.11 at a solar zenith angle of 601 and 801, which quantifies the effect of aerosol on surface UV irradiance. The relationship between the surface UV level and atmospheric aerosols can be more complex than the UV and ozone correlation since the aerosol effect is somewhat localized and time variant. Longterm UV monitoring in Belsk, Poland showed that 1% change in AOD forces a change of up to 0.15% in the UV dose (Krzyscin and Puchalsky, 1998). It has been reported that UV irradiance is reduced by 5–35% in comparison to aerosol-free conditions and is dependent on aerosol optical depth and wavelength weight function (Liu et al., 1991; Zerefos, 1997; Kylling et al., 1998). UV-B irradiance measured in a suburban area was 20% higher than that measured in downtown Mexico City, which corroborated the aerosol effect on surface UV irradiance (Acosta and Evans, 2000). Biomass burning aerosol is usually dominated by accumulation mode aerosols, and also often results in the elevation of coarse mode aerosols of soil particles near the fire due to ash (Kaufman et al., 1994; Artaxo et al., 1994). It was also reported that elevated concentrations of organic particles (radius Ro0:4 mm) and soil/ash particles (R41 mm) were usually associated with the presence of biomass burning (Kaufman et al., 1994). It was found that the A˚ngstro¨m exponent (440–1020 nm) ranged between 1.0 and 2.5 during biomass burning (Kaufman et al., 1994). Surface UV and visible irradiances have been measured using an ultraviolet multifilter rotating shadow-band radiometer (UV-MFRSR) and a multifilter rotating shadow-band radiometer (visible-MFRSR) at Gwangju, Korea. MFRSRs can provide several aerosol optical parameters such as aerosol optical depth (AOD) and the A˚ngstro¨m exponent, which can provide useful information about aerosol size distribution. In this study surface UV and visible irradiance data were analyzed for a biomass burning aerosol characterization campaign period at Gwangju Institute of Science and Technology (GIST), Gwangju, Korea around the harvest season in October and November of 2002. Spectral AOD results were compared with aerosol mass concentration and chemical composition. The goals of this study are to (i) investigate the variation of
aerosol radiative properties, such as the AOD and A˚ngstro¨m exponent during a biomass burning period in Gwangju, and to (ii) correlate these aerosol optical parameters with aerosol chemical characteristics. 2. Methodology 2.1. Radiation measurement A visible-MFRSR and UV-MFRSR were used in order to measure the global and diffuse spectral irradiances, AOD and A˚ngstro¨m exponent from direct normal irradiances. UV-MFRSR is a sevenchannel UV version of the visible-MFRSR and is described by Harrison et al. (1994) and Gao et al. (2001). Data were acquired from October 4 to November 12, 2002 on the rooftop of the Department of Material Science and Engineering building of Gwangju Institute of Science and Technology (GIST) in Gwangju (351130 N, 1261500 E), Korea. The UV-MFRSR measures the spectral UV irradiances centered at wavelengths 304, 311, 317, 323, 331, and 367 nm with 2 nm FWHM while the visible-MFRSR measures the spectral irradiances centered at 415, 500, 615, 673, and 870 nm with 10 nm FWHM. The measurement outputs from the UV-MFRSR and visible-MFRSR sampled every 20 and 15 s, respectively. These values were averaged for every 1-min period and saved into two separate data loggers. The UV-MFRSR was calibrated prior to initial installation in February 2002. The visibleMFRSR was recalibrated in October 2001 by the manufacturer. 2.2. Aerosol sampling Aerosol sampling was conducted at Gwangju, Korea from October 8 to November 12, 2002 with significant agricultural activities in the southern parts of Korea. Sampling, analysis procedure, and site description are described by Ryu et al. (2004). 2.3. Aerosol optical depth and A˚ngstro¨m formula Total atmospheric optical depth was determined by analyzing direct normal solar irradiance from total and diffuse solar irradiances measured by MFRSRs. Direct normal irradiance (DNSI) is calculated by subtracting diffuse irradiance (Imin) (measured when the shadow-band blocks the sundisk) from the global irradiance (Imax) (measured
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without blocking of the sun-disk) and dividing the result by the cosine of the solar zenith angle (y) as presented in Eq. (1) cos y is equal to the inverse of the optical air mass for a plane, parallel, nonrefracting and uniformly mixed atmosphere with a solar zenith angle below 801 (Iqbal, 1983; Harrison and Michalsky, 1994; Bigelow and Slusser, 2000). I ¼ DNSI ¼ ðI max I min Þ= cos y.
(1)
The Beer–Lambert law provides the total atmospheric optical depth. AOD is determined by subtracting the contributions by Rayleigh scattering, total column ozone absorption, and water vapor absorption. Water vapor absorption is not considered since the channel affected by water vapor was excluded in this study. I ¼ I 0 expðttotal mÞ
(2)
ttotal ¼ tozone þ twatervapor þ tRayleigh þ taerosol
(3)
where I0 is the solar irradiance at the top of the atmosphere, I is the direct normal solar irradiance, t is the atmospheric optical depth, and m is the relative optical air mass. I0 is retrieved using the Langley regression method when the relative optical air mass is between 1.5 and 3.0 for the UV-MFRSR due to its poor angular response when the solar zenith angle is below 701 and between 2 and 6 for the visible-MFRSR since the rate of change of the air mass is small at lower air masses (Harrison and Michalsky, 1994; Bigelow and Slusser, 2000; Kim et al., 2006). The Rayleigh scattering optical depth is well modeled by Hansen and Travis (1974). Ozone optical depth is a product of total column ozone and ozone absorption cross-section. The Earth Probe/TOMS satellite provides the total column ozone data with a 1.251 (lat.) 1.001 (long.) resolution in the form of one scan a day at around noon (local) with an uncertainty of 75% (http://toms.gsfc.nasa.gov/). Comparison of TOMS ozone data with the ground-based ozone measured by a Dobson spectrometer at Seoul, Korea during the measurement period showed a 5% difference in total column ozone. The ozone absorption crosssection at 226 K by Molina and Molina (1986) was used to calculate the ozone absorption optical depth. The effective ozone temperature of 226 K with variability of 220–231 K was determined from the radiosonde data obtained over Gwangju, Korea. Therefore, total uncertainty in ozone optical depth due to errors in total column ozone and effective ozone temperature is determined to be 70.12,
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70.05, 70.02, and 70.01, at 304, 311, 317 and 323 nm, respectively. tðlÞ ¼ bla ,
(4)
where t (l) is the AOD at each wavelength, l, b is a turbidity coefficient, and a is the A˚ngstro¨m exponent (A˚ngstro¨m, 1961). In this study a was calculated from spectral AODs by the linear regression fit in the visible/near IR ranges (415–870 nm), as well as the UV (311–367 nm) and UV-A (331–367 nm) ranges. AOD and a were only computed for conditions involving cloudless skies to exclude the effect of cloud. A cloudless sky was determined by the following criteria: I std o0:03, I avg
(5)
where Iavg and Istd are the 3-min average and standard deviation of irradiance measured by the broadband channel (300–1040 nm). 3. Results and discussions The correlation between daily mean AOD at 870 nm and PM mass concentrations is plotted in Fig. 1. Correlation coefficients between PM mass concentration and spectral AOD at eleven wavelengths are summarized in Table 1. Average correlation coefficients were determined to be 0.5870.15 and 0.5270.14 for PM2.5 and PM10 mass concentrations, respectively. Spectral AOD variation is plotted in Fig. 2 during the measurement period, October 4 – November 12, which shows high values on October 25 and November 12 and the lowest value on October 27. Open field burnings have been observed throughout the study period. The study site was affected by various types of aerosols as well as local biomass burning. In this study, various contributions from these sources to the total aerosol loading were examined by measurements of surface irradiance, chemical composition measurement and backward trajectory analysis for selected days (October 14, 25 and 27, and November 12). October 27 was selected as the background case when the air was relatively clean after precipitation on the previous day. 3.1. Surface irradiance Total, diffuse, and normal direct UV irradiances are plotted in Fig. 3 for the selected cases. As the direct-to-diffuse ratio is independent of absolute
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AOD870nm
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y=0.0028x+0.075 R=0.88
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20
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PM2.5 mass concentration (ug/m3)
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0.25 0.20 0.15 0.10 0
20
40
60
80
100
120
140
160
PM10 mass concentration (ug/m3)
Fig. 1. Correlation between aerosol mass concentration and AOD at 870 nm and correlation coefficient (R).
Table 1 Correlation coefficients (R) between aerosol mass concentrations (mg/m3) and spectral AODs on October 8–10, 14, 25, 27, 30 and November 12
PM2.5 PM10
t304
t311
t317
t323
t331
t367
t415
t500
t617
t673
t870
avg.
0.57 0.47
0.56 0.49
0.45 0.40
0.46 0.39
0.48 0.40
0.48 0.43
0.49 0.46
0.58 0.54
0.70 0.65
0.77 0.71
0.88 0.83
0.5870.15 0.5270.14
1.2
1.0
AOD304 AOD310 AOd317 AOD323 AOD331 AOD367 AOD415 AOD500 AOD615 AOD673 AOD870
AOD
0.8
0.6
0.4
0.2
0.0 4-Oct
8-Oct
9-Oct
10-Oct 14-Oct 25-Oct 27-Oct 30-Oct 12-Nov date
Fig. 2. Spectral AOD variation during the measurement period.
calibration and absolute solar spectrum, it can be a good indicator showing total atmospheric attenuation. The direct-to-diffuse ratio was high on
October 14 and 27, except under cloudy conditions, which means that the aerosol loading was low. Diffuse irradiance is also related to aerosol size,
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single scattering albedo, surface albedo as well as aerosol amount. However, the ratio was much lower due to high aerosol loading on October 25 and November 12. 3.2. Aerosol optical depth and aerosol chemical composition Daily mean values of spectral AODs under cloudless sky conditions are summarized in Table 2. AODs for the selected days varied between 0.6970.40 at 304 nm and 0.2870.18 at 870 nm on
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average. The mean A˚ngstro¨m exponent was 0.727 0.20, 1.4170.36 and 1.0970.82 in the visible (415–870 nm, avis), UV (311–367 nm, auv) and UV-A (331–367 nm, auvA) ranges, respectively. Iqbal (1983) reported an a value of 1.3070.50, which is comparable to our results. Wavelength dependence of AODs is represented as a. An A˚ngstro¨m exponent determined from the wavelength dependence of AODs can provide basic information on aerosol size distribution. Fig. 4 shows the spectral dependence of AODs in the UV and visible ranges. In Fig. 5, A˚ngstro¨m exponents
Fig. 3. UV surface irradiance at 368 nm and direct-to-diffuse ratio on (a) October 27, (b) October 14, (c) October 25, and (d) November 12.
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Fig. 3. (Continued)
Table 2 Spectral AOD at 11 channels and A˚ngstro¨m exponent in the visible (415–870 nm), UV (311–367 nm), and UV-A (331–367 nm) regions
14-Oct 25-Oct 27-Oct 12-Nov
t304
t311
t317
t323
t331
t367
t415
t500
t616
t673
t870
avis
aUV
a*UVA
0.40 0.85 0.34 1.18
0.44 0.82 0.32 1.20
0.41 0.73 0.30 1.08
0.39 0.72 0.30 1.00
0.36 0.69 0.29 1.00
0.35 0.63 0.23 0.97
0.32 0.55 0.20 0.81
0.27 0.47 0.17 0.79
0.23 0.40 0.15 0.77
0.21 0.37 0.14 0.68
0.17 0.30 0.11 0.52
0.7870.10 0.8670.10 0.8370.04 0.4270.16
1.2170.44 1.5370.08 1.9370.32 1.1770.06
0.8570.26 0.9170.05 2.2570.07 0.3370.12
computed in the UV and visible ranges are plotted against AOD311 nm and AOD415 nm. The A˚ngstro¨m exponent in the UV (auv) and visible ranges (avis) varied between 0.30–2.40 and 0.02–1.10, respec-
tively, and showed no clear relationship between a and AOD. The lowest mass PM concentration and AOD values were observed on October 27 as summarized
ARTICLE IN PRESS J.E. Kim et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 1609–1621
Fig. 4. Spectral AOD variation and wavelength dependence of AOD (A˚ngstro¨m exponent) on October 14, 25, 27 and November 12 in the (a) visible wavelength range and (b) UV wavelength range.
in Table 2. Daily mean AODs ranged between 0.34 and 0.11 at 304 and 870 nm with A˚ngstro¨m exponents 0.8370.04, 1.9370.32, and 2.2570.07 for the visible, UV and UV-A ranges, respectively. Fine and coarse mass concentrations were 11.9 and 8.5 mg/m3, respectively. Fig. 6 shows comparison of AOD and a values along with aerosol mass concentration. In Table 3, the aerosol characteristics obtained in this study are compared with those of a previous study (Ogunjobi et al., 2004) at the same site, which retrieved daily AOD from MFRSR data collected from January 1999 to August 2001 at the same site and correlated it with aerosol chemistry data measured in downtown Gwangju located about 10 km away from the study site. AOD values obtained on October 27 were higher than
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those obtained on clean days of the previous study (Ogunjobi et al., 2004) at the Gwangju site. Biomass burning observed around the study site in the late autumn involved the open field burning of agricultural waste, mostly in the form of rice straw. High values of AOD were observed on October 25 due to field burnings around the study site, which were 1.4–1.8 times higher than those on October 27. Fine and coarse mode aerosol mass concentration on October 25 increased to 72.7 and 79.1 mg/m3, respectively. The higher fraction of organic carbon (OC) observed in the fine mode aerosol mass fraction was the main effect of biomass burning at Gwangju, Korea during the study period as detailed in Ryu et al. (2004). OC contributed to 22% of the total fine mode mass concentration measured on October 27 (background case) while biomass burning increased the OC concentration so that it represented 42% of the total fine mode mass concentration on October 25 as shown in Fig. 7. The coarse mode aerosol concentration also increased on October 25. This might be due to the fact that coarse mode aerosol can increase during biomass burning due to the elevation of soil particles near the fire (Kaufman et al., 1994; Artaxo et al., 1994; Harrison et al., 1994) and aging or coagulation of fine particles at high concentration (Reid et al., 1998; Eck et al., 2005). As noted by Kaufman et al. (1998) smoke particles might increase their radius during their stay in the atmosphere by condensation and coagulation. A higher fraction of coarse mode aerosols is related to a low avis on this date. Ogunjobi et al. (2004) shows about a four times higher mass concentration in fine mode compared to coarse mode aerosol and a higher avis in comparison to values obtained during the BG period as shown in Table 3. A lower mass concentration of coarse particles in their study might be attributed to the aerosol measurement site, which was located downtown and affected to a less degree by coarser particles raised from biomass burning. Rare Asian dust events in autumn were recorded over the Korean peninsula during November 11–12, 2002. However, AOD and a could not be determined on November 11 due to thick cloud cover, when heavier Asian dust was observed. A three-day backward trajectory shows the transport of the air mass from Inner Mongolia and the Gobi Desert (Ryu et al., 2004). Biomass burning was also observed on November 12 and affected the air quality over the study area. Due to the simultaneous
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Angstrom exponent (UV)
3.0 2.8
10/14
2.6
10/25
2.4
10/27
2.2
11/12
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 (a) AOD at 311 nm 3.0 2.8
Angstrom exponent (Visible ranges)
2.6 2.4 2.2
10/14 10/25 10/27 11/12
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 (b) AOD at 415 nm
Fig. 5. Scatter plots of spectral AODs versus A˚ngstro¨m exponent for selected days.
occurrence of Asian dust and biomass burning on November 12, fine and coarse mode aerosol mass concentrations increased to 52.1 and 92.9 mgm3, respectively, which represent respective values that are 3.4 and 9.9 times higher than those measured on October 27. The highest AOD values up to 1.18 at 304 nm and 0.52 at 870 nm were observed on that day. Asian dust and biomass burning aerosol increased AOD by 2.3–3.2 times in the UV range and 3–4 times in the visible range, in comparison to
measurements obtained in the absence of these events. Measurements of aerosol chemical composition on this date show the characteristics of both dust particles and biomass burning aerosol. K+ concentration, a good tracer of biomass burning aerosol, increased to 0.43 mg/m3 on November 12, which was about five times higher than that on October 27. Organic carbon concentration also increased to 21.4 mg/m3 and contributed to 41% of the fine aerosol as shown in Fig. 7. Concentrations
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mass concentration(ug/m3)
of crustal species on that day were also very high. Concentrations of aluminum (Al), calcium (Ca), iron (Fe), and titanium (Ti) were 1.37 mg/m3, 0.89 mg/m3, 0.73 mg/m3, and 18.7 ng/m3, respectively. Even though surface aerosol mass concentration from biomass burning was slightly lower than that of October 25, total columnar aerosol loading on November 12 was much higher due to Asian dust, which is generally known to be dominated by coarse particles, contributing to even higher AOD at all wavelengths. This is because biomass burning usually occurred around the measurement site and affected the near surface, while Asian dust was usually transported above the boundary layer as well as near surface. Thus, the effect of Asian dust Oct14
Oct27
Nov12
fine coarse
2.0
alphaUV alphavis
1.6 alpha
Oct25
180 160 140 120 100 80 60 40 20 0
1.2 0.8
AOD
0.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
AOD304 AOD670
Oct14
Oct25
Oct27
Nov12
date
Fig. 6. Comparison of AOD and A˚ngstro¨m exponent along with fine and coarse mode aerosol mass concentration.
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on the columnar air quality was more significant than local biomass burnings. Due to the influence of abundant coarse Asian dust aerosols, low A˚ngstro¨m exponent values of 0.4270.16, 1.1770.06 and 0.3370.12 were observed on November 12 in the visible, UV and UV-A ranges, respectively. auvA retrieved from AODs at 331 nm and 367 nm in the spectral region free from ozone absorption showed the lowest value on November 12 as did the value of avis. However, auv was quite high on that day compared to auvA and avis. That might be due to high uncertainties associated with the retrieval of AOD in the UV-B region due to ozone errors (Kim et al., 2006). Wavelength dependence of AOD in the visible range on this date is less linear than in the other three cases as shown in Fig. 4a. The Angstrom exponent for the 415–615 nm and 615–870 nm ranges were 0.13 and 1.1, respectively, indicating that AOD changes more sensitively at longer wavelength and that larger particles are abundant in the column atmosphere. Values for AOD and avis observed on an Asian dust day in this study were higher than previously observed values at the same site (Ogunjobi et al., 2004) as shown in Table 3. This might be due to the simultaneous occurrence of biomass burning and Asian dust. avis shows greater scattering out than other cases shown in Fig. 5, which indicates the effects of various types of aerosol size distribution from both biomass burning and Asian Dust events on November 12. On October 14, moderate values of AODs were measured and ranged from 0.40 at 304 nm to 0.17 at 870 nm, as summarized in Table 2. A comparison presented in Fig. 6 shows that both the aerosol mass concentration and AOD were higher than that associated with the BG case. AODs increased by
Table 3 Comparison of AOD at 500 nm, A˚ngstro¨m exponent and PM mass concentration with a previous study at the same region Parameter
BG
UP
AB
BB
AOD500 nm A˚ngstro¨m exponent Fine mass (mg/m3) Coarse mass (mg/m3)
0.09 0.69 6.3 11.7
0.21 1.70 43.4 29.2
0.57 0.33 27.6 126.1
0.36 1.19 43.2 10.4
Ogunjobi et al. [2004] (27)
AOD500 nm A˚ngstro¨m exponent Fine mass (mg/m3) Coarse mass (mg/m3)
0.17 0.83 11.9 8.5
0.27 0.78 39.7 84.2
0.79 0.42 52.1 92.9
0.47 0.86 72.7 79.1
This study
This study: BG (background, October 27), UP (urban pollution, October 14), BB (biomass burning, October 25), AB (Asian Dust plus biomass burning, November 12) categorized by Ryu et al. (2004).
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Fig. 7. Fine mode aerosol chemical composition on (a) 14, October (b) 25, October (c) 27, October and (d) 12, November 2002.
18–60%, while fine and coarse mode aerosol mass concentrations increased by 230–890%, respectively, when comparing values on October 14 and October 27. The AOD increment was much smaller than aerosol mass concentration measured on the ground. This might be attributed to the fact that aerosol mass concentration represents a near-surface value whereas AOD represents vertically integrated aerosol optical characteristics. When the aerosol vertical distribution is known, the relationship between the various aerosol properties could be better examined. The three-day backward trajectories on October 14 shown in Fig. 8 indicate that air mass originated from the heavily populated and polluted eastern part of China and then slowly transported over the
west coast of the Korean peninsula. Thus, the air mass arriving in the sampling site is affected by transported urban pollutants. However, the aerosol mass concentration data in Fig. 6 show that coarse mode aerosol still dominated even though the fine mode concentration increased. Trace element analysis results on this date show a high fraction of crustal species, which are mostly dominant in the coarse mode. Concentrations of Al, Fe, Ca and Ti in PM10 were 1.15 mg/m3, 0.78 mg/m3, 0.53 mg/m3, and 12.5 ng/m3, respectively, which are comparable to the Asian dust and biomass burning case of November 12. As shown in Fig. 5, avis, auv, auvA values on October 14 were 0.7870.10, 1.2170.44 and 0.8570.26, respectively. Low values of alpha also support coarse mode dominance of the air mass
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90˚
55˚
45˚
Changchun
Guangzhou
20°
150˚
50˚
Seoul
Lanzhou
Chengdu
140˚
Ulan Bator
45°
35°
130˚
2002-021025-03U12L (500,1000 and 2000m agl)
Ulan Bator
40°
120˚
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120˚
130˚
140˚
150˚
Three-day back-trajectories at Gwangju (021112.03U12L)
90˚
100˚
110˚
120˚
130˚
140˚
20˚
150˚
Fig. 8. Backward trajectories on (a) 14, October (b) 25, October (c) 27, October and (d) 12, November 2002.
on that day. Therefore, it can be concluded that the aerosol on October 14 was a mixture of dust aerosols and transported anthropogenic pollutants. 4. Conclusions Ground-based spectral AOD measurements were carried out at Gwangju, Korea to investigate the variability of AOD and the A˚ngstro¨m exponent during biomass burning periods using a visibleMFRSR and UV-MFRSR. Intensive measurements of aerosol properties were also conducted to compare fine and coarse aerosol chemical components with AOD and the A˚ngstro¨m exponents. AOD and A˚ngstro¨m exponents in the visible range were dependent on wavelength and aerosol type while AOD in the UV range showed a lesser dependence on aerosol type. This study analyzed AOD and chemical characteristics on four days. The cleanest day was October 27, after precipitation. Local field burning was severe on October 25 and increased PM mass concentration by 5–8 times and AOD by 1.4–1.8 times. Local biomass burnings that occurred even on small scales on October 25 increased the columnar aerosol loading by up to 180% and
consequently affected the spectral dependence of AOD. The simultaneous occurrence of Asian dust and biomass burning on November 12 resulted in the highest AOD level of 0.52–1.18 depending on the wavelength and the lowest A˚ngstro¨m exponent of 0.42 and 0.33 in the visible and UV-A ranges, respectively. However, the A˚ngstro¨m exponent measured for the UV region was relatively high due to the higher retrieval uncertainty of AODs in the shorter wavelength range. Fine and coarse mode aerosol mass concentrations for the Asian dust plus biomass burning case increased by approximately 3.4 and 9.9 times, respectively, while AOD increased by 2.3–4.0 times in comparison to values obtained for a clean day (October 27). Stagnant conditions during October 14 resulted in a high aerosol mass concentration over the study site. A mixture of transported urban pollutants and dust aerosols increased both fine and coarse mode aerosol mass concentrations. Due to a relative dominance of coarse mode aerosol on October 14, visible AOD increased more disproportionately than UV-AOD in comparison to those of the background case. Information on aerosol vertical distribution retrieved by LIDAR measurement may
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shed valuable light on the correlation between AOD and aerosol chemical component as well as the optical contribution of the aerosol component to AOD. Acknowledgments This work was supported by KOSEF through ADvanced Environmental Monitoring Research Center (ADEMRC) at Gwangju Institute of Science and Technology (GIST) and in part by the Brain Korea 21 (BK21) project.
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Gwangju, Korea. Journal of the Air & Waste Management Association 54, 1124–1137. Stolarski, R., Bojkov, R., Bishop, L., Zerefos, C., Staehelin, J., Zawodny, J., 1998. Measured trends in stratospheric ozone. Science 256, 342–349. Tang, X., Madronich, S., 1995. Effects of increased solar ultraviolet radiation on tropospheric composition and air quality. Ambio 24 (3), 188–196. Wenny, B.N., Saxena, V.K., Frederick, J.E., 2001. Aerosol optical depth measurements and their impact on surface levels of ultraviolet-B radiation. Journal of Geophysical Research 106 (D15), 17311–17319. Zepp, R.G., Callaghan, T.V., Erickson, D.J., 1995. Effects of increased solar ultraviolet radiation on biogeochemical cycles. Ambio 24 (3), 181–187. Zerefos, C., 1997. Factors influencing the transmission of the solar ultraviolet irradiance through the Earth’s atmosphere. In: Zerefos, C.S., Bais, A.F. (Eds.), Solar Ultraviolet Radiation, Modeling Measurements and Effects, ATO ASI Series I, vol. 52. Springer, New York, pp. 133–141.
Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 1609–1621 www.elsevier.com/locate/jastp
Spectral aerosol optical depth variation with different types of aerosol at Gwangju, Korea Jeong E. Kim, Seong Y. Ryu, Zhuanshi He, Young J. Kim Advanced Environmental Monitoring Research Center (ADEMRC), Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Republic of Korea Received 24 December 2004; received in revised form 18 March 2006; accepted 5 May 2006 Available online 21 July 2006
Abstract Simultaneous ground-based measurements of aerosol chemical composition and atmospheric aerosol optical depth (AOD) in the UV and visible regions were carried out at Gwangju (35.131N, 126.501E), Korea during a biomass burning period, October 4–November 12, 2002. An Asian dust event and biomass burning events were observed during the study period. The correlation coefficients (R) between AOD and PM2.5 and PM10 mass concentration during the study period are 0.5870.15 and 0.5270.14, respectively. Aerosol optical properties (AOD and A˚ngstro¨m exponent) and chemical characteristics were presented for selected days and their impacts on aerosol loading were examined. Both fine and coarse mode particle mass concentrations increased on biomass burning day resulting in high AOD values. The simultaneous occurrence of Asian dust and biomass burning resulted in the highest AOD of 1.20 at 311 nm and the lowest A˚ngstro¨m exponent of 0.42 in the visible range. When both transported urban pollutants and land dust aerosols affected the air quality over the study area, AOD and surface PM mass concentration increased 18–60% and 230–890%, respectively. r 2006 Elsevier Ltd. All rights reserved. Keywords: UV irradiance; Aerosol optical depth; MFRSR; Biomass burning aerosol; Asian dust
1. Introduction Studies on depletion of the stratospheric ozone have heightened the interest in the study of surface UV-B radiation because of associated anticorrelation aspects (Bais et al., 1997; Bernhard et al., 1997; Dickerson et al., 1997; Stolarski et al., 1998; Cappellani and Kochler, 1999). Positive UV trends Corresponding author. Tel.: +82 62 970 3401; fax: +82 62 970 3404. E-mail addresses: [email protected] (J.E. Kim), [email protected] (S.Y. Ryu), [email protected] (Z. He), [email protected] (Y.J. Kim).
1364-6826/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2006.05.008
can harm the ecosystem, such as terrestrial and aquatic biogeochemical cycles, and may also have adverse effects on human health (Tang and Madronich, 1995; Zepp et al., 1995; Caldwell et al., 1995; Longstreth et al., 1995). Total column ozone is the main factor affecting the surface UV-B level, but other factors such as cloud and aerosol can also affect the surface UV irradiance. The anticorrelation between ozone and the UV-B level has been well documented through the use of RAF (radiation amplification factor) for various spectral response functions (Madronich, 1993; Blumthaler et al., 1995). The aerosol effect on surface UV radiation has been actively studied during the last decade (Kylling et al.,
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1998; Meleti and Cappellani, 2000; Wenny et al., 2001) di Sarra et al. (2002) obtained RAF with higher accuracy by taking into account the aerosol effect with a sensitivity of 0.07–0.27. Krzyscin (2004) calculated RAF of aerosol to be 0.10–0.11 at a solar zenith angle of 601 and 801, which quantifies the effect of aerosol on surface UV irradiance. The relationship between the surface UV level and atmospheric aerosols can be more complex than the UV and ozone correlation since the aerosol effect is somewhat localized and time variant. Longterm UV monitoring in Belsk, Poland showed that 1% change in AOD forces a change of up to 0.15% in the UV dose (Krzyscin and Puchalsky, 1998). It has been reported that UV irradiance is reduced by 5–35% in comparison to aerosol-free conditions and is dependent on aerosol optical depth and wavelength weight function (Liu et al., 1991; Zerefos, 1997; Kylling et al., 1998). UV-B irradiance measured in a suburban area was 20% higher than that measured in downtown Mexico City, which corroborated the aerosol effect on surface UV irradiance (Acosta and Evans, 2000). Biomass burning aerosol is usually dominated by accumulation mode aerosols, and also often results in the elevation of coarse mode aerosols of soil particles near the fire due to ash (Kaufman et al., 1994; Artaxo et al., 1994). It was also reported that elevated concentrations of organic particles (radius Ro0:4 mm) and soil/ash particles (R41 mm) were usually associated with the presence of biomass burning (Kaufman et al., 1994). It was found that the A˚ngstro¨m exponent (440–1020 nm) ranged between 1.0 and 2.5 during biomass burning (Kaufman et al., 1994). Surface UV and visible irradiances have been measured using an ultraviolet multifilter rotating shadow-band radiometer (UV-MFRSR) and a multifilter rotating shadow-band radiometer (visible-MFRSR) at Gwangju, Korea. MFRSRs can provide several aerosol optical parameters such as aerosol optical depth (AOD) and the A˚ngstro¨m exponent, which can provide useful information about aerosol size distribution. In this study surface UV and visible irradiance data were analyzed for a biomass burning aerosol characterization campaign period at Gwangju Institute of Science and Technology (GIST), Gwangju, Korea around the harvest season in October and November of 2002. Spectral AOD results were compared with aerosol mass concentration and chemical composition. The goals of this study are to (i) investigate the variation of
aerosol radiative properties, such as the AOD and A˚ngstro¨m exponent during a biomass burning period in Gwangju, and to (ii) correlate these aerosol optical parameters with aerosol chemical characteristics. 2. Methodology 2.1. Radiation measurement A visible-MFRSR and UV-MFRSR were used in order to measure the global and diffuse spectral irradiances, AOD and A˚ngstro¨m exponent from direct normal irradiances. UV-MFRSR is a sevenchannel UV version of the visible-MFRSR and is described by Harrison et al. (1994) and Gao et al. (2001). Data were acquired from October 4 to November 12, 2002 on the rooftop of the Department of Material Science and Engineering building of Gwangju Institute of Science and Technology (GIST) in Gwangju (351130 N, 1261500 E), Korea. The UV-MFRSR measures the spectral UV irradiances centered at wavelengths 304, 311, 317, 323, 331, and 367 nm with 2 nm FWHM while the visible-MFRSR measures the spectral irradiances centered at 415, 500, 615, 673, and 870 nm with 10 nm FWHM. The measurement outputs from the UV-MFRSR and visible-MFRSR sampled every 20 and 15 s, respectively. These values were averaged for every 1-min period and saved into two separate data loggers. The UV-MFRSR was calibrated prior to initial installation in February 2002. The visibleMFRSR was recalibrated in October 2001 by the manufacturer. 2.2. Aerosol sampling Aerosol sampling was conducted at Gwangju, Korea from October 8 to November 12, 2002 with significant agricultural activities in the southern parts of Korea. Sampling, analysis procedure, and site description are described by Ryu et al. (2004). 2.3. Aerosol optical depth and A˚ngstro¨m formula Total atmospheric optical depth was determined by analyzing direct normal solar irradiance from total and diffuse solar irradiances measured by MFRSRs. Direct normal irradiance (DNSI) is calculated by subtracting diffuse irradiance (Imin) (measured when the shadow-band blocks the sundisk) from the global irradiance (Imax) (measured
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without blocking of the sun-disk) and dividing the result by the cosine of the solar zenith angle (y) as presented in Eq. (1) cos y is equal to the inverse of the optical air mass for a plane, parallel, nonrefracting and uniformly mixed atmosphere with a solar zenith angle below 801 (Iqbal, 1983; Harrison and Michalsky, 1994; Bigelow and Slusser, 2000). I ¼ DNSI ¼ ðI max I min Þ= cos y.
(1)
The Beer–Lambert law provides the total atmospheric optical depth. AOD is determined by subtracting the contributions by Rayleigh scattering, total column ozone absorption, and water vapor absorption. Water vapor absorption is not considered since the channel affected by water vapor was excluded in this study. I ¼ I 0 expðttotal mÞ
(2)
ttotal ¼ tozone þ twatervapor þ tRayleigh þ taerosol
(3)
where I0 is the solar irradiance at the top of the atmosphere, I is the direct normal solar irradiance, t is the atmospheric optical depth, and m is the relative optical air mass. I0 is retrieved using the Langley regression method when the relative optical air mass is between 1.5 and 3.0 for the UV-MFRSR due to its poor angular response when the solar zenith angle is below 701 and between 2 and 6 for the visible-MFRSR since the rate of change of the air mass is small at lower air masses (Harrison and Michalsky, 1994; Bigelow and Slusser, 2000; Kim et al., 2006). The Rayleigh scattering optical depth is well modeled by Hansen and Travis (1974). Ozone optical depth is a product of total column ozone and ozone absorption cross-section. The Earth Probe/TOMS satellite provides the total column ozone data with a 1.251 (lat.) 1.001 (long.) resolution in the form of one scan a day at around noon (local) with an uncertainty of 75% (http://toms.gsfc.nasa.gov/). Comparison of TOMS ozone data with the ground-based ozone measured by a Dobson spectrometer at Seoul, Korea during the measurement period showed a 5% difference in total column ozone. The ozone absorption crosssection at 226 K by Molina and Molina (1986) was used to calculate the ozone absorption optical depth. The effective ozone temperature of 226 K with variability of 220–231 K was determined from the radiosonde data obtained over Gwangju, Korea. Therefore, total uncertainty in ozone optical depth due to errors in total column ozone and effective ozone temperature is determined to be 70.12,
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70.05, 70.02, and 70.01, at 304, 311, 317 and 323 nm, respectively. tðlÞ ¼ bla ,
(4)
where t (l) is the AOD at each wavelength, l, b is a turbidity coefficient, and a is the A˚ngstro¨m exponent (A˚ngstro¨m, 1961). In this study a was calculated from spectral AODs by the linear regression fit in the visible/near IR ranges (415–870 nm), as well as the UV (311–367 nm) and UV-A (331–367 nm) ranges. AOD and a were only computed for conditions involving cloudless skies to exclude the effect of cloud. A cloudless sky was determined by the following criteria: I std o0:03, I avg
(5)
where Iavg and Istd are the 3-min average and standard deviation of irradiance measured by the broadband channel (300–1040 nm). 3. Results and discussions The correlation between daily mean AOD at 870 nm and PM mass concentrations is plotted in Fig. 1. Correlation coefficients between PM mass concentration and spectral AOD at eleven wavelengths are summarized in Table 1. Average correlation coefficients were determined to be 0.5870.15 and 0.5270.14 for PM2.5 and PM10 mass concentrations, respectively. Spectral AOD variation is plotted in Fig. 2 during the measurement period, October 4 – November 12, which shows high values on October 25 and November 12 and the lowest value on October 27. Open field burnings have been observed throughout the study period. The study site was affected by various types of aerosols as well as local biomass burning. In this study, various contributions from these sources to the total aerosol loading were examined by measurements of surface irradiance, chemical composition measurement and backward trajectory analysis for selected days (October 14, 25 and 27, and November 12). October 27 was selected as the background case when the air was relatively clean after precipitation on the previous day. 3.1. Surface irradiance Total, diffuse, and normal direct UV irradiances are plotted in Fig. 3 for the selected cases. As the direct-to-diffuse ratio is independent of absolute
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AOD870nm
0.30
y=0.0028x+0.075 R=0.88
0.25 0.20 0.15 0.10
10
20
30
40
50
60
70
80
PM2.5 mass concentration (ug/m3)
AOD870nm
0.30
y=0.0011x+0.082 R=0.83
0.25 0.20 0.15 0.10 0
20
40
60
80
100
120
140
160
PM10 mass concentration (ug/m3)
Fig. 1. Correlation between aerosol mass concentration and AOD at 870 nm and correlation coefficient (R).
Table 1 Correlation coefficients (R) between aerosol mass concentrations (mg/m3) and spectral AODs on October 8–10, 14, 25, 27, 30 and November 12
PM2.5 PM10
t304
t311
t317
t323
t331
t367
t415
t500
t617
t673
t870
avg.
0.57 0.47
0.56 0.49
0.45 0.40
0.46 0.39
0.48 0.40
0.48 0.43
0.49 0.46
0.58 0.54
0.70 0.65
0.77 0.71
0.88 0.83
0.5870.15 0.5270.14
1.2
1.0
AOD304 AOD310 AOd317 AOD323 AOD331 AOD367 AOD415 AOD500 AOD615 AOD673 AOD870
AOD
0.8
0.6
0.4
0.2
0.0 4-Oct
8-Oct
9-Oct
10-Oct 14-Oct 25-Oct 27-Oct 30-Oct 12-Nov date
Fig. 2. Spectral AOD variation during the measurement period.
calibration and absolute solar spectrum, it can be a good indicator showing total atmospheric attenuation. The direct-to-diffuse ratio was high on
October 14 and 27, except under cloudy conditions, which means that the aerosol loading was low. Diffuse irradiance is also related to aerosol size,
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single scattering albedo, surface albedo as well as aerosol amount. However, the ratio was much lower due to high aerosol loading on October 25 and November 12. 3.2. Aerosol optical depth and aerosol chemical composition Daily mean values of spectral AODs under cloudless sky conditions are summarized in Table 2. AODs for the selected days varied between 0.6970.40 at 304 nm and 0.2870.18 at 870 nm on
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average. The mean A˚ngstro¨m exponent was 0.727 0.20, 1.4170.36 and 1.0970.82 in the visible (415–870 nm, avis), UV (311–367 nm, auv) and UV-A (331–367 nm, auvA) ranges, respectively. Iqbal (1983) reported an a value of 1.3070.50, which is comparable to our results. Wavelength dependence of AODs is represented as a. An A˚ngstro¨m exponent determined from the wavelength dependence of AODs can provide basic information on aerosol size distribution. Fig. 4 shows the spectral dependence of AODs in the UV and visible ranges. In Fig. 5, A˚ngstro¨m exponents
Fig. 3. UV surface irradiance at 368 nm and direct-to-diffuse ratio on (a) October 27, (b) October 14, (c) October 25, and (d) November 12.
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Fig. 3. (Continued)
Table 2 Spectral AOD at 11 channels and A˚ngstro¨m exponent in the visible (415–870 nm), UV (311–367 nm), and UV-A (331–367 nm) regions
14-Oct 25-Oct 27-Oct 12-Nov
t304
t311
t317
t323
t331
t367
t415
t500
t616
t673
t870
avis
aUV
a*UVA
0.40 0.85 0.34 1.18
0.44 0.82 0.32 1.20
0.41 0.73 0.30 1.08
0.39 0.72 0.30 1.00
0.36 0.69 0.29 1.00
0.35 0.63 0.23 0.97
0.32 0.55 0.20 0.81
0.27 0.47 0.17 0.79
0.23 0.40 0.15 0.77
0.21 0.37 0.14 0.68
0.17 0.30 0.11 0.52
0.7870.10 0.8670.10 0.8370.04 0.4270.16
1.2170.44 1.5370.08 1.9370.32 1.1770.06
0.8570.26 0.9170.05 2.2570.07 0.3370.12
computed in the UV and visible ranges are plotted against AOD311 nm and AOD415 nm. The A˚ngstro¨m exponent in the UV (auv) and visible ranges (avis) varied between 0.30–2.40 and 0.02–1.10, respec-
tively, and showed no clear relationship between a and AOD. The lowest mass PM concentration and AOD values were observed on October 27 as summarized
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Fig. 4. Spectral AOD variation and wavelength dependence of AOD (A˚ngstro¨m exponent) on October 14, 25, 27 and November 12 in the (a) visible wavelength range and (b) UV wavelength range.
in Table 2. Daily mean AODs ranged between 0.34 and 0.11 at 304 and 870 nm with A˚ngstro¨m exponents 0.8370.04, 1.9370.32, and 2.2570.07 for the visible, UV and UV-A ranges, respectively. Fine and coarse mass concentrations were 11.9 and 8.5 mg/m3, respectively. Fig. 6 shows comparison of AOD and a values along with aerosol mass concentration. In Table 3, the aerosol characteristics obtained in this study are compared with those of a previous study (Ogunjobi et al., 2004) at the same site, which retrieved daily AOD from MFRSR data collected from January 1999 to August 2001 at the same site and correlated it with aerosol chemistry data measured in downtown Gwangju located about 10 km away from the study site. AOD values obtained on October 27 were higher than
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those obtained on clean days of the previous study (Ogunjobi et al., 2004) at the Gwangju site. Biomass burning observed around the study site in the late autumn involved the open field burning of agricultural waste, mostly in the form of rice straw. High values of AOD were observed on October 25 due to field burnings around the study site, which were 1.4–1.8 times higher than those on October 27. Fine and coarse mode aerosol mass concentration on October 25 increased to 72.7 and 79.1 mg/m3, respectively. The higher fraction of organic carbon (OC) observed in the fine mode aerosol mass fraction was the main effect of biomass burning at Gwangju, Korea during the study period as detailed in Ryu et al. (2004). OC contributed to 22% of the total fine mode mass concentration measured on October 27 (background case) while biomass burning increased the OC concentration so that it represented 42% of the total fine mode mass concentration on October 25 as shown in Fig. 7. The coarse mode aerosol concentration also increased on October 25. This might be due to the fact that coarse mode aerosol can increase during biomass burning due to the elevation of soil particles near the fire (Kaufman et al., 1994; Artaxo et al., 1994; Harrison et al., 1994) and aging or coagulation of fine particles at high concentration (Reid et al., 1998; Eck et al., 2005). As noted by Kaufman et al. (1998) smoke particles might increase their radius during their stay in the atmosphere by condensation and coagulation. A higher fraction of coarse mode aerosols is related to a low avis on this date. Ogunjobi et al. (2004) shows about a four times higher mass concentration in fine mode compared to coarse mode aerosol and a higher avis in comparison to values obtained during the BG period as shown in Table 3. A lower mass concentration of coarse particles in their study might be attributed to the aerosol measurement site, which was located downtown and affected to a less degree by coarser particles raised from biomass burning. Rare Asian dust events in autumn were recorded over the Korean peninsula during November 11–12, 2002. However, AOD and a could not be determined on November 11 due to thick cloud cover, when heavier Asian dust was observed. A three-day backward trajectory shows the transport of the air mass from Inner Mongolia and the Gobi Desert (Ryu et al., 2004). Biomass burning was also observed on November 12 and affected the air quality over the study area. Due to the simultaneous
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Fig. 5. Scatter plots of spectral AODs versus A˚ngstro¨m exponent for selected days.
occurrence of Asian dust and biomass burning on November 12, fine and coarse mode aerosol mass concentrations increased to 52.1 and 92.9 mgm3, respectively, which represent respective values that are 3.4 and 9.9 times higher than those measured on October 27. The highest AOD values up to 1.18 at 304 nm and 0.52 at 870 nm were observed on that day. Asian dust and biomass burning aerosol increased AOD by 2.3–3.2 times in the UV range and 3–4 times in the visible range, in comparison to
measurements obtained in the absence of these events. Measurements of aerosol chemical composition on this date show the characteristics of both dust particles and biomass burning aerosol. K+ concentration, a good tracer of biomass burning aerosol, increased to 0.43 mg/m3 on November 12, which was about five times higher than that on October 27. Organic carbon concentration also increased to 21.4 mg/m3 and contributed to 41% of the fine aerosol as shown in Fig. 7. Concentrations
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of crustal species on that day were also very high. Concentrations of aluminum (Al), calcium (Ca), iron (Fe), and titanium (Ti) were 1.37 mg/m3, 0.89 mg/m3, 0.73 mg/m3, and 18.7 ng/m3, respectively. Even though surface aerosol mass concentration from biomass burning was slightly lower than that of October 25, total columnar aerosol loading on November 12 was much higher due to Asian dust, which is generally known to be dominated by coarse particles, contributing to even higher AOD at all wavelengths. This is because biomass burning usually occurred around the measurement site and affected the near surface, while Asian dust was usually transported above the boundary layer as well as near surface. Thus, the effect of Asian dust Oct14
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on the columnar air quality was more significant than local biomass burnings. Due to the influence of abundant coarse Asian dust aerosols, low A˚ngstro¨m exponent values of 0.4270.16, 1.1770.06 and 0.3370.12 were observed on November 12 in the visible, UV and UV-A ranges, respectively. auvA retrieved from AODs at 331 nm and 367 nm in the spectral region free from ozone absorption showed the lowest value on November 12 as did the value of avis. However, auv was quite high on that day compared to auvA and avis. That might be due to high uncertainties associated with the retrieval of AOD in the UV-B region due to ozone errors (Kim et al., 2006). Wavelength dependence of AOD in the visible range on this date is less linear than in the other three cases as shown in Fig. 4a. The Angstrom exponent for the 415–615 nm and 615–870 nm ranges were 0.13 and 1.1, respectively, indicating that AOD changes more sensitively at longer wavelength and that larger particles are abundant in the column atmosphere. Values for AOD and avis observed on an Asian dust day in this study were higher than previously observed values at the same site (Ogunjobi et al., 2004) as shown in Table 3. This might be due to the simultaneous occurrence of biomass burning and Asian dust. avis shows greater scattering out than other cases shown in Fig. 5, which indicates the effects of various types of aerosol size distribution from both biomass burning and Asian Dust events on November 12. On October 14, moderate values of AODs were measured and ranged from 0.40 at 304 nm to 0.17 at 870 nm, as summarized in Table 2. A comparison presented in Fig. 6 shows that both the aerosol mass concentration and AOD were higher than that associated with the BG case. AODs increased by
Table 3 Comparison of AOD at 500 nm, A˚ngstro¨m exponent and PM mass concentration with a previous study at the same region Parameter
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UP
AB
BB
AOD500 nm A˚ngstro¨m exponent Fine mass (mg/m3) Coarse mass (mg/m3)
0.09 0.69 6.3 11.7
0.21 1.70 43.4 29.2
0.57 0.33 27.6 126.1
0.36 1.19 43.2 10.4
Ogunjobi et al. [2004] (27)
AOD500 nm A˚ngstro¨m exponent Fine mass (mg/m3) Coarse mass (mg/m3)
0.17 0.83 11.9 8.5
0.27 0.78 39.7 84.2
0.79 0.42 52.1 92.9
0.47 0.86 72.7 79.1
This study
This study: BG (background, October 27), UP (urban pollution, October 14), BB (biomass burning, October 25), AB (Asian Dust plus biomass burning, November 12) categorized by Ryu et al. (2004).
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Fig. 7. Fine mode aerosol chemical composition on (a) 14, October (b) 25, October (c) 27, October and (d) 12, November 2002.
18–60%, while fine and coarse mode aerosol mass concentrations increased by 230–890%, respectively, when comparing values on October 14 and October 27. The AOD increment was much smaller than aerosol mass concentration measured on the ground. This might be attributed to the fact that aerosol mass concentration represents a near-surface value whereas AOD represents vertically integrated aerosol optical characteristics. When the aerosol vertical distribution is known, the relationship between the various aerosol properties could be better examined. The three-day backward trajectories on October 14 shown in Fig. 8 indicate that air mass originated from the heavily populated and polluted eastern part of China and then slowly transported over the
west coast of the Korean peninsula. Thus, the air mass arriving in the sampling site is affected by transported urban pollutants. However, the aerosol mass concentration data in Fig. 6 show that coarse mode aerosol still dominated even though the fine mode concentration increased. Trace element analysis results on this date show a high fraction of crustal species, which are mostly dominant in the coarse mode. Concentrations of Al, Fe, Ca and Ti in PM10 were 1.15 mg/m3, 0.78 mg/m3, 0.53 mg/m3, and 12.5 ng/m3, respectively, which are comparable to the Asian dust and biomass burning case of November 12. As shown in Fig. 5, avis, auv, auvA values on October 14 were 0.7870.10, 1.2170.44 and 0.8570.26, respectively. Low values of alpha also support coarse mode dominance of the air mass
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on that day. Therefore, it can be concluded that the aerosol on October 14 was a mixture of dust aerosols and transported anthropogenic pollutants. 4. Conclusions Ground-based spectral AOD measurements were carried out at Gwangju, Korea to investigate the variability of AOD and the A˚ngstro¨m exponent during biomass burning periods using a visibleMFRSR and UV-MFRSR. Intensive measurements of aerosol properties were also conducted to compare fine and coarse aerosol chemical components with AOD and the A˚ngstro¨m exponents. AOD and A˚ngstro¨m exponents in the visible range were dependent on wavelength and aerosol type while AOD in the UV range showed a lesser dependence on aerosol type. This study analyzed AOD and chemical characteristics on four days. The cleanest day was October 27, after precipitation. Local field burning was severe on October 25 and increased PM mass concentration by 5–8 times and AOD by 1.4–1.8 times. Local biomass burnings that occurred even on small scales on October 25 increased the columnar aerosol loading by up to 180% and
consequently affected the spectral dependence of AOD. The simultaneous occurrence of Asian dust and biomass burning on November 12 resulted in the highest AOD level of 0.52–1.18 depending on the wavelength and the lowest A˚ngstro¨m exponent of 0.42 and 0.33 in the visible and UV-A ranges, respectively. However, the A˚ngstro¨m exponent measured for the UV region was relatively high due to the higher retrieval uncertainty of AODs in the shorter wavelength range. Fine and coarse mode aerosol mass concentrations for the Asian dust plus biomass burning case increased by approximately 3.4 and 9.9 times, respectively, while AOD increased by 2.3–4.0 times in comparison to values obtained for a clean day (October 27). Stagnant conditions during October 14 resulted in a high aerosol mass concentration over the study site. A mixture of transported urban pollutants and dust aerosols increased both fine and coarse mode aerosol mass concentrations. Due to a relative dominance of coarse mode aerosol on October 14, visible AOD increased more disproportionately than UV-AOD in comparison to those of the background case. Information on aerosol vertical distribution retrieved by LIDAR measurement may
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shed valuable light on the correlation between AOD and aerosol chemical component as well as the optical contribution of the aerosol component to AOD. Acknowledgments This work was supported by KOSEF through ADvanced Environmental Monitoring Research Center (ADEMRC) at Gwangju Institute of Science and Technology (GIST) and in part by the Brain Korea 21 (BK21) project.
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