Conversion of SO 2 to Particulate Sulfate during Transport from China to Japan – Assessment by Selenium in Aerosols –

To examine transportation and oxidation states of sulfur compounds from China to Japan, the SO2 concentration in the atmosphere and the nss-SO4 concentration and selenium (Se) concentration in aerosols were observed at four sites in western Japan. High SO2 concentrations in the atmosphere and the nss-SO4 concentration and Se concentration in aerosols were found when the air mass passed through the planetary boundary layer (PBL) over large cities or industrial areas in China during winter with winds from the northwest. A bibliographic survey of Se and S concentrations in aerosols over eastern Asia during 1991–2004 revealed that the average Se and S concentrations in aerosols at seven sites of urban or industrial areas in China were, respectively, 15.7 ng/m and 7.4 μg/m. The S concentration and Se concentration in aerosols in China were, respectively, about 3 times and 10–20 times higher than those of Korea and Japan. The Se/S ratios of aerosols in China were higher (avg. 2.5 × 10) than that of Korea (avg. 0.6 × 10) or Japan (avg. 0.7 × 10). The SO2 conversion rate was ascertained as 0.1% h to 1.5% h using the Se(sum)/S ratio technique, when the air mass passed over large combustion sources in China and later reached Japan. The Se/S (or its reverse) technique provides useful information to ascertain the SO2 to SO4 conversion rate.


INTRODUCTION
High-nss-SO 4 2concentrations in the atmospheric particulate matter in the atmosphere (rain, snow, wet and dry deposition, and aerosols) are often detected in winter at sites facing the Sea of Japan (e.g., Yamaguchi et al., 1991;Satake and Yamane, 1992;Wakamatsu et al., 1996).High selenium concentrations are also detected in wet and dry depositions during winter (Yokoo et al., 1995).These phenomena are believed to result from long-range transport of particulates from China, borne by winds from the northwest.Ohara et al. (2007) described that 82% of SO 2 emitted in eastern Asian area in 2000 originated from China.Streets and Waldhoff (2000) predicted that SO 2 emissions in China will increase from 25.2 Mt in 1990 to 30.6 Mt in 2020: a 22% increase during those 30 years.The increased amounts of coal consumption and SO 2 emissions are thought to be closely related (e.g., Ohara et al., 2007).Sulfur contained in coal and sulfur compounds is emitted into the atmosphere during combustion.The trend is divisible into three distinct time periods.The amount of SO 2 emitted from China roughly doubled during 1980-1995. Later, during 1996-2000, SO 2 emissions decreased by 13% from 24.3 Tg to 21.2 Tg (Lu et al., 2011).After 2000, SO 2 emissions in China increased dramatically by 61% from 21.2 Tg in 2000 to 34.0 Tg in 2006 (Lu et al., 2011).Nevertheless, recent reports have described that the amount of SO 2 emitted from China decreased from 2006 in spite of increased coal consumption because the rate of flue-gas desulfurization (FGD) in coalfired power plants increased in China (e.g., Lu et al., 2010).Trends of SO 2 and nss-SO 4 2-concentrations detected in Japan resembled those of SO 2 emissions in China (Lu et al., 2010).These results indicate that sulfur compounds observed in Japan are associated with SO 2 emissions in China.Nss-SO 4 2-, converted from the SO 2 components among aerosol particles, affects cloud physics and optical properties.When oxidized, the sulfur compounds produce nss-SO 4 2-aerosols, which can act as cloud condensation nuclei (CCN) (Wigley, 1989).The CCN changes affect climate by altering the number density and size distribution of droplets in clouds, and consequently their albedo (Wigley, 1989).In terms of sulfate aerosols, both the direct radiative effects and the indirect effects on clouds have been acknowledged (Solomon et al., 2007).They can influence precipitation mechanisms and the heat balance in the atmosphere.Consequently, obtaining information related to the conversion rate of SO 2 to SO 4 2-is important for investigation of the meteorological behavior of aerosol particles.
Selenium is a trace element that is richly present in coal.Tian et al. (2010) estimated the amount of Se emitted to the atmosphere from coal combustion in China.They calculated the amount of Se emissions as roughly doubling during 1980-1995, maintaining an approximately constant level during 1995-2000, and increasing again during 2000-2007.Growth of coal combustion has led to greater amounts of S and Se emitted to the atmosphere.In fact, S and Se in atmospheric aerosols are influenced strongly by coal combustion.During 2000-2005, the ratio of coal consumption to total fossil fuel consumption in China was about 60% (unit: PJ) (Ohara et al., 2007).However, the average FGD penetration rate in China increased from 1%-78% during 2000-2010 (Lu et al., 2011).Kido et al. (2012)  The Se/S ratios in fossil fuels have different values.Therefore, the ratio is used as a tracer to infer combustion sources (e.g., Hashimoto et al., 1970).Hashimoto et al. (1970) and Chiou and Manuel (1986) reported that the Se/S ratio in coal is higher (4-6 × 10 -4 ) than the Se/S ratio of heavy petroleum (0.63 × 10 -4 ) or in petroleum (0.3-1 × 10 -4 ).Dodd et al. (1991) reported that the Se/S ratio in aerosols derived from coal-fired power plants is higher (2.5 × 10 -2 ) than the Se/S ratio in aerosols derived from oil-fired power plants (4.8 × 10 -5 ).These Se/S ratios in the aerosols are useful to infer their particulates' combustion sources.
A previous study compared the Se/S ratios (or its reverse) in aerosols in air masses derived from combustion sources and those derived from observation sites in the U.S. to estimate the degree of oxidation of SO 2 that occurred during transport (e.g., Husain and Dutkiewicz, 1992).They estimated the homogeneous gas phase oxidation of SO 2 using a method of comparing the SO 4 2-/Se ratio in the aerosols in combustion source areas and those at observation sites.It can be inferred that using more details of SO 2 to SO 4 2-conversion rates will provide a better estimate than that obtained using the nss-SO 4 2-/(nss-SO 4 2-+ SO 2 ) ratio for combustion sources in China and an observation site in western Japan.
We observed nss-SO 4 2-concentrations and Se(0), Se(IV), and Se(VI) concentrations in atmospheric aerosols, and SO 2 concentrations in western Japan.We investigated the distributions of Se concentrations and the Se/S ratios in the atmospheric aerosols of eastern Asia using data from reports of studies conducted during 1990-2005.This study was undertaken to estimate the extent of the SO 2 oxidation using SO 2 to SO 4 2-conversion ratio (Fs ratio = SO 4 2-/(SO 2 + nss-SO 4 2-)) and the Se/S ratio in aerosols sampled in western Japan.Moreover, this report describes controlling factors of Se concentration and SO 2 to SO 4 2-conversion rates during transport from combustion sources in China to western Japan.

METHODS
Field observations were performed at Mikuni (35°15′N, 136°08′E), Nagoya (35°15′N, 136°98′E), Mihonoseki (35° 35′N, 133°13′E), and Amami oshima (28°47′N, 129°72′E).Both Mikuni and Mihonoseki are rural coastal sites facing the Sea of Japan.A local city, Fukui (pop.270,000) is located about 30 km south-southeast of Mikuni.A local city, Yonago (pop.150,000) is located about 20 km south of Mihonoseki.Amami oshima is a remote island facing the East China Sea.A local city, Amami (pop.45,000) is located about 20 km southeast of Amami oshima.Mikuni, Mihonoseki, and Amami oshima are all distant from large urban cities.Atmospheric samples from the Asian continent can be sampled under prevailing westerly or northerly winds.Nagoya, an urban area (pop. 2 million), has an industrial area located about 20 km south-southwest of it.Therefore, atmospheric samples might be influenced by industrial pollution carried along with winds from the south.The Se, nss-SO 4 2-, and SO 2 concentrations in atmospheric samples collected at Nagoya can indicate Se, nss-SO 4 2-, and SO 2 concentrations in urban areas in Japan.
Sampling periods at Nagoya, Mikuni, Yonago, and Amami oshima were, respectively, 2-4 and 25-28 February, 1999, 1-5 and 25-27 February, 11-19 December, 1999, and 16-22 December, 2000.Atmospheric aerosols were collected on PTFE membrane filters (0.2 μm pore size, 47 mm diameter; Advantec Toyo Kaisha Ltd.) using a suction pump at flow rates of 10-20 L/min.Sampling was performed continuously during 6:00-18:00 in the daytime and during 18:00-6:00 at night.The PTFE membrane filters were cut into halves.Aerosol samples for major ionic species were extracted from one half of the filter soaked in Milli-Q water using an ultrasonic method.Se (IV) and Se (VI) were extracted from aerosols on half of each PTFE membrane filter.Concentrations of major ionic species (5 cations, 3 anions) in the extracts were determined using ion chromatography (cation, DX-320 J and anion, DX-300; Dionex Corp.).The level of nss-SO 4 2-was estimated using the following equation.
In preliminary experiments for Se (IV and VI) extraction, only 54-67% of Se was extracted with Milli-Q water, whereas 83-93% was extracted with 0.1 to 1.0 N HCl.Therefore, we used 0.1 N HCl and an ultrasonicator for 30 min to extract Se (IV) and Se (VI).Selenium (IV) was derivatized to piazselenol and was determined using high-performance liquid chromatography (HPLC, RF-10Axl; Shimadzu Corp.) according to the fluorescence detection method (Ishikawa and Hashimoto, 1988).A mixture of cyclohexane and ethyl acetate (95/5, v/v) was used as the effluent (Nakaguchi et al., 1985).Piazselenol in the effluent was detected using a fluorometric detector (excitation at 380 nm; fluorescence at 530 nm) with a detection limit of 40 pg and accuracy of better than 4%.Selenium (VI) was reduced to Se (IV) with KBr in 12 N HCl at 90°C for 20 min (Nakaguchi et al., 1985), and was adjusted to pH 1 using 6M NH 4 OH.The amount of Se(VI) was ascertained by subtracting the amount of Se(IV) (analyzed using the method described above) from the sum of Se(IV) and Se(VI), as obtained using this reduction method of Se(VI).
Se( 0) is insoluble by 0.1 N HCl.Se(0) was separated from Se(IV) and Se(VI) by filtration using PTFE membrane filters (0.2 μm pore size, 25 mm diameter; Advantec Toyo Kaisha Ltd.).Se(0) was dissolved using 5 mL conc.HNO 3 (Nakamuro et al., 1973), after which it was concentrated to 1 mL in a boric acid glass vial at about 120°C on a hot plate.The silica gel powder was spread on a hot plate to buffer the rapidly simmering HNO 3 .After Se(0) was dissolved, a 6M NH 4 OH solution was added.Then pH was adjusted to 1.The solution was analyzed using the same method as that used for Se(IV).During the procedure for the determination of Se(IV) and Se(VI), more than 98% of Se(IV) was recovered from a sample, although no Se(VI) was recovered from another sample.The SO 2 data of Mikuni and Nagoya were obtained, respectively, from the Fukui Air Quality Authorities and the Aichi Air Quality Authorities.We measured SO 2 in Mikuni and Nagoya respectively using conductometric method and fluorescence method.SO 2 data of Mihonoseki were obtained for the Oki site from the Acid Deposition Monitoring Network in East Asia (EANET).SO 2 in Oki was measured using pulsed fluorescence (Model 43C-TL; Thermo Electron Corp.).The SO 2 data of Amami oshima were obtained by Yonemura et al. (unpublished data).Actually, SO 2 in Amami oshima was measured using pulsed fluorescence (Model 43S; Thermo Electron Corp.).Air mass courses at each site were analyzed for isentropic backward trajectories (Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model ver. 4 developed by the National Oceanic and Atmospheric Administration (NOAA) Air Resources Laboratory (ARL) with FNL (Draxler and Rolph, 2009;Rolph, 2009).These trajectories were calculated using the following conditions: Start altitude, 1000 m above ground level; calculation time, 120 hr (five days); start time, 0:00 Japan standard time (JST) and 12:00 JST, which are in the middle of every sampling time.Meteorological conditions under the influence of Asian continental outflow and those under the local pollution from Japan were defined respectively as Type A and Type B.

Geophysical Distribution of SO 4 2-, Se in Aerosols over Eastern Asia
We assessed Se and S concentrations and the Se/S ratios in aerosols at 16 sites throughout northeastern Asia (China, Taiwan, Korea, and Japan).Table 1 shows the Se and S concentrations and the Se/S ratios in aerosols observed in these northeastern Asian cities during 1991-2005.The quantities of Table 1 (1-14) correspond with the sampling site quantities in Fig. 1.The average concentration of Se observed in seven cities of China was 15.7 ng/m 3 The respective Se concentrations in large cities in China--Beijing, and Shanghai--were 11.7 ng/m 3 and 19.5 ng/m 3 The Se concentrations in large cities in Korea (Seoul, Chongju and Kwangju) and in Japan (Tokyo and Osaka) were 0.9-1.5 ng/m 3 The respective Se concentrations in Nagoya and Mikuni were 0.5 ng/m 3 and 0.4 ng/m 3 Therefore, the Se concentrations in China during 1991 and 2005 are 10-20 times higher than those of Korea and Japan.The Se concentration in Taiwan showed a high value (30 ng/m 3 ).The air quality in Taipei City improved during 1994-2003 (Chang and Lee, 2007).The various primary air pollutant concentrations such as CO, NO x , SO 2 , and PM 10 decreased during 1994-2003 (Chang and Lee, 2007).It might be inferred that the high concentration of Se in Taiwan was observed before improvement of air quality in Taipei City.

Nss-SO 4
2-, Se, and SO 2 Concentrations in Western Japan Next, we examined samples taken at four sites in western Japan, leeward of China, in 1999 and 2000 to ascertain the effects of sulfur compounds transported from China to Japan.We investigated the following: (1) nss-SO 4 2-and Se in aerosols, SO 2 concentrations, and the factors controlling these concentrations; (2) the characteristics of Se concentrations and oxidation states; (3) Fs and SO 2 to nss-SO 4 2-conversion rates (% h -1 ) during transport from China to Japan.In earlier studies, Se had been used as an indicator element of atmospheric pollutants from coal combustion (e.g., Chiou and Manuel, 1986).Additionally, the Se/S (or its reverse) ratios in aerosols were used in earlier studies in tracer techniques to estimate the homogeneous gas phase oxidation of SO 2 in the atmosphere (e.g., Husain and Dutkiewickz, 1992).We investigated Fs and SO 2 conversion rates during transport from China to Japan by examining Se/S ratios in aerosols.

Controlling Factors of Concentrations of nss-SO 4
2-and Se in Aerosols and SO 2 Gas Fig. 2(a) presents diurnal variations of concentrations of Se(0), Se(IV) and Se(VI) in aerosols at Mikuni, Nagoya, Mihonoseki, and Amami oshima sites.The concentrations of Se(0 + IV + VI) varied respectively at Mikuni and Nagoya: 0.02-1.04ng/m 3 and 0.07-1.52ng/m 3 The respective concentrations of Se(IV + VI) varied at Mihonoseki and Amami oshima: 0.01-0.73ng/m 3 and 0.09-0.39ng/m 3 The respective concentrations of nss-SO 4 2-in aerosols at Mikuni, Nagoya, Mihonoseki and Amami oshima varied: 0.83-5.43μg/m 3 , 0.87-3.01μg/m 3 , 0.57-2.94μg/m 3 , and 0.54-4.87μg/m 3 (Fig. 2(b)).We defined the periods of continental outflow as Type A, and the periods of local contamination as Type B. The high Se(0 + IV + VI) or high Se(IV + VI) concentration episodes were apparent not only at observation sites facing the Sea of Japan (Mikuni and Mihonoseki) in Type A (Kagawa et al., 2003) but also in Nagoya and Amami oshima.During this observation period, Se and nss-SO 4 2concentrations varied widely among different meteorological    1. conditions (Fig. 2).The controlling factors of aerosol concentrations are thought to be affected by (1) the effective source intensity of air pollutant and (2) removal processes such as precipitation scavenging during transport.Urban areas in Japan are possible sources of Se and nss-SO 4 2-.Concentrations of Se and nss-SO 4 2-increased when the air mass received local pollution (Type B).Here, we analyzed data of Type A, which were influenced by continental outflow.Observation sites of Mikuni, Yonago, and Amami oshima were less influenced by local contamination in this study.In addition to controlling factors, Nishita et al. (2007) described that particle concentrations of the accumulation mode range (100-300 nm) decreased by an order of magnitude when the cumulative precipitation amount was greater than 5 mm.Consequently, the trajectory data, which represent total cumulative precipitation amounts from 120°E to the sampling sites greater than 5 mm were eliminated from all trajectory data.
3(b) and 3(c), the character of K, M, and A respectively express hours before arrival and altitude when the air mass of Mikuni, Mihonoseki, and Amami passed through 120°E.area in Fig. 3(a) shows the total SO 2 emissions from coal combustion in China in 2000 (> 30 kt/yr per 0.5° × 0.5° grid cell) (Ohara et al., 2007).Although these areas are distributed in urban areas in Japan, Korea, Taiwan, and industrial areas in inland China, they were located especially near 120°E along coastal areas of China.The area in which Se emissions from coal combustion in China were found in 2005 (> 5 t/yr per 1° × 1° grid cell) (Tian et al., 2010) had also spread from 110° to 120°, near coastal areas of China (Tian et al., 2010).Fig. 3(a) portrays the horizontal backward trajectory: air masses passed over a gray area (large SO 2 and Se emission sources) containing high concentrations of nss-SO 4 2-and Se.However, the air masses passing over gray areas did not always show high concentrations of nss-SO 4 2-and Se.The ordinary planetary boundary layer (PBL) altitude is approximately 2 km (Stull, 1988).Generally, atmospheric aerosols show high concentrations in PBL and show low concentrations in the free troposphere (FT) at altitudes greater than 2 km (Jaenicke, 1993).Fig. 3(b) and Fig. 3(c) show the vertical isentropic backward trajectory for five days from the sampling site.The ordinary top of PBL is near 2000 m.We classified the air mass into two types.Fig. 3(b) and Fig. 3(c) respectively show air masses that passed through the area below the 2000 m a.g.l.over 120°E (Type I) and those that passed above 2000 m a.g.l.over 120°E (Type II).Remote continental and maritime aerosols fit nicely with the background aerosol at altitudes greater than 2 km (Jaenicke, 1993).often accompanied with highly concentrated episodes of nss-SO 4 2-and Se by continental outflow (Wakamatsu et al., 1996;Mori et al., 1997;Kagawa et al., 2003).Zhang et al. (2009) reported that the vertical profiles of aerosol concentrations showed high concentrations in the PBL and low concentrations in FT when a cold front passed through Beijing in China because the top of the inversion layer formed an extremely strong barrier and prevented exchange aerosol particles from the PBL height to the free troposphere.Ren et al. (2012) measured PM 2.5 by aircraft over eastern coastal areas in winter from December 25, 2002-January 6, 2003.They described that the highest concentrations of PM 2.5 were observed at the lowest altitude, indicating the influence of ground-level sources.Ichikawa et al. (1998) reported that SO 2 concentrations and particulate matter including sulfates were distributed uniformly at mixing height of approximately 1000-1200 m in ocean areas northwest of Kyushu Island.Consequently, high concentrations of Se(sum) and nss-SO 4 2appeared at Type I.For this study, low concentrations of Se(sum) and nss-SO 4 2-appeared at Type II.These results show that the Se concentrations in aerosols observed in western Japan were controlled by the vertical pathway (i.e., PBL or FT) of the air mass over industrial areas in China near 120°E.It can be inferred that aerosols derived from the boundary layer over the industrial area in China near 120°E reflect the aerosol characteristics of Chinese industrial areas.

SO 2 Conversion Rates of during Transport from China to Japan
Field studies of SO 2 oxidation were conducted mostly in downwind regions of sufficiently isolated sources (e.g., Elshort et al., 1978, Hegg andHobbs, 1980;Meagher et al., 1983).The contents of SO 2 and particulate sulfates were measured.Then the data were analyzed in terms of the concentration ratio ]) as a function of time or as a combination of wind speed and distance (Warneck, 1999).Sampling was accomplished with aircraftbased or ground-based measurements (Warneck, 1999).Husain and Dutkiewicz (1992) used Se as a tracer to infer the lifetime of SO 2 against gas phase oxidation by measuring SO 4 2-particle concentrations.Selenium-containing particles have the same sinks as SO 4 2-aerosols have, but they have no atmospheric sources.Therefore, as SO 2 is oxidized, the nss-SO 4 2-/Se ratio increases.Husain and Dutkiewicz (1992) conducted measurements of nss-SO 4 2-and Se in aerosols in summer at Mayville and Whiteface Mountain in the U.S, and subsequently produced a simplified model for the SO 2 oxidation rate using the nss-SO 4 2-/Se ratio.They found homogeneous SO 2 oxidation rates of 3% h -1 in summer.For this study, we applied the technique of Husain and Dutkiewicz (1992) and assessed SO 2 oxidation rates using Se/S ratios in aerosols during transport from China to Japan.Coal has been used as a main fuel in China.The Se/S ratio in coal (4.85-11 × 10 -4 ) is higher than the Se/S ratio in heavy petroleum (0.58-1.3 × 10 -4 ) (Hashimoto et al., 1970;Chiou and Manuel, 1986).We sought to compare the Se/S ratios in the aerosol particles and Se/S ratio in the coal.Therefore, we used not nss-SO 4 2-/Se but Se/S in the discussion so that it might be easy to compare values with the Se/S ratio in air aerosol, and the Se/S ratio in coal.
The Se(sum)/S nss-SO 4 2-ratio in the aerosols of Type I was 2.4-8.8 × 10 -4 (avg.5.4 × 10 -4 ).These ratios were lower than the ratios measured at large cities and industrial areas (Table 1).Husain and Dutkiewicz (1992) reported that SO 4 2-/Se ratios in the aerosols were double the ratio at the source because of SO 2 to SO 4 2-conversion during transport.For this study, it can be inferred that the Se(sum)/S nss-SO 4 2-ratio in the aerosols in Type I was lower than the Se(sum)/S nss-SO 4 2-ratio in the aerosols in China (Table 1).The conversion ratio of SO 2 to nss-SO 4 2-can be estimated using nss-SO 4 2-/(SO 2 + nss-SO 4
Atmospheric particulate matter pollution is an important issue in Beijing, China (Yu et al., 2013).Next, we estimate how much Se(sum)/S nss-SO 4 2-.changes or its quantitative evaluation during transport from China to Japan.The initial model conditions assumed for Beijing were set to the Se concentration and nss-SO 4 2-concentration in 2001 of Okuda et al. (2008) (Se: 9.07 ng/m 3 , S nss-SO 4 2-: 4.78 μg/m 3 , Se/S nss-SO 4 2-: 1.9 × 10 -3 ) and the SO 2 concentration reported by the Beijing Municipal Environmental Protection Bureau in 2001 (SO 2 : 64 μg/m 3 ).Fig. 6 shows a scatter plot of the travel time and the Se(sum)/S ratio after the air mass leaving 120°E.Almost always, the Type I air mass reached  ) ratios and Se(IV + VI)/S nss-SO 4 2-ratios (Se(sum)/S nss-SO 4 2-ratios) in aerosols.
Japan within 24-42 hr, as inferred from the isentropic backward trajectory.Fig. 7 shows a scatter plot of the travel time and the Fs ratio after the air mass leaving 120°E.The initial Fs value was 0.17.Yao et al. (2002) and Zhou et al. (2012), respectively observed sulfate and SO 2 in Beijing in winter in 1999-2000 and 2006.They reported the average Fs as about 0.2 in winter.This study's initial Fs value was close to the previous studies' value found for winter in Beijing.The solid line, dotted line, dashed line, dashed twodotted line, long dashed line and dashed one-dotted line respectively show the time variation of the Se/S ratios in the aerosols from Beijing, as calculated using 0.1% h -1 , 0.5% h -1 , 1.0% h -1 , 1.5% h -1 , 2% h -1 and 3% h -1 .Table 3 presents some results from previous studies in winter.Calvert et al. (1978) reported that the SO 2 oxidation rate by OH radical was 0.1% h -1 for winter.Meagher et al. (1983) reported that the average SO 2 oxidation rate was 0.15% h -1 in non-aqueous phase for winter.Newman (1981) reported that the diurnal average SO 2 oxidation rate was quite low, probably less than 1% h -1 .Takami et al. (2007) reported that the SO 2 oxidation rate of heterogeneous conversion was 2.0% h -1 for early spring in Cape Hedo, a remote island in Japan.Uno et al. (1997) produced an atmospheric transport model in eastern Asia.Then they reported that most of the observation data located within the heterogeneous reaction rate were 0.5-2.0%h -1 in winter.Uno et al. (2003) and Phadnis et al. (1988) produced a model of long range transport in eastern Asia, reporting respectively that the SO 2 conversion rates were 2.8 × 10 -4 s -1 (1% h -1 ) and 1% h -1 .Calvert and Stockwell (1983) reported that the SO 2 to nss-SO 4 2-conversion rate by model calculation in the typical mixed air mass during long-range transport was approximately 13-24% day -1 (avg.0.54-1.0%h -1 ).Additionally, they described that the SO 2 to nss-SO 4 2-conversion rates are higher with increased relative humidity.In this observation, the relative humidity varied from 53%-91% in Type I.The relation between the Se/S ratio 10 -5 10 -4 10 -3 10 -2 0.1 % h -1 0.5 % h -1 1.0 % h -1 1.5 % h -1 2.0 % h -1 3.0 % h -1 Type I (Se(sum)/S nss-SO 4 2-) Fig. 6.Scatter plot of travel time and Se/S ratio in the aerosols (start point: 120°E).The lines present results of a model of SO 2 conversion rate (0.1% h -1 , solid line; 0.5% h -1 , dotted line; 1% h -1 , dashed line; 1.5% h -1 , dashed two-dotted line; 2%, long dashed line; 3%, dashed one-dotted line).
Type I (Fs)  SO 2 conversion rate and relative humidity is not clear.As reported by Calvert et al. (1978) and Uno et al. (1997), SO 2 to SO 4 2-conversion rates were ranged from 0.1% h -1 to 2.0% h -1 .In this study, the SO 2 to SO 4 2-conversion rate estimated by the Se(sum)/S nss-SO 4 2-in Type I was within 0.1% h -1 -1.5% h -1 (Fig. 6).Furthermore, the SO 2 to nss-SO 4 2-conversion rate estimated by the Fs in Type I was almost 0% h -1 -3.0% h -1 (Fig. 7), yielding results that were within the limits expected in general.It can be inferred that SO 2 to nss-SO 4 2-conversion during transport from China to Japan not only causes a homogeneous reaction but also a heterogeneous reaction and an aqueous reaction.
Considering these results comprehensively, the Se(sum)/ S nss-SO 4 2-ratio is low compared to that of China because SO 2 that passed through the boundary layer over large cities or industrial areas in China was oxidized and converted to nss-SO 4 2-at a rate of 0.1-1.5% h -1 during transport from China to Japan.

CONCLUSIONS
This study was conducted to investigate the correspondence of transport conditions of Se concentrations, nss-SO 4 2concentrations and Se(sum)/S nss-SO 4 2-ratios in the aerosols in western Japan in winter.Observation of nss-SO 4 2-and Se in aerosols, and SO 2 was performed at four sites in western Japan (Mikuni, Nagoya, Mihonoseki, Amami) in winter in 1999 and 2000.The nss-SO 4 2-concentration and Se concentration in aerosols showed high concentrations from an air mass that passed through the inside of the PBL over urban or industrial areas in China.Results obtained by calculating the generation speed of SO 4 2-by oxidization of SO 2 until the air mass emitted from the large sources arrives at Japan using Se(sum)/S nss-SO 4 2-show values of 0.1-1.5%.This result is a reasonable value that is comparable to results obtained using models reported by Uno et al. and others.The result calculated using Fs was in the range of 0-3.0%, yielding results that were within generally expected limits.It can be inferred that SO 2 to SO 4 2-conversion during transport from China to Japan causes not only a homogeneous reaction but also a heterogeneous reaction and an aqueous reaction.The Se/S (or its reverse) technique provides useful information to ascertain the SO 2 to SO 4 2conversion rate.

Fig. 2 .
Fig. 2. Concentrations of Se(IV), Se(VI), and Se(0) (a), the nss-SO 4 2-concentrations (b), and the SO 2 concentrations (c) at Mikuni, Mihonoseki, Nagoya, and Amami oshima sites.Periods of Type A and B are depicted respectively as black and gray bars at the top of Fig. 2(a).

Fig. 3 .
Fig. 3. Horizontal (a) and vertical (b and c) backward trajectories for five days: Type I (solid lines) and Type II (broken lines) in horizontal backward trajectory.Gray areas show the grid of total emissions of SO 2 more than 30 kt from fuel combustion and industrial sources in China for 2000, 0.5° × 0.5° (unit: kt/yr per grid cell)(Ohara et al., 2007).In Figs.3(b) and 3(c), the character of K, M, and A respectively express hours before arrival and altitude when the air mass of Mikuni, Mihonoseki, and Amami passed through 120°E.

Fs
Fig. 5. Scatter plot of nsss-SO 4 2-/(SO 2 + nss-SO 4 2- Itahashi et al. (2012)reported that the optical thickness in eastern Asia is decreasing from its peak of 2005-2006.It can be inferred that the time around 2005 marked an important transition for SO 2 emissions in China.

Table 1 .
Se and S concentrations in aerosols in eastern Asia.

Table 2 .
Se (IV), Se (VI), and Se (0) concentrations in aerosols at four sites in western Japan.

Table 3 .
SO 2 to SO 4 2-conversion rate derived from field observations and model calculations in winter.