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 Aerosol chemical composition and gaseous species were measured at Cape Hedo, Okinawa, Japan (CHO), during the ABC/EAREX project period in March 2005, using high-time-resolution instruments including an Aerodyne quadrupole aerosol mass spectrometer (Q-AMS), a tapered element oscillating microbalance (TEOM), and gas monitors in order to investigate the transport and subsequent chemical transformation of aerosol in the east Asian region. Sulfate was the dominant species in fine aerosol mode and the average concentration of ammonium, sulfate and organics was 1.25, 6.37 and 2.16 μg m−3, respectively. The sulfate concentration observed at CHO in 2005 was about 1.5–2 times higher than that in 1994. For the majority of high-sulfate observations, the air mass was transported from the central east Chinese region (between Shanghai and the Shandong Peninsula). Sulfate transport was intermittent and strongly correlated with the passage of synoptic-scale high-/low-pressure systems. Chemical components and their concentration showed significant change on 17–18 March, which is accounted for by the change in air mass origin and the synoptic-scale weather system. In addition, it is suggested that the difference of air quality at the air mass origin reflects the chemical composition at CHO. The high sulfate concentration required heterogeneous conversion of SO2 to sulfate at a rate of 2.0% h−1. A pronounced signal at m/z = 44 confirmed that organic compounds were oxidized.
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 Rapid economic development has brought about increased energy consumption along with a higher rate of emission. For example, although SO2 emission in 2000 in China decreased by 16% compared to 1994 because of reduced coal use and switching to low-sulfur coal [Streets et al., 2003], recent data on SO2 emission shows an increase [State Environmental Protection Administration of China (SEPA), 2000, 2005]. In addition, because of transportation growth and increased use of chemicals, solvents and oil, emission of NOx and nonmethane volatile organic compounds (NMVOC) in 2000 increased by about 10% compared to 1994. Increase of emission leads to the increase of pollutants outflow from Asia, and may affect regional climate change through both direct and indirect effects of aerosols. Thus transport of anthropogenic aerosols and subsequent chemical transformation are of great interest.
 Observation of outflow from Asia has been conducted since the early 1990s. In 1990, the Perturbation by East Asian Continental Air Mass to Pacific Oceanic Troposphere (PEACAMPOT) project was implemented to monitor outflow [Hatakeyama et al., 2001]. Then, NASA conducted the Pacific Exploratory Mission (PEM)-West B in 1992 and 1994 [Arimoto et al., 1997]. In both missions, aerosol chemical composition was measured at Cape Hedo, Okinawa, and the sulfate concentration was reported to be about 3 μg m−3 [Arimoto et al., 1997]. In the 2000s, several projects investigated the direct and indirect effects of aerosols on regional climate: Aerosol Characterization Experiment – Asia (ACE-Asia [Huebert et al., 2003]), Transport and Chemical Evolution over the Pacific (TRACE-P [TRACE-P Science Team, 2003]), Pacific Exploration of Asian Continental Emission (PEACE [Parrish et al., 2004]), Atmospheric Particulate Environment Change Studies (APEX) and Atmospheric Environmental Impacts of Aerosols in East Asia (AIE). In 2003, we monitored aerosol chemical composition at Fukue Island, Japan (Figure 1), using an Aerodyne aerosol mass spectrometer during the APEX project period [Takami et al., 2005]. We observed high sulfate concentration (about 15 μg m−3) when the air mass was transported from the Qingdao region in China. Our results are in good agreement with those observed by a British group who first measured aerosol chemical composition at Jeju Island, Korea using an Aerodyne aerosol mass spectrometer during the ACE-Asia project period [Topping et al., 2004]. These observations revealed that aerosol chemical composition varies very much in the East China Sea region.
 In order to monitor outflow and subsequent chemical transformation of aerosols, we set up our aerosol monitoring system at Cape Hedo, Okinawa in autumn 2003. In this paper, we report the results from the ABC/EAREX project (March 2005), a period in which high sulfate concentration was frequently observed. We discuss the concentration level, the transport patterns and variation of aerosol chemical composition (mainly sulfate) in relation to back trajectory analysis and weather systems.
2. Experimental Method
 Our observation station was located at Cape Hedo, Okinawa, Japan (26.87°N, 128.25°E, 60 m asl.) as shown in Figure 1. The Cape Hedo Observation Station (CHO) is located at the north end of Okinawa Island. CHO is far away from any populated areas of the island, and has been used to study the outflow of pollution from east Asia [Kanaya et al., 2001a, 2001b; Kato et al., 2004].
 Several instruments were deployed for the intensive campaign. In this paper, we use mainly the data on aerosol mass concentration, aerosol chemical composition and gaseous species (SO2, CO, O3). An Aerodyne quadrupole aerosol mass spectrometer (Q-AMS; Aerodyne Research, Inc., MA), employed to monitor aerosol chemical composition, is capable of measuring both the mass spectra and mass distribution of nonrefractory ambient aerosols. Setup conditions for ambient measurement were the same as for our previous observation at Fukue [Takami et al., 2005]. A detailed description of the Q-AMS can be found elsewhere [Jayne et al., 2000; Allan et al., 2003a, 2003b; Jimenez et al., 2003]. Briefly, aerosols are separated from gaseous species by an aerodynamic lens and vaporized at 600°C on a vaporizer. The size cut of the aerodynamic lens is approximately PM1.0 [Jimenez et al., 2003]. The vaporized molecules are then ionized by the standard 70-eV electron impact ionization, and positive ions are analyzed by quadrupole mass spectrometer, which gives the mass spectra of aerosol components. The aerosol mass distribution against aerosol size is calculated from aerosol flight time, which is defined as the traveling time from the chopper to the detector. Flight time is converted to mass distribution using the results of size calibration [Jayne et al., 2000]. Mass distribution and mass spectra of aerosols were alternately measured every 30 s. The Q-AMS data are analyzed as described by Allan et al. [2003a, 2003b, 2004] and Jimenez et al. . The relative ionization efficiency (RIE) value for ammonium, nitrate, sulfate and organics used in this paper was 4.0, 1.1, 1.2 and 1.4, respectively. Organic mass is obtained by subtracting both gaseous species and inorganic species (sulfate, nitrate, ammonium, chloride, etc) from total mass. The organic mass reported by the Q-AMS is the same as the “organic matter” reported by other instruments. It includes all elements which are part of the organic species and not just organic carbon (OC).
 Collection efficiency (CE) of unity and IE/AB = 0.8 × 10−12 Hz were adopted for analysis of Q-AMS data obtained at CHO. IE is the ionization efficiency, which is calibrated using ammonium nitrate aerosol, and AB is the fraction of ambient nitrogen gas monitored by Q-AMS. The detailed description of IE and AB can be found elsewhere [Jayne et al., 2000; Allan et al., 2003a, 2003b; Jimenez et al., 2003]. IE/AB = 0.8 × 10−12 Hz with a standard deviation (1σ) of 0.1 × 10−12 Hz is an average value for 3-a measurements with our Q-AMS. IE/AB during this campaign was 0.8 (±0.06) × 10−12 Hz. Although CE depends on composition and phase of aerosols sampled by the AMS and varies from location to location, the value of CE = 0.5 is more commonly used for analysis of Q-AMS data. This number is based on the comparison between a PILS and Q-AMS [Takegawa et al., 2005]. In order to check the CE value, we compared sulfate concentrations measured using Q-AMS (Q-AMS-sulfate) with those measured using a low-pressure impactor (LPI-sulfate) in the spring of 2006. The low-pressure impactor (Tokyo Dylec Co.) consists of 13 stages, each one collecting size-resolved aerosols. Aerosol sampling was carried out at the top of the observation site, and the sampling period was about a week. Collected samples were extracted with deionized water using an ultrasonic bath. Ions were analyzed using an ion chromatography system (Dionex 500). The results are shown in Figure 2. The Q-AMS-sulfate analyzed using CE = 0.5 was about twice larger than the LPI-sulfate. CE = 1 seems suitable for analyzing the Q-AMS data obtained at CHO in March 2006.
 Since CE = 1 is found to be suitable for the data obtained at CHO in March 2006, we have to examine the possibility of CE = 1 for the data obtained in March 2005. Although there are several factors for CE to be determined, we consider that CE is mainly determined by the relative humidity of the inlet and aerosol composition. In Table 1, inlet relative humidity, inlet temperature, molar ratio of ammonium to sulfate and mass ratio of organic to sulfate are shown. Comparing these values in 2005 with those in 2006, they seem to be similar. In 2006, the organic-to-sulfate mass ratio is about 0.4. As shown in Figure 3a, the organic-to-sulfate mass ratio in 2005 is also about 0.4, indicating that sulfate is a dominant species. Although not conclusive, there is evidence that CE becomes greater than 0.5 when sulfate fraction is much greater than organics [Onasch et al., 2005], which does not exclude the possibility of CE = 1. The molar ratio of ammonium to sulfate is often less than two and sometimes even less than unity (Figure 3b). This indicates that sulfate compounds are mixtures of ammonium sulfate ((NH4)2SO4, molar ratio = 2), ammonium bisulfate (NH4HSO4, molar ratio = 1) and sulfuric acid (H2SO4, molar ratio = 0), and that ammonium bisulfate is a major component since the average molar ratio of ammonium to sulfate is 1.14. Deliquescence point of ammonium bisulfate is at relative humidity (RH) of 40% at 25°C [Tang and Munkelwitz, 1977; Imre et al., 1997]. At CHO, the range of RH in the ambient air in spring is between 40 and 90%, and the RH in the inlet of Q-AMS is between 20 and 60%. Taking hysteresis into account, ammonium bisulfate is considered to be in the liquid phase. It is known that aerosol in the liquid phase shows CE = 1 [Allan et al., 2004]. Therefore we guess that aerosol measured at CHO in 2005 mainly consists of the liquid phase aerosol and CE = 1 is suitable. CE = 1 is used for all species, since mass distribution was similar for ammonium, sulfate and organics, suggesting that they are all on the same aerosol particle.
Table 1. Comparison of Inlet Relative Humidity (RH), Inlet Temperature (T), Molar Ratio of Ammonium to Sulfate, and Mass Ratio of Organics to Sulfate Between March 2005 and March 2006
Inlet RH, %
Inlet T, °C
Molar Ratio of NH4/SO4
Mass Ratio of Org/SO4
 Aerosol mass concentration was measured by tapered element oscillating microbalance method (TEOM, RP1400, R&P/Thermo Electron Corp., MA), which measures total PM2.5 mass. Aerosol sampling was conducted through a half-inch copper tube projecting from the building roof. The sampling height was approximately 4 m from the ground. A PM2.5 cyclone or impactor was used to cut off coarse particles. CO was measured by a nondispersive infrared instrument (Model 48C, Thermo Electron Corp., MA) [Kato et al., 2004]. SO2 and ozone data was provided by the Acid Deposition and Oxidant Research Center (ADORC). SO2 and ozone were measured by ultraviolet fluorescent method (Model APSA-360, Horiba, Kyoto) and ultraviolet photometric method (Model APOA-360, Horiba, Kyoto), respectively [Acid Deposition Oxidation Center (ADORC), 2005].
3.1. Aerosol Mass Concentration and Variation
 Aerosol mass concentration and chemical composition were measured by TEOM and Q-AMS in March 2005, as shown in Figure 4. Average concentration and standard deviation are shown in Table 2. The average aerosol mass concentration (PM2.5) measured by TEOM was 15.1 μg m−3 and the maximum concentration was 81.6 μg m−3 at 1530 local time (LT) on 24 March. Sulfate was a major species, and organics and ammonium were present in lower amounts than sulfate. Nitrate and chloride concentrations were very low throughout the observation period.
Table 2. Average Concentration and Standard Deviation of Chemical Species
NH4, μg m−3
SO4, μg m−3
Org, μg m−3
TEOM, μg m−3
EC, μg C m−3
All Periods: 4 Mar, 1210 LT, to 25 Mar, 2350 LT
Period A: 11 Mar, 1800 LT, to 12 Mar, 0900 LT
Period B: 13 Mar, 0600 LT, to 14 Mar, 0000 LT
Period C: 14 Mar, 0600 LT, to 15 Mar, 1200 LT
Period D: 18 Mar, 0000 LT, to 18 Mar, 0600 LT
Period E: 18 Mar, 0600 LT, to 18 Mar, 1200 LT
Period F: 23 Mar, 0900 LT, to 23 Mar, 1800 LT
Period G: 24 Mar, 1200 LT, to 24 Mar, 1800 LT
 The TEOM results show sharp peaks on 11–12, 18, 23 and 24 March. During the same period, sulfate concentration was high. The maximum sulfate concentration was 27.2 μg m−3 recorded at 1600 LT on 24 March, which is almost the same time as the maximum aerosol mass concentration was measured by TEOM. The concentration of ammonium and organics was also high (8.46 and 7.54 μg m−3, respectively).
 The average ratio of organics to sulfate was 0.39, and as mentioned in the Experimental section, it was less than unity for most of the observation period (Figure 3a). The molar ratio of ammonium to sulfate was 1.14. The average ratio of the total mass concentration (equal to the sum of ammonium, nitrate, sulfate, chloride, organics) measured by Q-AMS to the aerosol mass concentration measured by TEOM was 66%. The average ratio of sulfate mass to aerosol mass measured by TEOM was 43%. These values indicate that sulfate is a dominant species in the fine particle mode measured at CHO.
3.2. Mass Distribution and Mass Spectra of Aerosols Measured by Q-AMS
 Typical mass distribution and mass spectra during high-sulfate events (24 March) are shown in Figure 5. The peaks in vacuum aerodynamic diameter for ammonium, sulfate and organics are located at 500–600 nm, which is in the accumulation mode. Mass distribution did not change significantly and similar mass distribution was obtained throughout the observation period.
 The mass spectra show only sulfate and organics for clarity. Typical sulfate fragments were observed at m/z = 32, 48, 64, 80, 81 and 98, where m/z is mass-to-charge ratio. For organics, m/z = 44 is one of the largest fragments. This signal corresponds to a carboxyl functional group (COO) and/or a fragment of thermally unstable oxygenated organics, which rearrange and fragment to give signal at m/z = 44. There are many small peaks corresponding to organic fragments at m/z > 50. Mass distributions and mass spectra patterns are similar to those observed at Jeju and Fukue [Topping et al., 2004; Takami et al., 2005].
3.3. Gaseous Species Mixing Ratio
 The mixing ratios of SO2, ozone and CO are shown in Figure 6. Peaks for ozone and CO mixing ratio were observed on 12, 17–18, and 23–24 March, and relatively large SO2 peaks were observed on 18 and 24 March.
4.1. Increasing Tendency of Sulfate at CHO
 Sulfate concentration at CHO was measured during the PEM-West B and PEACAMPOT project periods. Measurements were conducted using a filter sampler in the PEM-West B project. Average sulfate concentration during March 1992, 1993 and 1994 was 3.2 μg m−3 [Arimoto et al., 1997]. Murano et al.  used the filter pack method to measure sulfate from 26 February to 17 March in 1994 during the PEACAMPOT project. The sulfate concentration ranged from 0.147 to 6.97 μg m−3 and the average was 2.8 μg m−3. The average concentration for both measurements was in good agreement, and the sulfate concentration in the spring of early 1990s was considered to be about 3 μg m−3. In contrast, the average sulfate concentrations at CHO in March 2005 measured using Q-AMS were 6.37(±4.3) μg m−3. The Q-AMS data in April 2004 recorded the average sulfate concentration of 5.06 (±3.7) μg m−3 (A. Takami et al., Aerosol chemical compositions measured at Cape Hedo Observatory, Okinawa, Japan, in spring 2004, manuscript in preparation, 2007). ADORC measured sulfate at CHO using the filter pack method for samples collected once every two weeks. The monthly mean sulfate concentration in March 2005 and April 2004 was reported to be 5.01 and 5.82 μg m−3, respectively [ADORC, 2005, 2006]. Although there are some discrepancies between the Q-AMS data and the filter pack method data, the sulfate concentration is about 5–6 μg m−3. These values indicate a recent increasing tendency in sulfate concentration at CHO, with concentrations about 1.5–2 times higher than in 1994.
 1. A high concentration of sulfate was observed over the East China Sea, where air pollutants spent a relatively long time (approximately 3 d) and were well oxidized. The estimated residence time for SO2 was less than 50 h in this region in the boundary layer [Galloway, 1990].
 2. The polluted air mass was observed when it was transported from central China (around Shanghai) via the East China Sea to Japan. It was driven by a rapidly moving low-pressure system.
Uno et al.  studied the role of the synoptic-scale weather system and the importance of aqueous reaction of SO2 to sulfate using numerical model simulation called STEM. They revealed that the intermittent transport of pollutants was strongly correlated to the passage of synoptic-scale high-/low-pressure systems. They also found that most of the observed data including sulfate concentrations were in good agreement with simulation results by setting the aqueous reaction rate between 0.5 and 2.0% h−1.
 In order to investigate the transport pattern, we calculated the back trajectories and obtained the weather charts for the observation period. Back trajectories were calculated using HYSPLIT4/NOAA ARL with FNL starting at 500 m [Draxler and Rolph, 2003; Rolph, 2003]. The weather charts were obtained from data published by the Japan Meteorological Agency. Tables 2 and 3 show the average concentration and ratio of aerosol chemical species during high-sulfate periods, which were divided into seven periods labeled A to G. Figure 7 shows the back trajectories and weather charts corresponding to these periods.
Table 3. Average Molar Ratio of Ammonium to Sulfate and Mass Ratio of Other Chemical Species
 Back trajectories showed that the air mass was transported from the Shandong Peninsula except for the cases in which the air mass was transported from Shanghai (11 March, 1800 and 2100 LT, in period A; 18 March, 0000 LT, in period D; 23 March, 0900 LT, in period F; 24 March, 1500 LT, in period G) and from the Korean Peninsula (23 March, 1200–1800 LT, in period F). This indicates that, in the majority of cases, sulfate and its precursor, SO2, were transported from the central east China region (between Shanghai and the Shandong Peninsula), which is a similar pattern described in statement 2 above. Figure 7 shows that all the trajectories pass over the East China Sea, suggesting that high sulfate concentration would be observed over the East China Sea, which is also a similar pattern described in statement 1. It is confirmed that transport of sulfate from central east China via the East China Sea is a typical pattern to observe high sulfate at CHO.
 The weather charts in Figure 7 show a frontal system passing over Okinawa Island on 11, 18 and 23 March (Figure 7, periods A, D and F). The sulfate concentration was elevated after the frontal system passed, indicating that pollutants were transported behind the frontal system associating with the low-pressure system. In contrast, on 13–14 March (Figure 7, periods B and C), a high-pressure system was located over southern inland China and a low-pressure system was located at the northeast side of Japan. This is a typical winter weather pattern observed in this region, which brought a strong clockwise wind pattern around the high-pressure system [Uno et al., 1998]. It is considered that high sulfate observed on 13–14 March was due to the high-pressure system. The same pattern was observed on 24 March. It is confirmed that the intermittent transport of sulfate observed here is strongly correlated with the passage of synoptic-scale high-/low-pressure systems.
4.3. Transport Pattern of Other Species During High-Sulfate Period
 The concentration variation of other chemical species sometimes differs from that of sulfate. During the high-sulfate period of 13–15 March (periods B and C in Table 1 and Figure 7), the mixing ratio of CO was about 190 ppbv, which is a background level at CHO. The lifetime of CO is about one month, which is long enough to be transported in the east Asia region with little chemical change, since it takes less than one week for the air mass to be transported over the East China Sea. This means that the air mass reaching CHO during this period was aged and/or was transported from a relatively clean region, such as the Pacific Ocean. However, back trajectories in Figure 7, periods B and C, show that the air mass was transported from the Shandong Peninsula. We calculate the SO4/SOy ratio, where SO4 is sulfate and SOy is the sum of SO2 and sulfate, and the molar ratio of ammonium to sulfate (hereafter referred to as NH4/SO4) for the periods B and C. When the SO4/SOy ratio was high (0.96 and 0.92 in Table 2), the molar ratio of NH4/SO4 was low (0.70 and 0.76). According to Hatakeyama et al. , the NH4/SO4 ratio tends to be low in the case of long transport distance and/or long residence time over the sea. One of the reasons is that ammonium is lost by deposition while sulfate increases by SO2-to-sulfate conversion. The same reason may be applied to account for the low organics to sulfate ratio (0.27 and 0.19). In addition, cloud processing plays an important role. Conversion of SO2 to sulfate during cloud processing is efficient and results in the high SO4/SOy ratio without the need to have a very aged air mass. Also ammonia could be scrubbed from the air mass by clouds. High sulfate with low ammonium during periods B and C is considered to be the results of deposition and cloud processing of chemical species during the transport.
 In the D and E periods on 18 March, the chemical composition showed significant change even though the high-sulfate event lasted for only a short period (12 h). Figure 8 shows the variation of chemical species on 17–18 March. Concentration of aerosol chemical species started increasing late at night (2300 LT) on 17 March, and only sulfate concentration showed a peak, recorded at 0300 LT on 18 March (period D). The SO4/SOy ratio was relatively high (0.79). While the sulfate concentration stayed at around 15 μg m−3, CO, SO2, ammonium, organics and aerosol mass concentration (measured by TEOM) was elevated and peaked between 0700 and 0800 LT on 18 March (period E). At that time, the air mass appeared relatively fresh since the SO4/SOy ratio was low (0.51) and CO was high (401 ppbv). Chemical compositions and their concentration showed significant change for the short period. This can be explained by the dynamics of the atmosphere, i.e., shift of air mass origin from Shanghai to the Shandong Peninsula (Qingdao) and change of synoptic-scale weather pattern (Figure 7, periods D and E). During period E, when the air mass origin moved to the Shandong Peninsula where dust storms are often observed, sampled air masses probably contained dust aerosols since the aerosol mass measured by TEOM doubled but AMS nonrefractory mass did not. Thus we conclude that the air mass origin strongly influenced the chemical composition.
4.4. Conversion of SO2 to Sulfate
 The conversion of SO2 to sulfate is an important factor to account for the high-sulfate period observed at CHO. The average SO4/SOy ratio through the observation period is 0.83. During the A, B, C and F periods, the SO4/SOy ratio is greater than 0.9, which is higher than the average ratio, and the mixing ratio of SO2 in these periods was less than 0.4 ppbv, which is less than the average value of 0.45 ppbv. These values are compared to those measured at other locations to estimate the progression of SO2 oxidation during transport. The SO4/SOy ratio measured at Qingdao, Shandong Peninsula in winter/spring period of 2001 and 2002 was 0.47 and 0.27, respectively [Takami et al., 2006]. In the winter of 1999–2000, Yao et al.  measured the ratio of SO4/SO2 at Beijing. This is converted to a SO4/SOy ratio of 0.21, which is also low compared with that at CHO. Yao et al.  stated that the aqueous phase reaction is a minor reaction path in winter. The SO4/SOy ratio in the northern East China Sea was about 0.5, which was calculated using the results of aerial observation carried out near Fukue Island in March 2001 [Hatakeyama et al., 2004]. Comparing these ratios, the average SO4/SOy = 0.83 at CHO is much higher than that in China and in the northern East China Sea, indicating that most of the SO2 was converted to sulfate during transport over East China Sea to CHO.
 According to the back trajectories shown in Figure 7, the time it took for the air mass to leave the Chinese coastal region and reach CHO ranged from 24 to 55 h, which is relatively short compared to statement 1. The air mass stayed over the East China Sea for 1 to 2 d, indicating the importance of aqueous reaction of SO2 to sulfate. The rate constants between OH and SO2 in the gas phase are 9.7 × 10−13 or 8.8 × 10−13 cm3 molecules−1 s−1 [Atkinson et al., 1997; DeMore et al., 1997]. Assuming that the concentration of OH radicals is on the order of 1 × 106 cm−3, the pseudo first-order rate constants are 9.7 × 10−7 and 8.8 × 10−7 s−1, respectively [Finlayson-Pitts and Pitts, 2000]. Thus the estimated lifetime of SO2, reacting with OH radicals in the gas phase, is 11.9 and 13.2 d, respectively, which seem too long to account for the high sulfate observed at CHO. The rate constant in the CMAC model is 1.36 × 10−6 s−1, which is used for the reaction SO2 → SULF + SULFAER, here “SULF” is gas phase sulfate and “SULFAER” is aerosol phase sulfate [Gispon and Young, 1999, Table 8A-4, equation (84)]. The lifetime of this rate constant is about 8.5 d, which also seems too long to account for the high sulfate observed at CHO. The CMAC rate constant, 1.36 × 10−6 s−1, is similar to the lower limit of the conversion rate of SO2 to sulfate used by Uno et al. . Their simulation revealed that a higher aqueous reaction rate, i.e., 2.0% h−1, brought better results in the model calculation, which were compared to the observation data for higher sulfate concentration. The aqueous reaction rates 0.5 and 2.0% h−1 correspond to the rate constants 1.39 × 10−6 s−1 and 5.61 × 10−6 s−1, respectively, and also correspond to the lifetimes (1/k) 8.3 and 2.1 days, respectively. Since the air mass transport time from the Chinese coastal region to CHO ranged from 24 to 55 h (1 to 2 d), aqueous reaction likely occurs to produce sulfate during transport over the East China Sea, and the conversion rate of SO2 to sulfate is considered to be close to 2.0% h−1, as suggested by Uno et al. .
 The SO4/SOy ratios for the E and G periods (18 and 24 March) are much lower than the average value of 0.83. The mixing ratio of SO2 for the E, and G periods (5.16 and 3.46 ppbv, respectively) is much higher than the average SO2 value of 0.45 ppbv. In these cases, the air mass was transported within 30 h from the central east China region (between Shanghai and the Shandong Peninsula) (Figure 7, periods E and G). Short transport time is one of the reasons for the low SO4/SOy ratio at CHO. However, the air mass origin and the synoptic-scale weather system are similar between the E and G periods and the A and B periods. The frontal system passed over Okinawa Island and the high-pressure system moved eastward. A similar ratio SO4/SOy is expected under the similar transport and weather pattern. This means that the weather system alone cannot account for the low SO4/SOy ratio on both 18 and 24 March. We suggest that the day-to-day air quality in the region where the air mass passes may play some role in the SO4/SOy ratio. Air mass was mainly transported from near Qingdao, Shandong Peninsula, where the mixing ratio of SO2 has been changed significantly at times. For example, measurements made at Qingdao during 4 February to 15 March in 2002 show a range of SO2 mixing ratios from 0.17 ppbv to 66.8 ppbv with corresponding SO4/SOy ratios of 0.68 to 0.22 [Takami et al., 2006]. If the air mass happened to contain a high SO2 mixing ratio, and if the transport time was relatively short, a low SO4/SOy ratio would most likely be observed at CHO. Thus it is suggested that the air quality at the air mass origin would reflect the SO4/SOy ratio observed at CHO.
4.5. Oxidation of VOCs During Transport
 As mentioned above, the SO4/SOy ratio was generally high at CHO. This suggests that the air mass has aged and other chemical components could have also been oxidized. In urban areas, the mass distribution of aerosol is often bimodal and peak positions differ for organics and sulfate, with organics having the fresh smaller mode associated with them in urban areas [Canagaratna et al., 2004]. The mass distributions observed at CHO (Figure 5) are unimodal and show similar peak positions for ammonium, sulfate and organics (500–600 nm, accumulation mode). This indicates that the aerosols were aged, and is further supported by the organic mass spectra (Figure 5). The signal at m/z = 44 (hereafter refereed to as m44) is one of the largest fragments in the spectra. This signal corresponds to a carboxyl functional group (COO) and/or a fragment of thermally unstable oxygenated organics, which rearrange and fragment to give signal at m/z = 44. Both types of species are produced by the oxidation of organic compounds. In urban areas, the average ratio of m44 to organics is less than 0.1 [Canagaratna et al., 2004]. At CHO, it was 0.15 (±0.05) for the entire observation period. Therefore it was concluded that organics observed at CHO were oxidized.
 The m44/organics ratio ranged from 0.11 to 0.16 during high-sulfate events, and showed the less variation compared to the variable SO4/SOy ratio (Table 2). Organic compounds are considered to be oxidized in the gas phase by OH radicals in the daytime and ozone and nitrate mainly at night. Little variation in the m44/organics ratio indicates that the aqueous reaction does not play a major role in the oxidation of organic compounds, in contrast to the SO2-to-sulfate conversion.
4.6. Increase of Sulfur Emission
 The sulfate concentration observed at CHO in 2005 was about 1.5–2 times higher than in 1994. However, SO2 emission in China in 2005 was 25.5 Tg, which is a slight increase (7.5%) compared to the 1995 level (23.7 Tg). This alone cannot account for the increased sulfate at CHO. According to SEPA, China, SO2 industrial emission increased from 16.1 Tg in 2000 to 21.7 Tg in 2005 (35%). This indicates that rapid industrial growth resulted in the increased SO2 emission. In the previous section, it was shown that, in most of the high-sulfate observations, the air mass was transported from the central east Chinese region (between Shanghai and the Shandong Peninsula). It is likely that SO2 emission has recently shifted to the coastal region such as Shanghai and the Shandong Peninsula, where industry development is underway. It is inferred that the increased sulfate at CHO may have some relation to the industrial growth in China's coastal region. Further investigation of emission and transport is required.
 Aerosol chemical composition and gaseous species were measured at Cape Hedo, Okinawa, Japan during the ABC/EAREX project period in March 2005, using high-time-resolution instruments. Sulfate was the dominant species in fine aerosol mode and the average concentration of ammonium, sulfate and organics was 1.25, 6.37 and 2.16 μg m−3, respectively. The concentration of sulfate observed at CHO in 2005 is about 1.5–2 times higher than in 1994. In most cases of high-sulfate observation, the air mass was transported from the central east Chinese region (between Shanghai and the Shandong Peninsula). Sulfate transport was intermittent and strongly correlated with the passage of synoptic-scale high-/low-pressure systems. The suggested aqueous reaction rate of SO2 to sulfate was close to 2.0% h−1 to account for the high sulfate concentration observed at CHO. A pronounced signal at m/z = 44 indicates that organic compounds at CHO were oxidized. Chemical components and their concentration showed significant change at CHO, which is accounted for by the dynamics of the atmosphere. It is suggested that air quality at the air mass origin reflects chemical composition such as the SO4/SOy ratio observed at CHO.
 This work is supported by GERF/MOE (B-8, C-51), MEXT (AIE418), APEX (FY1999-2004), NIES_SR (FY2000-2005), which is deeply appreciated. Gas and aerosol data provided by ADORC are also greatly appreciated.