3.1.2. Ion Species in Aerosols
 The concentrations of water-soluble ionic species in aerosols are summarized in Table 1. Figure 3 shows the temporal variations and frequency of the concentrations of non-sea salt sulfate (nss-SO42−), nitrate (NO3−), ammonium (NH4+), and non-sea salt calcium (nss-Ca2+) in aerosols. The concentrations of nss-SO42− and nss-Ca2+ were calculated from the ratios of SO42−/Na+ and Ca2+/Na+ in seawater, respectively. Temporal variations in the concentrations of four chemical species are similar to one another and to those of the particle number densities.
Figure 3. Temporal variations and histograms of the concentrations of (a) nss-SO42−, (b) NO3−, (c) NH4+, and (d) nss-Ca2+ in aerosol. The solid triangles show the arithmetic mean, and the open triangles show the median value.
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Table 1. Concentrations of Ionic and Carbonaceous Compounds in the Aerosols From 29 March to 31 Maya
 The mean concentration of nss-SO42− was determined to be 2.48 μg m−3, which is close to the values previously reported from the east Asian Pacific Rim region during the winter and/or spring. For instance, Uematsu et al.  reported a mean concentration of nss-SO42− as 2.69 μg m−3 from Sapporo (about 250 km south of Rishiri) in the springtime, and Matsumoto et al.  obtained a mean concentration of 2.44 μg m−3 for nss-SO42− on Hahajima Island over the western North Pacific during outbreaks of continental air masses. On the other hand, higher concentrations of nss-SO42− (with a mean value of 7.90 μg m−3) have been found on Jeju Island, near the Korean Peninsula, during the spring [Carmichael et al., 1997]. In Rishiri, when the site was affected by the outflows of continental air masses, nss-SO42− concentrations intermittently increased to 5 μg m−3 or more. On the contrary, lower concentrations of nss-SO42− ranged within 0.5–1.0 μg m−3 were most frequently observed as shown by the histogram of nss-SO42− (Figure 3a), and these are close to the values previously reported from the remote ocean; for instance, 0.55 μg m−3 in the oceanic high-pressure region of northwestern Pacific during the summer [Matsumoto et al., 1998].
 Mean concentration of NO3− was found to be 0.64 μg m−3, which is close to the value reported from Hahajima Island during outbreaks of continental air masses [Matsumoto et al., 1998]. The samples with the concentrations below 0.2 μg m−3, however, were most frequently observed, and these lies within the range of values measured in the remote marine background atmosphere; for instance, 0.18–0.35 μg m−3 [Prospero et al., 1985] and 0.11–0.36 μg m−3 [Savoie et al., 1989] over the remote Pacific.
 Mean concentration of NH4+ was 0.72 μg m−3. Previous observations in the east Asian Pacific Rim region found the mean concentrations of NH4+ to be 0.37 μg m−3 on Hahajima Island during outbreaks of continental air masses [Matsumoto et al., 1998], and 1.32 μg m−3 on Jeju Island in the springtime [Carmichael et al., 1997].
 Figure 4 shows the temporal variation in the ratio of Cl−/Na+ in aerosols. This ratio was found to decrease when the site was hit by continental polluted air masses. Chlorine depletion in aerosols is caused by the reactions of acid gases such as sulfur dioxide and nitric acid with sea-salt particles [e.g., Zhuang et al., 1999]. The outflow of continental air masses containing abundant acid gases may cause chlorine-loss reactions in aerosols, thus leading to depletion of Cl− in sea-salt particles and generation of hydrochloric acid gas.
 The concentrations of particulate species showed large fluctuations during the observation period. The air quality on Rishiri showed drastic alternations according to air mass origins; the concentrations of particulate species were frequently higher because of outbreaks of continental polluted air masses, whereas under background conditions, they decreased to lower values similar to those observed over the remote ocean. As shown in Table 1, arithmetic mean values of the concentrations of all compounds are larger than the median values, which reflect the intermittent influence from the outflows of continental air masses. The ratios of mean/median values are higher especially for NO3− and nss-Ca2+, but are lower for nss-SO42− and NH4+. More pronounced enhancement in the outflows was found in the concentrations of NO3− and nss-Ca2+ than those of nss-SO42− and NH4+. The histograms and temporal variations for nss-SO42− and NH4+ show similar patterns, and those for NO3− and nss-Ca2+ also resembles each other. It is inferred from these results that the transportation of these compounds is done in the following order: nss-SO42− together with NH4+, and NO3− together with nss-Ca2+.
 Table 2 summarizes mean concentrations of ionic compounds in the fine- and coarse-particle ranges. Most of nss-SO42− (92%) and NH4+ (100%) exist as fine particles (d < 2.5 μm), which is consistent with many previous studies. Homogeneous nucleation and subsequent condensation of sulfuric acid derived from the photo-oxidation of sulfur dioxide, and aqueous oxidation reaction of sulfur dioxide absorbed in cloud droplets are the main generation processes of nss-SO42− in aerosols [McHenry and Dennis, 1994], which subsequently react with gaseous ammonia and transform to ammonium salt [Yamato and Tanaka, 1994]. Absorption of sulfur dioxide on sea-salt particles and subsequent oxidation are also a significant generation process of nss-SO42− in marine aerosols [Sievering et al., 1992, 1999; Yvon and Saltzman, 1996]. The size distributions of nss-SO42− and NH4+ obtained in this study can be explained on account of these generation processes. NO3− is mainly included in coarse particle range (d > 2.5 μm), as well as nss-Ca2+. Mineral dust particles act as an important carrier for NO3− in this region, as will be explained later.
Table 2. Mean Concentrations of Ionic Species in Fine and Coarse Particlesa
| ||Coarse (d > 2.5 μm)||Fine (d < 2.5 μm)|
|Mean||Ratio to Total||Mean||Ratio to Total|
3.1.3. Particulate Carbonaceous Species
 Figures 5a and 5b show the temporal variations and frequency of EC and OC, respectively. Unfortunately, because of the failure of the thermal control, the measurement of OC evolved below 200°C was unsuccessful until 28 March; therefore the data of OC during this period are not available. The results for carbonaceous species measurements from 29 March to 31 May are also summarized in Table 1. The mean concentrations of TC, EC, and OC are 1.05, 0.25, and 0.80 μg m−3, respectively.
Figure 5. Temporal variations and histograms of the concentrations of (a) EC, (b) OC, and (c) OCHT in fine particles (d < 2.5 μm). The solid triangles show the arithmetic mean, and the open triangles show the median.
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 During the outflow events of continental air masses, the concentrations of carbonaceous species drastically increased to higher values: around 1 μg m−3 and 2 μg m−3 or more for EC and OC, respectively. On the contrary, under background conditions, the carbonaceous species showed lower values, that is < 0.1 μg m−3 for EC and 0.3–0.9 μg m−3 for OC. These temporal variations in the concentrations of particulate carbonaceous species clearly show the alternation of air quality on Rishiri according to the origin of air masses. This pattern was also true for the ionic species and the particle number densities.
 Recently, the ambient carbon particulate monitor has been used for continuous measurement of particulate OC and EC in urban [Höller et al., 2002] and remote marine atmosphere [Uematsu et al., 2001; Matsumoto et al., 2001]. Höller et al.  determined the annual mean concentrations of TC and EC in fine particles (d < 2.5 μm) in Uji, Japan, as 4.76 and 2.73 μg m−3, respectively. Matsumoto et al.  reported that the concentrations of TC and EC in fine particles (d < 2.5 μm) were about 0.70 and 0.08 μg m−3, respectively, over the remote tropical North Pacific, whereas they increased to 1.14 and 0.34 μg m−3, respectively, over the oceanic region close to the Japan's main island.
 The measurements of particulate carbonaceous species by our method have significant uncertainties as with many previous studies. Matsumoto et al. [2003a] reported that a positive artifact probably caused by the adsorption of organic gases on the collectors of the ambient carbon particulate monitor could significantly lead to an overestimation of OC, especially the OC evolved at temperatures below 200°C. Similar results have also been reported by the measurements using quartz fiber filters, which have traditionally been used to collect and analyze particulate carbon [Novakov et al., 1997]. In marine atmosphere, however, the OC evolved at high temperatures between 200 and 340°C (OCHT) and EC may be not largely affected by the positive artifact in our method [Matsumoto et al., 2003a]. The concentrations of OCHT are also given in Table 1. The ratio of OCHT/OC was about 0.51, indicating that 49% of OC could be affected by positive artifact. The temporal variation and frequency of the OCHT concentrations are shown in Figure 5c, which are similar to those of OC. The concentrations of OCHT measured in this study should be considered as lower limited values, since they may be subject to underestimation because of a negative artifact induced by the evaporation of collected particulate organic matter during sample collection. Besides, in the thermal analysis used in our method, it is possible that the pyrolysis of some portion of OC also result in the underestimation of OC [Novakov et al., 2000a; Chow et al., 2001].
 Another problem associated with the ambient carbon particulate monitor is that particulate carbonaceous substances smaller than 0.14 μm in diameter are not collected by the impactor, and this may result in the underestimation of carbonaceous substances, especially of EC [Höller et al., 2002]. Although OC is mainly concentrated in the submicron size range, only a small fraction of OC is expected to be smaller than 0.14 μm [McMurry and Zhang, 1989; Höller et al., 2002]. Höller et al.  argued that even though a large fraction of EC exists as particles smaller than 0.14 μm in the urban area, it concentrates in accumulation-mode particles larger than 0.14 μm in the rural sites. The underestimation of EC due to missing the particles smaller than 0.14 μm should be small on Rishiri, since the site is mostly affected by chemically aged air masses.