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The average deposition fluxes of nss-SO₄^2-, NO₃^-, NH₄⁺, nss-Ca²⁺, Na⁺ and Cl^- by all types of rain (i.e., sum of the annual fluxes by the rainwater of Cy, SF, Ty and Other) over the eight years were 37.6±7.3, 16.3±4.2, 19.0±3.4, 9.6±4.8, 97.0±38.2, and 115.2±48.2 meq m^-2 yr^-1, respectively (Fig. 2). The contributions by different types of rain to the flux of an ion differed with the types. Cy contributed 22.2±3.5 meq m^-2 yr^-1 to nss-SO₄^2-, 9.7±2.1 meq m^-2 yr^-1 to NO₃^-, 11.1±2.6 meq m^-2 yr^-1 to NH₄⁺, and 5.6±0.8 meq m^-2 yr^-1 to nss-Ca²⁺, which were 58-60% of the average deposition fluxes of the respective ions. The fluxes of nss-SO₄^2-, NO₃^-, NH₄⁺ and nss-Ca²⁺ caused by SF were 8.2±5.6, 3.6±2.4, 4.3±3.2, 1.6±2.0 meq m^-2 yr^-1, respectively and the contribution was 17-23%. The fluxes of the terrigenous ions caused by Ty were 1.0±1.0, 0.4±0.5, 0.6±0.5 and 0.3±0.3 meq m^-2 yr^-1, respectively and the constitution to any of the ions was less than 4%.

Na⁺ and Cl^-, i.e. the major ions in sea water, were extremely abundant in Ty rainwater and the contribution of Ty to the wet deposition fluxes of the two ions was not small, with 24% to the flux of Na⁺ and 25% to the flux of Cl^-. Cy contributed approximately 44% to the fluxes of both ions, while SF approximately 14%. These results revealed that Cy type of rain was the largest contributor to the fluxes of all ions. The terrigenous ions nss-SO₄^2-, NO₃^-, NH₄⁺ and nss-Ca²⁺ were dominated by Cy, and the contributions of Ty were small. Na⁺ and Cl^- were substantially contributed by Ty, in addition to the predominant contribution by Cy.

The results were consistent with the characteristics of the rain types. It has been proved that cyclone-associated air masses around Japan were abundant in anthropogenic pollutants and mineral dust from the Asian continents (Zhang et al., 2005; Kaneyasu et al., 2000). Stationary front-associated air masses were affected by both continental air mass and maritime air mass, and were expected to be less abundant in anthropogenic pollutants and mineral dust (Zhou et al., 2004). Moreover, the annual rainfall of Cy was larger than that of SF (Fig. 2). Therefore, the fluxes by Cy were higher than that by SF. Typhoons are strong low-pressure systems initially generated by tropical maritime air masses (Su et al., 2012). As a consequence, the Ty rainwater is extremely abundant in sea salt components (Sakihama and Tokuyama, 2005) and it could significantly contribute to the fluxes of Na⁺ and Cl^-, although the rainfall in a year was much less than that of Cy and SF.

The year-by-year variations of the annual fluxes, sum of the fluxes from January to December in a year, of nss-SO₄^2-, NO₃^- and NH₄⁺ did not have clear trends (Fig. 3). There were likely two-year repeats of increase and decrease from 1996 to 2002. However, the fluxes of the ions in 2003 were the largest in the eight years. Cy contributed more than 50% to the flux of each ion in most of the years, and the trends of these ions caused by Cy were similar to the variations caused by all rain types together. The portion contributed by SF was small and less than 30% in years except for 1996 (up to 39%) and 2003 (up to 44%). Ty’s contribution constituted less than 10% in all years.

Fig. 3. 
Year-by-year variation of annual wet deposition fluxes of ions, rainfall amount and frequency caused by Cy, SF, Ty and Other.

The year-by-year variation of the annual flux of nss-Ca²⁺ was different from those of nss-SO₄^2-, NO₃^- and NH₄⁺. The flux was high in 1996 and 2003, although it was low and did not change largely in the period 1997-2002. This variation was similar to the variation of the portion contributed by SF. The portion by Cy was approximately stable and was more than 30% of the total nss-Ca²⁺ flux in all years.

The annual fluxes of Na⁺ and Cl^- were high in 1996 and 1997, and then they decreased until 2001. After that, the fluxes increased. The variation was similar to that of the portions contributed by Ty. But in some years, the portions by Ty were smaller than 10%. Variations of Na⁺ and Cl^- fluxes caused by Cy rain were different from the variations caused by all rain types together, and the portions by SF were less than or close to 30% in all years.

Therefore, Cy was the largest contributor to the annual fluxes of the six ions. In details, the variation of the fluxes of nss-SO₄^2-, NO₃^-, NH₄⁺ was mainly governed by Cy. SF substantially contributed to variation of the flux of nss-Ca²⁺, and, in some years, episodic Ty could largely contribute to the amount and variation of the fluxes of Na⁺ and Cl^-. Temporal variation of wet deposition flux has been usually studied and discussed by annually-, seasonally- or monthly-integrated data (e.g. Fowler et al., 2007; Lehmann et al., 2005). Those studies provided useful information on the long-term variation of wet deposition fluxes but did not explain the mechanisms for the variation. The results of the present study indicate that the fluxes by different types of rain need to be carefully investigated in order to accurately elucidate the long-term variations of the fluxes. In addition, some single rain case significantly contributed to the annual fluxes. For example, high annual fluxes of nss-Ca²⁺ in 2003 was actually caused by two SF cases and one Other case. Detail analyses revealed that the SF cases were abundant in nss-Ca²⁺ due to the influence of continental air masses for a long duration. The Other case was a complex case which was affected by both stationary front and typhoon, and brought large rainfall with large amount of nss-Ca²⁺. The flux of nss-Ca²⁺ caused by the three cases was 10.6 meq m^-2, which was corresponding to approximately half of the annual fluxes by all rain types in 2003. These results indicate that extreme rainfall events such as these should be treated carefully in discussing the temporal variation of the fluxes.

The rainfall and the ionic concentrations in rainwater are the factors determining the fluxes and their variations (Vet et al., 2014; EANET, 2006). To evaluate the relationship between rainfall and the fluxes, regression analysis was conducted. There was an approximately positive relationship between the annual fluxes of terrigenous-origin ions such as NO₃^- and the annual rainfall caused by Cy (R=0.74, Slope=0.008) (Fig. 4). Weak positive relationship existed for SF rain (R=0.48, Slope=0.005). This is because Cy and SF were the major types of rainfall contributors and the concentrations of the ions in the rainwater of Cy and SF were larger than in other types of rain (Toyonaga and Zhang, 2016). In contrast, the annual fluxes of nss-Ca²⁺ caused by Cy were negatively correlated with the annual rainfall caused by Cy (R= - 0.71, Slope= - 0.003) (Fig. 4). Detailed analyses revealed that the negative correlation was due to the extremely high concentration of nss-Ca²⁺ and low rainfall of the Cy in spring when the Cy was influenced by Asian dust, which could also be confirmed in spring rain cases around the East China Sea in previous studies (e.g. Seto et al., 2007). An exception is the Ty type of rain which had very high concentrations of Na⁺ and Cl^-, but Ty had limited influence on the annual fluxes and the year-by-year variation. Typhoons are episodic phenomena in the northwest Pacific areas and several typhoons may arrive at and pass the East China Sea every summer (Ho et al., 2004). At a place of more frequent typhoons’ passage, such as the areas in the south of Okinawa, Ty’s influence on ion fluxes would be more serious (Sakihama and Tokuyama, 2005).

Fig. 4. 
Annual wet deposition fluxes of NO₃^- (upper) and nss-Ca²⁺ (lower) vs. annual rainfall amounts caused by Cy and SF. Closed and open circles indicate the cases of Cy and SF, respectively. Linear regressions are marked by solid and dashed lines with the slope and correlation coefficient (R).

The average wet deposition flux of the nitrogen compounds, i.e. NO₃^- and NH₄⁺, was 35.3 meq m^-2 yr^-1 at the site of the present study. It is in the level similar to those observed at remote islands, such as Hedo of Okinawa (EANET, 2006) (Table 1). The fluxes at ocean areas, including the Pacific, the Atlantic and the Indian Ocean, were 2.1-10.1 meq m^-2 yr^-1 (Baker et al., 2010; Duce et al., 1991). Therefore, the East China Sea is actually the region with largest wet deposition flux of nitrogen compounds. Several previous studies have reported the large wet deposition fluxes of ions at the East China Sea areas (e.g. Zhang et al., 2007), and the cause has been attributed to the emission of relevant compounds and precursor gases in the continent, particularly in China (Vet et al., 2014; Uno et al., 2007). The present results show that the contribution by SF, Ty and Other together to the flux was approximately 14.5 meq m^-2 yr^-1, which is close to the maximum at ocean areas. This suggests that the major contributor of the extreme large flux at the East China Sea areas was Cy rain. The increase or decrease of Cy rainfall in a year could largely influence the annual flux of nitrogen compounds to the ocean areas, in addition to the change of emission.

The above results also suggest the necessity of taking into account the synoptic weather types for an accurate interpretation of wet deposition fluxes and their comparison with other areas. It has been reported that westerlies and meteorological conditions, such as the strength of cyclones, significantly influence the transport of air pollutants from the Asian continent and their deposition via rain (Morino et al., 2011; Zhang et al., 2011; Baker et al., 2010). The results of the present study strongly indicate that the ignorance of the differences that were caused by individual types of rain masked the underlying variations with rain types. This was also supported by a comparison between the sites where frequency of typhoon’s passage was apparently different. The flux at Hedo of Okinawa Island, where typhoons pass more frequently and is also influenced by SF every year, was 37.5 meq m^-2 yr^-1 (Table 1) (EANET, 2006). Sakihama et al. (2008) reported that the annual deposition flux of nitrogen compounds caused by typhoon-associated rain (i.e. as the Ty rain of the present study) constituted approximately 10% of the total flux in Okinawa Island. According to these values, the flux of nitrogen compounds by non-Ty rainwater at Hedo should have been approximately 33.8 meq m^-2 yr^-1. This is very close to the annual flux at the present study site after the extraction of the contribution by Ty, which was 34.3 meq m^-2 yr^-1. Therefore, the wet deposition fluxes of nitrogen compounds to the ocean caused by Cy and SF at different sites in the East China Sea were approximately similar and the differences of the fluxes between the sites were mainly due to Ty.

These results indicate Cy was the most important rain type for nitrogen compound supply to the East China Sea from the air in the point of view of long-term effects. Cyclones occur at the intrusion of cold polar air from northwest or west, and move eastward in the midlatitude westerly wind flows (Barry and Chorley, 2003). The postfrontal or prefrontal air is abundant in terrigenous substances or anthropogenic pollutants in recent years (Zhang et al., 2005; Kaneyasu et al., 2000). As discussed in section 3.1, these characteristics were consistent with the fact that the fluxes of nitrogen compounds by Cy were larger than the fluxes by SF and Ty, because SF and Ty rains are mainly affected by maritime air and contain less anthropogenic air masses compared to Cy rain. Regarding the fact that cyclones are typical and constant meteorological phenomena causing rain in the East China Sea, the supply by Cy is common, and can play an constant role in the air-sea interaction in the sea areas.