Abstract

The total suspended particulate (TSP) samples over the Bohai Sea and the Yellow Sea were collected during two cruises in spring and autumn in 2012. Concentrations of water-soluble ions {Na+, K+, NH4+, Mg2+, Ca2+, Cl, NO3, SO42−, and CH3SO3 [methanesulfonic acid (MSA)]} and trace metals (Al, Pb, Zn, Cd, Cu, and V) were measured. Mass concentrations of TSP samples ranged from 65.2 to 136 μg m−3 in spring and from 15.9 to 70.3 μg m−3 in autumn, with average values of 100 ± 22.4 and 40.2 ± 17.8 μg m−3, respectively. The aerosol was acidic throughout the sampling periods according to calculation of equivalent concentrations of the cations (NH4+, nss-Ca2+, and nss-K+) and anions (nss-SO42− and NO3). Correlation analysis and enrichment factors revealed that the aerosol composition in the coastal marine atmosphere had a feature of a mixture of air masses: that is, crustal, marine, and anthropogenic emissions. Trace metals were enriched by a wide range of 1–103, and enrichment factors for crustal source (EFc) were relatively higher in spring. Species like Cd, Zn, and Pb had an overwhelming contribution from anthropogenic sources. In addition, the concentrations of MSA varied from 0.0075 to 0.17 and from 0.0019 to 0.018 μg m−3 during the spring and autumn cruises, respectively, with means of 0.061 and 0.012 μg m−3, respectively. Based on the observed MSA and nss-SO42− concentrations in spring and autumn, the relative biogenic sulfur contributions to nss-SO42− were estimated to be 8.0% and 3.5% on average, respectively, implying that anthropogenic sources had a dominant contribution to the sulfur budget over the observational area.

1. Introduction

Natural and pollutant materials can reach the coastal oceans via a number of pathways, and atmospheric deposition serves as one of main supplies from continents (Arimoto et al. 1996; Zhang et al. 2004). As the Gobi Desert and Loess Plateau areas are supposed to be important sources of mineral aerosols over East Asia and the North Pacific Ocean (Zhang et al. 1993; Gao et al. 1997). Gao et al. (1992) had reported that the estimated atmospheric deposition of mineral aerosol to the Yellow Sea (YS) was 9–76 g m−2 yr−1 based on the representative crustal elements, which accounted for 20%–70% of the total input of mineral to the YS. Atmospheric deposition is also a significant source for chemical components to the marine system over the remote ocean. Previous studies have been conducted to obtain a better understanding of the geochemical cycling of combined nitrogen and trace elements such as Se, Zn, and Fe (Vandermeulen and Foda 1988; Bruland et al. 1991; Duce et al. 1991; Falkowski et al. 1998; Morel and Price 2003; Nakamura et al. 2005; Lam and Bishop 2008).

Sulfate is the major contributor of condensation nuclei, and the climate forcing caused by sulfate-rich aerosol particles has stirred up much interest. According to previous study models, aerosol climate forcing acts in a way that tends to offset the temperature increases caused by greenhouse gases—this is not only a result of the aerosol particles’ direct scattering of shortwave solar radiation but also their capacity to act as cloud condensation nuclei (CCN). Changes in CCN concentration affect the cloud droplet number and then influence the albedo and extent of cloud (Charlson et al. 1987; Wu et al. 2006).

Sea salt, mineral aerosols, and secondary aerosols (non-sea-salt sulfate, nss-SO42−, NO3, and NH4+) are the main components of water-soluble ions, and the secondary inorganic components mainly result from the interaction of SO2, NOx, and NH3 with particles from the continent. Su et al. (2011) have indicated that China has become the largest emitter of SO2 in the world and the total emission has doubled from 15.4 to 30.8 Tg from 1990 to 2007 because of the rapid pace of economy and industry. Thus, the rapid industrialization and urbanization in the eastern coastline of China have caused regional air pollution, changed acidic and/or alkaline nature of atmospheric aerosols, and affected the pH value of surface water according to dry–wet deposition (Huang et al. 2008).

The YS is a semiclosed water body of the northwestern Pacific Ocean with an area of 3.8 × 105 km2. The Bohai Sea (BS) is the innermost gulf of the YS. Their favorable geographical locations make the eastern China coastal seas a good transition zone for mineral particles and pollutants from Asia to the western North Pacific through atmospheric long-range transport. As a result, the input of pollutants to the region is considerable. Several previous observations were made mainly over eastern and southern China coastal seas (the East China Sea and the northern South China Sea) or at several local monitoring sites—for instance, Qianliyan and Cheju Island (Chen et al. 1997; Kim et al. 2000; Zhang et al. 2001; Hsu et al. 2007; Uematsu et al. 2010; Zhu et al. 2013)—but little work has been done on the characteristics of total suspended particulate (TSP) over the BS and YS, especially in spring and autumn on a large scale. In this study, TSP samples were collected over the BS and YS in spring and autumn, and the concentrations of aerosol components, including major water-soluble inorganic ions, selected trace metals and methanesulfonic acid (MSA) were determined. As the input from distant sources varies with time and space, special emphasis has been placed on discussing the characteristics and ionic correlations of aerosol compositions. In addition, we assessed the main source of measured ions qualitatively and quantitatively in combination with enrichment factors and provided valuable information for understanding the input pathway of aerosols over the study area. Overall, these data establish quantitative budgets of crustal and anthropogenic inputs to the marine boundary, which would be helpful to understand the geochemical cycling of nutrients in the eastern marginal seas of China. Moreover, it is very useful as a guideline for the government to formulate policies to solve the problems of environmental pollution.

2. Materials and methods

a. Sampling areas and sampling facilities

Two cruises were conducted in the BS and YS on board the R/V Dong Fang Hong 2 from 2 to 20 May (spring) and from 2 to 19 November (autumn) 2012 (Fig. 1). A total of 12 TSP samples were obtained in the spring cruse and 11 samples were obtained in the autumn cruise. The atmospheric particulates were continuously collected using a high-volume sampler (Model KB-1000, Jinshida Electronic Technology Co. Ltd., Qingdao, China) with Whatman 41 filters (Whatman International Ltd., Maidstone, England) used as substrate. Whatman 41 filters have been widely used to collect TSP aerosol (Gao et al. 1996; Baker et al. 2006; Zhang et al. 2007). The sampler was mounted windward on the upper deck of the ship approximately 10 m above the sea surface. Wind direction and ship driving direction were monitored in real time by the onboard monitoring system. Additionally, TSP samples were captured only when the wind direction was suitable for sampling and the ship speed was over 2 knots (kt; 1 kt = 0.51 m s−1) to avoid the deposition of waste gas from the ship. The sampler was turned off immediately when the speed was below 2 kt and anchored for other research activities. The sampling duration was about 24 h with a mean flow rate of 1.0 m3 min−1. After collection, the samples were folded in half according to the sampling mark line in clean polyethylene bags and stored in a refrigerator (−24°C) before dry weighting and chemical analysis. Clean plastic nippers and disposable gloves were used to avoid contamination during the whole operation process. To minimize the biases caused by the ambient conditions, all of the filters were first dried in the glass driers for 24 h before and after sampling and then weighed with an analytical balance under constant room temperature (25° ± 1°C) and humidity (40% ± 1%). In addition, the sample filters were weighed at least twice to estimate the net mass of filters until the weighing differences were controlled within ±1.0 mg.

Fig. 1.

Ship tracks in the BS and YS in (top) the first cruise (spring) and (bottom) the second cruise (autumn).

Fig. 1.

Ship tracks in the BS and YS in (top) the first cruise (spring) and (bottom) the second cruise (autumn).

b. Ion analyses

1) Analyses of trace elements

For the concentrations of trace metals, a strong digestant was employed by exposing the samples to a mixture of concentrated acids (Zhuang et al. 2001). Approximately 0.1 g of the filters were placed in clean dry Teflon containers, then concentrated acid mixtures (5 mL HNO3 + 1 mL HClO4 + 0.5 mL HF) were added and heated sequentially to dryness at 150°C. After digestion, 1 mL HNO3 was added to dissolve the residues and left for several days before being filtered through cleaned 0.2-mm filter units. Then the sample was adjusted to 25 mL with Milli-Q water.

The concentrations of trace metals including Al, Pb, Zn, Cd, Cu, and V in aerosols were determined using an inductivity coupled plasma atomic emission spectrometer (ICP-AES; ICAP 6300, Thermo Fisher Scientific Co., United States). The detection limits for trace elements were 50 ng mL−1 for Al, 4.0 ng mL−1 for Pb, 8.0 ng mL−1 for Zn, 1.5 ng mL−1 for Cd, 1.5 ng mL−1 for Cu, and 0.2 ng mL−1 for V.

2) Analyses of ionic species

Water-soluble ions in aerosols were ultrasonicated for 40 min and extracted into 25 mL high-purity water made from the Milli-Q system (resistivity greater than 18 MΩ·cm) and concentrations were determined by ion chromatograph (Model ICS-3000, Dionex Co., United States). A CS12A separator column was used to determine cations (Na+, K+, NH4+, Mg2+, and Ca2+) and a AS11-HC separator column was used to determine anions (Cl, NO3, SO42−, and MSA). The detection limits were 0.01 mg L−1 for the cations and 0.02 mg L−1 for the anions. During the two cruises, a total of eight filter blanks were taken and stored to analyze the anions and cations along with the samples. The background concentrations of those ions were detected to correct the real concentrations in the samples. The non-sea-salt component was calculated using the equation

 
formula

In this study, mass ratios (m/m) of 0.0370, 0.0382, and 0.251 were used for (K+/Na+)seawater, (Ca2+/Na+)seawater, and (SO42−/Na+)seawater, respectively (Millero 2006).

c. Backward-trajectory analysis

We adopted 3-day backward-trajectory analyses to trace the sources of air masses using the hybrid single-particulate Lagrangian integrated trajectory (HYSPLIT 4) model (NOAA Air Resources Laboratory; http://www.arl.noaa.gov/ready/hysplit4). At three different altitudes of 500, 1000, and 1500 m AGL, trajectories were obtained to identify the origin and flow of typical air masses.

The synoptic conditions presented relatively similar circulation patterns during the two cruises. Gansu, Xinjiang, and Inner Mongolia Provinces were the main birthplace for the dust. Historical dust data were found in the Summary of Environmental Quality in Gansu Province in 2012 (www.gsep.gansu.gov.cn) in which dust weather occurred 44 times and evolved into dust storms mainly in April and May. Large-scale dust storms also occurred in Hohhot City from April to May according to the 2012 Inner Environment Bulletin (http://www.nmgepb.gov.cn/). These records were well consistent with the backward trajectories.

According to the backward trajectories, sampling sites were strongly influenced by the continental air masses and the dust storms were measured according to the above government records. Moreover, it also illustrated that air masses mainly originated from the continental areas and the northwest monsoon system was the major driving force of wind transport during spring (March–May) and midautumn (October–November), which drove the air masses directly to the adjacent sea areas (Fig. 2).

Fig. 2.

The 3-day backward trajectory analysis for typical days during the two cruises in 2012: (a) 3 May, (b) 9 May, (c) 15 May, (d) 18 May, (e) 3 Nov, (f) 7 Nov, (g) 11 Nov, and (h) 17 Nov. For a detailed discussion of origins of the air mass, refer to the text.

Fig. 2.

The 3-day backward trajectory analysis for typical days during the two cruises in 2012: (a) 3 May, (b) 9 May, (c) 15 May, (d) 18 May, (e) 3 Nov, (f) 7 Nov, (g) 11 Nov, and (h) 17 Nov. For a detailed discussion of origins of the air mass, refer to the text.

3. Results and discussion

a. Meteorological conditions

Meteorological parameters including temperature, wind speed, and relative humidity were obtained simultaneously by the onboard monitoring system. During sampling, the average wind speed and temperature in the spring cruise were 4.8 ± 2.2 m s−1 and 15.3° ± 2.1°C, respectively, while data in the autumn cruise were relatively higher, with average values of 7.3 ± 3.8 m s−1 and 11.4° ± 3.9°C. In addition, the relative humidity ranged from 45.7% to 100.5% in spring and from 41.8% to 70.1% in autumn, respectively, with average values of 80.3% and 61.0%.

b. Overview of the TSP samples

The concentrations of detected ions in TSP samples during spring and autumn are summarized in Table 1. Concentrations of water-soluble ions ranged from 11.9 to 43.1 and from 9.29 to 37.5 μg m−3, respectively, with averages of 22.6 ± 10.5 and 25.0 ± 9.11 μg m−3. These results are comparable with data from other marginal seas (Lee et al. 2002; Nakamura et al. 2005) but much higher than remote oceanic regions. As presented in Table 1, of all TSP samples, the concentrations of the most abundant species followed the order SO42− > NO3 > NH4+ > Cl > Na+ > Ca2+ > Mg2+ > K+, where SO42− and NO3 were dominant. Total concentrations of secondary species (nss-SO42−, NO3, and NH4+) averaged 19.0 and 15.2 μg m−3 and occupied about 79.7% and 60.4% of the analyzed ionic species during spring and autumn, suggesting that the investigated areas during the two cruises were under strong continental influence.

Table 1.

Statistical summary of ions in TSP aerosols collected during the spring and autumn cruises (units for TSP mass concentration and water-soluble ions are μg m−3, and units for trace metals are ng m−3).

Statistical summary of ions in TSP aerosols collected during the spring and autumn cruises (units for TSP mass concentration and water-soluble ions are μg m−3, and units for trace metals are ng m−3).
Statistical summary of ions in TSP aerosols collected during the spring and autumn cruises (units for TSP mass concentration and water-soluble ions are μg m−3, and units for trace metals are ng m−3).

For metal elements, the average concentrations were 1.37 ± 1.83 μg m−3 in spring and 1.80 ± 1.86 μg m−3 in autumn, accounting for 1.37% and 4.48%, respectively, of the TSP mass concentrations. Al had dominant proportions, ranging from 278.5 to 3024 ng m−3, with higher levels than Zn by an order of magnitude and higher levels than others by three or four orders of magnitude. In addition, Pb, Cd, Cu, and V had average concentrations (spring and autumn) in the order Pb (37 and 61 μg m−3) > Cu (8.8 and 10 μg m−3) > V (12 and 3.9 μg m−3) > Cd (0.84 and 0.88 μg m−3).

c. Temporal variations

1) Water-soluble species

Most species except SO42− and NO3 exhibited relatively higher concentrations in autumn than in spring and the sum of ions contributed an average of 24.1% and 58.5% to TSP mass concentration, respectively. This might be explained by the sampling period in autumn accompanied by the higher wind speed, which could bring more continental particles and sea-salt species to the atmosphere over the observational area.

The ratio of the total water-soluble ions to TSP mass concentration revealed obvious seasonal variations. In spring, the variation was not distinct, from 11.3% to 40.0%, while it ranged from 20.9% to 78.4% in autumn. The high TSP mass concentrations and low ratios of the total water-soluble ions to TSP in spring were probably due to the dust intrusion from the continent, in which a large part was water insoluble. Under the influence of cold-frontal systems and Mongolian cyclonic depression (Sun et al. 2001), dust storms occurred frequently during spring, especially in March–May. Elevated levels of dust particles and relatively lower water-soluble particulates corresponded well with the frequency of dust events. Even though the dust strength abated during the spring collecting period, influence of dust storms on the atmosphere over coastal areas could not be ignored.

2) Trace metals

Some of the trace metals exhibited a similar seasonal trend to water-soluble species. The highest value of Pb (190 ng m−3) was observed in autumn, higher than the lowest one in spring by two orders of magnitude. Al, Pb, Zn, and Cu also exhibited the same trend, indicating the strong influence from the land. Taking average values into consideration, the concentration of Cu in autumn was 20.3% more than in spring, while Al was 31.5% more than in spring. Al in autumn might be contributed not only from the crustal source but also from anthropogenic emissions, especially coal combustion (Fan 2006). On the contrary, the seasonal distribution of V was threefold higher in spring than in autumn.

d. Neutralization of aerosol acidity

The ion balance expressed by the ratio of the equivalent concentrations of main cations to anions is a good indicator of the acidity of the TSP samples. The role of Cl in acidity and the role of Na+ and Mg2+ in alkalinity are negligible since they primarily originate from the ocean in the form of sea salt that is neutral. In addition, the fractions of ionic components that originate from the sea do not play any role in deciding acidity or alkalinity of the aerosols as well. Thus, the acidic property of aerosol samples is dominated by the presence of NH4+, non-sea-salt Ca2+ (nss-Ca2+), non-sea-salt K+ (nss-K+), nss-SO42−, and NO3. Here, cations (NH4+, nss-Ca2+, and nss-K+) and anions (nss-SO42− and NO3) were converted to equivalent concentrations to compare the relationship of the species.

Figure 3 shows a positive correlation between the two groups (r = 0.58) and the slope is 0.20. It clearly indicates that the acidic components were dominating over the alkaline components, rendering an acidic nature to aerosols throughout the sampling periods, and the acidic nature might ascribe to the larger input of nss-SO42− and NO3, which were considered to be major acidifying anions in the aerosols.

Fig. 3.

Correlation between the sum of acidic ionic species (nss-SO4 2− and NO3) and the sum of alkaline ionic species (NH4 +, nss-Ca2+, and nss-K+) in the form of equivalent of concentrations over the observational areas.

Fig. 3.

Correlation between the sum of acidic ionic species (nss-SO4 2− and NO3) and the sum of alkaline ionic species (NH4 +, nss-Ca2+, and nss-K+) in the form of equivalent of concentrations over the observational areas.

e. Source identification of TSP composition

1) Element source identification by correlation analysis

Table 2 lists the results of correlation coefficient matrix of water-soluble ions and Al to show the internal relationships among different species. Although the individual ion concentrations were quite different, the correlations between internal species within each sampling season showed similar patterns. Significant correlations were observed among the secondary ions. NH4+ corresponded well with nss-SO42− and NO3, while nss-SO42− was less correlated with NO3. Neither Al nor Ca was strongly correlated with the secondary species in correlation matrix, and all of them were less or even negatively related with sea-salt-dominated Na, suggesting that the sea-salt input and crustal source may not be the dominant source.

Table 2.

Correlation coefficient matrix of aerosol composition over the BS and YS. Correlation coefficients with statistical significance of p < 0.01 are in boldface.

Correlation coefficient matrix of aerosol composition over the BS and YS. Correlation coefficients with statistical significance of p < 0.01 are in boldface.
Correlation coefficient matrix of aerosol composition over the BS and YS. Correlation coefficients with statistical significance of p < 0.01 are in boldface.

Mg was recognized as another reliable indicator element of marine source (Keene et al. 1986). However, there was no significant internal correlation among Mg, Al, and sea-salt components in the spring cruise. K exhibited strong correlations with secondary ions. Nevertheless, in the autumn cruise, Mg was found to be significantly correlated not only with Na, but also with Al. Our observation suggested that aerosols collected in the spring cruise were strongly interfered by continental transport, especially continental anthropogenic activity.

2) Element source identification by enrichment factor (EF)

(i) EF for water-soluble components

The main source for elements in atmosphere aerosols can be estimated by using enrichment factors. Here, Al and Na are regarded as conservative elements and used as reference indices of crustal and sea-salt components, respectively (Chester et al. 2000; Millero 2006). The enrichment factor for a crustal source is estimated by the equation

 
formula

where (Cx/Al)aerosol represents the mass concentration ratio of element x (Cx) to Al in aerosols and (Cx/Al)crust stands for the corresponding ratio of element x to Al in crust materials. Similarly, the contribution of sea salt can be calculated by the equation

 
formula

where (Cx/Na)aerosol is the mass concentration ratio of element x (Cx) to Na in aerosols and (Cx/Na)seawater is the ratio of element x to Na in seawater. Data of EF are shown in Table 3. By convention, if EF ≤ 10, then it is considered that the element in aerosols has a significant crust and/or marine contribution, while EF > 10 indicates that it has an important proportion of noncrustal and/or nonmarine source and is considered the enriched element.

Table 3.

Enrichment factors of major aerosol components.

Enrichment factors of major aerosol components.
Enrichment factors of major aerosol components.

In the spring and autumn cruises, average EFc values less than 5 were found for Ca (2.79 and 2.46) and K (1.21 and 1.19), while the average EFs values were greater than10 (except K in autumn). It indicated that these elements had a primary contribution of crustal materials during these two seasons. Cl had high average EFc values up to 574 and 1016 in spring and autumn, respectively, showing an important proportion of noncrustal source. Similarly, Na had a dominant noncrustal character with EFc values up to 57.9 and 20.2 in spring and autumn, respectively. It revealed that characters of aerosols possessed a considerable proportion of marine sources. In addition, enrichment factors of Mg relative to crust and seawater sources were comparable (Table 3) and exhibited nonenriched character with respect to soil and sea salt. Combined with the correlation coefficient, Mg was found to be significantly correlated not only with Na, but also with Al, implying the mixed input of marine and crustal sources.

When normalized against Al, EF values of the secondary ions were extremely high with minimum value of 203.7 for NH4+, while the average EFs value of nss-SO42− ranged from 15.0 in autumn to 58.1 in spring, illustrating its non-sea-salt source. Airmass trajectories (Fig. 2) suggested that the air mass primarily passed through northern China (e.g., Hebei and Shandong Provinces), where population is huge and economies are well developed. Therefore, emission from human activities might contribute to the dominant source of the secondary species.

The above comparison suggested that different components exhibited significantly different sources and the water-soluble compositions in coastal marine atmosphere had a feature of neither crustal nor marine, but rather, a mixture of air masses.

(ii) EF for trace metals

EFs values of the trace metals were very high, reaching 104–107. However, little information was presented on the strength of marine aerosol over the observational areas based on the equations from Liu et al. (2002). In this case, only EFc values of trace metals are discussed below.

Trace metals in aerosols over the BS and YS had the EFc values of 10–30 times as high as those over the remote ocean, like the North Pacific (Duce et al. 1983). With respect to the average EFc values, the metals were divided into two groups:

  1. V. The enrichment factor values of V were about 1, far less than 10. This result is comparable with that observed over the YS in the same season by Liu et al. (2002). Zoller et al. (1973) and Rahn et al. (1981) pointed out that the burning of heavy fuel oil that contained V-porphyrin compounds was the main source for V. However, during the abated dust storm period, the inputs of V had somewhat changes and other input sources became apparent (Duce et al. 1983).

  2. Cu, Pb, Zn, and Cd. The average EFc values of Pb, Zn, and Cd were 170, 78.2, and 582, respectively, while the average value of Cu (19.9) was much lower but still markedly higher than 10.

To estimate contributions from crustal materials (Rc) and the pollutant source (Rp) to trace elements, EFc values were used as variable in the calculation based on the equations from Liu et al. (2002) and the results are shown in Table 4.

Table 4.

Source contributions to the trace metals in aerosols.

Source contributions to the trace metals in aerosols.
Source contributions to the trace metals in aerosols.

Overall, The contributions from pollutant source for Cd, Zn, and Pb were greater than 97%, and the contribution fraction from the anthropogenic emission for Cu was found slightly lower than Zn, but still greater than 90% in the atmospheric aerosols in the two seasons, suggesting that these trace metals were overwhelmingly controlled by anthropogenic emission. By contrast, data of V showed that the relative contribution from the anthropogenic emission was a bit lower than, or comparable to, that from the crustal source, suggesting a combination of crust and anthropogenic sources.

f. MSA and contribution of biogenic SO42−

MSA in the marine boundary layer was one of major end products from the oxidation of dimethylsulfide (DMS) produced by marine biota, and factors like concentrations of OH and NO3 radicals and temperature could also affect its formation from DMS oxidation (Gao et al. 1996). In springtime, especially when there is a bloom, ocean primary productivity has a great influence on the DMS concentration level and then indirectly affects MSA. Kim et al. (2000) conducted a 1-yr-period observation at Cheju Island in 1997 and 1998 and summarized that atmospheric DMS concentration ranged from 19 to 1140 pptv, with maximum occurring in spring and minimum in fall. Factors like light, temperature, and nutrients carried by dust storms are more suitable for the growth of phytoplankton in spring and summer. Therefore, more DMS are emitted into the atmosphere, resulting in higher atmospheric MSA concentrations. Over the observational areas, the concentrations of MSA varied from 0.0075 to 0.17 μg m−3 in spring and from 0.0019 to 0.018 μg m−3 in autumn, with respective means of 0.061 and 0.012 μg m−3, which were comparable with those obtained from other adjacent areas (Gao et al. 1996; Nakamura et al. 2005; Yang et al. 2009). Furthermore, the MSA level in spring was fivefold the observed level in autumn, exhibiting a significant temporal variation. This seasonal tendency is consistent with the result reported by Atkinson et al. (1984).

In contrast, the concentrations of nss-SO42− in spring varied from 2.47 to 17.5 μg m−3 with an average of 8.14 μg m−3, while those in autumn ranged from 1.63 to 12.1 μg m−3 with a mean of 5.70 μg m−3. The magnitude of nss-SO42− agreed well with the previous results obtained over adjacent areas—for example, 4.07 μg m−3 over the East China Sea (Gao et al. 1996) and 7.40 μg m−3 over the northern YS (Yang et al. 2009). However, it is markedly higher than those over remote oceanic regions. For instance, the mean value of nss-SO42− was only 0.37 μg m−3 over the remote midlatitude Pacific Ocean under the minimum influence of Asian dust and it was 0.67 μg m−3 over the equatorial Pacific Ocean regions (Savoie et al. 1989).

MSA is used as a marker for the fraction of nss-SO42− attributable to biogenic DMS since the ratio of MSA to nss-SO42− was suggested to be constant (18–20) in clean marine air masses at latitudes between 30°S and 30°N according to the studies carried out by Saltzman et al. (1983, 1985, 1986) and Savoie and Prospero (1989) over the remote Pacific Ocean. Despite uncertainties related to the application of the ratio from remote oceans to coastal and shelf regions, the numerical value of 19 was adopted to approximately assess the biogenic contribution to total nss-SO42− at coastal areas of East Asia (Arimoto et al. 1996; Gao et al. 1996). Our result showed that biogenic nss-SO42− contribution to the total nss-SO42− ranged from 1.1%–15% (average 8.0%) in spring and from 1.1% to 7.2% (average 3.5%) in autumn. The spring values were in agreement with the results obtained over the offshore region of the East China Sea in spring 1992 (Gao et al. 1996). Nakamura et al. (2005) reported that the average contribution from biogenic source appeared to be 7.9% in autumn, which was higher but still comparable with the autumn values in this study.

4. Conclusions

In the present study, the chemical characteristics and sources of water-soluble ions and trace metals in atmospheric aerosols were investigated over the BS and YS during spring and autumn. In spring, high TSP mass concentrations but low mass ratios of water-soluble ions to TSP were found, which could be attributed to the great influence from dust intrusion from mainland. Based on the portion of the secondary ions and calculation of equivalent concentrations of the cations and anions, the marine aerosols presented the acidic properties during the two cruises.

Overall, the correlations between internal ions in each observational season showed similar patterns. The enrichment factors of the secondary ions were extremely high relative to crustal and marine sources, indicating the major influence by anthropogenic activity.

Characters of aerosols indicated that Cd, Zn, and Pb were enriched elements, while V was not apparently enriched. The pollution source contributions (Rp) of Cd, Zn, and Pb were greater than 97%, suggesting the overwhelming contribution by anthropogenic emission. V has a mixed source from crustal and anthropogenic emissions, and the contribution from crustal source was greater than 55% in spring and autumn. In addition, relatively high nss-SO42− and low biogenic SO42− contribution suggested that anthropogenic SO42− is the dominant component for total nss-SO42− in aerosols over the BS and YS.

Acknowledgments

We wish to thank the captain and crew of the R/V Dong Fang Hong 2 for help and cooperation during the two cruises. We also wish to thank three anonymous reviewers for valuable comments and suggestions, which greatly improved the manuscript. This work was financially supported by the National Natural Science Foundation of China (41320104008), the Natural Science Foundation for Creative Research Groups (41221004), the Changjiang Scholars Programme, Ministry of Education of China, and the Taishan Scholars Program of Shandong Province.

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