Chemical Composition and Sources of Marine Aerosol over the Western North Pacific Ocean in Winter

Abstract

Atmospheric deposition of long-range transported continental substances from natural and anthropogenic sources affects biogeochemical processes in marine systems. Emissions of sea spray contribute aerosol particles to the marine atmosphere. Despite the importance of continental dispersion and atmospheric processes involving aerosol particles within remote marine atmosphere, knowledge of the sources of various water-soluble ions is limited because of insufficient observations. Concentrations of Total suspended particulates (TSPs) and major inorganic ions (Cl⁻, Na⁺, SO₄²⁻, Mg²⁺, Ca²⁺, K⁺, NO₃⁻, NH₄⁺), as well as organic nitrogen (ON-N) values, were measured in marine aerosol collected over the western north Pacific (WNP) during a cruise from 3 December 2014 to 13 March 2015. Aerosol samples were analyzed to determine their chemical characteristics and a source apportionment for this region and the continental influence on the open ocean when air masses are from continent in winter. TSP mass concentrations ranged from 14.1 to 136.0 μg/m³ with an average of 44.8 ± 28.1 μg/m³. Concentrations of TSPs and major ions were higher near the coast (close to Qingdao and Xiamen) and lower over the open ocean. The total mass of inorganic ions and organic nitrogen accounted for 51.1% of the total TSP. Cl⁻ had highest concentrations among the major inorganic ions, followed by SO₄²⁻, NO₃⁻, Mg2⁺, Ca²⁺, K⁺, and NH₄⁺, respectively. However, Cl⁻ showed a deficit relative to Na⁺ in most samples, likely related to heterogeneous reactions within the marine atmosphere. Most SO₄²⁻, Mg²⁺, Ca²⁺, and K⁺ were from sea salt, while other major ions were from continental sources. The non-sea-salt (nss) fractions of Ca²⁺, Mg²⁺ and K⁺ were derived from continental crust, while nss-SO₄²⁻ and NO₃⁻ were derived from anthropogenic sources. ON had several sources, including reactions of NO_x with volatile organic compounds (anthropogenic sources) or NH₃ with gaseous hydrocarbons, as well as crustal and marine biogenic sources.

1. Introduction

The long-distance transport of atmospheric aerosol particles from natural and anthropogenic sources affects ocean ecosystems, climate, and atmospheric chemical processes [1,2,3]. Atmospheric deposition is an important pathway in which nutrients fertilize the open ocean, stimulating phytoplankton growth and influencing the marine biogeochemical cycles [3]. Marine aerosols are comprised of both primary and secondary inorganic and organic components [4]. Primary aerosol particles are emitted directly from natural and anthropogenic sources, while secondary aerosol particles form from condensable atmospheric gases [2]. In the remote marine atmosphere, ions from sea salt make significant contributions to the aerosol mass of Na⁺, Cl⁻, SO₄²⁻, Mg²⁺, K⁺ and Ca²⁺ [1,2,3]. Both natural and anthropogenic components from continental sources also may affect NO₃⁻, SO₄²⁻, Ca²⁺, K⁺, and trace metal concentrations over open ocean [1,5,6,7]. Duce et al. [5] suggested that reactive nitrogen from anthropogenic sources accounts for a third of the ocean’s external nitrogen supply. Nitrogen deposition to the open ocean is assumed to be 80–100% from anthropogenic sources [8]. In addition, the atmospheric inputs of sulfur and nitrogen oxides from anthropogenic or natural sources to the ocean may accelerate rates of ocean acidification [9]. Other atmospheric substances may also influence the ocean, such as iron, phosphorus, cadmium, and persistent organic pollutants. However, studies suggest that the effect of anthropogenic substances on oceans may not be as great as previously expected [10,11,12].

During the East Asian winter monsoon, strongly flowing cold air masses bring large amounts of mineral dust and anthropogenic pollutants from the Asian continent out over the western Pacific Ocean [13,14,15]. As a result of ongoing rapid economic and industrial development the western Pacific Ocean has received more anthropogenic substances from the Asian continent via long-range transport [2,13,14,16]. It has been reported that higher concentrations of NO₃⁻ and NH₄⁺ were observed within air masses from the Asian continent in winter, compared with air masses from the Pacific Ocean in summer at subtropical Okinawa Island [16]. Both NO_x and NH_y can absorb onto dust or crust particles from the Asian continent to transport to the western North Pacific Ocean (WNP), producing higher concentrations of non-sea-salt (nss) SO₄²⁻, nss-K⁺, and nss-Ca²⁺ in winter and spring. Such levels contrast the low levels in summer, when air masses are from the Pacific Ocean [2,13,17]. Thus, it is necessary to study the continental influence on the open ocean when air masses are from continent in winter.

In this study, we measured concentrations of major ions (Cl⁻, Na⁺, SO₄²⁻, Mg²⁺, Ca²⁺, K⁺, NO₃⁻, NH₄⁺) and organic nitrogen (ON) in total suspended particulates (TSP) sampled over the WNP in winter from 3 December 2014 to 13 March 2015 (winter) to identify the spatial variation and sources of major ions and ON. We used a back-trajectory analysis model to assign source regions to these components, with a particular focus on identifying continental (or nss) sources in this region.

2. Methodology

2.1. Aerosol Collection and Chemical Analysis

The TSP samples were collected using a special high-volume KC-1000 sampler (1.05 ± 0.03 m³/min; Laoshan Institute for Electronic Equipment, China), which was fixed on the front uppermost observation deck of the Research Vessel Kexue to avoid contamination from the ship exhaust, as described in our previous study [18]. The cruise covered a period of 101 days from 3 December 2014 to 13 March 2015. The cruise occurred during the winter period of the East Asian monsoon. The aerosol optical thickness (AOT), dust, SO₂ and NO₂ show marked spatial variation during this period (Figure 1), with higher values nearshore to China and lower ones over the Pacific Ocean. The average wind streamlines of 1000 hPa over the western North Pacific Ocean (WNP) indicate a wind speed varying from 4 to 12 m/s. Wind speed increased from about 7 to 12 m/s, as the vessel traveled away from the shore. Sampling sites are shown in Figure 2.

All samples (n = 31) were collected on precombusted Pallflex membrane filters (Type: Tissuquartz™ Filters, 2500 QAT-UP, reorder No. 7204; size: 8 × 10 in; Pall Corp.; Port Washington, NY, USA). Collection time was 48 h (or 2 d/sample). Four blank filters were taken in this cruise. Once a sampling interval was completed, the sample holder and filter were placed in a clean zip-locked plastic bag, folded in two with exposed sides face to face. All samples and blank filters were stored in a refrigerator at −20 °C until analysis in the laboratory. Detailed information about sampling procedures and analysis of all major components are described in Xiao et al. [2].

Briefly, concentrations of major anions (Cl⁻, SO₄²⁻, NO₃⁻), as well as water-soluble metal and nonmetal elements (Ca, K, Mg, Na) were determined using a Dionex™ ICS-90 ion chromatography system (Thermo Fisher Scientific Inc.; Waltham, MA, USA) and a Vista-MPX™ simultaneous inductively coupled plasma optical emission spectrometer (ICP-OES; Varian Inc.; Paolo Alto, CA, USA), respectively. The NH₄⁺ concentrations were analyzed using spectrophotometry, following treatment with Nessler’s reagent. The detection limits of Cl⁻, NO₃⁻, and SO₄²⁻ were 0.03, 0.08, and 0.075 mg/L, respectively, and the relative standard deviation of these ions of standard samples were 2.55%, 1.16%, and 1.36%, respectively [19,20]. The detection limits of Ca, K, Mg, and Na were 0.003, 0.06, 0.0005, and 0.02 mg/L, respectively, and the relative standard deviation of these ions of standard samples were less than 1.5% [19,20]. The detection limit of NH₄⁺ was 0.1 mg/L and its relative standard deviation was less than 5.0% [19,20]. A good ionic balance between cations and anions in μeq (y = 1.03x, R² = 0.92) in our samples suggested that no major ionic components were missed [21]. Detailed information about ON analysis is described in Jickells et al. [22]. The ON-N fraction in the aerosol samples was determined by subtracting the amounts of NO₃⁻ and NH₄⁺ from total nitrogen (TN), i.e., ON-N = TN − NH₄⁺ − NO₃⁻. The total nitrogen and carbon (TN and TC) concentrations in aerosols were determined by Elemental Analyzer (EA-IRMS, Flash 2000). Briefly, six circular filters with a diameter of 4.5 mm were weighted and placed in a tin cup, and then combusted subsequently at 980 °C in combustion tube within the EA. During combustion, all the nitrogen- and carbon-containing species were converted to N₂ and CO₂, respectively. These gaseous components were separated on a packed column of a gas chromatograph installed within the EA. Analytical precision determined from multiple measurements of various standards was and 5% for TN and TC concentrations.

2.2. Concentrations of the Non-Sea-Salt Fraction

To determine the source contributions of major ions within aerosol particles, the concentrations of the nss fractions were calculated, using Na⁺ as a reference element for a marine origin, as follows [2,19]:

$$nss-X_{aerosol}=X_{aerosol}\times (1-{(X/N{a}^{+})}_{sea}/{(X/N{a}^{+})}_{aerosol})$$

(1)

where X represents various ionic components, and (X/Na⁺) values for seawater are from Keene et al. [23].

2.3. Back Trajectories and Aerosol Optical Thickness

To determine the long-distance transport of atmospheric pollutants, 5-d (120 h) back trajectories of air masses arriving at the location at the start of each sampling period were computed using the open source software TrajStat (Version 1.2.2.6; MeteoInfo), with the top of the model set to 1000 m above sea level, as described in Xiao et al. [2,18,19,24]. The back trajectories of all starting locations are shown in Figure 2. Modeled AOT, dust, SO₂ and NO₂ data are shown in Figure 1 (data from https://giovanni.gsfc.nasa.gov). Meanwhile, to provide a regional context to this study, Navy Aerosol Analysis and Prediction System Global Aerosol Model data (https://www.nrlmry.navy.mil/aerosol/#aerosolobservations) for SO₂, dust, and smoke surface concentrations are shown in Figure S1.

3. Results and Discussion

3.1. Total Suspended Particulate and Major Ion Concentrations of Marine Aerosol Samples

The average TSP concentration in samples was 44.8 ± 28.1 μg/m³, with a range from 14.1 to 136.0 μg/m³. The highest TSP concentration was measured in the Bashi Channel near Xiamen, followed by coastal marine areas near Qingdao (Figure 2). The average TSP concentration over the WNP was much lower than those values in winter at Yongxing Island in the South China Sea (158.9 μg/m³) [2] and over the Northern Yellow Sea (145.2 μg/m³) [1], consistent with their respective AOT distributions (Figure 1), both which are marginal seas of northwestern Pacific Ocean. The average TSP concentration over the WNP was similar to the value obtained in previous study in this region (36.7 μg/m³) [3]. The percentage of total major inorganic ions contributing to the TSP over the WNP was similar to values for the Pacific Oceans [3], but higher than over the South China Sea [2].

The total mass concentration of major inorganic ions (Cl⁻, Na⁺, SO₄²⁻, Mg²⁺, Ca²⁺, K⁺, NO₃⁻, NH₄⁺) and ON-N was 22.9 μg/m³; it had a similar spatial variation to TSP (Figure 1). These components accounted for 51.1% of the TSP. Cl⁻ had the highest concentration among these ions (Figure 3 and Figure 4), ranging from 2.1 μg/m³ to 21.3 μg/m³, with an average value of 9.9 ± 4.6 μg/m³. It was followed by Na⁺ (2.0–10.9 μg/m³; 5.8 ± 2.2 μg/m³), SO₄²⁻ (0.8–12.5 μg/m³; 3.3 ± 3.0 μg/m³), NO₃⁻ (0.1–10.3 μg/m³; 1.2 ± 2.3 μg/m³), Mg²⁺ (0.24–1.71 μg/m³; 0.78 ± 0.34 μg/m³), Ca²⁺ (0.15 ± 2.44 μg/m³; 0.49 ± 0.50 μg/m³), K⁺ (0.08–0.94 μg/m³; 0.36 ± 0.24 μg/m³), and NH₄⁺ (0.02–1.81 μg/m³; 0.21 ± 0.45 μg/m³), as shown in Figure 3. The average value of ON-N was 0.94 ± 1.02 μg/m³, with a range from 0.05 μg/m³ to 4.64 μg/m³.

Sea salt (ss) and nss contributions to Cl⁻, SO₄²⁻, Mg²⁺, K⁺, and Ca²⁺ were determined using Na⁺ as the reference element for sea salt [2,19]. Generally, most of Cl⁻ values were deficit relative to Na⁺, with an average deficit of 14.8 ± 16.4%, and a range of 0.9–85.9% (Figure 5). On average, fractions of 39.7 ± 12.3%, 41.8 ± 17.0%, 30.0 ± 19.0% and 13.1 ± 6.9% of the total SO₄²⁻, Ca²⁺, K⁺, and Mg²⁺ were derived from a non-sea-salt origin (Figure 6), indicating that most of the SO₄²⁻, Ca²⁺, K⁺, and Mg²⁺ was derived from sea salt. The concentrations of nss-SO₄²⁻, nss-Ca²⁺, nss-K⁺ and nss-Mg²⁺ were 1.89 ± 2.92, 0.27 ± 0.47, 0.15 ± 0.19 and 0.12 ± 0.10 μg/m³, respectively (Figure 6).

3.2. Major Ion Characteristics and Sources of Aerosol over the Western North Pacific

Typical marine ions (Na⁺ and Cl⁻) accounted for 71.5% of total inorganic ions over the WNP, which was higher than levels reported over the South China Sea in winter [2]. The concentrations of Na⁺ and Cl⁻ were close to those of a previous study of the Pacific Ocean (5.8 and 9.0 μg/m³, respectively) [3], but higher than those of other marine areas in winter, such as Northern South China Sea, and Northern Yellow Sea [1,25]. The average molar ratio of Cl⁻ to Na⁺ was 1.14, which was slightly smaller than the value of 1.17 for seawater, indicating that Cl⁻ was in deficit. This may reflect Cl⁻ evaporation to the atmosphere. In the marine atmosphere, sea salt aerosol can react with inorganic acids, such as HNO₃ and H₂SO₄, as well as NO₂ and SO₂ in heterogeneous reactions to release gaseous reactive chlorine compounds, including HCl, Cl₂, ClNO₂, and HOCl to the atmosphere [2,3,13,18,23,26,27]. Such reactions result in a deficit of Cl⁻ relative to Na⁺, when compared with fresh sea salt aerosol. The value of the Cl⁻ deficit measured on this cruise (Figure 5) was slightly lower than that reported for the Atlantic and Pacific Oceans by Zhang et al. [3], suggesting that the aerosol samples in this study were also influenced by anthropogenic emissions. In this study, Cl⁻ concentrations were correlated with SO₄²⁻ when air masses were from the open ocean (R = 0.51, p < 0.01;

Table 1. Correlation coefficients among major ions in aerosol over the western North Pacific with air masses from continent ((a), n = 8) and open ocean ((b), n = 23).

	Na+	Cl−	SO42−	nss-SO42−	K+	nss-K+	Mg2+	nss-Mg2+	Ca2+	nss-Ca2+	NH4+	NO3−	ON
(a)
Na+	1	0.65	0.21	0.17	0.73 *	0.49	0.99 **	0.92 **	0.52	0.40	0.44	0.05	−0.26
Cl−		1	0.59	0.37	0.38	0.19	0.64	0.56	0.45	0.38	0.18	0.36	−0.08
SO42−			1	0.98 **	0.55	0.61	0.24	0.29	0.67	0.69	0.57	0.92 **	0.66
nss-SO42−				1	0.62	0.71	0.20	0.29	0.63	0.65	0.68	0.93 **	0.78
K+					1	0.95 **	0.75 *	0.77 *	0.89 **	0.83 *	0.91 **	0.57	0.56
nss-K+						1	0.52	0.57	0.90 **	0.89 **	0.96 **	071 *	0.77
Mg2+							1	0.96 **	0.52	0.40	0.45	0.07	−0.24
nss-Mg2+								1	0.50	0.39	0.45	0.10	−0.16
Ca2+									1	0.99 **	0.94 **	0.79 *	0.80
nss-Ca2+										1	0.94 **	0.84 **	0.87 *
NH4+											1	0.74 *	0.86 *
NO3−												1	0.89 *
ON													1
(b)
Na+	1	0.97 **	0.61 **	0.25	0.67 **	0.38	0.98 **	0.60 **	0.80 **	0.30	0.15	0.13	−0.44 *
Cl−		1	0.51 **	0.18	0.61 **	0.33	0.95 **	0.56 *	0.74 **	0.23	0.09	0.05	−0.41 *
SO42−			1	0.92 **	0.86 **	0.80 **	0.66 **	0.56 *	0.76 **	0.61 **	0.75 **	0.71 **	−0.35
nss-SO42−				1	0.72 **	0.79 **	0.33	0.42	0.53 **	0.60 **	0.84 **	0.80 **	−0.21
K+					1	0.95 **	0.75 **	0.72 **	0.66 **	0.41	0.78 **	0.50 *	−0.39
nss-K+						1	0.50 *	0.62 **	0.48 *	0.38	0.91 **	0.57 **	−0.30
Mg2+							1	0.81 **	0.80 **	0.31	0.25	0.21	−0.41
nss-Mg2+								1	0.49 *	0.22	0.45 *	0.46 *	−0.22
Ca2+									1	0.81 **	0.34	0.50 *	−0.26
nss-Ca2+										1	0.39	0.68 **	0.02
NH4+											1	0.61 **	−0.13
NO3−												1	−0.10
ON													1

*: p < 0.05; **: p < 0.01.

a) because they both originate from sea salt. However, it was not correlated with nss-SO₄²⁻ (R = 0.59, p > 0.05; R = 0.18, p > 0.05, respectively, Table 1) or NO₃⁻ (R = 0.36, p > 0.05; R = 0.05, p > 0.05, respectively, Table 1), suggesting that the Cl⁻ deficit cannot be fully explained by the formation of inorganic salts, but organic acids, such as oxalic acid, fulvic acid and malonic acid, and atmospheric processes must also contribute to the Cl⁻ deficit [13,18,26,28]. Generally, inorganic acids play a major role in chloride depletion, but in some cases this role may be played by organic acids, especially in the absence of continental anthropogenic emissions [13,28]. Over the open ocean, water-soluble organic acids can be transported from polluted continental regions or formed in situ by photochemical oxidation of precursor biogenic volatile organic compounds (VOCs) and unsaturated fatty acids [28]. We found that in some samples (e.g., 1, 2, 6, 7, 19 and 27) there was enrichment of Cl⁻, suggesting that a small fraction of Cl⁻ is derived from anthropogenic sources [19]. Another study observed that dry particles were devoid of acetate but enriched in Cl⁻ related to evaporation of acetic acid [26].

Mg²⁺, another typical ion in the marine atmosphere [2], was well-correlated with Na⁺ (R = 0.99 and 0.98, p < 0.01, respectively; Table 1). The ratios of Mg²⁺ to Na⁺ were close to that of seawater, indicating that Mg²⁺ was mainly derived from this source. The concentrations of ss-Mg²⁺ versus nss-Mg²⁺ are shown in Figure 6d; these ratios further support this view. In this study, ss-Mg²⁺ and nss-Mg²⁺ accounted for 86.9% and 13.1% of the total Mg²⁺ in the TSP, respectively. Thus, the ratio of nss-Mg²⁺ to total Mg²⁺ was higher than ratios measured over the Pacific Ocean [3]. The nss-Mg²⁺ may originate from crustal sources, given that nss-Mg²⁺ is often treated as a crustal-derived ion and element in continental studies [19,29]. Ca²⁺ ions are also treated as tracers of crustal material [2,19]. No relationship between Ca²⁺ and Mg²⁺ was found when air masses were from continent (p > 0.05; Table 1a) and a strong relationship was found when air masses were from the open ocean (R = 0.80, p < 0.01; Table 1b), although a much larger portion of nss-Ca²⁺ (41.8 ± 17.0%) than nss-Mg²⁺ (13.1 ± 6.9%) was observed (Figure 6). Larger contributions of nss-Ca²⁺ were found over the South China Sea [2]. The average molar ratio of nss-Mg²⁺ to nss-Ca²⁺ (0.760) in samples of this cruise was higher than the average ratio for continental crust (0.569) [30], indicating that nss-Mg²⁺ is enriched relative to nss-Ca²⁺ within the marine atmosphere. K⁺ is commonly used as a tracer for biomass and biofuel burning [31]; however, we found that K⁺ showed little influence from biomass and biofuel burning sources over the remote WNP (Figure 2 and Figure S1). Typically, in remote marine environments, sea salt provides 70% of K⁺, e.g., in the Pacific marine atmosphere, while nss-K⁺ accounts for 30% of the total water-soluble K⁺ (Figure 6). In this study, strong correlations between K⁺ and Ca²⁺ (both R = 0.89, p < 0.01; Table 1), as well as nss-K⁺ and nss-Ca²⁺ (R = 0.66, p < 0.01; Table 1a) when air masses from continent were observed. These relationships suggest that nss-K⁺ over the WNP is primarily derived from the crust. In Figure 1 and Figure S1, we show that Asian dust influences the remote WNP, which may provide crustal sources of Ca²⁺, Mg²⁺ and K⁺. We found higher contributions of nss-Ca²⁺ to total Ca²⁺, nss-Mg²⁺ to total Mg²⁺ and nss-K⁺ to total K⁺ in coastal regions (near Qingdao and Xiamen), compared with open ocean environments, suggesting that coastal regions were more affected by dust (Figure 1 and Figure S1). Higher contributions of these nss ions, especially nss-Mg²⁺ and nss-K⁺, were also observed when air masses originated from near continent rather than from open ocean regions (Figure 2). Our results indicate that it is more difficult for Ca²⁺ (compared with Mg²⁺ and K⁺) to transport to open ocean environments.

The fraction of nss-SO₄²⁻ accounted for 39.7% of total SO₄²⁻ in samples of this cruise. This percentage is close to that observed over the Northern and Southern Atlantic Oceans, but higher than that in a previous study of the Pacific Ocean [3]. However, the average concentration of nss-SO₄²⁻ was much higher than over the Atlantic Ocean, Pacific Ocean, and Mediterranean Sea [3,15], but lower than over the Yellow Sea, East China Sea, Yongxing Island in the South China Sea [1,2,32], Sea of Okhotsk, and Sea of Japan [15]. The concentrations of nss-SO₄²⁻ recorded near the coast on this cruise were close to values over the Yellow Sea and East China Sea [32]. Like our results for nss-Mg²⁺ and nss-K⁺, we found a higher percentage of nss-SO₄²⁻ to total SO₄²⁻ in coastal regions and in air masses originating from near the continent, compared with those from open ocean regions. However, Figure S1 clearly shows that continental SO₄²⁻ is transported to and affects these open ocean environments. The average concentration of NO₃⁻ was 1.2 ± 2.3 μg/m³ in samples from our cruise; these are higher than those observed over the Northern Atlantic and Pacific Oceans [3], but lower than those from the China Sea [3,18,25,32]. Very high concentrations of NO₃⁻ were observed near the coast close to Qingdao and Xiamen (Figure 4), whereas much lower values were recorded over the open WNP in December 2014, especially in samples of air masses from the open ocean (Figure 2). Zhang et al. [3] surmised that the background level of NO₃⁻ was about 0.12 μg/m³ in this region. The average concentration of NO₃⁻ was 0.20 μg/m³ in December between 8.5° N and 14.0° N, which is close to this background level, suggesting that NO₃⁻ was not markedly influenced by continental surprising. When air masses were from the continent (in February 2015), the average concentration of NO₃⁻ reached 0.49 μg/m³. In this case, continental sources appear to have contributed majorly (76%) to the total NO₃⁻. NO₃⁻ was also well correlated with SO₄²⁻ (R = 0.92 and 0.71, respectively, p < 0.01; Table 1) and nss-SO₄²⁻ (R = 0.93 and 0.80, respectively, p < 0.01; Table 1), indicating that the relatively high concentrations of NO₃⁻ and nss-SO₄²⁻ were related to continental sources. In contrast, we found low concentrations of NH₄⁺ over the open ocean in samples from our cruise, with average values of 0.04 μg/m³ in December and 0.07 μg/m³ in February. Other studies also reported low values of NH₄⁺ over the WNP [4,33,34], and over the South China Sea [2]. These results indicate that gaseous ammonia or particulate ammonium are difficult to transport long distance, and that NH₄⁺ over the WNP may be from local emissions. Zhao et al. [35] suggested that atmospheric nitrogen deposition to the northwestern Pacific decreased rapidly downwind of the Asian continent. In addition, ON-N (0.94 μg/m³) had much higher concentrations than NO₃⁻-N (0.26 μg/m³) and NH₄⁺-N (0.16 μg/m³), accounting for 74.0% of the total nitrogen (NO₃⁻-N, NH₄⁺-N and ON-N), with a range of 28.0–97.4%. The average concentration of ON-N over the WNP was markedly lower than at other sites, including inland cities (Baoji), coastal cities (Chennai and Rondonia), and marine sites (Gosan, Okinawa Island, Bermuda, Northern South China Sea, Yellow Sea, and North Pacific Ocean) [4,10,36,37,38,39,40,41]. The ON content was correlated with nss-Ca²⁺ (R = 0.87, p < 0.01; Table 1a) when air masses from continent, indicating that part of this ON was derived from primary crustal material. A good correlation was also found between ON and both NO₃⁻ and NH₄⁺ (Table 1a), suggesting that secondary formation was a major source of ON. The most common secondary ONs are organic nitrates, such as hydroxyalkyl nitrates produced by the gas phase reactions of NO_x and VOCs or water-insoluble ON produced by the reactions of gaseous hydrocarbons with NH₃ [22,38]. Altieri et al. [10] and Pavuluri et al. [38] also suggested that ON may be derived from marine biogenic emissions. A high concentration of protein in western North Pacific Ocean in the winter was found [42]. So, a weak relationship with Na⁺ when air masses were from the open ocean (Table 1b) was found.

4. Conclusions

Cl⁻ and Na⁺ made the greatest contributions to total inorganic ions, followed by SO₄²⁻, NO₃⁻, Mg²⁺, Ca²⁺, K⁺, and NH₄⁺. ON-N had higher average concentrations than NO₃⁻ and NH₄⁺ in all samples. A Cl⁻ deficit in samples was likely caused by formation of acids and/or other atmospheric processes. Relate to most SO₄²⁻, Mg²⁺, Ca²⁺, and K⁺ were derived from sea salt, with the remainder from continental sources, including crustal and anthropogenic sources. When air masses were from continental regions, NO₃⁻ concentrations were significantly higher, indicating that the WNP was influenced by anthropogenic sources. In contrast, NH₄⁺ may reflect local oceanic emissions. ON had several sources, including reactions of NO_x with VOCs, gaseous hydrocarbons with NH₃, and both crustal and marine biogenic sources.