Geographical distribution and sources of phosphorus in atmospheric aerosol over the North Pacific Ocean



[1] Atmospheric aerosol samples with two size fractions (Da ≤ 2.5 μm and Da ≥ 2.5 μm) were collected in the subtropical region of the western North Pacific (WNP) and over the central North Pacific (CNP) Ocean in summer and geographical distributions of particulate concentrations of total phosphorus (TP), water soluble fraction of phosphorus, and tracer elements (Fe, Zn, Na) were investigated. Average concentrations (in ng m−3) of TP, Fe, Zn, Na in the WNP region were 7.3 ± 4.3, 16.2 ± 17.1, 5.5 ± 7.5, and 1450 ± 700, while they were 2.5 ± 1.2, 5.9 ± 5.1, 0.9 ± 1.2, and 1480 ± 610 along the 160°W line in the CNP, respectively. Source apportionment using the tracer elements exhibited that contribution of anthropogenic source was estimated to be 38% and 18%, while that of crustal source was estimated to be about 5% for both WNP and CNP regions, respectively. Oceanic source was estimated to have a little contribution. Half or less fraction of the TP could be accounted for by the anthropogenic and crustal sources. Water solubility of the TP was found to be higher in the WNP region than the CNP region. The similar fractions of water soluble and anthropogenic contribution to the TP found in both WNP and CNP regions suggested that anthropogenic phosphorus was water soluble and dominant in the WNP region, although the unaccounted fraction occupied a large portion of the TP (47% and 76% for WNP and CNP, respectively) as insoluble form.

1. Introduction

[2] Phosphorus (P) is an essential nutrient for all forms of life since phosphorus is used universally to maintain biological processes and construct organisms. In the oligotrophic open oceans, where essential nutrient(s) (e.g., nitrate, phosphate) is deficient, it has been demonstrated that the deposition of the essential nutrients from atmosphere to the surface of such oceans can play an important role to sustain and stimulate primary production in the ocean [Uematsu et al., 2004; Jickells et al., 2005; Duarte et al., 2006; Duce et al., 2008]. Recent observations show that bioavailability of the phosphorus actually limits primary production and abundance of bacteria [Wu et al., 2000; Krom et al., 2004; Mills et al., 2004; Thingstad et al., 2005; Hashihama et al., 2009]. Since atmospheric phosphorus mainly resides in aerosol phase [Duce et al., 1991], characteristics of aerosol-phase phosphorus, such as concentration range, size distribution, geographical distribution, emission sources, source strengthen, atmospheric transformation, are essential to estimate the impact of atmospheric input of the phosphorus to the ocean ecosystem. However, phosphorus composition in the atmospheric aerosols has been poorly characterized [Duce et al., 1991; Mahowald et al., 2008]. The North Pacific Ocean is one of the least characterized regions about the aerosol-phase phosphorus [Mahowald et al., 2008]. Although several land-based measurements have been carried out in the North Pacific Rim region [Cohen et al., 2004; Chen et al., 2006; Chen and Chen, 2008; Chen et al., 2008], there is no measurement in the open ocean region after the pioneer work in the late 1970s [Graham and Duce, 1979].

[3] In order to fill the gap, we have conducted two shipboard measurements in the eastern region of East Asia and over the North Pacific Ocean. Purposes of the current study are to provide the geographical distribution of aerosol-phase concentrations of total phosphorus (TP) and water soluble phosphorus (WSP) and water insoluble phosphorus (WIP = TP − WSP) in the western North Pacific (WNP) and in the central North Pacific Ocean (CNP), and to estimate a relative contribution of different phosphorus sources to the TP using tracer elements over the North Pacific Ocean.

2. Results and Discussion

[4] Ship tracks of the two cruises are shown in Figure 1a. The KH-05-2 cruise was divided into three transects which are generally related to the distribution of oceanic nutrients and biological activities. We refer the KH-06-2 cruise area as the western North Pacific (WNP) region and the KH-05-2 cruise area as the central North Pacific (CNP) region. Details of the cruises, air mass types encountered during the cruises, aerosol sampling, and analytical methods are described in the auxiliary material.

Figure 1.

(a) Ship tracks and geographical distributions of aerosol-phase concentrations of total phosphorus (TP), iron (Fe), zinc (Zn), and sodium (Na) in ng m−3 observed during two ship cruises in the coast of East Asia (KH-06-2, red ship track, June 2006) and over North Pacific Ocean (KH-05-2, green, blue, and brown ship tracks, August–September 2005). Each ship track with different color represents a division where regional average concentrations were calculated. Vertical bars with red color were the data points which were considered to be heavily influenced by local sources. Vertical bars with blue color were the data points where Zn > 15 ng m−3 in the open ocean region. Both data points were not used in the calculation of the regional averages. (b) Estimated contributions of different phosphorus sources considered in this study (anthropogenic, gray; crustal, red; seawater, blue; and others, yellow) to the total phosphorus (TP) for the WNP and along 160°W line in the CNP regions. (c) Average contributions of water soluble (blue) and water insoluble (gray) fractions to the total phosphorus over the WNP and along 160°W line in the CNP.

[5] In Figure 1a, geographical distributions of total aerosol-phase phosphorus (TP), iron (Fe), zinc (Zn), and sodium (Na) are shown. The height of each bar represents aerosol-phase elemental concentration in each 12 hours sampling period. Fe, Zn, and Na are selected for analysis to conduct a source apportionment analysis for TP since these elements generally represent different aerosol sources (Fe: crustal, Zn: anthropogenic, Na: oceanic). The TP concentration found to be higher in the WNP region than the CNP region by a factor of three (Table 1). But the high TP concentration in the WNP region rapidly declined as being away from the land. This suggests that there is a large terrestrial P source(s) on the land which is more locally effective to the composition of atmospheric aerosol. In the open ocean, TP concentration was low and less variable except for several points in the vicinity of the Hawaii and Japanese islands. Similar geographical distributions were also observed for Zn and Fe. Since Zn and Fe are the tracers for anthropogenic and crustal aerosol source, respectively, the similar trends indicate a larger contribution of anthropogenic and crustal sources to the TP in this region from the Asian continent. Sodium concentration varied regardless of distance from the land, reflecting in-situ formation by air bubble bursting at the surface of ocean, which is regulated by wind speed [Gong, 2003].

Table 1. Average Concentrations of Aerosol-Phase Total Phosphorus, Iron, Zinc, and Sodium for Different Divisions Observed During Two Research Cruises in This Studya
Tropical and subtropical North PacificAug 200512.40.9218.77.5211.21.821130056021
160°W line central North Pacific (CNP)Aug–Sep 200512.51.2285.95.1270.91.228148061028
Northern North PacificSep 200512.83.6125.63.9121.12.1122590119012
Western North Pacific (WNP)Jun 200617.34.33416.217.1345.57.634145070034
Tropical and subtropical North PacificMay–Jun 197520.400.5721- -------
TaiwanSep 2003–Dec 2004320∼70-12- -------
Cheju island, KoreaJan–Jun 2002422.020.0 173282 3235 438439 
Hong KongJan–Jun 2002450.046.0 187215 116125 935828 

[6] Table 1 summarizes average concentrations of particulate TP, Fe, Zn, and Na measured in the four different transects. For comparison, values obtained in previous observations conducted in the North Pacific region are also listed. Measured aerosol composition in Hong Kong predominantly reflected anthropogenic source emission [Cohen et al., 2004], while the composition measured at Jeju Island in Korea reflected both anthropogenic and crustal source emissions in the Asian continent since Jeju Island is located downstream of Asian outflow and the observation covered the period of dust outflow events [Cohen et al., 2004]. The open ocean region was divided into three transects (1) the tropical and sub-tropical North Pacific, (2) along the 160°W Line in the central North Pacific, and (3) the northern North Pacific as depicted with different colors in the ship track. As already shown in Figure 1a, TP, Fe, and Zn concentrations were relatively constant in the all three transects of the North Pacific. Therefore, we will further focus on the comparison between the western North Pacific region and the central North Pacific region along the 160°W line.

[7] The TP and Fe concentrations in the WNP region were about three times higher than those in the CNP region, while Zn concentration was higher by a factor of five. Sodium concentration was almost the same in the two regions. In the main island of Taiwan, TP concentration during August and September, when oceanic air parcels were dominant, was reported to be ∼20 ng m−3 [Chen et al., 2006, 2008]. Although our “average” TP concentration in the WNP was about one third of the TP observed in Taiwan, similar high TP concentrations (14 ∼ 22 ng m−3) were also observed over the proximity of the lands or islands during this study. We thus consider that this study well represented a regional mean with little local influence over the WNP region during summer. In the CNP region, the only previous TP measurement was the work by Graham and Duce [1979]. As compared in Table 1, current TP concentration was about six times higher than that by Graham and Duce [1979]. Using a preliminary global TP model study, Mahowald et al. pointed out that the modeled TP in the remote North Pacific Ocean was significantly overestimated compared to the observation by Graham and Duce [Mahowald et al., 2008]. The TP concentrations observed in this study were in the range of the modeled TP concentrations. The discrepancy may be simply due to the fact of variable nature of atmospheric aerosol concentration in time, space, and season, however, it should be noted that the TP concentration reported by Graham and Duce [1979] in the North Pacific is also significantly lower even when it is compared to those they observed in different oceans. Clearly, more measurements of the TP concentration over the remote regions of the Pacific Ocean are required to address the discrepancy. Comparing with the global model simulation of atmospheric phosphorus [Mahowald et al., 2008], the measured geographical distribution of the TP (high concentration in the closer vicinity of the East Asia and sharp decrease as being away from the land) showed a qualitatively similar trend. The measured TP concentrations in the mid-latitude and close to the land were also within the range of the modeled ones. However, there is a large discrepancy around the equatorial zone: Modeled TP was about one order of magnitude lower than the measured concentration in this study. One should keep in mind that current observations were conducted in June, August and September, while the modeled concentration is an annual average.

[8] Figure S1 shows a relative abundance of coarse and fine fractions of TP, Fe, Zn, and Na in the WNP and CNP regions. About 59% of TP resided in the fine fraction in the WNP region, while the fraction decreased to 48% in the CNP region. Such decrease in the fine fraction from the WNP to CNP region was also observed for Zn and Fe. The simultaneous decrease in the fine fraction indicates that anthropogenic (Zn) and crustal (Fe) sources have more fine fraction than coarse one. It is noteworthy that the TP has larger fine fraction than those of Zn, Fe, and Na in both WNP and CNP regions, which suggests that phosphorus sources considered and represented by Zn, Fe and Na in this study may not be enough to fully account for the observed fine fraction of the TP.

[9] In order to estimate a relative contribution of crustal, anthropogenic, and seawater phosphorus sources to the TP, a crude source apportionment analysis was carried out using Fe, Zn, and Na as tracers for the phosphorus sources in the WNP and CNP regions. Using the average concentrations of the tracers, phosphorus from the different sources can be estimated as

equation image

Paerosol is the measured average TP. Px (x = crustal, anthropogenic, oceanic) is the estimated phosphorus concentration from source x. Pothers is concentration of the unaccounted portion of the observed TP. (P/Z)x, where Z = Fe, Zn, and Na, is the characteristic ratio of mass concentrations of phosphorus and compound Z in source x, which we assume unique to each source. For the (P/Fe)crustal, average concentrations of P and Fe in the upper crust were used [McLennan, 2001]. For the (P/Zn)anthro, since there is no established P/Zn value for anthropogenic P source, we calculated the (P/Zn)anthro using mass concentrations measured in Hong Kong (Table 1). Although aerosol composition in Hong Kong mainly reflects emission from anthropogenic P sources, continental dust and/or biomass burning sources may also contribute to the observed TP to some extent. For the (P/Na)oceanic, we took a conservative total phosphorus concentration in the bulk surface seawater (2.8 μM, see auxiliary material for details), although phosphate concentration varies more than one order of magnitude in the region of this study [Conkright et al., 2000; Hashihama et al., 2009].

[10] Figure 1b shows a contribution of considered P sources to the average TP in the WNP and CNP regions. In both regions, the largest fraction was the unaccounted “others” (77% and 57% for WNP and CNP, respectively) and only 25 ∼ 45% of the TP could be accounted for by the oceanic, crustal, and anthropogenic sources. Anthropogenic source was the largest contributor among the accounted portion. In the WNP region, the estimated TPanthro was about 2.8 ng m−3, which constituted about 38% of average TP; while in the CNP region, the estimated TPanthro was about 0.4 ng m−3 (18% of the TP). The relative contribution of the anthropogenic P to the TP in the WNP was twice larger than that in the CNP region, while the concentration in the WNP was seven times larger than that in the CNP. The contribution of crustal source was the second largest next to the anthropogenic source, but its contribution to the TP was estimated to be only about 5% in both regions. TPcrustal concentration in the WNP region was 0.34 ng m−3, which was about 2 times larger than that in the CNP region (0.13 ng m−3). The clear contrast in these two regions showed that anthropogenic and crustal sources in the Asian continent are important phosphorus sources for the North Pacific Ocean and the coastal region of East Asia. Oceanic source was estimated to have a very small contribution (< 1%) in the both regions. In the current estimate, we assumed that (P/Na) ratio in seawater was conserved in sea salt aerosols, however, it has been shown that TP in sea salt aerosols are enriched relative to Na when the sea salt aerosols are produced through bubble bursting at the surface of ocean by a factor of 4 ∼ 170 [Graham et al., 1979]. Taking this enrichment into account, the oceanic source may considerably contribute to the TP particularly in the Northern North Pacific Ocean where TP concentration in seawater is high [Conkright et al., 2000]. But this oceanic TP will not supply “new” phosphorus as nutrient to the ocean when the aerosols are deposited to the ocean since the oceanic TP is a recycled component. It may be possible that long-range transport of phosphorus-rich sea salt aerosol may bring phosphorus to the ocean with low phosphorus concentration, however, it is unlikely since the sea salt aerosol has its most of the mass in the coarse fraction as shown in Figure S1 and thus have shorter atmospheric lifetime and transportation distance.

[11] It was surprising that anthropogenic, crustal, and oceanic sources, which are generally considered as major P emission sources, accounted for only 25 ∼ 45% of aerosol-phase TP in the North Pacific Ocean during this study. Using a preliminary chemical transportation model, a global distribution of aerosol-phase TP and their relative source contribution were estimated recently [Mahowald et al., 2008]. In the model study, various P emission sources, such as crustal, biomass burning, fossil fuel combustion, biofuel combustion, volcano, primary biogenic particle, and ocean, were considered. The model study suggested that crustal source contributed to 50 ∼ 90% of the TP over the North Pacific Ocean except for the equatorial region and the coastal region of Asia, while anthropogenic sources (assume 90% of biomass burning is also anthropogenic) contributed only up to 30% even in the close vicinity of the Asian continent and over the North Pacific [Mahowald et al., 2008], which is quite different from the estimate in this study. One possible reason is the large seasonal variation of Asian outflow and dust events during spring [Parrington et al., 1983; Uematsu et al., 1983, 2003]. Present study did not cover the season with frequent Asian outflow and dust event, which may be, at least in part, the reason for the observed small contribution of continental crustal source. However, the large seasonal variation still does not explain the large unaccounted portion of the TP. The (P/Fe)crustal varies for different source soils of Asian dust by −35 ∼ +30% relative to the value used in this study (see auxiliary material), although this only increases the estimated crustal contribution from 5% to 6.5%. In the current source apportionment study, biomass burning, volcanic emission, and primary biogenic aerosol are not considered as phosphorus sources. We consider that volcanic emission and primary biogenic aerosol sources are unlikely as the unaccounted portion. Volcanic emission is usually a point emission source, thus it should create a large geographical gradient in TP. However, our results show relatively constant TP concentration over the North Pacific Ocean. As for the primary biogenic aerosol emission, this terrestrial emission should be localized near the land and have little contribution to the TP in the remote open ocean region like CNP region in this study. The primary biogenic aerosol is quite unlikely as an explanation for the unaccounted portion. Currently, we consider biomass burning may be one of the unaccounted sources. As already mentioned, the unaccounted fraction had more fine fraction than the coarse one. Such larger fine fraction of TP in biomass burning aerosols over coarse fraction has been reported in the biomass burning aerosols in Brazil [Mahowald et al., 2005]. Further detailed study to estimate the contribution of biomass burning source by referring black carbon concentration or non-sea salt potassium is required. Gas phase phosphorus like PH3, which is primary gaseous phosphorus, may contribute to the aerosol-phase TP through its uptake and nucleation. It has been reported that PH3 concentration was quite high (200 ∼ 1000 ng m−3) in the coastal area compared to the aerosol-phase TP, however, the concentration sharply declined to zero as being away from coastal area [Zhu et al., 2007]. It is unlikely that the PH3 gas actually contributes to the present unaccounted portion of the TP, particularly in the open ocean region like CNP region.

[12] In Figure S2, the relative fraction of WSP and WIP obtained by 4-day sampling is presented over the North Pacific Ocean. Insert is the averaged fractions of WSP and WIP in the WNP region and the open ocean region in the North Pacific. The fraction of WSP was higher in the WNP than that in the CNP region. Such lower water soluble fraction in TP has been also observed in Taiwan for the air parcels of oceanic origin [Chen et al., 2008]. Appling the average fractions of the WSP to the average TP concentrations in the WNP and CNP regions (Table 1), mass concentrations of the WSP were estimated to be 0.4 and 3.0 ng m−3 for the WNP and CNP, respectively. Figures 1b and 1c show the accounts for the average TP based on source apportionment and water solubility for the WNP and CNP regions. Interestingly, water soluble fractions well agreed with those of anthropogenic fraction in both the WNP and CNP regions. This suggests that anthropogenic fraction of the TP is more water soluble than the TP from other sources and the increased WSP in the coastal region was due to the increased contribution of anthropogenic phosphorus source. “Others” mainly consists of insoluble compositions which are expected to exist rather uniformly both in the coastal and open ocean regions. The geographical distribution of WSP fraction in the North Pacific (Figure S2) also supports the large contribution of anthropogenic phosphorus source in the WNP region. The WSP fraction decreased as being away from the dust source region (Asian continent) in the North Pacific, while it increased as being away from the Saharan desert in the Atlantic Ocean [Baker et al., 2006], where Saharan dust is dominant particulate phosphorus source [Mahowald et al., 2008]. This clear contrast also suggests that different types of particulate phosphorus sources are dominant in the Atlantic Ocean and the North Pacific.

3. Conclusion

[13] The TP concentrations observed in the CNP region were about one order of magnitude higher than those previously observed in tropical and sub-tropical region in the North Pacific Ocean by Graham and Duce [1979]. The high concentrations in the WNP region declined steeply as being away from the land and became relatively constant over the North Pacific Ocean.

[14] Source apportionment using the tracer species (Fe, Zn, Fe) as representatives of crustal, anthropogenic, and seawater sources could account for only half or less fraction of the TP. The contribution of crustal source was relatively small although a model study suggested that crustal source was the dominant contributor to the aerosol-phase TP in this region [Mahowald et al., 2008]. The similar fractions of water soluble and anthropogenic contribution in both WNP and CNP regions suggest that anthropogenic phosphorus is more water soluble and contributed to the increase of water soluble fraction in the WNP region.

[15] Current study shows that various phosphorus sources are contributing to the aerosol-phase phosphorus with varying degree in the North Pacific Ocean, which contrasts the different relative source contribution in North Atlantic Ocean where Saharan dust is considered to be a dominant aerosol-phase phosphorus source [Mahowald et al., 2008]. Further detailed observations with different season and region are desired to understand the behavior and sources of the atmospheric aerosol-phase phosphorus in the Pacific Ocean.


[16] Authors gratefully acknowledge captain and crew of R/V Hakuho for their assistance during KH-05-2 and KH-06-2 research cruises. This study was supported partially by funds from the Grant-in-Aid for Scientific Research in Priority Areas “Western Pacific Air-Sea Interaction Study (W-PASS)” under Grant No.18067005 from Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This research is a contribution to the Surface Ocean Lower Atmosphere Study (SOLAS) Core Project of the International Geosphere-Biosphere Programme (IGBP).