Longitudinal distributions of WSON. Figures 1a and 1b show the longitudinal distribution of the concentrations of WSON in fine particles (WSONF) with particle diameter (DP) < 0.95 µm and in coarse particles (WSONC) with DP > 0.95 µm, from samples collected at 23ºN in the subtropical North Pacific (Fig. S1). The average concentrations of WSONF and WSONC were 6.1 ± 6.2 ngN m− 3 and 22 ± 21 ngN m− 3, respectively, during the cruise measurement. The sum of these values is within a range of the WSON concentrations in total suspended particles (TSP) collected over the western North Pacific (3.0–35 ngN m− 3) (Miyazaki et al., 2011). Here, along the cruise track at 23ºN, the oceanic region at 200–240ºE is defined as the eastern North Pacific (ENP), whereas the region at 135–200ºE is defined as the western North Pacific (WNP). Concentrations of WSON both in the fine and coarse particles showed a distinct longitudinal gradient, with substantially higher concentrations in ENP. The average concentrations of WSONF and WSONC in ENP were approximately 3–6 times as large as those in the WNP (Table 1).
Table 1
Average concentrations of nitrogen species and stable carbon isotope ratios in the fine (DP < 0.95 µm) and coarse (DP > 0.95 µm) aerosol particles collected along 23ºN in each oceanic region.
| The western North Pacific (WNP, 135–200ºE) | | The eastern North Pacific (ENP, 200–240ºE) | | All (135–240ºE) |
| Fine | Coarse | | Fine | Coarse | | Fine | Coarse |
WSON (ngN m− 3) | 2.4 ± 1.9 | 7.9 ± 6.0 | | 7.5 ± 6.6 | 49.0 ± 12.6 | | 6.1 ± 6.2 | 21.6 ± 21.3 |
NH4+ (ngN m− 3) | 42.6 ± 15.8 | 3.6 ± 3.2 | | 35.5 ± 22.2 | 17.5 ± 8.3 | | 37.4 ± 20.9 | 8.3 ± 8.5 |
NO3− (ngN m− 3) | 0.5 ± 0.4 | 154.6 ± 65.8 | | 1.8 ± 1.8 | 373.2 ± 91.1 | | 1.5 ± 1.7 | 227.5 ± 127.6 |
NO2− (ngN m− 3) | 0.4 ± 0.2 | 32.9 ± 54.0 | | 1.4 ± 1.4 | 9.4 ± 9.4 | | 1.1 ± 1.3 | 25.0 ± 45.8 |
δ13CWSOC (‰) | −23.9 ± 1.4 | NA | | −22.1 ± 1.6 | NA | | −22.8 ± 1.7 | NA |
Table 2
Estimated net flux of WSON and NH4+ in fine particles from the ocean to the atmosphere using an empirical equation given by Ceburnis et al.42.
| The western North Pacific (WNP, 135–200ºE) | The eastern North Pacific (ENP, 200–240ºE) | All (135–240ºE) | Emission flux in the subtropical North Pacific Paulot et al. (2015) |
WSON (ngN m− 2 d− 1) | 0.7 ± 1.2 | 5.2 ± 7.7 | 4.0 ± 7.0 | No data |
NH4+ (ngN m− 2 d− 1) | 28.6 ± 55.8 | 23.0 ± 42.8 | 24.5 ± 46.6 | 0–342.5 |
Average wind speed (m s− 1) | 4.5 ± 1.8 | 5.3 ± 1.8 | 5.1 ± 1.8 | --- |
WSON:NH4+ | 0.08 ± 0.08 | 0.33 ± 0.34 | 0.26 ± 0.31 | No data |
The longitudinal distributions of the mass concentrations of inorganic nitrogen (NH4+, NO3−, and NO2−) generally showed similar patterns to those of WSON (Fig. S2). The sum of the mass concentrations of the inorganic nitrogen in ENP was approximately twice as large as those in WNP. Overall, NH4+ was the dominant component of water-soluble total nitrogen (WSTN) (~ 78%) in the fine-mode aerosols, followed by WSON (16%) (Fig. 1c). In contrast, NO3− was the most abundant WSTN component of the coarse-mode aerosols, which accounted for 83% of the WSTN mass on average. Notably, the mass fraction of WSON in ENP was significant, accounting for up to ⁓50% (average: 16 ± 14%) and ⁓19% (average: 7 ± 4%) of the WSTN in the fine and coarse particle mass, respectively (Fig. 1c and 1d). This result indicates the importance of the abundance of WSON, particularly in the fine particles in ENP.
Figure 1e presents the concentrations of chlorophyll (Chl) a in surface seawater (SSW) samples obtained during the cruise, as well as the average concentrations of Chl a during August–September 2017 derived from the MODIS-Aqua ocean color data (https://neo.sci.gsfc.nasa.gov/view.php?datasetId=MY1DMM_CHLORA&year=2017). The two different measurements of Chl. a generally showed similar longitudinal distributions, which resemble the longitudinal trend of the WSON concentrations. Moreover, 3-day back trajectories, calculated by HYSPLIT (https://www.ready.noaa.gov/HYSPLIT_traj.php), showed that the sampled air masses were transported primarily over the oceanic regions in the Pacific (Fig. S1). These results suggest that most of the observed aerosols were transported within the marine boundary layer with less influence of terrestrial sources prior to the sampling. The overall results suggested that the observed aerosols, particularly those in ENP, were largely influenced by marine sources associated with phytoplankton.
Isotopic characterization of aerosol organic carbon and formation processes of WSON. Previously, a method using stable carbon isotope ratios (δ13C) of aerosol organic carbon has been successfully used to determine the contributions of marine and terrestrial sources to organic aerosol in the marine atmosphere20,21. The δ13C of water-soluble organic carbon (WSOC; δ13CWSOC) as a function of the WSONF concentrations is shown in Fig. 2. The average δ13CWSOC observed in this study was − 22.8 ± 1.7‰, with 71% of the data (12 out of 17 data points) in the range of typical marine origin (between − 24‰ and − 18‰)21. This isotopic characterization supports that most of the organic carbon fraction of the observed aerosols were of marine origin.
To elucidate the possible formation processes of the observed WSON, we used several tracers of marine origins. Figures 3a and 3b show scatter plots between WSON and Na+ concentrations. The concentrations of Na+, used as a tracer of marine primary emission, did not show any significant correlations with those of WSONF or WSONC in the study area (R2 < 0.1). Moreover, concentrations of glucose, a molecular tracer of marine primary aerosols22, were below the lower detection limit for most of the samples during the cruise. These results suggest insignificant contributions of direct emissions from sea surface to the observed WSON.
The above results also imply that secondary formation was likely the dominant process underlying for the formation of WSON observed in this study. Figures 3c and 3d present scatter plots between WSON and methanesulfonic acid (MSA) concentrations in each particle size category. MSA has been widely used as a tracer of marine SOA, because it is an oxidation product of dimethyl sulfate (DMS). MSA is either produced by gas-phase MSA directly scavenged by aerosols or rapidly produced in the aqueous phase from scavenged dimethylsulfoxide (DMSO) and methanesulfinic acid (MSIA)23. The concentrations of WSON did not show any significant correlations with those of MSA in each size range regardless of the oceanic region. The insignificant correlation suggests that the origin of the observed WSON aerosols differed from DMS or the formation pathways of WSON were different from oxidation processes of DMS.
Distributions of nitrogen fixation and aerosol WSON. To further explore the origin and possible formation process of the observed WSON aerosol associated with phytoplankton, we focus on N2 fixation in SSW as a possible source of atmospheric reactive nitrogen. N2 fixation is the biological conversion of N2 to NH4+ or dissolved ON (DON), which represents the main external source of bioavailable nitrogen in marine environment. A significant fraction of fixed N2 can be directly released by N2-fixing microorganisms as dissolved inorganic nitrogen (NH4+, NO3−, and NO2−) and DON in the ocean. Luo et al.24 estimated that on average, spatially integrated N2 fixation over the Pacific accounted for ~ 50% of that in the global ocean, pointing to the importance of the Pacific in terms of this process.
In the subtropical North Pacific, previously measured nitrogen isotope ratios (δ15N) in particulate organic matter (POM) from seawater suggested that POM was significantly affected by nitrogen supplied from N2-fixing microorganisms25. Indeed, in the current study, the average δ15N of POM in the SSW samples was − 0.3 ± 1.0‰ (data not shown), which is within the range of δ15N values of diazotrophic cyanobacteria typically ranging from − 2‰ to 0‰26. This indicates that the organic matter in SSW collected in the study region is derived from nitrogen supplied by the N2-fixing microorganisms such as cyanobacteria.
Figure 4 shows the latitudinal distributions of the N2-fixation rate in the SSW samples compared with those of aerosol WSON concentrations in each size category. The average N2-fixation rate in the SSW samples collected in ENP was 38.6 ± 13.3 ngN L− 1 d− 1, which was significantly higher than that in the WNP (11.5 ± 10.6 ngN L− 1 d− 1). The observed range of the N2-fixation rate in SSW is similar to those reported for the same oceanic region in previous studies27–29. Moreover, the longitudinal gradient of the N2-fixation rate is similar to that along 23ºN in the tropical Pacific, as shown by previous field measurements and model simulations30–34. The higher N2-fixation rate in ENP compared to that in WNP could be attributed to the greater abundance of N2-fixing microorganisms such as Trichodesmium and a symbiotic unicellular cyanobacterium (UCYN-A) measured by quantitative polymerase chain reaction (qPCR) of nifH. In particular, the number of the nifH copy of UCYN-A in ENP was three orders of magnitude larger than that in WNP (data not shown).
The WSON concentrations in both fine and coarse particles showed positive correlations with the N2-fixation rates in the SSW samples (Figs. 4 and 5), with R2 of 0.27 (p < 0.05) and 0.60 (p < 0.05) in the fine and coarse modes, respectively. Meanwhile, the R2 values for the WSON concentrations and primary productivity in SSW were 0.08 (p = 0.24) and 0.58 (p < 0.05) in the fine and coarse modes, respectively (Fig. S3 and S4), which were lower than those for WSON and N2-fixation rate. The positive relation between the WSON mass concentrations and N2-fixation rate in SSW suggests that reactive nitrogen produced by N2-fixing microorganisms in SSW significantly contributed to the formation of WSON aerosols.
Nitrogen fixation as a possible source of WSON in marine aerosols. Previous laboratory experiments showed that DON and NH4+ are released in seawater through N2 fixation by microorganisms known as diazotrophs35,36. The average WSTN/WSOC ratio in all the size range of the observed aerosols was 1.70 ± 0.94, while those in ENP and WNP were 1.61 ± 0.99 and 1.87 ± 0.82, respectively. Yvon-Durocher et al.37 reported that the N:C ratios of marine algal assemblages over the subtropical Pacific ranged between 0.10 and 0.13, while Wannicke et al.35 obtained N:C ratios of Trichodesmium (0.21 ± 0.02) through a laboratory experiment. Furthermore, Berthelot et al.36 reported that the ratio of dissolved nitrogen (DN) to DOC released during N2 fixation was 0.07 ± 0.48. The currently observed WSTN:WSOC ratio in the aerosols was much higher than the N:C ratios of microbial and the DN:DOC ratios in seawater affected by N2 fixation. The higher WSTN:WSOC ratios in aerosols relative to the typical DN:DOC ratios in seawater suggest that nitrogen-containing aerosols are preferentially produced relative to organic carbon in the atmosphere.
Previous studies showed that oceanic regions at low latitude, including the subtropics act as a source of NH3 (the net flux of NH3 is out of the ocean to the atmosphere)38,39. Paulot et al.40 used two global ocean biogeochemical models to show evidence for a missing source of atmospheric ammonia (NH3 and NH4+) over the Equatorial Pacific that was attributable to photolysis of marine ON at the ocean surface or in the atmosphere. Indeed, NH4+ was the dominant component of the aerosol reactive nitrogen in the fine particles (Fig. 1c) in this study, whereas the correlations between NH4+ concentrations and N2-fixation rates were insignificant (R2 < 0.08). This insignificant correlation is partially attributable to phase-partitioning of ammonia into the gas phase in the subtropical region. The secondary formation processes of WSON include emissions of gas-phase ON from the ocean and/or marine VOCs reacting with NH341, although the exact mechanism of the formation is not apparent in this study. Nevertheless, the current results of the shipboard measurements suggest that N2 fixation in SSW has an important role, which could explain one of the missing sources of atmospheric WSON and ammonia indicated by previous modeling studies.
Here we roughly estimated sea-to-air fluxes of WSON and NH4+ in the fine particles, using an empirical equation given by Ceburnis et al.42. Ceburnis et al.42 derived PM1.0 mass fluxes including both primary and secondary processes, by vertical gradient measurements of marine aerosols following a power law relationship with the surface wind speed. In the current study, the average surface wind speeds were applied to the empirical equation followed by using the mass fractions of WSONF and NH4 + F in the observed fine particles. Table. 2 summarizes the calculated fluxes of WSONF and NH4 + F in the study area. The average net fluxes of WSON in this study were estimated to be 0.7 ± 1.2 ngN m− 2 d− 1 and 5.2 ± 7.7 ngN m− 2 d− 1 in WNP and ENP, respectively, with the average of 4.0 ± 7.0 ngN m− 2 d− 1. The average flux of NH4 + is 25 ± 47 ngN m− 2 d− 1, which is within the range of the emission flux previously estimated in the subtropical North Pacific (0–342.5 ngN m− 2 d− 1)40. The ratios of the average net flux of WSON to NH4+ estimated in this study were 0.08 ± 0.08 (WNP) and 0.33 ± 0.34 (ENP), with a maximum of 1.17. Although the average net flux of WSON corresponded to only 0.02 ± 0.04% of the N2-fixation rate with large uncertainty of the flux calculation, the current estimate implies that sea-to-emission of WSON, in addition to that of NH3/NH4+, is important.
To summarize, the current result suggests that N2-fixing microorganisms in the SSW contributed to the formation of aerosol WSON and possibly other reactive nitrogen species, such as NH3/NH4+, in the oceanic region of this study. Further field studies are required to elucidate the effect of N2 fixation in surface seawater on the emission of atmospheric reactive nitrogen in different oceanic regions. Additional laboratory studies are needed to evaluate the factors controlling the atmospheric emissions of reactive nitrogen associated with N2 fixation in surface sea water. Jiang et al.43 predicted that global ocean warming in the future would result in large increases in growth and N2 fixation by Trichodesmium. Consequently, the formation process and the amount of atmospheric WSON associated with N2 fixation in surface seawater are expected to change, which should be important from viewpoints of climate effect of marine atmospheric aerosol as well as air-sea exchange of nitrogen.