Atmospheric reactive nitrogen (N) deposition can be important for marine biological activity over large remote areas of the oceans where N supply by deep nutrient rich water is small. Although recent studies have estimated impacts of atmospheric N inputs, there are still large uncertainties regarding the global atmospheric N cycle since most studies are based on the results of several models. The atmospheric N cycle over the oceans contains the most uncertain part because the validation of model output was primarily based on comparisons to terrestrial sampling sites. Moreover, because of rapid Asian economic growth, emissions of anthropogenic substances (e.g., nitrogen oxides; NOx) from the Asian continent have significantly increased. Since the western North Pacific receives a large influx of mineral dust and pollution aerosol from the Asian continent through atmospheric transport, estimating deposition flux of atmospheric N and its impacts on biogeochemical cycles over the western North Pacific have become increasingly important.

Aerosol, rain and sea fog (only in the subarctic western North Pacific) samples were collected between 48°N and 55°S during the KH-08-2 (R/V Hakuho Maru, 29 July-17 September 2008, the subarctic and subtropical western North Pacific), the MR08-06 (R/V Mirai, 15 January-8 April 2009, the North and South Pacific) and the KT-09-5 (R/V Tansei Maru, 1 May-6 May 2009, the semi-pelagic western North Pacific) cruises conducted over the North and South Pacific Oceans, in order to estimate dry and wet deposition fluxes for atmospheric reactive N species, including ammonium (NH4+) and nitrate (NO3-), and evaluate their impact on marine biogeochemical cycle (Fig. 1).

Concentrations of NH4+ and NO3- in marine aerosols collected over the semi-pelagic western North Pacific Ocean varied from 59-182 nmol N m(-3) and(-1)3-86 nmol N m(-3), with averages of 117 ± 42 nmol N m(-3) and 36 ± 22 nmol N m(-3), respectively. Aerosol reactive N in our data was composed of ~77% NH4+ and 23% NO3- (median values for all data). Most NH4+ (~90%) was found in fine mode aerosols (Da < 2.5 μm), suggesting that it was formed by gas-to-particle conversion. In contrast, NO3- (~55%) was predominantly found in coarse mode aerosols (Da > 2.5 μm), indicating a chemical reaction between nitric acid gas (HNO3(g)) and sea-salt/crustal aerosol in the marine atmosphere. Both NH4+ and NO3- showed strong relationships with nss-SO4(2-) and nss-K+ (r = 0.73-0.96), suggesting that fossil fuel combustion and biomass burning are significant sources of NH4+ and NO3-, and/or that they experienced similar transport and removal mechanisms. The estimates of fractions of atmospheric reactive N species derived from specific sources using the tracer species (nss-SO4(2-), nss-K+, nss-Ca2+ and Na+) revealed that 97-99% (mean 98%) of NH4+ and 78-88% (mean 84%) of NO3- were derived from agricultural activity and fossil fuel combustion, respectively. Mean dry deposition fluxes for NH4+ and NO3- were estimated to be 31 ± 17 μmol N m(-2) d(-1) and 33 ± 17 μmol N m(-2) d(-1), respectively. Although the mean concentration of NH4+ was 3 times higher than that of NO3-, dry deposition fluxes of both were approximately the same since fluxes to the ocean are dominated by the coarse mode, resulting in NO3- being deposited much more rapidly. Atmospheric bioavailable N deposition flux (64 ± 31 μmol N m(-2) d(-1)) was found to be maximally responsible for the carbon uptake of 420 ± 210 μmol C m(-2) d(-1) (139-669 μmol C m(-2) d(-1)) by using the Redfield C/N ratio of 6.625, indicating that it can therefore support 0.05-1.5% of the primary production.

During the Leg 1 of KH-08-2 cruise, sea fog occurred predominantly when the dominant wind direction was southward and air temperature dropped to its dew point, indicating that the warm and humid air masses form the low and middle latitudes of the North Pacific passed over the cold sea surface of the northern North Pacific and they were cooled down to a saturation temperature, since the relatively cold sea surface temperature stabilizes the lower atmosphere, making a favorable condition for sea fog formation. Mean particle number densities during non sea fog events were 25 ± 31 cm(-3) for aerosols in the range of 0.3 < D < 0.5 μm(-2).6 ± 3.0 cm(-3) for 0.5 < D < 1.0 μm, 0.53 ± 0.70 cm(-3) for 1.0 < D < 2.0 μm, and 0.17 ± 0.27 cm(-3) for D > 2.0 μm. In comparison, the mean particle number densities during sea fog events decreased by 4% (mean particle number density 24 ± 20 cm(-3)) for aerosols in the range of 0.3 < D < 0.5 μm, 12% (2.3 ± 3.1 cm(-3)) for 0.5 < D < 1.0 μm, 55% (0.24 ± 0.52 cm(-3)) for 1.0 < D < 2.0 μm, and 78% (0.038 ± 0.091 cm(-3)) for D > 2.0 μm. This result suggests that the growth of aerosol particles to liquid droplets leads to the acceleration of particle removal from the atmosphere, and that particles with diameters larger than 0.5 μm could act preferentially as condensation nuclei for sea fog droplets and were more efficiently scavenged by sea fog. The pH values of rain and fog water collected over the North Pacific varied from(-3).4-5.9, whereas those of rainwater over the South Pacific ranged from 5.7-6.7. These results indicate strong influence of acidic substances in the northern hemisphere. Mean concentration of NO3- in fog water was approximately 6 times higher than that in rainwater, whereas those of NH4+ were almost similar in both fog water and rainwater. This result revealed that HNO3(g) was scavenged more efficiently by fog water as well as NO3- in aerosols, and suggested that particle scavenging mechanisms between fog and rain are different (e.g. in-cloud scavenging, below-cloud scavenging and hygroscopic property).

Total concentrations of NH4+ and NO3- in bulk (fine + coarse) aerosols during the KH-08-2 and MR08-06 cruises varied from 0.93-12 nmol m(-3) and 0.44-5.6 nmol m(-3), respectively. Aerosol reactive N in our data set was composed of ~68% NH4+ and ~32% NO3- (median values for all data), with ~81% and ~45% of each species being present on fine mode aerosol, respectively. The total NH4+ and NO3- concentrations showed similar trends, with higher concentrations in samples collected over the western North Pacific and lower values over the South Pacific. These distributions likely resulted from large terrestrial emission sources of N in the northern hemisphere, deposition during transport across the ocean, and the intertropical convergence zone (ITCZ) by which cross-equatorial transport is suppressed. Concentration of NH4+ (0.93-4.1 nmol m(-3)) in aerosols collected over the South Pacific were a factor of 1.2-6.3 higher than the results of model study (0.65-0.78 nmol m(-3) STP) calculated aerosol NH4+ concentrations without natural emissions in the South Pacific, suggesting that emissions of ammonia (NH3) from the ocean could become a significant source of aerosol NH4+ in the South Pacific because NH3 is emitted into the atmosphere from the ocean as a result of biological activity, and that much of observed aerosol NH4+ in the open ocean aerosols could be recycled oceanic NH3. Overall, NO3- mainly was found in coarse mode aerosols, while NH4+ was largely associated with the fine mode. Interestingly, 73 ± 4.2% of NO3- collected in the coast of Chile was found in fine mode aerosols, although it is known that NO3- in the marine atmosphere is predominantly associated with coarse mode aerosol, suggesting that NO3- accumulated in the coarse mode could have been removed more rapidly by dry or wet deposition during transport because of the larger particle size, and that NO3- could be produced by lightning in the free troposphere and by injection from the stratosphere because there are few chances to react with coarse mode sea-salt that is continuously supplied from the sea surface.

Concentrations of NH4+ and NO3- in rainwater during the KH-08-2 and MR08-06 ranged from 1.7-55 μmol L-1 and 0.16-18 μmol L-1, respectively. Reactive N in rainwater was composed of ~87% NH4+ and ~13% NO3- (median values for all data), suggesting that NH4+ is more abundant in rainwater collected over the North and South Pacific Ocean, and that it is a more important reactive N species supplied by wet deposition. A significant correlation (r = 0.74, p < 0.05, n = 10) between NH4+ and methanesulfonic acid (MSA) in rainwater samples collected over the South Pacific and the coast of Chile, where SeaWiFS satellite images revealed persistently high chlorophyll a levels because of upwelled sea water, suggesting that emissions of NH3 from the ocean could become a significant source of NH4+ over the South Pacific.

The estimated dry deposition fluxes for atmospheric reactive N species varied from 0.55-7.8 μmol m(-2) d(-1) for NH4+ to 0.22-8.6 μmol m(-2) d(-1) for NO3-, contributing ~46% by NH4+ and ~54% by NO3- to the dry deposition flux for total reactive N (TRN, i.e. TRN = NH4+ + NO3-) (median values for all data). Wet deposition fluxes of atmospheric reactive N species varied from(-3).5 to 119 μmol m(-2) d(-1) for NH4+ and from 0.30 to 36 μmol m(-2) d(-1) for NO3-, accounting for ~83% by NH4+ and ~17% by NO3- of TRN from wet deposition flux (median values for all data). While NO3- was the dominant reactive N species in dry deposition, reactive N supplied to surface waters by atmospheric wet deposition was predominantly by NH4+ (42-99% of the wet deposition fluxes for TRN). Total (dry + wet) mean deposition fluxes of atmospheric TRN in the Pacific Ocean from 48°N and 55°S were estimated to be 32-64 μmol m(-2) d(-1), with 66-99% of this in the form of wet deposition, indicating that wet deposition plays an important role in the supply of atmospheric reactive N to the Pacific Ocean compared to dry deposition, although the relative contributions are highly variable among regions. The total mean deposition fluxes of atmospheric reactive N over the Pacific Ocean were found to be maximally responsible for the carbon uptake of 210-420 μmol C m(-2) d(-1) in the Pacific Ocean, suggesting that reactive N deposited to the Pacific Ocean from the atmosphere can support 0.86-1.7% of the total primary production.

Fig. 1. Cruise tracks of the KH-08-2 (blue; Leg 1, red; Leg 2), the MR08-06 (pink) and the KT-09-5 (purple) cruises.

陸上で大気中に放出された窒素化合物は、エアロゾルあるいはガスとして大気中を輸送され海洋へと沈着する。窒素は、植物プランクトンの必須元素であり、特に窒素が枯渇する海域では、大気からの沈着(乾性・湿性沈着)が重要な供給源であることが示唆されている。陸から外洋へ飛来する大気中のエアロゾル、降水中の窒素化合物の海洋への沈着による海洋生態系への影響を定量的に評価することは、人間活動により増加している現状において極めて重要である。

本論文は、太平洋における大気エアロゾル、降水、海霧を船上で採取し、化学分析を行い、粒径分布などの船上大気観測データと併せて、海域による窒素化合物の組成や濃度の特徴を把握、その起源や除去過程を解析し、大気から海洋への窒素化合物の沈着量を見積もり、海洋生態系への大気からの寄与を議論したものである。

本論文は全6章からなる。第1章は、序章であり、大気中の反応性の高い窒素化合物の重要性と本研究の位置づけ・目的が述べられている。第2章には船舶観測の詳細と化学分析手法が記述されている。

第3章では、三陸沖におけるエアロゾル中の無機窒素化合物の測定結果から、アンモニウムイオンの90%が微小粒子として存在し、アンモニアガスが粒子化したことを示唆した。一方、硝酸塩の55%は粗大粒子として存在し、海洋大気中硝酸ガスが粗大粒子である海塩粒子に吸着し、硝酸ナトリウムを形成したことを示した。海洋大気では解離された硝酸ガスは海塩粒子に吸着するため、アンモニウムイオンと硝酸塩はそれぞれ違う粒径分布を示す。観測したアンモニウムイオンと硝酸塩の濃度を基に無機窒素化合物の沈着量を見積もった結果、観測されたアンモニウムイオンの平均濃度が硝酸塩の平均濃度より約3倍高かったにもかかわらず、アンモニウムイオンと硝酸塩の沈着量はほぼ同じであった。観測期間中に三陸沖へ沈着した無機窒素化合物は、約0.05-1.5%の一次生物生産に寄与すると見積もられた。

第4章では、西部北太平洋の亜寒帯海域における夏季(6月~8月)に最大50%の海霧の発生頻度率を持つ海域での研究成果である。海霧発生の時と非発生の時の粒子数濃度を比べた結果、粒径が0.5 μm以上の粒子が優先的に海霧の凝結核として働くことがわかった。西部北太平洋の亜寒帯海域で採取した雨水と霧水の平均pHは、それぞれ4.1、4.2とほぼ同じであった。雨水と霧水の酸性度に大きく寄与した成分は、それぞれ非海塩性塩素イオンと非海塩性硫酸塩であった。

霧水中の植物プランクトンが生産するジメチルスルフィドから由来するメタンスルホン酸と非海塩硫酸塩の結果から、海霧は植物プランクトンの活動によって放出される硫黄化合物を降水より効果的に除去したことと、海霧による大気中のエアロゾルやガスの除去過程は、降水による除去過程とは異なることが示唆された。海霧の凝結核として働く硝酸ナトリウムだけではなく、硝酸ガスも海霧によって効果的に除去されたことが示唆された。西部北太平洋の亜寒帯海域における全無機化合物の全沈着量(乾性+湿性+海霧による沈着量)に対する乾性・湿性・海霧による沈着の寄与率は、それぞれ11%、72%、17%であり、夏季に海霧による無機窒素化合物の沈着量が極めて重要であることが示唆された。

第5章では、南北太平洋における大気中の窒素化合物の差異を明らかにした。南北太平洋におけるアンモニウムイオンと硝酸塩は、北太平洋での濃度が南太平洋での濃度より高く、似たような濃度変化傾向を示したが、北太平洋が南太平洋より窒素化合物の陸起源の影響を強く受けていることを示した。南太平洋における大気エアロゾル中のアンモニウムイオンは、海洋生物の活動によって大気へ放出された可能性を示唆した。

南北太平洋では、湿性沈着によって太平洋へ供給される無機窒素化合物は、アンモニウムイオンが硝酸塩より重要であることが示唆された。また、南太平洋で採取した降水中のアンモニウムイオンは、メタンスルホン酸と高い相関関係を示し、この海域では海洋生物の活動によって大気へ放出されたアンモニアガスが重要な起源であることを示唆した。

南北太平洋における無機窒素化合物の海洋生態系への影響として、南北太平洋における約0.86-1.7%の一次生物生産に寄与すると見積もられた。しかし、大気擾乱に伴う突発的な窒素化合物の沈着は短期間に多量の窒素化合物を海洋へ供給することができる。また、人為起源の窒素化合物の放出量は増加し続けている。従って海の成層化による窒素が枯渇する海域では、大気からの窒素化合物の沈着は重要な窒素の供給源であり、海洋生態系を変化させる可能性がある。

本論文では、大気から南・北太平洋への窒素化合物の沈着量を見積もるため、北緯48度-南緯55度で得られたエアロゾル、降水、海霧試料を用いて大気から海洋への窒素供給にとって重要な反応性無機窒素化合物を測定した。海霧による海洋大気中での窒素の除去と海洋への供給の占める割合が大きく、南太平洋上での海洋生物起源の寄与の重要さを定量的に明らかにしたことは高く評価できる。また、これらの物理・化学データは、今後、大気海洋物質循環研究やモデル予測の高精度化にも不可欠なものである。

なお、本論文の第2章における観測と第3章は植松光夫教授、第4章と5章の各章は植松光夫教授、古谷浩志博士との共同研究であるが、論文提出者が主体となって研究を行ったもので、その寄与が十分であると判断できる。したがって、審査委員一同は本論文が博士(農学)の学位論文として価値あるものと認めた。