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博士 （環境科学） >
Role of marine nitrogen-fixing organisms in the formation of atmospheric reactive nitrogen
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| Title: | Role of marine nitrogen-fixing organisms in the formation of atmospheric reactive nitrogen |
| Other Titles: | 大気反応性窒素の放出生成に対する海洋窒素固定生物の役割 |
| Authors: | 土橋, 司 Browse this author |
| Issue Date: | 23-Mar-2023 |
| Publisher: | Hokkaido University |
| Abstract: | Marine atmospheric reactive nitrogen emitted from the sea surface is an important factor that affects air quality and climate over the ocean via influencing atmospheric photochemical field, particle formation, and cloud formation. Among atmospheric reactive nitrogen, water-soluble organic nitrogen (WSON) and ammonia/ammonium affect the particle formation, water-solubility, acidity, and light-absorbing properties of aerosol particles. First, size-segregated aerosol and surface seawater (SSW) samples were simultaneously collected over the subtropical North Pacific to investigate the origin of aerosol WSON in the marine atmosphere. The fine-mode WSON concentration at 200?240oE along 23oN defined as the eastern North Pacific (ENP) was significantly higher than that at 135?200oE, defined as the western North Pacific (WNP). Analysis of the stable carbon isotope ratio of water-soluble organic carbon (WSOC) (δ13CWSOC) together with backward trajectory indicated that most of the observed WSON in the fine particles in the ENP originated from the ocean surface. Positive relations were found among nitrogen fixation rate, dissolved organic nitrogen (DON) in SSW, and the WSON concentrations. The result suggests that reactive nitrogen (DON and ammonium), produced and exuded by nitrogen-fixing microorganisms in SSW, contributed to the formation of WSON aerosols.
To demonstrate the contribution of nitrogen-fixing microorganisms to the formation of atmospheric reactive nitrogen, including WSON, a laboratory incubation experiment was conducted, using Trichodesmium erythraeum IMS101 (hereafter referred to as Trichodesmium), which is one of the most representative nitrogen-fixing microorganisms. During the incubation period of approximately two months, the growth and decline of the nitrogen-fixing microorganisms were observed. During the exponential phase, the DN and DOC concentrations in seawater increased with increasing the chlorophyll-a concentration, suggesting that DN and DOC were exuded from Trichodesmium during that phase. Throughout the experiment, ammonium and ammonia were the most abundant reactive nitrogen (avg. 70%) in both particle and gas phases, followed by WSON (avg.
20%). During the decline and death phases, the concentrations of atmospheric ammonia and gas-phase basic WSON significantly increased, suggesting the decomposition of DN and dead Trichodesmium cells and/or photochemical reactions in seawater, followed by the production of low-molecular-weight, more volatile, basic DN that was more easily transferred from seawater to the atmosphere.
These results demonstrated sea-to-air emissions of WSON and ammonium/ammonia associated with the growth and decline of Trichodesmium for the first time. The basic-gas reactive nitrogen components significantly released from seawater can affect the acidity of the atmosphere, and their reactions with acidic gas species can promote the particle formation. This in turn leads to an increase in the particle number concentration, which can further affect the number of cloud particles and thus impact on climate over the ocean. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第15259号 |
| Degree Level: | 博士 |
| Degree Discipline: | 環境科学 |
| Examination Committee Members: | (主査) 教授 鈴木 光次, 教授 西岡 純, 准教授 山下 洋平, 助教 宮﨑 雄三, 教授 角皆 潤 （名古屋大学大学院環境学研究科） |
| Degree Affiliation: | 環境科学院（地球圏科学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/89471 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 環境科学院(Graduate School of Environmental Science)
学位論文 (Theses) > 博士 （環境科学）
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