The tropical and subtropical Indian Ocean (IO) is expected to be a significant source of water-soluble organic aerosols (WSOAs), which are important factors relevant to cloud formation of aerosol particles. Current atmospheric numerical models significantly underestimate the budget of organic aerosols and their precursors, especially over tropical oceans. This is primarily due to poor knowledge of sources and the paucity of observations of these parameters considering spatial and temporal variation over the tropical open ocean. To evaluate the contribution of sources to WSOA as well as their formation processes, submicrometer aerosol sampling was conducted at the high-altitude Maïdo observatory (21.1^∘ S, 55.4^∘ E; 2160 m a.s.l.), located on the remote island of La Réunion in the southwest IO. The aerosol samples were continuously collected during local daytime and nighttime, which corresponded to the ambient conditions of the marine boundary layer (MBL) and free troposphere (FT), respectively, from 15 March to 24 May 2018. Chemical analysis showed that organic matter was the dominant component of submicrometer water-soluble aerosol (∼ 45 ± 17 %) during the wet season (15 March–23 April). On the other hand, sulfate dominated (∼ 77 ± 17 %) during the dry season (24 April–24 May), most of which was attributable to the effect of volcanic eruption. Measurements of the stable carbon isotope ratio of water-soluble organic carbon (WSOC) suggested that marine sources contributed significantly to the observed WSOC mass in both the MBL and the FT in the wet season, whereas a mixture of marine and terrestrial sources contributed to WSOC in the dry season. The distinct seasonal changes in the dominant source of WSOC were also supported by Lagrangian trajectory analysis. Positive matrix factorization analysis suggested that marine secondary organic aerosol (OA) dominantly contributed to the observed WSOC mass (∼ 70 %) during the wet season, whereas mixtures of marine and terrestrial sources contributed during the dry season in both MBL and FT. Overall, this study demonstrates that the effect of marine secondary sources is likely important up to the FT in the wet season, which may affect cloud formation as well as direct radiative forcing over oceanic regions.

中文翻译：

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热带印度洋留尼汪岛 Maïdo 高空天文台水溶性有机气溶胶的来源

热带和亚热带印度洋 (IO) 预计将成为水溶性有机气溶胶 (WSOA) 的重要来源，这是与气溶胶粒子云形成相关的重要因素。当前的大气数值模型大大低估了有机气溶胶及其前体的预算，尤其是在热带海洋上空。这主要是由于对来源知之甚少，以及考虑到热带公海的空间和时间变化，对这些参数的观察很少。为了评估的源的贡献WSOA以及它们的形成过程中，亚微米气溶胶以高空Maïdo天文台（21.1进行采样^∘  S，55.4 ^∘ 乙; 2160 m asl)，位于 IO 西南部的拉留尼汪岛。气溶胶样品在当地白天和夜间连续收集，分别对应于 2018 年 3 月 15 日至 5 月 24 日的海洋边界层 (MBL) 和自由对流层 (FT) 的环境条件。化学分析表明有机物是 雨季（3 月 15 日至 4 月 23 日）亚微米水溶性气溶胶的主要成分（~  45  ± 17%）。另一方面，硫酸盐占主导地位（∼  77  ± 17 %）在旱季（4 月 24 日至 5 月 24 日），其中大部分归因于火山爆发的影响。水溶性有机碳 (WSOC) 的稳定碳同位素比的测量表明，海洋源对湿季 MBL 和 FT 中观测到的 WSOC 质量有显着贡献，而海洋和陆地源的混合对 WSOC 有贡献在旱季。拉格朗日轨迹分析也支持 WSOC 主要来源的明显季节性变化。正矩阵分解分析表明，海洋次生有机气溶胶 (OA) 对观测到的 WSOC 质量（~ 70 %) 在雨季，而海洋和陆地资源的混合在 MBL 和 FT 的旱季都有贡献。总的来说，这项研究表明，海洋二次源的影响可能对雨季的 FT 很重要，这可能会影响云的形成以及海洋区域的直接辐射强迫。