[A-AS09] Atmospheric Chemistry

[AAS09-10] Analysis of chemical transformation of organic aerosols transported over long distances from Tuoji Island, China to Cape Hedo, Okinawa

*SHIMADA KOJIRO¹, Nobuaki Yoshino¹, Eisaku ISHIKAWA¹, Michihiro Mochida², Ruichen Zhou², qianzhe sun², chenran wei², Akinori Takami³, Atsushi Shimizu³, Kei Sato³, Tomoki Nakayama⁴, Masahiro Yamaguchi⁴, Keiichi Sato⁵, Shiro Hatakeyama⁵, Shuichi Itahashi⁶ (1.University of the Ryukyus, Faculty of Science, Department of Marine Science, Department of Chemistry, 2.Nagoya University, 3.National Institute for Environmental Studies, 4.Nagasaki University, 5.Asia Center for Air Pollution Research, 6.Central Research Institute of Electric Power Industry)

Keywords:organic aerosol, photochemical oxidation reaction, photolysis, long range transport

In recent years, the health effects of the toxicity of aerosol produced by chemical reactions called secondary organic aerosols (SOA) have been pointed out. The composition of SOA transported over long distances is broadly divided into two debates based on the results of laboratory experiments. Firstly, SOA generates organic substances with relatively large molecular weights, such as oligomers and humic substances. Second, decomposition through ozone oxidation and photochemical oxidative reactions is accelerated, resulting in lower molecular weight. Therefore, we aimed to elucidate the formation and decomposition processes of SOA transported over long distances in the real atmosphere using Touji Island, Nagasaki and Cape Hedo.
SOA in the same air mass at three sites collected using high volume air sampler based on Shimada et al. (2020). Measurements were conducted in the spring and winter, 2021, 2022 and 2023. We collected total of 128 daily samples. Water-soluble organic components (WSOC) were analyzed using a TOC analyzer. Organic carbon (OC) and elemental carbon (EC) were analyzed using an OC/EC analyzer. Hydrophobic organic components (WIOC) were calculated by the difference between OC and WSOC. In order to investigate which samples collected SOA in the same air mass, we used an analysis method that combines a satellite, a three-dimensional atmospheric chemical model and observation data such as EC which does not cause chemical reaction. In particular, high polar water-soluble organic matter (HP-WSOM), which is a low molecular weight organic matter, and humic-like substance (HULIS), which is a high molecular weight organic matter, of the identified sample were extracted into two fractions using the method of Zhou et al. (2022). Quantitative analysis was performed using an aerosol mass spectrometer.
In this study, in order to investigate the generation and decomposition of WSOC classified into the same air mass, we formulated the following by normalizing EC (generation and decomposition rate = WSOC/EC(Hedo or Nagasaki) / WSOC/EC(Tuoji) × 100). As a result, the average value of WSOC/EC during transport from Tuoji Island to Nagasaki showed a production rate of 105%. The average value of WSOC/EC during transportation from Nagasaki to Cape Hedo showed a production rate of 152%. This suggests that WIOC was oxidized and became water-soluble.
In order to investigate whether the ratio of the low molecule HP-WSOM or the high molecule HULIS in WSOC is increasing, HP-WSOC and HULIS are formulated using EC normalization in the same way as the above formula. The average values of HP-WSOC/EC and HULIS/EC during transport from Tuoji Island to Nagasaki were 172% and 250%, both indicating generation. During transportation from Nagasaki to Cape Hedo, the average value of HP-WSOC/EC was 94%, indicating decomposition, and HULIS/EC was 285%, indicating formation. Therefore, it was found that high molecular weight organic matter such as HULIS is mainly chemical component in SOA.