Ms. Yoshida is currently employed at the Institute for Environmental Sciences.

• Journal: Science of the Total Environment

• Paper title: Sources identification of ammonium in PM₂.₅ during monsoon season in Dhaka, Bangladesh

• Authors: Hiroto Kawashima, Otoha Yoshida, Khaled Shaifullah Joy, Rasel Ahammed Raju, Kazi Naimul Islam, Farah Jeba, Abdus Salam

• Website: https://doi.org/10.1016/j.scitotenv.2022.156433

Comment from Otoha Yoshida

In my undergraduate research, I analyzed the nitrogen stable isotope ratio (δ¹⁵N) of ammonium ions in PM₂.₅ collected in Bangladesh, where air pollution is one of the most severe worldwide, and investigated their characteristics. I collaborated with Professor Salam at Dhaka University. Since the isotope approach had rarely been used in this field, Professor Kawashima invited me to take on this theme for my graduation research.

During this research, I had to establish part of the analytical method. Initially, I developed a separation method using ion exchange resin to isolate ammonium ions from nitrate ions, which enabled a stable analysis of the nitrogen isotope ratio of ammonia. I used the denitrifier method for analysis, but it was unstable at first and required a lot of time to achieve consistent results. Using the developed separation method, I measured ion concentrations and δ¹⁵N of ammonium ions in 33 atmospheric samples collected during the monsoon season in Dhaka. Many issues arose before getting reliable results, but through repeated discussions with my advisor and lab members, I was able to overcome them.

Although I did not perform the source analysis using the isotope mixing model myself, my advisor processed the data and estimated the contributions using the model. We actively communicated with Professor Salam during the preparation of the international journal paper, and I managed to write some comments in English, which is not my strength. I am relieved and very happy to have this paper published. It was a truly valuable experience.

I am currently working at the Institute for Environmental Sciences, where I handle pretreatment, analysis, and interpretation of atmospheric samples. Although the challenges differ from those in my graduation research, the fundamentals remain the same, and I enjoy my work while applying what I learned at university.

Comment from Associate Professor Hiroto Kawashima

Dhaka, Bangladesh, is one of the most densely populated areas in the world and faces serious air pollution problems. In this study, we analyzed the nitrogen stable isotope ratio of ammonium ions in PM₂.₅ in Dhaka and conducted source identification of these ammonium ions. Our results show that ammonium ions in PM₂.₅ come about half from agricultural sources (volatile components such as chemical fertilizers and livestock excreta) and half from non-agricultural sources. It is generally thought that ammonia mainly originates from agriculture, but our study shows significant variation depending on location. Ammonia emissions are expected to increase in the future, unlike many other air pollutants, making ammonia management an important international issue.

We started this research seven years ago through a personal contact, aiming to clarify the sources of severe air pollution in Dhaka. Since there was almost no prior research in this region, including setting up samplers, actual sampling, chemical analysis, and mixing model calculations, we faced many challenges, but we are relieved to have completed the paper.

Ms. Otoha Yoshida, who conducted the chemical analysis for this study, has published three international journal papers based on her graduation research. She has excellent experimental skills and data management abilities, and her Excel sheets are used as learning resources by her juniors.

This research was supported by the Comprehensive Environmental Research Promotion Fund, KAKENHI (Grant-in-Aid for Scientific Research B: Overseas Academic Survey), Nissay Foundation, Heiwa Nakajima Foundation, and the Akita Prefectural University President’s Project (Creative Research). Research assistants Raku Fujishima, Yuya Tsuda, Momoka Sudo, and Rui Kato also supported denitrifier management and chemical analysis.

Lastly, over the seven years of joint research with Bangladesh, Professor Salam and students at Dhaka University have always been very helpful, and we have built excellent relationships. Compared to my first visit seven years ago, Dhaka has greatly developed, with increased traffic, improved roads, and widespread use of smartphone apps, all surrounded by youthful energy and enthusiasm rarely seen in Japan. We hope to continue and expand this collaborative research in the future.

★ Comment from Professor Abdus Salam (Dhaka University)

Ms. Yoshida working at the Center for Environmental Science and Technology

Professor Salam and the sampler 

Cityscape of Dhaka 

Visit to the Independence Monument

Dinner with Professor Salam’s Family