Seasonal trends of the stable nitrogen isotope ratio in particulate nitrogen compounds and their gaseous precursors in Akita, Japan

Abstract

Particulate matter (PM) can have adverse effects on human health. Moreover, because the mechanisms of PM formation and behavior in the atmosphere are notably complicated, to reduce PM concentrations effectively and meet environmental standards, source–receptor relationships must be clearly understood. Stable isotope ratios can be used to detect chemical processes and distinguish sources. In environmental science, especially in research on aerosols, stable isotope ratios have proven to constitute a powerful tool for source identification. However, there are few long-term studies of isotope fractionation during secondary aerosol formation. In this study, stable nitrogen isotope ratios (δ¹⁵N) of ammonia gas (NH₃), nitrogen dioxide gas (NO₂), nitric acid vapor (HNO₃), particulate nitrate (NO₃⁻), and ammonium (NH₄⁺) in suspended PM (SPM) were analyzed to investigate seasonal trends and isotope fractionation during aerosol formation for long term sampling in Akita, Japan. The results indicated that δ¹⁵N-NH₄⁺ in SPM and δ¹⁵N-NH₃ gas ranged from 1.3‰ to 38.5‰ (mean 16.1‰) and from −33.6‰ to −0.0‰ (−16.9‰), respectively. Furthermore, δ¹⁵N-NO₃⁻ (SPM) and δ¹⁵N-NO₂ and δ¹⁵N-HNO₃ (gaseous) ranged from −4.6‰ to 4.8‰ (mean −0.5‰), from −8.2‰ to −3.1‰ (−5.4‰), and from −7.5‰ to 2.7‰ (−5.0‰), respectively. The mean annual isotope fractionation factors for transformations from gaseous NH₃ to NH₄⁺ in SPM, from gaseous NO₂ to gaseous HNO₃, and from HNO₃ gas to NO₃^– in SPM in the atmospheric environment were +33.3‰, +0.5‰, and +4.9‰, respectively. Isotope fractionation of NH₄⁺ in SPM was much higher than that of NO₃^– in SPM. As the chemical reaction from gaseous precursors progressed, δ¹⁵N-NO₃^– in SPM became steadily heavier.

Keywords:

• stable nitrogen isotope ratio
• NH3
• NO2
• HNO3

Acknowledgements

Takahiro Kurahashi and Yuya Haneishi at Akita Prefectural University contributed greatly, including conducting sampling and participating in discussions related to this research. Dr. Sakata Masahiro of the University of Shizuoka helped with the distillation of ammonia and nitrate in particulate matter. The author offers sincere thanks to all of these people.

Additional information

Funding

This study was supported by the Nippon Life Insurance Foundation, the Tostem Foundation for Construction Materials Industry Promotion, the Japan Securities Scholarship Foundation, and a Grant-in-Aid for Scientific Research on Innovative Areas, No. 23120704, a Grant-in-Aid for Scientific Research (B) No. 15H05121 from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and the Environment Research and Technology Development Fund (5RF-1401) of the Environmental Restoration and Conservation Agency of Japan.