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.