Inorganic ion and nitrogen isotopic compositions of atmospheric aerosols at Yurihonjo, Japan: Implications for nitrogen sources

We studied the suspended particulate matter (SPM) collected in Akita Prefecture, Japan from April 2008 to January 2009 for inorganic ion composition and nitrogen isotopic ratio (δ15N) of NH4+ and NO3−. The results showed an average SPM concentration of 15.6 μg m−3. The seasonal trend for SPM was higher values in the spring, lower in the winter. The major cations were Na+, NH4+, Ca2+ and major anions were SO42−, NO3−, Cl−. The annual correlation coefficient of ions indicates a very high value with NH4+ and SO42− (R = 0.93), NO3− and K+ (R = 0.65), NO3− and Ca2+ (R = 0.62). The high springtime values are the apparent result of the dust stream from Asia. Average δ15N–NH4+ and δ15N–NO3− were 16.1‰ and −0.69‰, respectively. δ15N–NH4+ increased slightly in summer, and δ15N–NO3− increased considerably in winter. The trends indicated conversely. The heavy δ15N–NH4+ in summer appears to be from agricultural sources such as animal waste and fertilizer. In addition, according to the difference in isotopes of NOx sources as the precursor of NO3−, the dominant origin of heavy δ15N–NO3− in winter could be NOx emitted from fossil fuel combustion at low temperature. Moreover, the average δ15N–NO3− seemed to be made to baseline (approximately 0%) by vehicle emissions at high temperature. These results are considered to be very reasonable.